Tires pressures and understeer
10-14-2003 | 03:04 PM
#1
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From: California
Tires pressures and understeer
http://www.tirerack.com/tires/tirete...n/racepres.htm
This was on the 993 board and seems a good read to me.
Joey Bagadonuts did a nice post sometime ago about his experiences with the 964 and Pirellis which is in the archives
This was on the 993 board and seems a good read to me.
Joey Bagadonuts did a nice post sometime ago about his experiences with the 964 and Pirellis which is in the archives
10-14-2003 | 05:38 PM
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From: Albuquerque, New Mexico, USA
Not sure I concur with their rear engine/rear drive suggestions.
With Kumho autocross rubber, I find I need to bias the rear pressure compared to the front a bit to get balanced handling.
The thing with these suggestions is that, as they say, its really only a starting point. There are too many variables for these numbers to work for most drivers in most situations.
And as for their chart, increasing front pressure to reduce understeer is only valid if you are underinflated. If you are close to properly inflated, adding pressure will cause you to push more, not less. As the tire stiffens, its as though you were increasing the spring rate, and increasing the spring rate on the end of the car that is already loosing traction will make it worse, not help.
Plus, if you go too far, you will cause the center of the tire to bulge out, further reducing traction.
The best way to set pressures is with a pyrometer, and failing that, chalk on the shoulders coupled with a sensitive butt-o-meter.
BGL
With Kumho autocross rubber, I find I need to bias the rear pressure compared to the front a bit to get balanced handling.
The thing with these suggestions is that, as they say, its really only a starting point. There are too many variables for these numbers to work for most drivers in most situations.
And as for their chart, increasing front pressure to reduce understeer is only valid if you are underinflated. If you are close to properly inflated, adding pressure will cause you to push more, not less. As the tire stiffens, its as though you were increasing the spring rate, and increasing the spring rate on the end of the car that is already loosing traction will make it worse, not help.
Plus, if you go too far, you will cause the center of the tire to bulge out, further reducing traction.
The best way to set pressures is with a pyrometer, and failing that, chalk on the shoulders coupled with a sensitive butt-o-meter.
BGL
10-14-2003 | 10:27 PM
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Using a pyrometer is definitely the key. Agreed, Brian! That said, here's what I've learned since my previous tests. YMMV.
I've been running MPSC's and Victo's (225/255) at autocross and track events for the past year. The best grip, based on the pyro, falls in the 34-36 psi range HOT.
For the track, I start around 28 psi all around and I usually end up around 35-36 at each corner at the end of a session. For autocross, I start at 34f/32r and usually end at 36 all around. The absence of hard braking and high-speed corners at autocross keeps the fronts relatively cool, so that's why I need to start at higher pressures.
Keep in mind, I run a C4, with a full tank and employ a fair bit of trail braking to rotate my car at autocross. Perhaps the C2's handle differently but that's the only way I can "toss it around" the course with my current suspension and tire combo. And of course, different tracks/courses produce different pressure changes, so there's no hard and fast rule on cold pressures. That's why the pyro is so handy.
With so many variable at work, I think any DE or autocross regular should own a pyro--the probe type. It will eliminate any doubts you have about tire pressures and allow you to focus on your driving.
I've been running MPSC's and Victo's (225/255) at autocross and track events for the past year. The best grip, based on the pyro, falls in the 34-36 psi range HOT.
For the track, I start around 28 psi all around and I usually end up around 35-36 at each corner at the end of a session. For autocross, I start at 34f/32r and usually end at 36 all around. The absence of hard braking and high-speed corners at autocross keeps the fronts relatively cool, so that's why I need to start at higher pressures.
Keep in mind, I run a C4, with a full tank and employ a fair bit of trail braking to rotate my car at autocross. Perhaps the C2's handle differently but that's the only way I can "toss it around" the course with my current suspension and tire combo. And of course, different tracks/courses produce different pressure changes, so there's no hard and fast rule on cold pressures. That's why the pyro is so handy.
With so many variable at work, I think any DE or autocross regular should own a pyro--the probe type. It will eliminate any doubts you have about tire pressures and allow you to focus on your driving.