I have talked several times before about dynamic geometry and how rake and trail change as the motorcycle goes around the racetrack. Under braking, weight is transferred to the front end, compressing the front suspension and decreasing rake and trail measurements. Likewise, weight transfer to the rear under acceleration causes the front suspension to extend and the rear to compress, increasing rake and trail.
While an ideal suspension setup must address many aspects, a large portion is devoted to controlling that pitch under acceleration and braking. As motorcycles become more powerful and are equipped with better brakes, pitch control becomes more of an issue.
With telescopic forks, there is not much that can be done to address the braking side of pitch. Part of the issue here is that the front end gets doubly loaded under braking: Vertical load is added due to the weight transfer from deceleration, and at the same time braking forces are fed horizontally into the forks. One option is to decrease rake so that the braking forces transmit less directly into the forks, but with decreased rake the side loads become greater and chatter can result. Using stiffer springs, another option, leads to reduced suspension compliance in other areas of the track. Braking forces and the associated suspension loads can be significantly higher than cornering loads, and compensating for braking often sacrifices too much in other areas.
Squat, or pitch under acceleration, can be controlled at least partially by some manipulation of the swingarm angle and chain geometry. When the motorcycle accelerates, weight is transferred to the rear, compressing the rear suspension. But at the same time, forces in the chain and swingarm cause the suspension to extend, an effect referred to as anti-squat. I will discuss the mechanics involved and how to change anti-squat in a later blog; for now, know that it is a setup variable that can be modified to a certain extent.
The desired goal here is a specific amount of squat under acceleration, controlled using the anti-squat effect. To find that desired amount of squat, it helps to know what happens at the extremes. If we were to remove the anti-squat effect entirely, all the weight transfer under acceleration would act to compress the rear suspension. With the rear suspension compressed and the front extended, even more static weight is added to the rear end. At some point, the front suspension is unloaded to the point that front traction suffers and the motorcycle runs wide at the corner exit.
At the opposite extreme, with a lot of anti-squat effect the rear suspension can actually extend under acceleration. With both front and rear suspension extended, the center of gravity is quite high and the wheelie tendency increases; at the same time, less static load on the rear tire can also reduce traction. More problematic is what happens in a slide: If the rear tire loses traction suddenly, some of the anti-squat forces are still in effect and act to extend the rear suspension quickly, which can lead to a highside crash.
Somewhere between those two extremes is an ideal amount of squat that keeps geometry consistent for good steering, and allows just the right amount of pitch and static weight transfer to maximize traction under acceleration while at the same time discouraging wheelies.