Auto-cross help
04-11-2013, 09:05 PM
#1
Melting Slicks
Thread Starter
Auto-cross help
A couple of days ago I attended an auto-cross event at Limerock Park.
I usually drive a vette but I took my Shelby Mustang to the track. The car pushed (understeered ) badly which I understand is common for the heavy 500 hp rear drive car. The Instructers had us air up to 45lbs all around. I used 7 year old Pirelli P ZERO NERO 235/55R17's front and 2 year old 255/45R18's on the rear. The rear was sideways often but very predictable and controllable but the front just plain sucked. My shocks and sway bars seemed to work well. I have had this car on the full track and had fun with it but on the tight track it was hell. I dropped the pressure in front 4 lbs but as luck would have we switched to the newer track, then combined the old and new tracks together so I never got a direct comparison to see if the drop helped. I have no plans to seriously auto cross this car but I would like to improve this situation and try again. I was thinking of putting the 255/R18's on the front and buying 2 more wheels and getting some more Pirellis in either the same size or go to 275's rear or buying some Hankook Ventus V12's to try. What do the experts say??
It was also suggested that I dial in 2 degrees of negative camber in front but this would probably chew up tires also!
I usually drive a vette but I took my Shelby Mustang to the track. The car pushed (understeered ) badly which I understand is common for the heavy 500 hp rear drive car. The Instructers had us air up to 45lbs all around. I used 7 year old Pirelli P ZERO NERO 235/55R17's front and 2 year old 255/45R18's on the rear. The rear was sideways often but very predictable and controllable but the front just plain sucked. My shocks and sway bars seemed to work well. I have had this car on the full track and had fun with it but on the tight track it was hell. I dropped the pressure in front 4 lbs but as luck would have we switched to the newer track, then combined the old and new tracks together so I never got a direct comparison to see if the drop helped. I have no plans to seriously auto cross this car but I would like to improve this situation and try again. I was thinking of putting the 255/R18's on the front and buying 2 more wheels and getting some more Pirellis in either the same size or go to 275's rear or buying some Hankook Ventus V12's to try. What do the experts say??
It was also suggested that I dial in 2 degrees of negative camber in front but this would probably chew up tires also!
Last edited by BPHORSEGUY; 04-11-2013 at 09:34 PM.
04-12-2013, 11:26 AM
#2
Drifting
Camber doesn't wear tires as much as toe. Two degrees should be fine if you can get it. My guess is the old tires in the front were killing you. 45 psi also seems really high, even for a car as heavy as a Mustang. I think I square tire setup would be the way to go, since you can rotate them more easily to maximize wear as well as needing fewer spares. I'd give that a try as the first place to start.
04-12-2013, 07:30 PM
#3
Melting Slicks
Thread Starter
Camber doesn't wear tires as much as toe. Two degrees should be fine if you can get it. My guess is the old tires in the front were killing you. 45 psi also seems really high, even for a car as heavy as a Mustang. I think I square tire setup would be the way to go, since you can rotate them more easily to maximize wear as well as needing fewer spares. I'd give that a try as the first place to start.
04-12-2013, 07:56 PM
#4
Burning Brakes
A square setup should help a lot and newer stickier tires will also make it more neutral. I autocross and track an older 99 Mustang with 315 square V710s and it is very neutral like that. If I put the street tires on which is a staggered 315 rear 275 front it pushes terribly if I drive it hard. I set my tire pressures around 30 front and 27 rear cold and forget about them.
45 lbs is an old school approach to keeping the tires on the rims. It's unnecessary and kills any chance of grip. Was the instructor in his 80s?
45 lbs is an old school approach to keeping the tires on the rims. It's unnecessary and kills any chance of grip. Was the instructor in his 80s?
04-12-2013, 08:05 PM
#5
Melting Slicks
Thread Starter
A square setup should help a lot and newer stickier tires will also make it more neutral. I autocross and track an older 99 Mustang with 315 square V710s and it is very neutral like that. If I put the street tires on which is a staggered 315 rear 275 front it pushes terribly if I drive it hard. I set my tire pressures around 30 front and 27 rear cold and forget about them.
45 lbs is an old school approach to keeping the tires on the rims. It's unnecessary and kills any chance of grip. Was the instructor in his 80s?
45 lbs is an old school approach to keeping the tires on the rims. It's unnecessary and kills any chance of grip. Was the instructor in his 80s?
Last edited by BPHORSEGUY; 04-12-2013 at 08:07 PM.
04-16-2013, 10:53 PM
#7
Max G’s
with Han Solo, 45 is probably too high and the most important part of your handling is the tires. You need to know how to read them. You want the tires to roll over to the edge of the sidewall to get max grip. Too much psi and the tire cannot get to it's ideal slip angle which is where max grip will occur.
