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XT225 Alpina

 

Yamaha XT225 Alpina - Project Plan                     Yamaha XT225 Alpina – Project Log 

Yamaha XT225 Alpina - Project Plan  

Of the two air cool platforms that we find particularly interesting, the Suzuki DR 350 is potentially the most versatile. The larger engine, both stock and enlarged to as much as 450cc is inherently more suitable for the combination of high-speed roadwork and trail riding at high-altitude. The Yamaha XT 225 shares many basic, desirable design characteristics with the DR. Other than the smaller engine, the most significant difference is that the XT weighs about 50 pounds less, and has a considerably more compact chassis, with stock frame geometry and suspension travel closer to our ideal for an Alpina concept. Oddly enough, although just fine by us, it's overall transmission ratio spread is wider than the DR's and most other dirt bikes for that matter. Below we reiterate the design characteristics of the stock bike that make it a good foundation for an Alpina project.

- long production history so there are many available used in the marketplace

- mechanical reliability is well-established

- wide ratio six speed gearbox

- broad power bands oriented towards low-speed torque

- electric start

- exceptionally easy to maintain

- can be fitted with large capacity, aftermarket fuel tanks

  The issues of comfortable road speed and high mountain trail ability are not insurmountable individually but difficult to resolve in a single design execution. The XT will maintain adequate highway cruising speed but there is not much in reserve so being incessantly tailgated or becoming a traffic hazard is a likely possibility, especially if you're trying to maintain a reasonable speed on a mountain highway. On the other end of the scale you can gear the XT low enough to keep chugging away on a trail at 11,000 feet. However, the engine doesn't put out enough power to do both with a single gearing package so the prospect of riding from your garage up the highway to a high mountain trail isn't terribly appealing. If you live in an area that doesn't put these kinds of dual demands on a single bike, or focus your build on just one end of the range, then the XT can make a very satisfying trail mount. Keep in mind, even when optimized for mountain trail riding, the XTs little engine is challenged at very high-altitude. Due in large part to the combination of a very wide ratio gear box and good low-speed engine torque you can still go almost anywhere though you may be making frequent use of first gear, occasionally with throttle pinned just to keep going, and you won't be popping any power wheelies over trail junk. But for moseying along at a leisurely pace and just exploring the neighborhood, the relatively light weight and very maneuverable XT can be a lot of fun.

In our design brief for the XT we will concentrate on the following areas to improve its performance.

- increased fuel capacity

- suspension performance

- engine throttle response

- weight reduction

Fuel capacity. Until recently, and through most of the XT's production life, the only larger fuel tank solutions were homebrews. The only one we are personally familiar with is fitting the 3.5 gallon plastic tank from a 1984 Honda XR 350. Takes a little fiddling but it does lash up reasonably well and is definitely preferable to retaining the stock tank. Within the last year Clark manufacturing has introduced a 4 gallon plastic tank designed to fit the XT. This is probably the simplest solution by far and gives the XT a very good range.

Suspension performance. When we speak of suspension performance in the context of the Alpina, our overriding concern is low-speed compliance over the nastiest of terrain conditions. Performance at high speed is secondary. Although the ideal setup is certainly to achieve both in a single suspension package, we suspect that achieving this combination requires suspension components much more sophisticated than those found on the XT. For low-speed work XT suspension is very compliant, absorbing roots, rocks, and assorted trail junk fairly well. Picking up the pace to even a moderate trail speed reveals shortcomings - soft spring rates, easily bottomed and easily deflected by obstacles at speed. We are always keeping our eyes peeled for swap that might get us closer to an ideal multipurpose suspension but for now, our initial efforts are focused on maximizing low speed compliance with the OEM suspension.

Tires. We believe it is reasonable to include the issue of tires as an integral part of suspension performance. If you think about it, the tire's ability to mold to the surface it contacts is the earliest stage of a motorcycles inclination, or not, to follow the terrain without being bounced off the line you have chosen. With this in mind, perhaps the single most effective change you can make early on is to replace the OEM tires with rubber intended for observe trials competition. The carcass construction, rubber compounds and tread patterns are the best available for this application. The side walls are extremely flexible and combined with soft sticky rubber permit the tire to “flow” over and around trail junk, giving excellent traction. Contrast this with a typical knobby tire of either a motocross or general off-road persuasion. The stiffer carcass and knob construction means that the point of contact between a motorcycle and the object may be as tenuous as a single knob or two. Put this as your sole point of contact on a wet log or muddy rock and you can envision why it is so easy to skitter sideways and end up in a heap, as a new obstacle in the trail. For a bike that sees trail use only, full competition trials rubber is fine. If however you envision some high-speed pavement in the mix, there are some DOT approved tires in the same general pattern but with rubber compounds that are durable enough to endure extended runs on pavement and have excellent service life as well. These would be the Pirelli MT 43 in an 18” rear tire and an IRC T-01 in a 21 inch front. According to most magazine and forum reviews, they don't give up a lot of performance compared to the competition rubber, and are a significant improvement over both other dual-purpose tires and off-road knobbys. It is generally accepted that the trials tires provide superior traction in most any off-road situation except for deep mud and perhaps deep sand, although the latter is still actively debated. If you're still convinced that a full knobby is the better all-around choice, then Pirelli MT 21 and Dunlop D606 are perhaps the most highly regarded for dual-purpose work.

 

Engine throttle response. The XT 225 comes equipped with a Mikuni BT 34 carburetor. This model falls into the category called constant vacuum design. You can find a more detailed explanation here, but the primary advantage of this design is that it removes direct control of the throttle slide from the hand of the rider. This provides two advantages, it generally proves fuel mileage and has some resistance to altitude induced jetting troubles. The trade-off however is that the carburetor is unable to satisfy the fuel demands of the engine when the rider snaps the throttle open. Under general cruising conditions this is not an issue of consequence, but when faced with the surprise of an trail obstacle and little time to prepare, instant engine response may be critical to negotiating the obstacle successfully. This may be more important with an engine of small displacement, having little low-speed grunt to carry it over a surprise. Although there is a moderate amount of work involved it is not fundamentally difficult to fit a pumper carb to the XT. The Mikuni TM 33, fitted to DR 350 dirt models for their entire 10 year production run will work, but a more suitable choice would probably be in the Mikuni TM 31. This was fitted to the DR 250 dirt model, unfortunately far less common than the 350.

Two other common ways to improve engine response is to increase the compression ratio and/or engine displacement. Unfortunately, the XTs basic engine architecture will not accommodate much in the way of displacement increase. Our concern with increases in compression ratio, although a very effective way to increase low-speed engine response, is that this change tends to demand higher octane fuel. As altitude increases octane requirements decrease so there is some offset here.  Since reliability is high on our list design requirements, our inclination is to steer clear of higher compression in our list of modifications.

 

Weight reduction. This is generally always worth the trouble of pursuing, especially if it is a matter of simply removing items rather than replacing them with lighter weight but more expensive substitutes. XT 225's sold in the United States have always been fully street legal DOT motorcycles. Therefore they are encumbered with lots of stuff you don't really need for trail riding. In regards to getting to the trail on public roads you need to choose a balance that you are comfortable with. As an example, in most jurisdictions is perfectly acceptable to use hand signals rather than turn signals to indicate your intent. You must judge whether you consider this trade-off of weight reduction versus convenience worth the effort. Passenger pegs are another obvious candidate. After you've exhausted deletions, you are left with substitutions. We believe the plastic fuel tank is a no-brainer, increase capacity, far more resistant crash damage ( save your OEM steel tank for resale time, so it still looks pretty ). The same rationale works for replacing the OEM taillight with a much more compact and crash resistant LED substitute.

 

In a more extreme vein, some of the design modifications on our long-term list include:

1) relocating the foot pegs to a position at or behind a swing arm pivot

2) fabricating a new subframe

3) fabricating a new, more effective airbox

4) replaced the rear drum brake system with a disc brake system

5) replacing or revising the wiring harness

6) adapting the fork assembly, front wheel and brake system from a modern observed trials bike

7) revising the rear suspension to incorporate a linkless shock design

8) replacing the XTs frame with that of the TTR 230

 

There is one instance in which we feel compelled to deliberately add weight. The OEM skid plate, although not a bad design, does not provide coverage for either the oil filter cavity in the clutch side cover or the magneto assembly. Impact damage to either of these areas might be a ride ending event. Although they might be available we aren't aware of any full coverage skid plates available in the aftermarket to fit the XT 225. Fortunately, is a relatively simple matter of fabricating extensions, or wings, that can be welded to the OEM skid plate and provide him with the necessary protection.

 

1) Most trials bike design positions the footpegs behind the swingarm pivot. Moving the footpegs rearward makes it easier to lift the front wheel over obstacles. This is especially useful when you do not have room to set up well in advance and must rely on a shift in body weight to get the front end light. This should be a relatively simple project but does alter the footpeg to shift lever and footpeg 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.

2&3) Items two and three really go together as a matched set. The air filter and airbox design is our opinion marginal. We liken the filter to a slice of toast slid into the slot of a toaster. You must grease all around the edges in order to assure a good dust proof seal, but you can't really see what is going on at the blind edges slid deep within the box. Any gap in the filter to airbox seal opens up the likelihood of accelerated engine wear and perhaps a ride ending failure in the field, so the inability to verify the integrity of the seal makes us uncomfortable. Redesigning the subframe gives us the possibility to fabricate a structure that is both lighter, quick detach, and configured to accommodate an airbox design of our choice -- all changes in our favor.

4)  Replacing the rear drum break with disc is neither essential nor high on our list. It would probably perform better over a wider range of conditions but I think we would do this change as much for the symmetry as anything else. It is a fairly extensive modification so we would not undertake it lightly.

5)  Revising the wiring harness would yield a minor reduction in weight but is not the point of this exercise. The benefits would arise from a generally cleaner package with elimination of wiring sections that serve no purpose on our revised platform. One could also make the case that there are fewer possibilities where the electricity in the system might go astray, some incremental improvement in reliability.

6) Replacing the entire fork, front wheel, and brake system would be away to significantly upgrade all these components with ones that are suitable for our application. Fork travel on most observed trials bikes is somewhat less than our ideal for Alpina, but we don't think he would be a debilitating loss. Would rather find out by giving it a spin then to assume its inadequacy. We feel pretty comfortable that the front wheel assembly and front brake system would be improvements over the OEM equipment. We would probably have to entertain revisions to the rear suspension design at the same time to keep both ends in balance.

7) Revising the rear suspension design would be a significant alteration. There is much to recommend a rear suspension configuration 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.  

8)  The TTR 230 is a very close relative of the XT 225. At casual glance it appears to be an XT simply stripped of its street legal equipment and intended for dirt riding only. In fact, it has an entirely different frame. We haven't yet determined if the geometry differs or not but the design and construction methods are much more appealing than that of the XT. Where the XT uses stamped sheet metal gusseting at the swingarm pivot, the TTR uses a tube to tube construction typical of more sophisticated dirt bike design. It is probably lighter, certainly more aesthetically pleasing and would also be easier to modify if we decide to fabricate a removable subframe. We have not yet looked closely enough to determine if the TTR retains the XT airbox or has a new design. 

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Yamaha XT225 Alpina – Project Log 

The core for this project is a 2000 model year bike with about 1,800 mostly street driven miles. 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. Because this bike has a small, low-power engine, and gearing selected for acceptable off-road performance at very high altitude, the bike is no longer capable of maintaining an adequate cruising speed on the highway. Although plated and legal for operation on public roads, component choices were made primarily with off-road performance in mind and considerations for pavement use are minor at best. The following items were removed or replaced:

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

- 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 OEM tail light assembly with a pair of LED marker lights, one wired as a taillight and one as a brake light. This pair of lights are much lighter and much more compact than the OEM assembly

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

- replaced the OEM 8.7 L (2.3 gal.) steel fuel tank with the OEM plastic tank from a 1984 Honda XR 350 9 L ( 2.4 gal ). Replacement tank is lighter, higher volume, and much less subject to damage from a crash.

- replaced the OEM tires with IRC T 11 competition observed trials tires both front and rear. The original tires performed very poorly off-road and the dirt orientation of this project permitted the choice of very specialized tires without consideration for the performance or durability on paved surfaces. 

 

Hole in the Rock, Utah 

Spring 2006 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.

- Change the final drive gearing from the stock 14/47, 3.36 to 13/50, a ratio of 3.85, approximately 13% percent lower than stock

-  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 XT performed very well in some riding situations and only adequately in others. It was very underpowered for high-speed use on fire roads and operation in deep sand washes. Operation at low speed in very technical training was just plain fun. The very low gearing was advantageous on steep, low speed uphills and downhills. The IRC trials tires grip very well, even when side hilling steep rock faces. Compared to prior rides in this area on much heavier, larger displacement bikes, the lighter weight XT was a joy to ride in even the most difficult train. The only real drawback was not being able to pull a speed on the fire roads to match other riders in the group, all mounted on horsepower rides. 

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 XT 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 neither the low-power engine nor the basic chassis geometry is well-suited to this type of riding, the light weight of the bike made it relatively easy to control, even when it required a lot of rider input. The most important thing was to maintain momentum at all costs because once you slowed down in the sand it was difficult to get back up to speed. There are no simple ways to either increase the power output of the engine or significantly lighten the overall weight of the bike. The less than ideal performance on the fire roads and in deep sand is more than offset by very good performance in the difficult, technical rocky terrain. 

Moderate trail speeds over choppy, rock infested two track was somewhere in the middle. The light weight of the bike was welcome and suspension absorbed the chop fairly well at lower speeds. As speeds rose however, the short wheelbase and soft suspension provided a very choppy, hobbyhorse ride with lots of impact felt through the handlebars. This created lots of blisters over the course of a long riding day. We wouldn't lengthen the wheelbase to solve this, because we think that would severely compromise the bikes performance in the technical terrain, which is perhaps the bikes strongest characteristic. We think the most profound improvement would come from modifying the suspension to better absorb the mid and high speed impacts while still retaining its good low-speed compliance. The solution for the rear suspension is the relatively simple, if expensive, purchase of high quality aftermarket shock absorber. The front suspension poses more difficult problem. We don't know of any high-performance aftermarket damper assemblies or modifications that are available to fit the stock XT fork assembly. The probable solution is to fit a more sophisticated fork assembly from a different brand and model of motorcycle. 

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 XT footpeg platforms are very 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 XT.

 

Mancos, Colorado 

Summer 2007 Field Test.  Locations for this series of single day rides were in the western foothills of the La Plata Mountains that lie east of Mancos, Colorado. The area is heavily forested and is laced with a complex network of fire roads, two track and single track trails, almost all dirt. The terrain generally ranges in altitude from about 7,000 to 9,000 feet with some isolated single track reaching up the slopes of the LaPlatas to perhaps 11,000 feet. Ambient temperatures range from the low 50s to mid 80°s F.

We did not make any modifications to the XT specifically in preparation for this series of tests. 

Mancos Field Test Summary. The XT was generally a pleasure to ride in these conditions, perhaps only lacking the gearing to take advantage of the higher speed tracks available, or the power to more easily maneuver the bike over obstacles on the higher altitude trails. On those higher altitudes trails though, the low gearing allow the engine to keep chugging away without stalling. It may have required first gear and very slow speeds a lot of the time but the bike kept moving ahead in any case. There is no question that it would have been more entertaining to have reserve power to loft the front wheel at will, but the riding was still a lot of fun.

Most of the terrain was relatively smooth with just occasional rocks and roots to deal with so the suspension performed well in most any situation I encountered. The narrow foot pegs still remain an issue but this was mitigated by more frequent opportunities to sit while riding and to give my knees a rest. Nonetheless, footpeg modifications should move to the top of the list.  

Silverton, Colorado 

Summer 2009 field test. Location for this 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 mid 50s to near 80°F. One modification made in preparation for this field test was to adapt wider foot pegs, pilfered from a KX 125, To the XT’s footpeg mounts. 

Silverton Field Test Summary. The performance of the XT was a mixed bag but I attribute this partly to riding with a group and partly to the XT’s unsuitability for the terrain. All of the other riders were on much more capable motorcycles and much of the riding ended up more as trail racing than exploratory trail riding. On the fire roads, I had to keep the XT’s throttle pinned to even remotely keep the group in sight. When the route turned to tight, technical single track, the XT performed well and the riding became more enjoyable. But when the train opened up and the speeds rose, the XT was out of its element. The short wheelbase and soft suspension had the XT pitching about and deflecting off of rocks and other obstacles in the trail. It required a lot of concentration and riding near the limit to try and keep up during the high-speed sections, which were also at high-altitude. Pretty exhausting day all in all. Had it been a day of trail riding at slower speeds and in more of an exploratory frame of mind, it probably would've been a much more relaxed ride. I still enjoyed the day immensely, but it had too many near misses for my taste which I attribute to the general speed range being beyond the capabilities of the XT. In addition to the high-speed trail work, the non-technical riding at the highest altitudes was quite a struggle for the XT. Although the low gearing could allow the XT to ultimately go anywhere with the other bikes, I spent a lot of time with the throttle pinned in first or second gear for long stretches to make some of the steeper climbs. At times, thoughts about the engine possibly expiring crossed my mind. This should be a clue that the bike really isn't suitable for the kind of riding being undertaken. The low gearing that did make the XT enjoyable on technical single track also meant that it was a struggle to keep up on mountain fire roads that connected the trails. This didn't carry the risk of crashing as did the high-speed trail work but was annoying nonetheless.

I suspect that a solo ride on much of the same route, speed adjusted to the capability of the XT would have been an altogether pleasant experience but I probably would have simply skipped some of the sections that I thought might seriously strain the XT's engine. I think though, that the XT simply lacks the power output to be truly enjoyable in many of the riding situations available in the high mountains. In contrast to foothills outside of Mancos, the additional 3 to 4000 feet seem to put the XT shy of the fun zone for too much of the time. 

The Silverton Field test did give us some good clues as to what to look for in a bike that would be more pleasurable to ride in the high mountains over a wide variety of riding situations. We liked the light weight, short wheelbase, steep steering geometry and moderate suspension travel in the tight, technical terrain. We think that a bike combining these characteristics with higher quality suspension components and steering damper that allowed good control at high trail speeds would be quite satisfactory. But the other half of this equation requires considerably more power as well. Our guestimate is that an engine displacement of 350cc would be a minimum suitable for these conditions.