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Suzuki DR450 Alpina

Suzuki DR450 Alpina - Project Plan                          Suzuki DR450 Alpina - Project Log


Suzuki DR450 Alpina - Project Plan 

The thought here is that a highly desirable but essentially lost conceptual theme for off road motorcycles is the Bultaco Alpina style of motorcycle. The Bultaco Alpina was essentially a trials motorcycle with a higher volume fuel tank, an “all day” sit down seat, lights, instrumentation and gearing suited to wide range of trail and road conditions – all in all an excellent platform for a trail bike. I use the term trail bike here to describe a vehicle suited for technically challenging terrain. By that I mean tight single track, roots, rocks, climbs with minimal approach run. Think of a tight “switchbacky” mountain trail. More of a platform for exploring the neighborhood rather than blitzing through it. 

Since the time of the Bultaco Alpina, dirt bikes have evolved down two essentially divergent evolutionary pathways. Currently, most versions of the trail bike have evolved as a derivative of the motocross platform rather than from the trials platform. This is understandable as the trials bike has evolve into a highly specialized device, characterized by miniscule fuel tanks and essentially non existent seats. While unsurpassed for dealing with highly technical terrain, they are essentially closed couse, limited “seat” time rides. Although the basic chassis geometry and engine characteristics are highly suitable for technical terrain, the whole trials package is really an unsuitable mount for most riders interested in all day exploration rides. not withstanding those riders who are quite capable and satisfied with a trials bike as their primary trail bike. There are a few exceptions, such as the Beta Alps and the Gas Gas Pamperas. The Scorpa T-Ride looks promising as well, but the reality is that most manufacturers offerings to fill the trailbike niche are essentially slightly reconfigured motocross platforms  - designed primarily to be competent at race speed in off road terrain. There are, of course, minor variations within that theme, a little bit of engine and gearbox ratio jiggling here, a bit of suspension jiggling there, but the basic conceptual design essentially remains optimized for going fast, not going slow. There is no question that given the choice of these two rather disparate platforms the motocross derivative is far more suitable for general trail riding than a trails bike. Exceed the speed limit on a trials bike and you may go ass over tea kettle. Go too slow on a motocross bike and you will probably just tip over. 

So, the goal here is to develop platforms which show promise as trail bikes along the pathway illuminated by the Bultaco Alpina. Development of engine characteristics, gearbox spreads, chassis geometry, appropriate suspension travel and compliance, long range fuel capacity - all combined with reasonable riding position ergonomics. 

One of the divisive issues in the choice of platform is the issue of water cooled or air cooled engines. There is no question that water cooling is superior for maintaining higher power outputs, more consistent power output over a range of ambient conditions and riding situations, and improved engine longevity. There is also no question that the water cooling system can severely compromise the fuel capacity potential of the motorcycle if you want to maintain a reasonable ergonomic riding position. It is also hard to argue with the idea that you can’t suffer a ride ending failure from a component that isn’t on the bike to begin with. In any case, this is an issue with clear benefits and liabilities for both options so our solution is to develop solutions for both platforms that capitalize on the advantages and mitigate the drawbacks of either system. 

We have chosen the Suzuki DR 350 as the platform for the air cooled version of the Alpina project. ( For you devotees of water cooling out there see our KTM RFS pages ) The Honda XR 400 might be good air cooled alternative, its chassis design and more sophisticated suspension are superior to the stock DR. However, it lacks electric start and has a five speed gearbox with a narrower overall spread than does the DR. We have deemed electric start as an essential feature, ( bad knees ). There are some smaller displacement e-start bikes that might make a good platform as well but our design parameters include very high altitude off road operation as well as maintaining adequate pace with highway traffic flow for dual sporting. The characteristic most likely to satisfy this muti purpose requirement is sufficient power to pull the gearing that allows both maintaining that highway pace and providing acceptable performance off road. In our opinion, these parameters demand a motorcycle engine of 350cc or larger. Our experience is that a 350cc air cooled engine is acceptable, although barely adequate, especially on the highway where inadequate performance subjects you to being tailgated, an exceptionally unpleasant experience.  Fortunately the DR engine can be fairly easily built into a very reliable configuration at 450cc. This should both resolve the highway pace issue and also improve performance on higher altitude trails. The only other electric start air cooled trail bikes with adequate power displace 650cc, with the lightest in excess of 25 lbs heavier than the Suzuki DR. Although the larger displacement handily solves the highway speed issue, the weight penalty is deemed too severe a compromise for acceptable performance off road. 

The DR’s basic power characteristics and gearbox ratios are acceptable, although the power is a bit under whelming for both highway and high altitude off road work. As mentioned earlier, it is fairly easy to upgrade the power. Although the overall gearbox ratio spread is fairly wide, the bike could make good use of a spread that is even wider yet. This is not easily accomplished. There is an indication in some source literature, which we haven’t yet verified, that the sibling DR250S may have a taller internal 6th gear pairing. This would provide some improvement, although fairly insignificant. The only remaining option is to investigate the feasibility of have custom internal gear sets fabricated. This is not uncommon in the motorcycle aftermarket world. It is assuredly an expensive proposition, but possibly an affordable option if there is a sufficient number of takers from the huge contingent of DR owners out there. 

The Chassis

The DR’s most glaring deficiencies concern the overall weight, the basic chassis design and the quality of suspension performance. Other than some minor tinkering, not much can be done about the weight unless the frame, suspension and wheel assemblies are replaced with alternate bits. Good chassis design and suspension performance can mitigate how that weight affects the riding feel. The goal of this project is to make the chassis perform better on the types of trails described in the opening paragraph. The DR’s chassis is relatively “lazy” compared to the most agile bikes available. The last generation Honda XR 250 ( starting in 1996 ) is probably the best example of the design bias we are looking for. The XR pushes the envelope towards the Alpina end of the trailbike curve, steep steering head, short trail, short swingarm and wheelbase, moderate suspension travel. The critical steering specs are 92mm of trail and head rake set at 24º. These numbers are a huge departure from the numbers typically found in the trail bike class. The swingarm length. at 20.5” is moderately shorter than the norm. This, combined with the tight steering head and appx. 10” suspension travel, yields a relatively short wheelbase of 55.1”. If that XR chassis came with a 400 - 500cc e-start engine we might not even feel compelled to chop up a perfectly fine DR. ( not really, this is a lot more fun ). An interesting and welcome discovery, even if very surprising, is that the distance from the swingarm pivot to the steering head centerline is significantly shorter on the DR than on the XR. This might imply that a reconfigured DR has the potential to be an even more compact chassis package than the XR - encouraging


Reconfiguring the DR towards the Alpina end of the spectrum involves these areas of design focus.

1)  alter the steering geometry to make the bike easier to maneuver on tight, technical terrain

2)  shorten the wheelbase for the same purpose

3)  shorten the swingarm to compliment the goals of #1 and  #3

4)  move the footpegs rearward to make it easier to lift the front wheel over obstacles

5)  shorten the suspension travel to make it easier to flat foot on the ground

6)  reconfigure the subframe to compliment the new suspension specs, and the goal of #5

7)  improve all facets of the suspension to achieve both compliance over obstacles at low speed and  resistance to loss of control at high speeds


1) Steering geometry may be the chassis characteristic most difficult to alter significantly. Rake and trail are essentially dictated by the design of two components; rake, which is essentially set the by the steering head’s angle of attachment to the frame and trail, which is essentially set by the combined offset of the fork triple clamps ( steering stem centerline to fork tube centerline ) and the offset of the front axle ( from front axle centerline to fork tube centerline ). The “as built” geometry numbers can be altered in a number of minor ways with adjustments to the moveable bits of the chassis, but those adjustments typically result in compromises to other handling characteristics, not necessarily desirable.

The most effective, although most radical, solution to changing the rake angle is to cut the steering head off of the frame and reattach it in a new position. That the DR’s backbone is also the engine oil reservoir severely aggravates the degree of difficulty in this approach. Depending on the specifics of a particular frame’s construction, even fabricators highly experienced with motorcycle frame modifications can have difficulty doing this type of alteration without experiencing oil leaks in the finished product.

An alternative is to reset the fork steering stem on a different angle than frame’s steering head. This can be accomplished in two ways, both of which involve extensive fabrication. The first is to incorporate intermediate cups between steering head bearing races and steering head which are adjustable in a manner that permits the fork steering stem to be set in a position not parallel to the frame’s steering head, effectively changing the effective angle of the forks relative to the frame. This allows the effective steering head angle to be steeper than stock, promoting quicker steering, partially from the resulting shorter wheelbase but mostly from the steeper steering angle. However, unless an off the shelf bearing set happens to fit the confines of the intermediate cups and steering stem diameter this solution probably requires fabricating and attaching larger head stock bearing race seats and or fabricating a different steering stem. Another method is to fabricate a new set of triple clamps that locate the fork tubes at a steeper angle than the steering stem.

Given that all of the methods to steepen the effective steering head angle involve difficult fabrication, adjusting the trail would appear to be a comparatively simpler pathway. This can be done in two ways or via a combination of both.

a)  change the offset of the fork tube bore centerline from the steering stem bore centerline. This typically involves purchasing or fabricating a new set of triple clamps To my knowledge, no aftermarket clamps specific to the DR’s forks are available. Perhaps there is an O.E.M. set or an aftermarket set that is adaptable, would likely take considerable research to track down a suitable set of clamps. An alternative is adapting a different set of forks, an exercise typically frought with its own set of complications

b)  change the offset of the axle bore on the fork sliders relative to the fork tube centerline. This is only feasible if the fork slider axle clamping bosses are sufficiently robust to accommodate a boring operation to remove sufficient material to permit fitting of an eccentric bushing set intermediate between axle and slider boss. With the stock DR fork assembly, this is highly unlikely. The commercial versions of the eccentric axle assembly are only available for USD forks, which otherwise have drawbacks for this application.


2) shorten the wheelbase. Numerous methods, separate or in combination

a)  a minor change can be accomplished here by keeping the rear wheel forward in the adjustment slots of the swingarm.

b)  minor change by sliding the fork tubes up through the triple clamps

c)  shorten the swingarm length ( moderate to extensive fabrication, dependent on swingarm design )

d)  reduce the triple clamp offset ( however, this alters trail in the wrong direction for our purposes )

e)  steepen the steering head angle ( extensive fabrication frought with perils )

f)  move the steering head back on the top tube towards the swingarm pivot ( see “e”, only more so )


3) Shortening the swingarm accomplishes two desirable things. It contributes to a shorter wheelbase, which generally accentuates manuverability in tight terrain. It also shifts the polar moment toward the rear of the bike which makes it easier to loft the front end. This change does reduce weighting on the front wheel’s contact patch, theoretically making it easier to wash out the front wheel when cornering. This compromise is most likely to be discovered when going fast and exploring the limits of front tire adhesion, a risk we are willing to take considering our performance goals. 

4)  Moving the footpegs rearward makes it easier to lift the front wheel over obstacles. This should be relatively simple project. Most trials bike design positions the footpegs behind the swingarm pivot. This alters the peg to shift lever and peg to brake pedal relationships. Need to assess severity of changes and difficulty in restoring acceptable peg to control relationships. Unknown yet how this affects the feel when riding in the standing position. 

5) In stock configuration the DR nominally sports 11+ inches of suspension travel. Stock spring rates are such that the DR settles into the suspension quite a ways so the effective travel is somewhat less and it is not too difficult to foot when necessary. The spring rates can be increased to keep the chassis up on the suspension, which would be beneficial if the DR was being used for high speed work where adsorbing impact from obstacle sat high speed is a useful characteristic. At high speed, putting your feet down is not a wise thing to do so as long as your feet reach the pegs, all is good. However, since the preferred design is biased toward slower trail speeds and negotiating obstacles, dabbing occasionally and/or deliberately is part of the game. In our opinion, suspension travel in the range of 8 – 10” will be entirely adequate and should yield significant improvement in the ability to foot for stability. The loss of ground clearance is not desirable but would be mitigated somewhat by stiffening the spring rates. The chassis will sit higher on the suspension and will also be better suite for those occasional bursts of speed across more open terrain. We are making a deliberate call here that, for our specialized application, shorter travel, if correctly sprung and dampened is superior to the long travel, too softly sprung suspension of the stock bike or even long travel when correctly sprung. The tricky part is choosing spring rates/damping characteristics that both keep the chassis up in the travel but is still allow exceptional compliance for low speed work over trail obstacles such as rocks and roots.

Ground clearance. Trials bikes typically combine both very high ground clearance and suspension travel in the neighborhood of 6-7’ or thereabouts. Achieving this particular combination of chassis traits compromise is really only available during original chassis design process. Essentially the steering head would be located lower on the main frame cradle and rear suspension geometry is designed to be complimentary. If one were to bite the bullet and relocated the DR’s steering head to adjust the rake angle, it would be relatively simple matter, within limits,  to lower the steering head at the same time. Barring that major surgery though means that we live with the reduced ground clearance in exchange for the shorter suspension travel and its attendant benefits.

Reducing wheel travel is generally simple in principal but variable depending on the actual mechanical components in the front forks and rear shock. The usual method is to reduce the stoke length of the front fork and rear shock by inserting travel limiting spacers or shortening internal components that determine stroke length. If one is lucky then an off the shelf fork spring of suitable length and rate will fit the altered space available. Alternately the stock springs can be shortened, although this operation increases the spring rate which may are not be compatible with the design goals for the finished bike. With a linkage rear suspension design, minor modifications in shock stroke yield proportionately larger changes  in wheel travel. Consequently, it might be possible to retain correct spring preload with the existing spring by repositioning the shock’s preload adjuster rings. This of course presumes that the spring rate is suitable. If not, it shouldn’t be difficult to acquire an alternative spring, for both length and rate. 

On this project we might go a step more complicated. There is much to recommend a rear suspension design that forgoes a linkage, less weight and far simpler maintenance. Somewhat more difficult to select a suitable spring rate since we are starting from scratch. This is complicated by the choice of angle at which the shock resides. If too shallow an angle, then the effective spring rate falls as the suspension progresses through the range of travel. Generally this is not a desirable characteristic since one is usually traveling at a pretty good clip to be working all of the way through suspension travel. Typically you would want the suspension to become stiffer at the extremes of travel to prevent hard bottoming and unsettling of the chassis. A sophisticated shock design can mitigate this with speed and/or travel sensitive damping changes but the role typically falls to the spring. This falling rate characteristic is strictly true only when a spring of straight rate is used. A spring of progressive rate will give the desired progressive stiffening as travel increases. 

6) reconfigure the subframe. Depending on the method of execution, the reduction in suspension travel may yield unnecessary clearance between the rear tire and inner fender surface at full compression. It would be beneficial if the subframe was reconfigured to eliminate some of the excess. This might also yield a seat at lower height and of flatter profile, both desirable changes. While we are at the reconfiguration will change subframe mounting from weld on to bolt on. 

7) Suspension performance is perhaps the simplest performance goal to describe but perhaps the most difficult to achieve. The goal, is to have suspension which is exceptionally compliant over any trail obstacles but still resistant to bottoming  and chassis upset when pushed to the limit in faster off road terrain. It would seem that the only way to achieve this is to choose an initial spring rate for compliance and then adjust the damping characteristics to add resistance to compliment the spring at the longer end of the suspension travel. I can’t think of any advantage to having the suspension sag into the travel range on too soft springs, seems a waste of travel. Need to research the principal behind suspension sag settings. Suspension performance is, in many regards, a highly knowledge and tool intensive endeavor. We may break down and send the suspension bits off to someone with both.


 The Engine

For our application, we want the engine to pull from the very lowest of rpm and with enough flywheel to resist stalling at low trail speeds over obstacles at low throttle openings. We have come to believe that the most effective way to improve the power characteristics of the DR engine is to increase the displacement by about 100cc with a larger cylinder bore. By all accounts, this significant change to displacement is free of any reliability issues if one refrains from trying to build a high horsepower engine on top of the displacement increase. Cool !! We aren’t after high horsepower any way. Our design goal is a broad spread of torque with bias toward improving low rpm throttle response. Our only concern is that the increased displacement may overcome the flywheels inertial ability to keep the engine turning over at the lowest of rpm - more prone to stalling at the lowest of engine speeds.

Generally, higher compression ratios boost low rpm torque. The trade off is that typically a higher octane fuel is required to prevent detonation and subsequent engine damage. That same higher compression ration should, in theory fight the piston as it rise towards TDC, promoting engine stalling. Unknown whether this a significant issue or not.

It would be very interesting to play with heavier flywheel to see what benefits might be available. Generally engines intended for slow sped operation benefit from changes here. Unfortunately, there are no commercially available aftermarket flywheels available, so custom would be the only option – expensive surely, may require a modified magneto cover as well.

Cylinder Head. Generally engines intended for low speed, high torque performance benefit from small valve sizes and related port cross sections.. I suspect the cylinder head from the sibling DR250, which is equipped with smaller valves, will fit the stud pattern of the DR350 crankcases, Remaining task would be to see if the resultant compression ratio  with the mm piston is excessively high. Keping imn mind that a higher than stock CR may yields low rpm torque gains, it may be possible to remove sufficient material from the DR250 head to bring the CR into an acceptable range.

Crankshaft. I suspect the crankshaft and connecting rod are adequate for this application. The only obvious deficiency is that the O.E.M. connecting rod small end does not utilize a bushing, the piston pin rides directly the small end of the rod. This is mostly an offense from an engineering POV, I doubt there are any serious  functional compromise for the majority of expected usage scenarios. The only solution is probably a Carillo rod, a very nice piece but also an expensive proposition to implement.

More Displacement.


Carburetion. The other significant player in throttle response is the carburetor. The DRS and DRES models are equipped with a constant velocity or CV type carb. The main advantage of this type for the rider covering a wide range of usage is that it does somewhat mitigate the effects of altitude increase on engine performance. The CV carb slide is raised only indirectly by the throttle. The throttle is mechanically linked only to the butterfly valve which opens the engine to air flow. The slide responds to the differential between the engine vacuum and ambient air pressure. Since this prevents the operator from lean bogging the engine by cracking the throttle, the carb can be jetted overall leaner than a conventional slide carb so the jetting may already be more suitable for higher altitude operation as  altitude increases. As altitude increases the CV carb reacts as if the engine were smaller in displacement, which, in effect it is by virtue of the altitude related power loss. The CV carb’s reduced fuel delivery coincides with the engine’s reduced fuel need – very handy coincidence. That the CV is generally more sharply attuned to actual engine requirements also improves fuel consumption characteristics. The major performance drawback of the CV is that its throttle response is decidedly inferior to a conventional slide carburator equipped with an accelerator pump. This feature can be adjusted to completely eliminate the lean bog of a carb without an AP. With the AP Adjusted correctly, the engine is very responsive to throttle input and so it is much easier to loft the front wheel on short notice – a very handy attribute in obstacle strewn terrain. Hamfistedness, however, can suck fuel at a disturbing rate. Unlike the CV, the slide carb has no intrinsic characteristic that favors high altitude operation so the trick here is understanding which jetting works best in which anticipated altitude ranges and prejet to match. Since WOT would be just as mall part of the operating range in our application, it may be possible to jet the carburetor generally lean and rely on the AP squirt for the demanding parts of the ride. Our operating parameters may also benefit from a smaller carb throat diameter. The pumper carb on a DR350 is a Mikuni TM33mm. The carb from a DR250 is TM31mm. Interesting possibilities here.

We will be using an air fuel ratio meter to dial in the jetting on our carb experiments.

Elimination of altitude jetting issues is perhaps the most appealing characteristic of fuel injection. Done correctly, this essentially solves this issue, not to mention that it probably optimizes fuel consumption under all operating scenarios. The power output may still be punk but at least the engine runs to its potential, however restricted that is.


Exhaust. For our application, a header pipe on the smaller diameter end of the range is probably desirable. The DR350 O.E.M. piece should be fine, but should determine if a DR250 header is a smaller diameter yet. The O.E.M. muffler is very quiet and very restrictive. There are some modifications to the muffler that apparently significantly improve flow with an inconsequential increase in decibel level. Especially with the planned increase in displacement, I suspect these mods will be worthwhile.

Oil Cooler. There is some debate on the value of oil cooling for this engine. The cylinder and head have relatively abbreviated finning. I suspect that during the normal ranges of expected operation, the cooling of the stock system is probably adequate. Both high horsepower, high load and low speed, high load would probably benefit from supplemental cooling. Although not nearly as effective at controlling engine temperature, an oil cooler will not have near the negative impact on riding ergonomics as does water cooling. The oil cooler will mount on the frame front down tube below the fuel tank. It is somewhat exposed to damage but careful design and fabrication of mounts and guards should largely mitigate the likelihood of damage. We will incorporate an oil temperature sender in the oil line junction block. Oil tempo info will be displayed on the Trail Tech Vapor computer. The cooler will be backed by a pair of 12V computer fans to augment cooling at low speeds. Ideally fan operation would be auto. controlled by relay tied into Vapor oil temp sensor circuit. Should speak with Trail Tech about if possible/how to. Need to design a QD bypass in case of cooler damage/leak on trail, possible at the bypass valve ports.



Suzuki DR450 Alpina - Project Log


The goal of this project is to gradually modify the base 1994 Suzuki DR 350SER into an all-road motorcycle that is comfortable and capable on pathways from unpaved highway to single track mountain trail, and in any kind of terrain, from desert to dense forest. The primary appeal of the DR 350SE street/trail model over the DR 350L off-road only model is the electric start found only on the SE. We deem this an essential feature for a comprehensive all-road motorcycle. However, since the goal of this project is a more suitable blend of on and off road capability than that provided by the SE model, we also purchased a basket case 1990 Suzuki DR 350L off-road model to provide an inexpensive source of more dirt oriented components to test on the base motorcycle as well as spares that can be modified while still retaining the original components as control pieces. 

Well, this is how a 1994 Suzuki DR 350SER came from the factory. The guy I purchased mine from pretty much stuck to street riding so it wasn't too badly buggered, just a few small problems due to neglect. Fortunately, he had already stripped off some of the garish graphics.



Hey Mikey, Let’s Make It a Real Dirt Bike !! 


To shift the bike towards a more off-road bias, a number of components were removed and some were replaced with items more suitable to an off-road orientation. The following items were removed or replaced:

- removed the passenger pegs. This is, after all, a solo ride.

- removed the side stand interlock switch. One less thing to fail at an inopportune time.

- removed the turn signal assemblies. Turn signal assemblies are very vulnerable to damage when off-roading and are not required for operating a motorcycle on public roads. Signaling turns by hand is a legal alternative, but you have to be exceptionally aware that other driver’s understand your intent.

-replaced the humongous OEM tail light assembly with a much lighter and much more compact unit modeled on the same pattern as a mid-70s Yamaha TY250



- replaced the two OEM mirrors with a single Acerbis folding mirror mounted on the clutch side

- replaced the SE tach/speedo instrument cluster with the much simpler and more compact trip odometer from the 1990 DR 350 dirt model


Acerbis Mirror  Up.JPG (1243340 bytes)

Acerbis Mirror Folded.JPG (1430593 bytes)

- replaced the rubber mounted footpeg assemblies of the SE model with the rigid mounted assemblies of the S model. At the loss of some comfort, this should provide more direct connection/feedback when using footpeg weighting to maneuver the bike

- replaced the OEM 9 L ( 2.4 gal ) steel fuel tank with an Acerbis 16 L ( 4.25 gal ) plastic fuel tank. Replacement tank is lighter, higher volume, and much less subject to damage from a crash.

- replaced the OEM tires with Pirelli MT 21. The original tires performed very poorly in the dirt.


image of the OEM rubber


Hole in the Rock, Utah 

Spring 2005 field test. Location for this multi-day ride was in southwest Utah, in the triangle of land bound by the Colorado and San Juan rivers, southwest of Hall’s Landing on Lake Powell. The central attraction of the area is the Mormon built Hole in the Rock Trail. The terrain is primarily bare, windswept sandstone interspersed with patches of sand, and water cut canyons with sand floors. Altitude is approximately 4,500 feet, plus or minus a few hundred feet. Ambient temperature was typically in the 60 to 80°F range. 

Riding was generally single track or open cross-country with the occasional sand wash although some days included a considerable portion of high-speed fire road to get to the more technical terrain. 

Previous riding experience in this terrain, lots of low to moderate speed over choppy, rock infested trail indicated that exceptionally low gearing and soft, compliant suspension would be desirable for most of the riding. Changes made in preparation for this field test.

- gearing selection was 13/50, a ratio of 3.85, approximately 13% percent lower than the stock 14/47, 3.36, of the off road L model and 30% lower than the 15/41, 2.733, of the streetable SER model.

- front fork springs and rear shock spring both came from the SER is model. Even though stiffer than the comparable pieces on the DR 350, they still seem to border on the soft side so were left in place

-  wings fabricated from aluminum plate were welded to the OEM tubular engine guard to protect both the oil filter housing and the magneto cover from impact with rocks. Serious damage to either could be a ride ending event.


Hole in the Rock Field Test Summary 

Generally speaking, the modified DR performed adequately but not spectacularly in any of the riding situations. Relatively heavy and underpowered, it was decidedly a handful in deep sand washes. The very low gearing was advantageous on steep, low speed uphills and downhills. In retrospect, the full knobby pattern and relatively hard rubber compound of the Pirelli MT 21's was not the ideal tire choice. Although the sandstone rock surface provides a very high traction surface, there were numerous instances when the MT 21s seemed to lose grip, especially on steeper climbs and over oddly shaped rocks. I strongly suspect that a set of soft, sticky, compliant observed trials tires would be more suitable in most of the terrain situations although probably providing inferior performance in deep sand, a relatively small portion of the total riding experience. 

A performance issue that appeared in more technical maneuvering was engine throttle response. At low speed, abrupt changes in the terrain such as obstacles or direction changes in the middle of a steep climb often require immediate engine response from very low engine speeds to lift, or it least lighten, the front wheel. The SER model is equipped with a constant vacuum ( CV ) carburetor. Throttle response is not one of it's noteworthy characteristics. This lack can be mitigated somewhat if you have enough forewarning to roll on the throttle smoothly, but this is often not possible given abrupt changes in terrain. The L model is fitted with a carburetor equipped with an accelerator pump. This typically provides excellent throttle response and would be more suitable in technical, low speed terrain. Definitely a worthwhile modification to make for a repeat visit to this type of terrain. 

Another performance issue appeared in deep sand. Although the basic chassis geometry, being relatively lazy, seemed okay in that the bike track fairly well once up to speed. However, the heavy weight of the DR and the relatively low power output seemed to cause some problems. If you lost momentum in the sand, such as slowing for turns or obstacles, it was difficult to get back up on plane so the bike tended to plow through the sand when reaccelerating rather than getting the front wheel back up on and floating over the sand. Reducing the DR's weight significantly is an exceptionally difficult/expensive task, which leaves increasing the power as the reasonable alternative. There are a variety of available approaches to this problem but for the riding situations we envision, the most suitable solution is probably an increase in displacement through the simplest method, a larger piston diameter. 

One unexpected, but useful discovery, appeared during a night riding stint. All of the other motorcycles on that ride were equipped with dirt model headlights, of significantly lower lumen output than that of the DR street/trail model. The DR was the only bike that illuminated the terrain well enough to ride at a reasonable clip. In fact, the other bikes could only be ridden at comparable speed by using the DR's illumination field. From this, I have decided that a headlight of lesser lumen output and/or inferior pattern than the DR's is not an acceptable replacement even though there can be a significant weight savings had with such a change. 

Another issue that arose that was mostly comfort oriented but also quite influential on the ease and effectiveness of maneuvering the bike. A significant contribution to controlling a motorcycle is made by selectively applying shifting body weight through the footpegs, especially when a standing position. At times this can provide the majority of input for direction or attitude changes and the more complex the terrain, the more important is the use of the footpeg weighting.  The DR footpeg platforms are relatively narrow, from inside to out, and reach only about halfway across the soles of the boots, not very comprehensive support. When riding in a straight line over mild terrain, standing up and centered over the bike, my feet tended to roll off of the ends of the footpegs which put an excessive side load on my knee joints. At the end of a riding session, both knees were sore, especially the one with damaged cartilage. When trying to shift body weight for maneuvering in more technical terrain, much of the weight shift through the foot pegs essentially transferred into open-air off the end of the pegs. This reduced the effectiveness of the attempted weight transfer and also aggravated the knee pain. The soles of my boots had deep indents partway across where my body weight was concentrated at the ends of the foot pegs, evidence of very poor weight distribution across the entire footpeg platform. A solution to this problem has a very high priority on the list of modifications to make to the DR. 



Baja California, Mexico 


Autumn 2006 field test. Location for this multi-day ride was Baja California, Mexico. The riding area was within a 100 mile radius of San Felipe, a coastal town on the Sea of Cortez. The terrain is primarily characterized by sand, lots and lots of sand. Sand washes, sand whoops, sandy trails, sandy two-track, sandy dry lakes, sandy roads, and beaches which are, of course, sand. Oh, I forgot - sandy uphills and down hills. Not all of the sand is soft as you typically think of sand. Much of the loose sand is interspersed with hard pack soil or has a healthy dose of embedded rocks. Sometimes the trails are littered with rocks. Getting to the sand from camp or from patch of sand to patch of sand requires riding on pavement. Although the riding is occasionally a relatively low speed such as tight trails or narrow meandering sand washes, the vast majority is relatively high-speed on roads, two track, cross country or the beach. The backcountry is comprised of a huge network of crisscrossing pathways and “roads”, very few of which come with any signage. Combine this with the relatively flat terrain and no available maps and it is very easy to lose your way. Other than knowing that the Sea of Cortez lies to the east and the mountains lie to the west you're pretty much bereft of reference points. So, getting lost is easy and finding water in the outback is exceptionally unlikely. Ambient temperatures ranged from the 60s to mid-80s Fahrenheit. 

Consequently, modifications to the DR in preparation for this trip were biased towards high-speed riding, extended time up on the foot pegs, navigation, and reliability. In anticipation of this field test a basket case 1994 Suzuki RMX 250 was purchased to provide an inexpensive source of alternative components more suitable than those found on the DR. Research indicated that the RMXs suspension components were adaptable to the DR and vastly superior for high-speed riding. Ultimately, it turned out that the adaptations would require compromises that were unacceptable for this application. However, the following changes were made in preparation for the Baja field test.

- replaced the DR foot pegs with the wider platforms of the RMX footpegs

- painted the front fender flat black, an old trick to prevent headlamp light from reflecting back off of the fender and interfering with your night riding vision

- replaced the SER model’s  rear wheel assembly with that from the L model. The replacement is lighter in weight, not having the cush drive of the SE, and coincidentally was already fitted with more suitable rear tire, a Metzler 4.50 x 18, having an aggressive knobby pattern and noted for excellent wear characteristics. The S model wheel also came equipped with a more suitable 47 tooth sprocket.

- installed Moose ultra heavy-duty inner tubes in both wheels since cactus thorn punctures seemed a reasonable, if not likely, possibility. An improvement in reliability was deemed an acceptable trade for the much heavier Moose tubes, a change otherwise in conflict with the design goal of reducing the bike’s weight.

-  replaced the SER’s rear shock with that from the L model which has both adjustable rebound compression damping, which allowed for a wider range of adjustment suit the conditions. Retained the SER’s shock spring since it is a stiffer rate than that of the L model

- fitted the Acerbis front disc brake guard from the RMX to protect the front brake rotor from rock damage. Expected that the high-speed riding would result in a lot of hidden surprises popping up with little time for avoidance maneuvers. A bent front rotor could ruin the entire trip. As it turned out, the inside diameter of the DR's hollow front axle is the correct size for a 7mm x 1.0 tap, so it was relatively easy to adapt the guard to the DR’s front fork.

- replaced the original rubber brake lines with braided stainless steel brake lines front and rear for two reasons. I have reasonable expectations of extended high speed riding with frequent and severe use of the brakes. Stainless steel lines are noted for improving consistency of brake performance, brake feel, and resistance to heat induced performance degradation. In addition they are much less likely to be damaged in a crash or by being snagged by trail obstacles. 

- replaced the minimalist OEM hand guards with Acerbis Rally Brush hand guards, which are more robust and utilize a much more secure mounting system. Had a reasonable expectation of encountering lots of really sharp pokey things in Baja and wasn't too keen on the idea of cactus thorns protruding from the backs of my hands

- fabricated a mount and fitted a Garmin Etrex GPS unit to the DR’s cockpit

- fabricated a tubular framework that attached to the front fork triple clamps and surrounded the odometer assembly and the GPS unit in a protective cage. Since getting lost in the Baja outback is very likely to have an unhappy ending, protection the navigation unit from damage in the event of a crash was high on my priority list


Baja Field Test Summary 

Generally speaking, the modified DR performed adequately but not spectacularly in any of the riding situations. However, in one specific situation, the DR was so unsuitable it almost died an untimely death.

- Cruising along at moderate pace in relatively sedate terrain, the DR was very comfortable for very long days in the saddle.

- As in the Utah field test, deep sand was the most challenging for the DR and for precisely the same reasons. An added wrinkle on this trip though was that we rode a portion of the Baja 1000 race course, used just a day or two prior to our ride. Part of our ride included absolutely the most brutal whoop sections I have ever encountered. The DR's suspension, stiffened to the maximum available with DR components was thoroughly overwhelmed by this terrain. Both front and rear suspension bottomed out repeatedly and the bike wollowed severely. I could not maintain high enough control to ride through the whoops fast enough to be smooth, so ended up riding up and down the faces and backsides of individual whoops. The high weight and low power output of the DR aggravated the difficulty of maintaining a suitable speed so, all in all, the whoops were an extended ordeal rather than being fun. One of my riding companions, who has years of expert level desert racing experience took the DR for a spin. He tried for the same speed through the whoops as his KTM520 MXC but he said the DR’s suspension blew through its travel so easily that he was afraid he was going to break the frame from casing the tops of the whoops. So, combine my inexperience with high-speed sand whoops and the bike's fundamental deficiencies, you get an exhausting slog rather than an entertaining blast of a riding experience - not cool.

The suspensions deficiencies were also apparent in the sections of hard packed, but still whooped terrain, in which the faces of the whoops were abrupt rather than rollers. This riding situation completely overwhelmed the DR's suspension, especially the front forks. They were unable to absorb the impacts with anything resembling smooth fluid control, even at moderate speed – an exhausting and, at times, painful experience.

- The extended range provided by the oversize Acerbis tank combined with the DR's low fuel consumption rate ( one of the few benefits of an engine that produces little power ) yielded a longer riding day than any of the other bikes in the group. I never had any concerns but being out of fuel and stranded.

- Fitting the DR with the wider foot pegs from the RMX made a huge difference in comfort and control in every riding situation.

- Okay, now for the near-death experience. On one of the warmest days, mid to upper 80s ambient, we were riding in a sand wash, a long steady moderate uphill pull. My riding companions were aboard a Honda CRF450X and a KTM 520 MXC, and I was trying to keep pace. I started to feel the engine struggling, demonstrably losing power, and I got the overwhelming feeling that the engine was about to melt - not good. I shut it down, listened to those little pinging noises an extremely hot engine makes as it cools and sat down in the shade to wait for my riding companions to return. When they got back, I told them I was probably done for the day and was just go slow and limp on back to port ( the campsite ). Fortunately, it was either downhill or flat ground all the way back to camp. When I got back to camp, I pulled the sparkplug – bone white, so I went up in main jet size above the sea level specification I was running. I didn't have a repeat of the near meltdown for the rest of the week but I also didn't find myself again in quite so demanding a riding situation. I suspect though that the lesson here is that a heavy motorcycle with a low powered air cooled engine is not suitable for demanding situations in the desert. It isn't just a matter of speed because I wasn't going that fast in the sand wash, in fact I couldn't go fast, which might have moved enough air to keep the engine cool. Either because the bike just had inadequate power to start with and couldn't get up on plane and adequate airflow or the situation so overloaded the engine that its power dropped off severely. In any case it had degenerated into a low-speed slog and was rapidly deteriorating.

Back from Baja, a little bruised but pretty much OK.




My conclusion though is that the DR is not really suitable for aggressive riding in the type of terrain we rode in Baja. Just moseying along at a leisurely to moderate pace would probably be fine and trouble-free. The weight of the bike, although undesirable, isn’t really a deal breaker. Lots of money might bring the suspension to a level that would handle both types of whoops, deep sand rollers and square edged hard pack. I am skeptical though that even additional power from the motor would solve the overheating issue in the sand washes. The DR cylinder and head are not generously finned and increased power, although easier to get the bike up on plane and into a faster fow of air past the engine, would likely aggravate the overheating issue in net. I have heard some anecdotal tales of frighteningly high oil temperatures in big bore Honda XR desert bikes. It is relatively easy to fit an oil cooler to the DR, which would probably help a great deal, but this involves an additional system which is subject to some of the same potential on-trail reliability issues as water radiators. Given this, as much as I prefer air cooled engines in off-road motorcycles, I think I have come to accept that water cooling provides a level of engine reliability and performance in the extremes of Baja riding that is perhaps essential for a trouble-free, no holds barred, riding experience.

Too bad, but so it goes. Besides, I have always been very impressed with KTM’s RFS series dirt bikes. My only serious objection to the design was the water cooling but I now see that as perhaps an essential feature for serious Baja adventuring, which I do really like a lot. So, I guess I'll add a new project to my list - a KTM Alpina, perhaps something around 550 cc or so.




Silverton, Colorado  

Summer 2007 field test.  Location for this multi-day ride was in the mountains around Silverton, Colorado. The terrain ranges in altitude from about 9000 feet to an excess of 12,000 feet and is laced with a network of steep mountain roads and trails, almost all dirt, frequently connected by high passes. Ambient temperatures range from the low 40s to near 80°F. An added element to this field test was the necessity of riding to and from Silverton from Durango, CO via a combination of high-speed paved roadway and dirt to track. Additionally, I had to carry the gear necessary to permit camping for three nights. Fortunately, I didn't have to pack food and cooking equipment, but that still left a lot of gear to pack.

Additional modifications to the engine and chassis of the DR for this particular field test weren’t deemed necessary. I did install the S model, with the cush drive wheel assembly and the Pirelli MT 21. This tire was more suitable for the combination of pavement and dirt riding in this field test. The rear sprocket at 43 teeth also provides more suitable gearing for this combination, especially the highway portion. I did take a selection of counter shaft sprockets with the intention of changing them to suit the riding plan for each day. Fortunately, changing the counter shaft sprocket on the DR is a very simple task.

I wasn't particularly concerned about the DR's suspension inadequacies on this field test. This was to be primarily a solo ride so would be easier to match the exuberance of my riding to the limits of the DR that it is when riding with a group. If anything, the DR’s suspension would be near ideal for the leisurely exploratory pace I was anticipating. Riding solo in such isolated terrain encourages staying well within your limits since rescue in the case of a serious riding mishap be more a matter of luck than anything else.

As mentioned, the equipment challenge for this field test and ultimately for the suitability of the DR as a long-range, multi-day, all-road mount is the ability to carry the necessary camping gear while not seriously compromising the DR's suitability as a trail bike. There are numerous “luggage” carrying solutions in the motorcycle world. Most of them involve panniers, either soft or hard cases mounted on either side of the motorcycle and behind the rider. Frequently these are supplemented by a duffel bag or other style of pack that is lashed to the rear fender loop. Panniers are available in varying volumes and sizes, limited only by your assessment of compromise between storage capacity and how the size and shape interferes with your riding. On roads, two tracks and most open single track, even large volume panniers are tolerable. Hard cases have somewhat of a reputation for causing injury when the rider’s lower leg is trapped between the hard case and the ground during a fall or when footing through a difficult section of terrain. Soft panniers likely eliminate the possibility of injury but typically are of much smaller cargo capacity. My other concern, perhaps not generally shared, is the possibility of snagging soft panniers on trail obstacles went on very narrow, tightly bordered, or highly technical single track.  

My solution for the DR is somewhat different. Rather than mounting luggage to either side of the bike, which I think is vulnerable to being snagged by trail junk or crushed in tip over, I have elected to utilize the space directly behind the rider and no wider than the rider. I achieve suitable cargo capacity by extending the packing area farther towards the rear than is generally deemed acceptable, primarily out of concern for ungainly weight distribution. In this design I have adapted a unique and very usual backpack/frame produced by the Gerry company in the late 1960s and early 1970s. The images will better illustrate the design than will my description but essentially the frame has a series of studs attached to the vertical rails and the pack has a series of matching grommets. Pack grommets snaps over the frame studs and the pack is divided into horizontal, zippered compartments so that packed items be located and isolated in the pack as you choose.

The pack frame is aluminum tubing,  the rear loop is half-inch EMT, and the vertical support pieces are .125” inch aluminum plate, so the entire rack assembly weighs just     At first glance, it may look like far too much of the pack weight is cantilevered out beyond the rear frame loop and over the taillight. However, with the compartmentalized design of the pack, one can place the lightest of items such as down sleeping bag and down clothing in the most rearward compartments, and items stay where they are initially packed so you have very good control over weight distribution of the entire load. This means that any weight placed out on the cantilevered part of the support rack amounts to less than 5 pounds, an inconsequential amount. The rest of the pack weight can be kept well centered and low over the supported section of the rack. My rationale behind this choice is that in typical trail riding the chances of tipping over to the side or sliding luggage side down are much higher than coming over backwards and snagging the extended loop of the support rack.

I have incorporated one other design feature to help minimize the conflict between demanding trail riding and carrying luggage. The rack is designed to be quickly detached from the motorcycle. The intention here is that the support rack and the pack may be left in the tent at base camp when the plan for the day riding includes difficult terrain.  



DR 350 Rack Final 010.jpg (1009315 bytes)






Silverton Field Test Summary


Generally, I was quite pleased with the DR's performance in most any of the situations I encountered. The suspension was never an issue, perhaps because I maintained a pretty moderate pace everywhere I went. The one situation where did have to ride aggressively actually suited the DR soft, compliant suspension quite well. During one of my day trips I rode up over Stony Pass Southeast of Silverton and down towards the Rio Grande reservoir. It started out as gravel road and gradually morphed into two track, then into open single track, followed by narrow wooded singletrack near Timber Hill. As I got deeper into the woods the trail became an increasingly steeper downhill. I came around a corner and see loose rock, mostly grapefruit sized and larger, filling the trail all the way down to the next turn, tens of yards away. It looked like the trail had been, or was still, a seasonal water course and had collected rocks caught up in the outflow. Anyway, I thought it might be a wee bit difficult coming back up through the rock field but the trail was starting to get really interesting so I tip in and down towards the next turn. I turned that corner and I see another leg down of about the same length, also comprised entirely of loose rock. I am kind of committed now since I can't really turn around and get any kind of run back up the hill. So, down I go to the next turn and behold yet another long leg down of nothing but rocks and I am thinking this is going to be really interesting coming back up. The remainder of the trail down to the Rio Grande reservoir is really nice dirt, a highly entertaining roller coaster of a ride with a really nice rhythm, threading through the trees and dense brush until I get to the fire road that borders the reservoir. The fire road is kind of boring and there's traffic so I decide to turn around and head back. The roller coaster is just as much fun on the way return trip but more and more the thought of the three stage loose rock uphill starts intruding into my consciousness.  As I contemplate this I realize that the worst thing would be to halt somewhere in the rocks and then try to regain momentum from a dead stop so carefully picking my way up at an observed trials pace is probably not a good idea. So, somewhere during the roller coaster ride I start picking up more and more speed, trying to get the into a rhythm before the rocks. I don't recall any distinctive landmarks for where the nice dirt ends, I just remember that the transition happened at a turn so I won't get much, if any, warning so it's likely to be a surprise. The trail is flowing really well. I come around a blind turn and I'm suddenly in the rocks, nicely in the power band in third gear. The first rock course go surprisingly well, no deflection of consequence so I'm able to maintain my speed and as I look up at the coming turn I see the outside is nicely banked, all dirt, and rock free. I rail that and launch up the next section, which also goes smoothly. The last turn doesn't have a banked outside line but goes well nonetheless. Engine RPM hardly drops during all three stages and I exit the top of the rock field in a wheelie, having cleaned the entire section. Absolutely makes my day. 

And here the benefits of the relatively soft, very compliant suspension come into focus. I think this is one of the major reasons why the climb back up to the rock sections went so smoothly. The suspension did a superb job of soaking up the incredibly choppy surface presented by the rock field and hardly deflected at all. This made it very easy to pick and maintain a line without getting bounced off line and losing momentum or even off of the trail altogether.  

The rest of the ride back to camp was pretty uneventful except for being herded under very close guard by a pair of sheepdogs through a herd of sheep clustered at Stony pass.  

On the last day of trail riding I spent most of my time riding to the very ends of the fire roads and trails in the mountains north of Silverton. The bike was comfortable and competent at the pace I was riding, soaking up most all of the road and trail chop without drama. The one change I wished for was more power, a recurring theme so it seems. There were numerous instances when I simply wanted to go faster or climb steeper and higher trails without the motor seemingly on the verge of grinding to a halt. Mind you, it did keep chugging away without fail, but it would've been a lot more fun with some reserve power to play with.

So, I got two major conclusions from the Silverton field test, and supported by the previous field tests. To be effective in the widest range of riding situations, the DR needs more power and I think the most desirable kind of power would be concentrated at low to middle RPM. An increase in low-speed torque is best achieved via an increase in displacement. Increasing the compression ratio also boosts torque at low rpm but unfortunately an engine modified in this way is typically less tolerant of low-grade fuels at higher rpm. For this round of engine modification we will leave camshaft, compression and exhaust changes out of the picture. These modifications typically compromise reliability, fuel tolerance and the nice quiet exhaust note of a stock system. The Suzuki DR 350 world has a long-established history with engine modifications. Displacement increases of nearly 100 cc are common and when done without concurrent changes in cams and compression, have a very good reliability record. My only concern is whether the stock head, cylinder, and oil system can deal with the additional heat generated by the oversize engine. My intent is to fit an oil temperature gauge before the engine modification, keep it in place after, and compare the temperature data collected in similar riding situations. If the increased displacement appears to cause unacceptable oil temperatures, and I will go ahead and get an oil cooler, accept the possibility of a new weak link and do my best to fabricate a guard for the oil cooler that minimizes the likelihood of damage.

The second conclusion is that I do not want to give up the very good low and medium speed suspension compliance in exchange for suspension that only performs well at fast speed over the whoops of Baja. The goal is to find a suspension setup that handles both situations with aplomb or it least strikes a satisfactory balance between the two types of riding. I suspect the rear suspension will be an easier task since it may be a simple ( if expensive ) matter of fitting a high-performance shock matched with the correct spring rate. There is a possibility that a sophisticated damper mechanism from an aftermarket company will fit within the DR’s shock body. This would be significantly less expensive than a complete new shock assembly. It would probably not be comparable to all new shock assembly at the extremes of demanding terrain but it might be perfectly acceptable in a far wider range of situations than what I currently have. The front suspension may be more challenging since the fundamental design of the internal damping mechanism is very unsophisticated and not amenable to significant modification. That leaves either finding a complete replacement damper system that will fit inside the OEM fork leg or fitting a complete, more sophisticated set of forks from a different motorcycle. This last solution is potentially very expensive and fraught with design compromises. Perhaps most apparent of these is that most of the modern fork damper designs come encased in USD ( upside down ) fork. The diameter of the fork at the triple clamps is much larger than that of the OEM fork so most USD forks may pose a serious interference problem with the oversize fuel tanks that are an essential part of the Alpina design mandate. I believe there are some earlier model Suzuki RM fork legs of conventional (right side up) format that contain fairly sophisticated damper assemblies - well at least for the late 80s, perhaps even as late as the mid-90s. Another possibility is the fork assembly from the Suzuki DRZ 400 off-road model, which has a reasonably sophisticated damping design. Not that encouraging at first glance, but perhaps these damper assemblies are amenable to state-of-the-art modification - need to ask around about that.



Pagosa Springs, Colorado  

Autumn 2009 field test. The location for this upcoming field test will be the forests and mountains surrounding Pagosa Springs, Colorado. I expect a lot of high mountain technical singletrack. This field test will be conducted while in the company of some very proficient riders so I have reason to suspect the trail speeds will generally be elevated compared to my pace when riding solo. Equipment modifications planned for this field test are:

- to try to shift the balance of suspension more towards competency at higher trails speeds. I believe the most significant gains can be made ( inexpensively ) by increasing the front suspension spring rate so the ride height is higher and the suspension is more resistant to bottoming at elevated speeds. The aftermarket Race-Tech fork springs are a triple rate at .40- .46 kg/mm compared to the SER’s OEM .390 kg/mm single rate spring. The initial rate of the replacement spring is similar to the rate of the OEM spring so one might expect initial compliance to be similar between the two. I'm crossing my fingers that the rising rate of the replacement is sufficient to keep the front suspension up at a higher ride height mid stroke and also provide greater resistance to bottoming than the OEM spring.

- the wheel assemblies of the DRSE will be replaced with those from the DRL. Both L wheels will be fitted with observe trials competition tires, the front a IRC TR-11, and the rear a Dunlop D803. These tires are significantly more compliant than the Pirelli MT21s currently fitted on the DRSE wheels. The advantages should become increasingly apparent as the terrain becomes more technically challenging. The DRS wheels are also fitted with Moose heavy duty tubes. One drawback to the L rear wheel assembly is that it will not accept the 50 tooth sprocket that fits the SE’s rear wheel. To achieve the low gearing I want for the Pagosa Mountain Field test, I will fit a 12 tooth counter shaft sprocket in combination with the 47 tooth rear sprocket on the L’s rear wheel which yields a final drive ratio of

3.92, close to the Utah 13/50 ratio of 3.85




- I also intend to fit the Mikuni TM33 accelerator pump carburetor from the L parts bike. I believe the improvement in throttle response will help mitigate the lack of power at high altitude. The CV carb fitted to the SER does have some ability to compensate for altitude. Maximizing the performance of the pumper carb requires jetting that is suitable for the conditions. To be more precise, I have decided to augment my seat of the pants feel by fitting a KOSO air/fuel ratio meter to help zero in on the optimal jetting. I installed the O2 sensor / sender in the spare header that came with the 1990 parts bike. Initial testing shows the DR with the CV carb to be running at about 13:1 at our local elevation of 6,500 feet. The ideal ratio is closer to 14.7:1, give or take a bit depending on whether you're in acceleration or cruising modes. Since the mixture is already a bit rich, it certainly shouldn't be surprising that the bike ran poorly at between 9,000 and 12,000 feet in the mountains around Silverton. The next step is to install the modified, more open, stock muffler and see how much leaner, if any, the mixture becomes. If the sound level of the modified muffler is still acceptable, then I will leave it in place and move on to installing the TM33 pumper carb. After fitting the TM 33 I will established some jetting specs over a range of altitudes. Those choices should be close to optimal for any riding at similar altitude, barring any extreme differences in weather conditions. When the jetting adjustments are finished, I will replace the modified header with the original so the sender and meter can be used in other projects. The jetting should be stable until the next stage of engine modification, a larger diameter piston, which should bring the displacement to around 450 cc. Then I will reinstall the modified header and A/F meter setup and establish the correct carburetor settings for the new engine.



KOSO Air / Fuel Ratio Sender


- we have come across a number of anecdotal reports indicating that a little minor surgery to remove some flow restrictions in the muffler would allow the stock engine to run more efficiently but with an inconsequential increase in sound level. This should also allow the engine to run cooler and the improved flow may be especially helpful at higher altitudes. The rearmost, cylindrical portion of the muffler is mostly empty space but for a small diameter, very restrictive perforated tube down the center that becomes the outlet. The modification involves removing the center tube and significantly increasing the diameter of the outlet. I also pointed the outlet down and slightly away from the bike. The pointing down should help mask any increase in sound level and combined with the out will redirect the hot exhaust gases away from the fabric pack when the pack and rack are being used. Since the OEM exhaust system is exceptionally quiet as built, I expect any increase in sound level to be acceptable.


- In addition to the KOSO A/F ratio meter and the Garmin GPS, I have installed a Trail Tech Vapor computer in the DR’s cockpit. The Vapor is a multifunction instrument, including speedometer, odometer, tachometer, oil temperature and miscellaneous other functions, it will also serve as replacement for the DRL’s odometer. The tachometer and oil temperature data in combination with the air/fuel ratio data should permit collection of comparative before and after data for the fuel and exhaust system modifications. This information should also establish baselines before and in between the next two modifications, fitting an oil cooler, and then followed by the long-awaited displacement increase. In fitting the Vapor, the OEM odometer cable is removed along with the odometer itself. The Vapor’s display is signaled via a magnet attached to the front brake rotor, which passes close by a sensor attached to the caliper bracket, sending a triggering signal up a wire to the instrument head or display. The OEM odometer drive is left in place as a front wheel spacer so the weight difference between the removed OEM pieces and the newly installed pieces is probably negligible.


- Fitting the Vapor, the air/fuel ratio meter and the Garmin GPS required fabrication of a bracket to cluster all three instruments within the space defined by the handlebars, the headlight assembly and the tubular protective cage previously fitted. While I was at it I incorporated a master power switch into the bracket assembly, but reasonably well hidden to casual inspection. The original key switch, as part of the OEM instrument cluster, was shelved long ago. I had been using a jumper wire to route power through a connector left over after removal of the cluster. It had the advantage of being extremely simple, extremely reliable, and its workings remotely obvious. It was a bit of a challenge to reach, especially while wearing gloves, all of which was aggravated with the new bracket and instrument cluster taking up most of the free space behind the headlight assembly. The bike couldn't be started unless you happen to have a suitable jumper and knew that the empty connector could be jumped -reasonably theft proof.




Pagosa Springs Field Test Summary


Well, um, uh - plans for the Autumn ride kind of fell apart. Hopefully, we will try again in the spring of 2010. If we're really lucky, perhaps we'll have the big bore kit installed and sorted by then. To be continued ………….