: Autocross Pressures
08-21-2002, 04:45 PM
Hey everyone. I am heading out to my 2nd autocross on Sunday morning and I am looking for a little help.
My car, B15 SE, is pretty stock and I want to get a better tire pressure set-up for the cross. I usually run about 35psi all the way around.
Is there a better pressure to shoot for in getting better turn-in on the course.
08-21-2002, 06:12 PM
Are you running the Firestones still? If so, you will have to pump the fronts way up, because the 'Stones have no sidewall stiffness to them at all.
If you're running a decent tire, try starting at 45/35, and see how it goes. (You're familiar with the chalk trick, right?) If you've got the Firestones on, bump them up a little higher -- I ran mine at 48 pounds until I wore them out.
If you do that, though, be damn sure you bleed the tires before you drive home, or you'll likely be in for an unpleasant surprise.
08-22-2002, 11:29 AM
I don't know what the stock tire pressure for the rear is on the B15, but say it's 29psi(like on the B14), well, you want to keep it there during your run. Only raise the fronts and leave the rears alone. You'll get better rotation that way(something sentras can always use).
08-22-2002, 12:19 PM
OK I will raise the fronts up to 40psi I think for some better turn-in grip, and leave the rears at 35psi or maybe bleed them down to 33psi or something. Don't want to fool with pressures all day.
08-22-2002, 02:59 PM
Basically, raise the fronts up until they're not rolling over on the sidewalls, while leaving the rears alone. Once you find the needed pressure, play with the rear if you want to see if you can get better rotation. No need to mess with them constantly, like you mention.
I run 40/38 as a base on my Azenis! driving illini runs the firestones still and if I remember correctly he runs the fronts at 50 and the rears at about 45! i'm sure he'll reply and let us know!
08-22-2002, 06:16 PM
Have you used the shoe polish trick yet? If not I'd highly recommend it. If you havent heard of it, thats where you take some white liquid shoe polish, and put it in 3-4 spots around each of your tires so you can see it no matter how the tire rest when you look at it afterwords. Have it extend down (an inch or so) on the sidewall, and a little bit up onto the tread. After your run, look at how much of the shoe polish has been worn off. Ideally that line would be right at the edge of your tread. With the firestones though, you may still have to accept a little bit of tire flex. After that, adjust tire pressure as necessary to get the desired results. This is why people bring portable air supplies like minicompressors or compressed air tanks. If you know somebody that autoxes, see if you can borrow theirs. If don't know anybody, you could ask around and most likely find someone willing to let you borrow theirs during the day. Otherwise if you plan to keep autoxing, you may want to consider investing in one. You can pick up mini compressors at pep boys for like $15.
I made the mistake of running stock off the lot tire pressures (33/30ish)? on my first autox. One of my tires got a 'u' shaped cut in the rubber...Half way down the sidewall! All the outside edges also had a ground beef look to them and swirl scrapes an inch or so down the sidewall. That was when I learned about the shoe polish. That little trick helped me find the setup I run. Depending on the course, I run 48-51 front and 39-41ish rear, on a dry day. Don't worry about "max psi" ratings. For a few runs, during a few hours, on a few days a year, your tires will handle it just fine. Having your tires fold under is far more damaging. The firestones need the extra pressure in front to keep from rolling over too much in hard cornering. Even at that pressure, after a little over 1 season under my belt, my outside edges are still worn to hell, and I need new tires. The outside edge of my tires are 1+ inch radius ¼ rounds instead of being squared off the way they come from the factory. Some of that is probably my driving style. Still being a relative rookie, I tend to push too fast into longer and/or tighter corners, which no amount of pressure up front may be able to save my front outside edges. The rear pressures I run are actually a little on the high side by "shoe polish" standards, and actually bow the tread out a little which decreases contact patch allowing me to get relatively neutral handling.
If you have any doubts or questions as to what to do and what changes to make, ask. Most anybody there will probably be more than willing to help you out and give you some advice/pointers.
08-22-2002, 06:36 PM
Driving Illini's advice is excellent. You can also use chalk, rather than shoe polish. it's less messy, won't fling onto your paint, and brushes off your tires nicely, whereas polish can leave a stain.
For reference, I ran 48/33 on my Firestones. The 33 in the back made it feel alot more willing to rotate.
08-22-2002, 07:27 PM
I will try the shoe polish deali out. I know last time I ran the 35psi all teh way around, and at teh end of my 5 timed runs all four tires had scuffs down the sidewall almost to the rim. I will crank it up further this time.