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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
- mechanical reliability is well-established
- wide ratio six speed gearbox
- broad power bands oriented towards low-speed torque
- exceptionally easy to maintain
- can be fitted with large capacity, aftermarket fuel tanks
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
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.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.
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.
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.
- 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
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.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.