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

BrianCunningham wrote:
OK here's the question

Since the uprights don't see engine torque

Can you even have anti-squat with and IRS, any IRS?

They'll see braking torque, since the brakes are mounted on
the uprights, unless you running inboards, but engine
torque is a 'pass through' deal.

CentralCoaster wrote:..

I have a feeling that somebody that does this stuff for a living is going to show up here and make us all feel stupid!

CentralCoaster wrote:
Sorry, you lost me at the knuckles "see" the engine torque.

They don't. They can't. There's a bearing in there, which can't transmit any torque to the knuckle.

CentralCoaster wrote:
The 3rd member on the other hand, does see the torque, because the pinion is trying to climb the ring gear, pushing the snout upwards. You can do that all day long, but the only torque it's going to transmit to the knuckles is some tiny amount from the twisting/binding of the lower camber rod bushings.

CentralCoaster wrote:
I have a feeling that somebody that does this stuff for a living is going to show up here and make us all feel stupid!

CFI-EFI wrote:
But there are wheel bearings in a solid wheel end also.

I don't think the pinion trying to climb the ring gear is the major force, here. Remember, the torque input at the pinion yoke is only approx 1/3 the torque at the spindle. If the bulk of the torque were at the pinion yoke, the "C" beam would be turned into a pretzel.

CentralCoaster wrote:
Now the dogbones, you could put the car on jackstands and
remove them entirely, put the car in gear and punch it, and
the knuckles wouldn't rotate at all.

CentralCoaster wrote:
My Corvette specs book shows 62% anti-squat for 87 and 88
vettes.

Now the dog bones, you could put the car on jack stands and remove them entirely, put the car in gear and punch it, and the knuckles wouldn't rotate at all. The only torque transmitted to them would be the bearing and brake drag, and the slight twisting of the lower camber rods with their glorious binding bushings.

*the tech drawing provided a few posts above appears to show the overall wheel base at 96. is that correct? *

Herb Adams book Chassis Engineering has a good chapter on IRS using the C4 as a example.... explains a LOT about this. Good book to have if you are a BSP/SM2 guy with a C4.

Only real downfall of a IRS is the anti-squat, practical limit is about 25%.

CFI-EFI wrote:

My 1984 Spec book says:
"Rr susp geometry - control arms pos to produce 51% anti-squat."

RACE ON!!!

by BrianCunningham on 2009/3/10 10:23:06 wrote:



Off the SCCA forum

Quote:

Herb Adams book Chassis Engineering has a good chapter on IRS using the C4 as a example.... explains a LOT about this. Good book to have if you are a BSP/SM2 guy with a C4.

Only real downfall of a IRS is the anti-squat, practical limit is about 25%.

CentralCoaster wrote:
I wonder if they're using the same baseline. Is 100% neutral, or 0% neutral?

I also have zero faith in the numbers in that Corvette specs book.

CentralCoaster wrote:
I believe that is his source. I recall somewhere there was criticism of the MVMA specs (maybe from the author himself), that GM wasn't very careful about the specs in the first place, and that some of them were pre-production.

I really don't know exactly. But there's no way in hell 87 and 88 have the same rear anti-squat if GM moved the dog bone brackets in 88.