Reducing rear tire pressure reduces oversteer?
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From: Ottaweenieville, Canada
Reducing rear tire pressure reduces oversteer?
I know it is backwards from the conventional wisdom, but it seemed to work last weekend, and I'm trying to understand why.
Car is a 944S2, caged, 400# front springs, 30mm rear torsion bars, 30mm front sway bar, 20mm rear sway bar. Toed-out in front a bit, toed-in at rear a bit, camber of -2.25 front, -2.75 rear
Last year, the car was set up, aligned, and corner-weighted for track with 245/45x16 on 7.5" rims front and 265/45x16 (tall!) on 9" rims rear. V710's all around. As one would expect, safe, predictable understeer, with equal tire pressures all around (34 psi). My primary objective last year was time trials, and the car was perfect for that.
This year I decided to get V710's only for autox, and put 215/40x16's on 7.5" rims all around. No other changes (yet). I started with 38psi cold all around. First event was last weekend, the course was full of slaloms of various kinds (offset gates, tight, open, equal length, decreasing length, etc. etc.). First run had some oversteer, but I use the first run to find my way around. Second run I changed nothing except I went for it, and had oversteer on corner exit, and some at mid-turn.
Now I forgot to bring my setup cheat sheet, and I never remember to add or subtract pressure. So I thought, lower rear pressure, rear tire gets more grip, ergo less oversteer. I dropped the rears by 2 psi, and, on the next run(s), I could put the power down nicely. Of course, I put down so much power that I went in too fast in a section and screwed up, but that's beside the point.
I get home and read I did it backwards ... I'm supposed to have raised the rear pressure to decrease oversteer.
Yet what I did ... worked ???
Car is a 944S2, caged, 400# front springs, 30mm rear torsion bars, 30mm front sway bar, 20mm rear sway bar. Toed-out in front a bit, toed-in at rear a bit, camber of -2.25 front, -2.75 rear
Last year, the car was set up, aligned, and corner-weighted for track with 245/45x16 on 7.5" rims front and 265/45x16 (tall!) on 9" rims rear. V710's all around. As one would expect, safe, predictable understeer, with equal tire pressures all around (34 psi). My primary objective last year was time trials, and the car was perfect for that.
This year I decided to get V710's only for autox, and put 215/40x16's on 7.5" rims all around. No other changes (yet). I started with 38psi cold all around. First event was last weekend, the course was full of slaloms of various kinds (offset gates, tight, open, equal length, decreasing length, etc. etc.). First run had some oversteer, but I use the first run to find my way around. Second run I changed nothing except I went for it, and had oversteer on corner exit, and some at mid-turn.
Now I forgot to bring my setup cheat sheet, and I never remember to add or subtract pressure. So I thought, lower rear pressure, rear tire gets more grip, ergo less oversteer. I dropped the rears by 2 psi, and, on the next run(s), I could put the power down nicely. Of course, I put down so much power that I went in too fast in a section and screwed up, but that's beside the point.
I get home and read I did it backwards ... I'm supposed to have raised the rear pressure to decrease oversteer.
Yet what I did ... worked ???
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I'm eagerly awaiting responses to this as well. My understanding is: dec oversteer by inc traction, and if we're talking just the tire pressure variable, then optimizing traction may mean inc pressures if your too low. But, if you're pressures are too high, then lowering pressure would be thing to do.
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Where's our brain trust?
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I'm no expert, but 38 psi cold seems somewhat high for V710s. Perhaps they were over 40 psi after the first run, thus reducing contact patch and grip for the second run. When you reduced pressure before the third run, you may have provided more contact patch and also put the tires more in their grippy range. If that's the case, however, by leaving the fronts so high, you should have experienced some push. The increase/decrease pressure guidelines for one end are relative to the other end. There is a point at which further increases in psi actually cause a reduction in grip.
Last edited by Jim Michaels; Aug 1, 2006 at 12:14 PM. Reason: correction
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From: Ottaweenieville, Canada
Yeah, 38 psi cold was high - 41 hot. I have a test'n'tune Friday in Toronto so I'm going to drop to 34psi cold all around and start over.
I also did a run in my new street tires (RE050A PP, 245/40x17 all around, 36psi) and the car (as expected) does oversteer. I'm changing sway bars to try and tune the oversteer out in that case as well, so we'll see.
I also did a run in my new street tires (RE050A PP, 245/40x17 all around, 36psi) and the car (as expected) does oversteer. I'm changing sway bars to try and tune the oversteer out in that case as well, so we'll see.
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Craig - as long as the tires are within the acceptable window of pressure/temp, reducing the air pressure does increase grip at that end of the car. If a tire is seriously underinflated, increasing pressure will also increase grip, that's why I underline the notion of a tire being withini an acceptable window of pressure and temperature. I didn't use the V710 a lot but I recall that they liked around 36 hot. For a short autocross run, 34-35 starting pressure sounds about right. Hey, are you ever coming back to Le Circuit?
Best,
Bob
Best,
Bob
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From: Phoenix, Az
Like Bob said.
More/less air can create understeer or oversteer depending on what part of the pressure vs grip point you are at.
In my race car I target my hot tire pressues at what I believe is their optimal grip. I don't use them to balance the car since any change in pressure wil reduce the tires grip. In stead I chose to adjust sway bars since this does a better job to shift balance and maintain my optimal tire pressure.
More/less air can create understeer or oversteer depending on what part of the pressure vs grip point you are at.
In my race car I target my hot tire pressues at what I believe is their optimal grip. I don't use them to balance the car since any change in pressure wil reduce the tires grip. In stead I chose to adjust sway bars since this does a better job to shift balance and maintain my optimal tire pressure.
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From: Blacksburg, VA
I think your suspension cheat sheet suggests a thicker sway bar in front, or a thinner bar in back, depending on which end you think is already closer to the edge. I autocrossed an '84 944 in stock classes (SCCA BS, DS, and CS - they kept moving it around) starting 20 years ago, but only had push to deal with; minimized by setting Koni shocks to full soft in front. Some oversteer is desirable for autocross, so later I disconnected the front sway bar to get oversteer on my RSA for autocross. Just had to remember to reconnect it before going on track. Yikes!
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From: Long Island, NY
Craig,
I too had the same experience. I was starting with the tires at about 36 cold which brought them to about 41 hot. One guy showed me that there are arrows on the sidewalls and that I can use the tire up to that point. I lowered the cold pressure to about 32-33 and it has increased me rear traction greatly. I still have not reached the arrows so I can take more air out if I want. By the way I am using the V710's also.
I too had the same experience. I was starting with the tires at about 36 cold which brought them to about 41 hot. One guy showed me that there are arrows on the sidewalls and that I can use the tire up to that point. I lowered the cold pressure to about 32-33 and it has increased me rear traction greatly. I still have not reached the arrows so I can take more air out if I want. By the way I am using the V710's also.
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Joined: Nov 2004
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From: Long Island , NY
Adjusting tire pressures will help, adjusting your driving style will improve your times a lot more. I am actually keeing up in my class with stock sway's and springs, KYB's with 20K on them and and and automatic transmission.
P.S. Dan I plan on posting laps 5 sec or more better than yours next Sunday !!!!!!!!!!!!!!
P.S. Dan I plan on posting laps 5 sec or more better than yours next Sunday !!!!!!!!!!!!!!
Race Car
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From: Long Island, NY
Originally Posted by Energy9
P.S. Dan I plan on posting laps 5 sec or more better than yours next Sunday !!!!!!!!!!!!!!
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From: Montreal
Craig - for your cheat sheet:
Oversteer - decrease rear tire pressure.
soften rear sway bar
soften rear shocks
stiffen front sway bar
Understeer - decrease front tire pressure
soften front sway bar
stiffen rear sway bar
soften front shocks
It can be summarized simply, to add grip soften the end of the car that needs more. Ross Bentley also points out that you should never use more roll stiffness than you need because body roll transfer weight to the outside (loaded) tires for more grip.
Naturally everything is a compromise and a track with a lot of fast transitions (autocross comes to mind) will call for more roll stiffness than a circuit with a lot of sweepeing turns.
Best,
Understeer
Oversteer - decrease rear tire pressure.
soften rear sway bar
soften rear shocks
stiffen front sway bar
Understeer - decrease front tire pressure
soften front sway bar
stiffen rear sway bar
soften front shocks
It can be summarized simply, to add grip soften the end of the car that needs more. Ross Bentley also points out that you should never use more roll stiffness than you need because body roll transfer weight to the outside (loaded) tires for more grip.
Naturally everything is a compromise and a track with a lot of fast transitions (autocross comes to mind) will call for more roll stiffness than a circuit with a lot of sweepeing turns.
Best,
Understeer
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From: Ottaweenieville, Canada
Bob - thanks, the pressure thing just goes against the "usual" recommendations, e.g., Tirerack's http://www.tirerack.com/tires/tirete....jsp?techid=58
... and Porsche, who raised the recommended rear pressure for my car to 44psi (front 36psi) to reduce oversteer.
However, the idea of being on one side or the other of the grip/pressure hump makes sense.
As for Tremblant, I thought only GT3's were allowed in RSR events?
Seriously though, I'm going back to Targa Newfoundland in a few weeks, and that's where the budget went this year. (Last year it was the Parade.) I may do some PCA DE next year, but Tremblant just isn't cost-effective for a non-instructor not-at-many-events time-off-during-the-week-costs-money person like me. 
M758: you're right, but this was the first time on this tire size, so I went with everything as-is to start. The car exhibited a lot less oversteer with 245/45x16 RA-1's on 9" rims all-around, but the V710's (and the different sizes/rims) seem to behave differently.
Energy9: You're right, but I ran out of talent 8 years ago, and it ain't coming back. Hack away I must.
Here's a few seconds of my first run that someone video'd, i.e., "where-the-h*ll-do-I-go-next" http://www.seko.ca/SII30Jul2006.wmv (right-click, save as ...)
... and Porsche, who raised the recommended rear pressure for my car to 44psi (front 36psi) to reduce oversteer.
However, the idea of being on one side or the other of the grip/pressure hump makes sense.
As for Tremblant, I thought only GT3's were allowed in RSR events?
Seriously though, I'm going back to Targa Newfoundland in a few weeks, and that's where the budget went this year. (Last year it was the Parade.) I may do some PCA DE next year, but Tremblant just isn't cost-effective for a non-instructor not-at-many-events time-off-during-the-week-costs-money person like me. M758: you're right, but this was the first time on this tire size, so I went with everything as-is to start. The car exhibited a lot less oversteer with 245/45x16 RA-1's on 9" rims all-around, but the V710's (and the different sizes/rims) seem to behave differently.
Energy9: You're right, but I ran out of talent 8 years ago, and it ain't coming back. Hack away I must.
Here's a few seconds of my first run that someone video'd, i.e., "where-the-h*ll-do-I-go-next" http://www.seko.ca/SII30Jul2006.wmv (right-click, save as ...)
Race Car
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From: Long Island, NY
Originally Posted by Energy9
Dan,
Car will be according to the rules, will pass tech with flying colors, and 5 sec is probably conservative.
Car will be according to the rules, will pass tech with flying colors, and 5 sec is probably conservative.
