This is all you have to worry about .
When the operator of a vehical, front or rear wheel drive, opens the throttle, three things could happen. The vehicle can move forward or the drive wheels can spin or the vehicle can spin about the driving wheels. Since the leverage created by the front weight of the vehical (Rear WD) or the leverage against the ground (Front WD) does such a good job resisting flipping over, forward motion is usually created. But some of the torque does go into attempting to flip the vehicle. This shifts more weight (or force) to the rear of the vehicle. This compresses the rear springs and extends the front springs as they each seek a new equalibriam position for the new loads. This is called Squat. On a motorcycle under power, the configuration of the driveline, wheel, and suspension components produce forces that work to extend the rear wheel. This is called Anti-squat. Under a given load, torquing force, and wheel position, the whole system will produce a Net Squat. Squat + (Anti-Squat) = Net Squat.
On motorcycles some net pro-squat is desirable for traction. To much net squat and the bike drives wide on turn exit, since the squatting relaxes the steering angle after the rider has already fixed his/her exit line. No net squat or net anti-squat and the tire spins. Anti-squat is affected by top chain angle, swingarm angle, countershaft sprocket position, and swingarm pivot position, as well as rear tire contact position. For a true detailed explaination see chapter 9-1 to 9-28 of the Tony Foale book (2002). Foyle is really THE GUY when it comes to chassis configuration issues and mathmatical models of suspension systems. If you do not have this book, order it right now.
TONY FOALE DESIGNS - Home page.
A great amount of confusion has been caused by using term "ride height adjustment" when talking about the rear end, it would be clearer if it was refered to as Squat Tuning , but convention holds. The end goal of setting the rear ride height is to reduce (but not eliminate) squat while accellerating on corner exit. Most stock street bikes are configured in a fasion that feels very net pro squat. This is not desirable for performance riding.
Max McAllister of Traxxion Dynamics recommends attaining an experimental rear ride height by setting the swingarm 12.5 degrees relative to the ground on a fully unladed bike (raised off the ground with tires just touching floor). This is a very crude approximation, but it is easy for everyone to understand and perform. It completely ignores how gearing affects squat. Later in the tuning process, the ride height will be raised or lowered based on rider feedback with a correctly tuned suspension system and gearing. 12.5 degrees is just a good starting point and will get you in the range to work on the suspension system.
I think that a better way exists for doing this. First, set you sag as you otherwise would and measure the eye to eye lenth of the rear shock while rider sagging. Then, go out riding with your preload tools to a turn that causes hard understeer under power. Add preload untill the understeer goes away. Go back home and measure the eye to eye length of the shock now while rider sagging. The difference of these two measurements should get you very close to how much the shock length needs to change to get proper squat. This technique works well for bikes with stock (non adjustable) shocks or bikes that need shims to adjust. It can also be used with the common ride height adjustment threads on most aftermarket shocks, since it gives infomation to the tuner that is a lot more relevent than unladen swingarm angles.
Traxxion Dynamics sells shim kits that may help to adjust ride height on some bikes. Other companies sell dogbones to adjust at the linkage, and Dan Kyle and Ammar Bazzaz even sell replacement linkage rockers for RC51. The most favored way of adjusting ride height is by using a feature available on almost every aftermarket shock sold. A thread and nut near the clevis changes the shocks overall length in small increments. Simple.
Do not adjust for corner entry or mid turn using the rear ride height. Many uneducated riders will say that raising the rear does the same as dropping the front, but this is flat out wrong. No matter how much you move the forks in the clamps you will not effect the squat characteristics of the rear end (essentially).
