Anti-dive and anti-squat

Anti-dive and anti-squat are expressed in terms of percentage and refer to the front diving under braking and the rear squatting under acceleration. They can be thought of as the counterparts for braking and acceleration as jacking forces are to cornering. The main reason for the difference is due to the different design goals between front and rear suspension, whereas suspension is usually symmetrical between the left and right of the vehicle.

Anti-dive and anti-squat percentage are always calculated with respect to a vertical plane that intersects the vehicle's center of gravity. Consider anti-dive first. Locate the front instant centers of the suspension from the vehicle's side view. Draw a line from the tire contact patch through the instant center, this is the tire force vector. Now draw a line straight down from the vehicle's center of gravity. The anti-dive is the ratio between the height of where the tire force vector crosses the center of gravity plane expressed as a percentage. An anti-dive ratio of 50% would mean the force vector under braking crosses half way between the ground and the center of gravity.

Anti-squat is the counterpart to anti-dive and is for the rear suspension under acceleration.

Anti-dive and anti-squat may or may not be desirable depending on the suspension design. Independent suspension using multiple control arms can be an issue if the percentage is too high (say over 30%). A percentage of 100% in this case would indicate the suspension is taking 100% of the weight transfer under braking instead of the springs. This effectively binds the suspension and turns the independent suspension into no suspension like a go-cart. However, in the case of leaf spring rear suspension the anti-squat can often exceed 100% (meaning the rear may actually raise under acceleration) yet because there isn't a second arm to bind against and the suspension can freely move. Traction bars are often added to drag racing cars with rear leaf springs to increase the anti-squat to its maximum. This has the effect of forcing the rear of the car in the air and the tires onto the ground for better traction.

Short answer - you can have anti-squat with IRS, though in practical application you are limited to a fraction of that available in a live axle.

Bit of a discussion on it here:

http://www.pro-touring.com/forum/showthread.php?t=12696

The geometric construction is only slightly different from what's been posted above, but it generally represents a huge difference in launch behavior (as you already know). The anti-squat line for IRS passes through the rear axle rather than the rear contact patch.

See the ** note in the November 28 post in this thread. Redraw the constructions to suit and you'll probably find the A/S line is at a fairly shallow slope. Perhaps somebody has C5/C6 rear suspension data; I suspect that it runs a bit more A/S than the RX7s did.

Other than doing things to make a big change in SVIC location - maybe some rear shocks with lots of bump damping will help?

Norm