Yamaha Tt350 Project Playbike
YAMAHA TT350 PROJECT PLAYBIKE
Stuffing a four-stroke engine into a two-stroke chassis makes for a fully serious half-breed...
RON GRIEWE
BUILDING A SUPER-SERIOUS, middle-displacement fourstroke dirt bike, one that is light and really handles, shouldn’t be a major engineering problem. But it apparently is, judging by the fact that the 350-to-400cc four-strokes turned out by the factories generally are heavy and don’t handle all that well.
One possible explanation for this situation is that when the factories build four-stroke dirt bilces, they usually design a completely new machine rather than simply starting with a proven two-stroke motocross chassis and modifying it to accept a fourstroke motor. The designers feel that such an approach isn’t the answer, because the differences in engine weight and mass between a twostroke and a four-stroke, and in the type and amount of power they each produce, affect the handling characteristics of the chassis.
But while that’s usually true, it’s not always true. And to help prove that point, we built the bike you see here, the Cycle World TT350 Special. It is little more than a 1985 Yamaha YZ250 chassis with a 1985 XT350 dual-purpose four-stroke engine stuffed in its midst. Yamaha, of course, has introduced an official TT350 in its 1986 lineup; but we built our bike before we knew that Yamaha was going to produce such a model. Besides, Yamaha took the typical route in building its TT350 by starting with a dual-purpose machine, the XT350, and making the changes needed to turn it into a pure off-road model.
We’ve never ridden Yamaha’s
TT350, but we’ve spent lots of time on ours; and it flat works. Ready to ride minus gasoline, the Special weighs 250 pounds—admittedly, not the lightest four-stroke ever built, but still pretty impressive when you consider that the engine, at 82 pounds, is kind of heavy for a 350cc four-stroke. And the engine is bone-stock, too, so the power output is not spectacular.
Despite being a tad heavier than we would have liked, though, the Special is a fabulous playbike. Its motocross-quality/quantity suspension allows the bike to pass over punishing terrain smoothly and quickly, and with a high degree of stability. The steering is so precise that you can assault corners like a madman and still maintain precise control all the way through. And the TT350 Special is one of the best bikes we’ve ever ridden for powersliding around the turns on fireroads.
We rode the Special in company with a Honda XR350R just to see how it compares to the best production bike in its class, and we were impressed with the results. The XR is a bit faster in drag races and roll-ons, although not by much; but we expected this to happen, simply because the XT’s dual-purpose engine is in a milder state of tune than the XR’s dirt-only motor. With appropriately hotter cams and a bit of typical hopping-up, the Special would undoubtedly have the edge in engine performance.
In every other way, though, the TT350 Special is superior to the XR just the way it sits. It easily outhandles the Honda, especially across big whoops and extremely cobby terrain, and it outsteers and outslides the XR, as well. It’s truly a fun motorcycle.
The people at Yamaha showed great interest in this project; they furnished the two bikes that were needed to build this one, and crawled all over the finished product and took lots of pictures. But that doesn’t necessarily mean that you’ll find such a bike in your Yamaha dealer’s showroom in the near future. A motorcycle like this one would not be cheap, and that could be a serious drawback in an area as price-sensitive as the middleweight four-stroke dirt-bike market.
Still, the TT350 Special—even with its overweight, underpowered engine —proves a very important point: The marriage between a twostroke chassis and a four-stroke engine can be a happy one.
