I decided for my rear suspension that I’d study five things: camber, toe, and anti-squat changes as the car hits a bump, and camber and roll centre changes as the car tilts in a roll. Each are discussed separately below, so a bag of Doritos might come in handy if you plan on sticking through this. Suspension theory can be a dry subject so I'll try to make it as interesting and informative as possible.
Anti-Squat
Squat is the tendency for the rear of the car to lower as the weight transfers rearward under acceleration. It isn't a particularly desirable characteristic because the more the rear squats, the less the rear wheels remain flat on the road resulting in less traction. To counter this tendency, anti-squat is a suspension characteristic that was designed to change the way acceleration forces act on the chassis. The following schematic shows the principle behind the forces at work:
In the drawings, the chassis of the car is represented by the rectangular box balanced on a pivot point located at the car's centre of gravity. As the car's rear wheels spin in the direction of the arrow marked "torque", the car accelerates toward the left of the drawing. The torque from the wheels is reacted by an opposing force transmitted through the suspension members represented by the long horizontal triangle attaching the wheel to the chassis. In both cases the spinning wheels cause the front of the triangular suspension to lift upwards. The difference between the two drawings is the location where the suspension attaches to the frame. In the upper drawing, it attaches forward of the centre of gravity, and results in the body of the car tipping backwards (or squatting) during acceleration. This represents less than 100% anti-squat. In the lower drawing, acceleration would cause the rear of the car to lift, which is equally bad because it would remove weight from the rear wheels and cause a loss of traction. This represents more than 100% anti-squat.
100% anti-squat would be achieved if the suspension could be made to act through the centre of gravity. In that case, the car would remain flat no matter how hard the car accelerated. One of the limitations of an independent suspension is that 100% anti-squat can't be attained, so there will always be a tendency to squat somewhat under acceleration.
To calculate the actual amount of anti-squat designed in to the stock suspension, the side view drawing is needed as well as a formula designed to take into account the fact that the suspension acts as though it's longer than it really is. Here is the actual '88 Fiero drawing depicting how the formula works (download the drawing to see the details more clearly):
It involves determining the longitudinal instant centre (IC) for the rear suspension by finding the intersection of the green dashed line (extended through the rear trailing arm angle) and the blue line (drawn perpendicular to the strut centre line at the upper strut bushing). Once the IC was located, a third line (red) was drawn between the centre of the axle to the IC. The angle of this red line to horizontal (7.85 degrees) was then used in a formula that included the height (H) of the car’s CofG and the wheelbase.
From the calculation on the drawing, you can see that the ’88 Fiero appears to have about 66% anti-squat at rest. The fact is, the amount of anti-squat changes as the car squats under acceleration. To see how much the anti-squat changes as the car squats, I used a simulation program that did the work for me. (The program belongs to a friend Zac Brown, who did the number crunching for me). The graph below shows how the amount of anti-squat diminishes as the rear suspension compresses.
For example, at 0 mm compression (ride height), the amount of anti-squat is 62% (pretty close to my free-hand calculation of 66%). But as the suspension compresses by say, 40 millimetres, the anti-squat drops off to about 45%. I'll use this benchmark later as I design my own suspension and superimpose the results on the same graph to see how they compare.