Wheel lift

[DJHache]

For the last couple years we've noticed that our inside rear tire likes to lift in skidpad (ie steady state cornering). What's worse, it happens that our 03 car seemed to perform better with a wheel off the ground. Basically to reduce wheel lift we had to make our car too understeer-happy.
I've seen a lot of other team with wheel lift and I wanted to know your take on it. We've tried a lot of things to combat it, but once the rear comes off the ground the RARB and rear damper/spring become useless.

[DJHache]

For the last couple years we've noticed that our inside rear tire likes to lift in skidpad (ie steady state cornering). What's worse, it happens that our 03 car seemed to perform better with a wheel off the ground. Basically to reduce wheel lift we had to make our car too understeer-happy.
I've seen a lot of other team with wheel lift and I wanted to know your take on it. We've tried a lot of things to combat it, but once the rear comes off the ground the RARB and rear damper/spring become useless.

[Charlie]

It was unreal how many teams were lifting tires in the endurance. A lot too! I have video of UMR and A&M getting BOTH inside wheels off the ground in endurance. That can't be good for grip. I'm not sure what to think of it, I don't know if those cars weren't set up well for conditions and just being driven the crap out of, or if there really is not much of a loss when things like that happen.

Our car was glued to the track, so was Cornells. I did find one picture of our front barely off the track though.

But lifting tires in skidpad, there's definitely an issue there.

[Kevin Hayward]

Our first car would occasionally lift an inside rear ... but not on a skidpan. We had just over 50% of weight on the front and it would only ever happen when driven really hard.

I would put the lift down to the static weight balance and the amount of caster jacking. Add to that a car with a COG not as low as it could have been and I think it explains the problem.

...

Our '03 car with a lot more weight on the rear and less caster will occasionally lift a front wheel when exiting a hard corner with the right foot down a fair amount. Didn't happen in US ... but then we never ran on a dry track (Autocross or Enduro).

As for suggestions, given that your weight balance is probably less alterable I would look at the amount of caster jacking and try and reduce it.

Hope that helps.

Kev
UWA Motorsport

[Frank]

before i rant on...

do you mind me asking what your static weight split is?

[Akos]

You really don't ever want to lift an inside rear wheel on the track. Its about the worst thing that can happen for good corner exit, that is trying to get on power and have the unloaded wheel just spin.

My guess would be you are running out of droop travel on the rear shock. The rear shock should be somewhere near the middle of travel with the car stationary and driver in the car.

A commom problem I found is that newbies when setting up the car for the first time use the preload on the spring to adjust the ride height or corner weights. You then usually end up with the shock either sitting on the bump stop or fully extended.

If your shock travel is ok, there could be a problem with your roll stifness. If the roll bars are too soft, the car can roll untill the shock becomes fully extended, and lift the wheel off the ground.

Or it can just simply be a cracked front arb, which has happened to me a number of times.

Cheers,

Akos

[Tire Guy]

What type of diff/driveline setup are you guys using? I am just curious as to how you are keeping power on the ground if you're lifting so frequently.

[Brian Smith]

What are the reasons for wheel lift? It seems like I should understand this, but I dont feel like I do.

Simple reasons:
1.Not enough droop travel
an obvious and dumb mistake.

2.Too much rebound damping not allowing the wheel to droop as fast as chassis roll (or as fast as the jacking from caster and offset becomes present)
seems like an easy fix (lower rebound damping and raise bump to keep same roll accel).

3.Very stiff rollbar at only one end.
possibly the toughest fix

Thats all I can think of right now.

Is there another reason I dont see? All of these problems could be fixed. Are teams that lift the rear running more rollbar in the rear?

[DJHache]

We have enough droop travel, it's easy enough to pick up the frame and see how far your droop is. It's also pretty quick to tune the shocks and we tried max compression and minimum rebound. We worked with roll bars too. The caster/kingpin effects are really hard to fight and if that is the cause then we are pretty much stuck with it.

How much roll do you think there is on an FSAE car? I don't think that we would max out our droop. How much would it take in terms of caster/kingpin effects to roll the frame enough that the wheel comes off the ground? I think we may also be fighting some binding which prevents the shock from moving around.

[Travis Garrison]

Well, ask some of the spool teams what they ran...

When I was up at WWU I measured some of our older spool cars to have ~9 deg caster and somewhere around 6-9deg kpi...which is pretty extreme.

What are you guys runing for your steering geometery?

As for the binding, have you lifted the car up and checked? Seems like there would have to be something very funny going on for that to actually hold a wheel off the ground in a situations such as a skidpad.

In any case if you can't fix it, save some weight, switch to a spool and tell the judges you planned it that way http://fsae.com/groupee_common/emoticons/icon_smile.gif

Travis Garrison
UW FSAE