CV axles and anti-squat....

[Cliff Beer]

The CV axles on my CAV are angled forward (relative to the 90 degrees on the longitudinal axis) approximately seven degrees. The car has an audi 016 transaxle on a Ford Racing 302 crate engine. The positioning is no doubt a function of the combined length of the engine and the transaxle output shaft location exceeding the comfortable distance between the bulkhead and the mid-point of the rear wishbones.

What I'm wondering is, is a little bit of forward angle helpful with regard to rear anti-squat? In other words, does the forward angle tend to provide lift to the rear of the car when accelerating by way of the torsional offset? I'm guessing it's not going to be a whole lot but maybe not the worst thing having some modest forward angle (well w/in the safe working range of the CVs). Any thoughts are appreciated.

 

[Dave Deerson]

Anti-squat is designed into the mounting locations of the suspension pick up points and the resulting angles of the control arm/radius rods. The forward angle of the axles would have no influence on anti-squat.

 

[Ian Clark]

Hi Cliff,

Interesting question, Daves correct about the anti-squat coming from suspension design not drivetrain placement. Also live axles and independant axles work entirely different as per driveline torque so it's easy to get befuddled.

In terms of vehicle dynamics, having the largest mass as close to the center of gravity and the middle of the wheelbase as possible reduces polar inertia so there would be less weight transfer to deal with in all directions, roll, squat and dive.

On the matter of rear anti-squat, the stock CAV GT as in the early pre s/n 100 cars could not tolerate anti-squat geometry as the inboard pickup points of the lower reversed A-Arms are solid rubber bushed. It trys to twist this bushing (because the upper trailing arm is shorter than the bottom) in bump or power all it really does is twist the a-arm and momentarily bind the bushing.

This is also the reason there is no rear castor on the "Series 100" CAV's - to protect or minimize forces on the bushings and arms. The lack of rear castor contributes adversely to bump steer as well.

However all this twisting, binding and momentary locking of the suspension makes the rear end twitchy under heavy application of the gas, brakes or turning. You don't do that, do you! Perfectly fine for a drive to the Diary Queen for an icecream though.

It's worth reading the posts on CAV Canadas' performance upgraded Steel Rear Uprights and Reversed A-Arm set-up. We addressed the geometry issues, bringing in anti-squat and rear castor while replacing all rubber bushes with monoballs or spherical joints. The rate of camber climb in bump is dramatically reduced as well, great for wide radial tires contact patch.

The difference in ride, handling and confidence can be felt even on the trip for an icecream - on the track it's a whole new animal. The chassis has been freed up to do it's job and the suspension is not fighting itself. You can concentrate on your line, braking, turn in, apex and exit rather than second guessing the car.

Call me biased, but these are essential parts. Links Here:http://www.gt40s.com/forum/cav-gt40-talk/21858-rear-uprights.html and Here:http://www.gt40s.com/forum/vendor-m...s/27323-cav-series-100-performance-parts.html . Hope that helps.

Cheers

 

[Bob Putnam]

In terms of vehicle dynamics, having the largest mass as close to the center of gravity and the middle of the wheelbase as possible reduces polar inertia so there would be less weight transfer to deal with in all directions, roll, squat and dive.

The only thing (location of mass-wise) that influences the final roll or pitch is the height of the CG. Polar moment influences the rate of change of roll in transition, but not the final state.

 

[Ian Clark]

Hi Bob,

Thanks for the adding to the discussion. I agree that the center of gravity height influences roll. Probably fair to say that if you raised the CG by 10% that would cause a greater effect on weight transfer in braking and acceleration than moving the CG 10% in the wheelbase. What do you think?

Also I agree that moving the major mass closer to the CG does not effect roll, at least not in the sense of total moments that need to be controled. The benefit of getting the major mass closer to the CG is that the roll stiffness front vs rear will be closer.

For example a Mustang has a much stiffer front sway bar than the rear because of the engine being over the front axle. The weight distribution is what, say 58/42 however I'm sure the front roll bar is way more than 38% stiffer than the rear. Probably 100%.

So if you lopped a big hole in the firewall and floor, moved the motor strait back so it was sitting over the CG instead of the front axle, I contend the front roll stiffness would have be lessend and the rear increased to maintain the same total roll resistance.

I almost talked myself out of that argument, the issue comes down to balance, in the Mustang example, after the engine is moved back the total roll resistance doesn't change, however the front end will be too hard and the rear too soft without altering the sway bars. I hope I got that right.

So you were right to call me up on roll resistance, the question of balance is always in my mind. Anti-squat geometry is one of many factors influencing "vehicle dynamics" and for me if the object is to get the "dynamic vehicle" balanced!

 

[Cliff Beer]

Hi fellas,

I do understand the logic that goes along with the weight placement and CG along with height and how that affects weight transfer and squat. And, I also understand the basics behind suspension geometry to lessen the effects of squat ie. placement of the mounting points for wishbones for example. That's all good stuff and, of course, the important stuff. What I was posing as a question is related, but different. Specifically, does the offset of the rotational axis (inboard v. outboard) of a CV axle canted forward some degrees provide any measure of anti-squat effect under acceleration? Said another way, if you think about the how the outboard end of the CV axle which is canted forward will want to move downward under acceleration, shouldn't this provide some limited amount of "lift" to the rear of the car, such lift acting as proxy for anti-squat? I would guess that the degree of this anti-squat effect is related to the coefficient of friction of the CV joints whereby greater friction would cause a higher torsional downward effect.

Maybe I'm just on drugs here however....

 

[Jac Mac]

Cliff, its 7.30pm friday night here in NZ, dont tempt my warped sense of humour with lines like that last one----- short answer, only the friction factor would cause a torque arm effect in the manner your suggesting , but since CV's torque 'should' be transmitted thru the shaft/CV centerline then - no!

 

[Ian Clark]

Hi Cliff,

Jac's got it right on the torque arm effect. This comes back to what I said about independant and lives axles behaving differently. Although not a drug induced state, the transaxle on power wants to walk up the crown gear in the diff, just like a live axle wants to wind up, all this torque load is controlled by the drivetrain mounts (or should be), none of it transfers to the uprights.

Cheers

 

[Cliff Beer]

Allright guys, super, thank you, I get it now. Was just trying to think of any positive reason to leave the position of the engine/transaxle as-is rather than squeezing it up a little closer to the bulkhead in an upcoming engine and transaxle swap.

Much appreciated.