Traction and Acceleration

by Adam

Very interesting material Tim. Thanks for taking the time to share with us. And thanks for participating in this forum, it's great to see the discussion.

I have a question about traction and acceleration in two-stroke motors. As I understand it, the traction advantage of the four-stroke single is largely due to the power driving the rear wheel in pulses. The additional time gap between power strokes in a four cycle engine cause it to gain additional traction by acting somewhat like anti-lock brakes (in reverse).

Delivering the power in pulses is the reason for the 'big bang' firing order on Yamaha's new R1 sport bike. Aprilia's new twin-cylinder 450 class motocross bike, the MXV 450, fires both cylinders simultaneously for the same reason.

Say we apply a traction control system to both a bike equipped with a two-stroke motor (as you have suggested) and a bike with a four-stroke motor. Wouldn't the four stroke still have a traction advantage due to the power pulsing?

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Jan 25, 2010
What's the ground?
by: Tim HIckox


First: You understand the problems and advantages of the Big Bang quite well. But the variables are confusing. First, not everybody responds the same way to the same conditions. Also, the four-stroke does not have an advantage under all conditions - sometimes the two-stroke has an advantage. So much depends on ground and tires.

Testing on icy ground in Colorado, I showed the guys that running the two-stroke in a higher gear gave it about the same traction as the four-stroke. If the race rules didn't give the four-strokes such a displacement advantage, the two-strokes would have much more power and so they could pull higher gears and turn the engine slower. But as it has been recently, the two-strokes have to run flat out to make the same power as the four-strokes, and it is under these (contrived) conditions that the two-stroke is at a disadvantage. Let a 250 run against a 250 - which is what the rules should be returning to - and it will be the four-stroke that has to be run at peak revs to keep up with the two-stroke, and then it will very seldom have any traction advantage, and it will much more often be at a disadvantage.

But the beauty of the traction control system that I described is that it continually varies the amount of power that goes to the wheel, and therefore puts the maximum to the ground - regardless of the conditions.

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