Basic question. What is the effect of tire air pressure on an axle's tendency to slip?
08-08-2005, 06:31 AM
#1
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Basic question. What is the effect of tire air pressure on an axle's tendency to slip?
I've had conflicting answers. With an Audi, the big concern is getting the rear end to rotate, even just a little.
I've been told to drop pressure to create slip angle, and I've been told to increase pressure to to loosen up the rear. Which is correct?
My last event at VIR I never got it right, although I did notice that I was generally overinflating at all four corners.
I've been told to drop pressure to create slip angle, and I've been told to increase pressure to to loosen up the rear. Which is correct?
My last event at VIR I never got it right, although I did notice that I was generally overinflating at all four corners.
08-08-2005, 10:05 AM
#2
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Answer given by TireRack.
Says that you reduce pressures to increase slip angles. Not what I've heard elsewhere, but I general trust TireRack.<ul><li><a href="http://www.tirerack.com/a.jsp?a=AR4&url=%2Ftires%2Ftiretech%2Ftechpage.jsp %3Ftechid%3D58">Less pressure = more slide.</a></li></ul>
08-08-2005, 01:49 PM
#3
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Be careful how you interpret any answer...
...as you don't really know where the median point is - 'less than WHAT ?' or 'more than WHAT ?'.
If you're at ideal tire pressures - where the tire generates the best stick and where you have a uniform spread of tire temps across the face of the tire - then I disagree that you should drop rear pressures to decrease traction.
When I want to loosen up a car, I raise the rear pressures, sometimes significantly. This works on FWD, RWD, front engined, rear engined etc. etc. etc. race cars.
Many times, if you are dropping pressures below an ideal point, then you're simply overheating the tire and it will get greasy, and you'll lose traction. I don't think that's a good way to manage grip.
My own opinion is that 1 lb. of air in the tires feels like 10-20 lbs. of spring in that same point of the suspension. Some tracks, some tire/car combinations, some conditions require extreme measures. We've run 24 hour races at abrasive tracks that really chew the front tires off the car, so we'll resort to blowing the backs up like bowling ***** to keep the car loose so the fronts won't have to do so much work (bowling ball = >75 psi). Sometimes, we'll just run different tires front & back, and trim pressures until we get the handling that we want (example - 710's on the front, Victoracers on the back).
Experiment. Do not take anyone's advice as gospel. Most people at race tracks are full of $hit.
If you're at ideal tire pressures - where the tire generates the best stick and where you have a uniform spread of tire temps across the face of the tire - then I disagree that you should drop rear pressures to decrease traction.
When I want to loosen up a car, I raise the rear pressures, sometimes significantly. This works on FWD, RWD, front engined, rear engined etc. etc. etc. race cars.
Many times, if you are dropping pressures below an ideal point, then you're simply overheating the tire and it will get greasy, and you'll lose traction. I don't think that's a good way to manage grip.
My own opinion is that 1 lb. of air in the tires feels like 10-20 lbs. of spring in that same point of the suspension. Some tracks, some tire/car combinations, some conditions require extreme measures. We've run 24 hour races at abrasive tracks that really chew the front tires off the car, so we'll resort to blowing the backs up like bowling ***** to keep the car loose so the fronts won't have to do so much work (bowling ball = >75 psi). Sometimes, we'll just run different tires front & back, and trim pressures until we get the handling that we want (example - 710's on the front, Victoracers on the back).
Experiment. Do not take anyone's advice as gospel. Most people at race tracks are full of $hit.
08-10-2005, 05:54 AM
#6
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I have KW coilovers with fairly high spring rates (don't have specs, but imagine solid steel pipes..
... in place of the coil springs). At my last track event, I tried lots of pressures (starting high and going down) and found out that I was gaining 4 or 5 lbs. from heat alone. I ended up at 36 lbs. (cold) all around. It still understeered like a container ship. It could be that the coilovers are setting up an irreversible understeer.
08-10-2005, 06:14 AM
#7
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You haven't told us what kind of tires...
...or what your starting pressures are, or if you have adjustable bars, or what your shock setting are, or spring rates. At best, we can throw darts...but we don't even have enough information to throw them in the proper direction.
BTW, if you're only building 4-5 lbs. of pressure in the tires, you're not even fully warming them up.
BTW, if you're only building 4-5 lbs. of pressure in the tires, you're not even fully warming them up.
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08-10-2005, 06:48 AM
#8
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OK.
1. Kumho Ecsta 255 40 17s.
2. 38 lbs (cold (as posted above).
3. No adjustable bars.
4. Shocks are not adjustable.
5. Springs are stiff (as posted above). I don't know rates).
<b>I just want to know if there is a general rule on varying tire pressures front-to-back in order to make one end looser or tighter.</b> TireRack offers a suggestion (posted below) but that's been contradicted. I suspect, now, that there is no hard and fast rule, and I'll just need to experiment.
2. 38 lbs (cold (as posted above).
3. No adjustable bars.
4. Shocks are not adjustable.
5. Springs are stiff (as posted above). I don't know rates).
<b>I just want to know if there is a general rule on varying tire pressures front-to-back in order to make one end looser or tighter.</b> TireRack offers a suggestion (posted below) but that's been contradicted. I suspect, now, that there is no hard and fast rule, and I'll just need to experiment.
08-10-2005, 07:37 AM
#9
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Here's our baseline on...
...Ecsta V700's -
FWD Showroom Stock car, 140hp @ the crank (when it was new...), ~2750 lbs. w/driver, stock suspension (cuz that's all that's allowed):
Fronts cold = ~34-36, fronts hot 43-46.
Rears cold = ~42-50+, rears cold ~52-56+.
If you run the same pressures front and back, you're just going to get understeer. Dropping the backs down 3+ lbs. will just roll them over the shoulders, and you'll burn the edges off the tires (look for tell-tale blue). Dropping pressures 1-2 lbs. on one end of a big heavy car won't give you that much different a feel (no Princess/pea thing going on here), so you have to make changed in big increments.
You've got a pix of VIR on your post...we ran Ecstas on all 4 corners at the VIR 13 Hour last October - first time we'd run a whole long enduro race on full sets of Ecstas (x4)- and we played with pressures all race long to hit the 'feel' we wanted. The surface at VIR seemd very 'Ecsta-friendly', where we've found other tracks feel aweful with those same tires. We were getting 3-4 hours of life from the fronts, before we were 'on the wires', and 5-8 from the rears.
Experiment. Experiment experiment experiment. Try unhooking the front bar, especially if you are running high spring rates. You don't have to do more than just disconnecting one bar link, as that will effectively remove it from the mix. Don't make 1-2 lb. pressure changes, as you won't feel them, session-to-session. Make big changes.
Really.
FWD Showroom Stock car, 140hp @ the crank (when it was new...), ~2750 lbs. w/driver, stock suspension (cuz that's all that's allowed):
Fronts cold = ~34-36, fronts hot 43-46.
Rears cold = ~42-50+, rears cold ~52-56+.
If you run the same pressures front and back, you're just going to get understeer. Dropping the backs down 3+ lbs. will just roll them over the shoulders, and you'll burn the edges off the tires (look for tell-tale blue). Dropping pressures 1-2 lbs. on one end of a big heavy car won't give you that much different a feel (no Princess/pea thing going on here), so you have to make changed in big increments.
You've got a pix of VIR on your post...we ran Ecstas on all 4 corners at the VIR 13 Hour last October - first time we'd run a whole long enduro race on full sets of Ecstas (x4)- and we played with pressures all race long to hit the 'feel' we wanted. The surface at VIR seemd very 'Ecsta-friendly', where we've found other tracks feel aweful with those same tires. We were getting 3-4 hours of life from the fronts, before we were 'on the wires', and 5-8 from the rears.
Experiment. Experiment experiment experiment. Try unhooking the front bar, especially if you are running high spring rates. You don't have to do more than just disconnecting one bar link, as that will effectively remove it from the mix. Don't make 1-2 lb. pressure changes, as you won't feel them, session-to-session. Make big changes.
Really.
08-10-2005, 10:24 AM
#10
AudiWorld Super User
Thread Starter
Thanks. 13 hours at VIR!? (PS: My Ecstas are not V700s, but 712 street tires.)
<center><img src="http://www.albeedigital.com/supercoupe/articles/images/ku_712.jpg"></center><p>
For fun, I made my sig into an animated GIF of the Oak Tree Turn at VIR. I never really nailed that turn, but it does show how rock hard the coilovers are.
For fun, I made my sig into an animated GIF of the Oak Tree Turn at VIR. I never really nailed that turn, but it does show how rock hard the coilovers are.