In previous blogs, and in the print version of the magazine, I have discussed anti-squat and how it relates to chassis setup. Anti-squat is a very important tool in making a motorcycle lap quickly at the racetrack, especially a powerful superbike, but at the same time it's one of the least understood setup parameters.
Some people claim that the rear end of the motorcycle must always compress, or squat, under acceleration to properly transfer load to the rear wheel for better traction. Others claim that the rear suspension must extend under acceleration, to "push" the tire into the ground and increase traction. People in the second group point to the experiment of putting the front tire of the motorcycle against a wall so that the bike can't move; when the clutch is gently released, applying power to the rear tire, the suspension extends significantly. But what really happens when the motorcycle is on the road or track and accelerating?
Here is what we know about anti-squat in theory: The three forces involved in compressing or extending the rear suspension as the motorcycle accelerates are the driving force, chain pull, and load transfer. Driving force refers to the rear wheel pushing the motorcycle forward, and generally acts to extend the rear suspension because of the swingarm angle. Chain pull is the force of the top run of the chain on the rear sprocket, also trying to extend the rear suspension under most conditions. And load transfer refers to the additional weight on the rear suspension due to acceleration.
There are a couple of key points to consider here: First, the load transfer component will occur whether or not the rear suspension compresses; in other words, acceleration will add weight to the rear wheel even if the rear suspension extends during that acceleration. The attitude of the motorcycle does affect the amount of load on the rear wheel, but to a very small extent. Second, the experiment of putting the front tire of the motorcycle against a wall removes load transfer from the equation; the rear suspension rises because only the chain pull and driving forces are present, the forces which serve to offset load transfer - which is eliminated here because the motorcycle is not accelerating.
Sum the three forces, and the math shows that the anti-squat effect decreases with more suspension travel, mostly because the swingarm angle changes through the stroke. At the top of the travel, acceleration will cause the rear end of the motorcycle to rise; at a certain point, equal to approximately the static sag setting on many bikes, the forces sum to zero and the suspension will neither compress nor extend on acceleration. As suspension travel increases, the anti-squat effect reduces further and the rear end will tend to squat on acceleration.
What happens in practice? Data that I have from Jodi Christie's superbike shows that in some corners, the rear suspension compresses during acceleration; in others, it extends; and in others, it remains constant from the moment Jodi applies the throttle to the end of the succeeding straight. The amount of compression or extension depends on traction, camber, elevation changes, and any number of variables.
The takeaway here is that, by adjusting various setup parameters as they relate to anti-squat, we can make the rear suspension do what we want on corner exits - extend, compress, or remain constant. This is usually a compromise to find a setting that works for the entire track, and we most often look at rear suspension in conjunction with other data, not on its own, for guidance on what that compromise should be.