Anti-Lift Geometry

[Drew Price]

On one of my other boards someone brought up the Subaru people using 'Anti-Lift' from suspension kits, which basically do the opposite of anti-dive in the front, by angling the lower control arm upwards (dropping the lower rear control arm mount).

Does anyone know if anti-lift is a rally term? I have never heard it used in road-racing terminology (but I haven't been studying it that long).

My thinking goes:

Anti-lift geometry would move the lower ball-joint forward on suspension droop (under nose lift from acceleration), increasing caster angle, which increases the self-aligning steer effect of the front tires. This would also feed more steering reaction into the steering wheel (seems like it's steering itself). This seems to me to be where the Subaru ads are coming up with reduced corner exit understeer figure, which is not entirely correct, it is more corner exit caster.

The tradeoff would be that there would be more fore/aft diving on braking, and reduced front suspension compliance under acceleration.

These all seem to me like good things on rally courses with soft springs and all-wheel-drive, pushing the front wheels into the ground on acceleration, but in road racing or on the street I think that the reduced compliance, especially under acceleration when not on completely smooth surfaces would be a big problem.

Thoughts?

Best,
Drew

[Matt N]

I think this answers your question.

Any of the anti-(squat, dive, lift) effects in side view, or additionally the roll center position-based jacking forces in front view, all come from some amount of 'bind'. With anti- geometry or roll centers above/below the CG all of the load transfer does not travel through the spring/damper units but some now travels through the suspension links. Logically we can gather that in order to have any of these binding or jacking forces, we first had to have load transfer, which means first of all we had to have grip.

So: the direct answer to your question is that anti-lift (under acceleration) in the front suspension has to be accompanied by some grip in the longitudinal direction at the front tire contact patches (accelerative tractive force in this case), and since your FSAE car probably is not all wheel or front wheel drive it doesn't have any accelerative force on the front tires so it doesn't apply.

Rear wheel drive vehicles can have anti-dive geometry in the front suspension (grip/load transfer provided by braking tractive force), anti-squat (drive tractive force) and anti-lift (braking tractive force again) in the rear suspension.

All wheel drivers can have anti-dive and anti-lift in the front and anti-squat and anti-lift in the rear.

Front wheel drivers can have anti-dive and anti-lift in the front and anti-lift in the rear.

It all gets down to where the forces from the tire grip came from and how they are being reacted into the chassis.

Matt

[Drew Price]

But acceleration in a rear wheel drive car lifts the nose, it's just partially a function of how much rear anti-squat there is. Assuming a little bit, or zero, then wouldn't anti-lift in the front still keep the nose down, regardless of whether there is accelerative tractive force at that end?

I guess I was just trying to wrap my head around why one would employ that geometry, since it seems like the dive in the front over braking would be so exaggerated that it would counteract any gain in acceleration, unless you were in a circuit, like rally or off-road racing, where you are traction limited under the brakes just as much as in acceleration, and it would be a positive tradeoff to be able to accelerate to pull through a corner, rather than after it.

And yes, all that is assuming front or all wheel drive, but one the subject, what does it do under acceleration with RWD?

Best,
Drew

[Alan]

Originally posted by Drew Price:
But acceleration in a rear wheel drive car lifts the nose, it's just partially a function of how much rear anti-squat there is. Assuming a little bit, or zero, then wouldn't anti-lift in the front still keep the nose down, regardless of whether there is accelerative tractive force at that end?

There needs to be a force applied through the suspension for any anti effects to take place. To answer your question, anti-lift on the front would not keep the nose down.

[Bill Kunst]

anti lift on a formula car? try a aero downforce, sounds like it will work http://fsae.com/groupee_common/emoticons/icon_wink.gif