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post #1 of 8 Old 10-02-2010, 10:47 PM Thread Starter
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NooB Autocross Question going back out sunday.

I have tire pressure question. I am new to auto crossing. I went last week and again today. I plan on going out Sunday also Here is my question i am running 38psi front 35 psi rear. This is because on the scca web site states that lower tire pressure can cause the tire to seperate from the wheel. In running two days i show outside edge wear on both fronts (understeer) with the drivers side being the worst. The car has quite a bit of neg camber. -3deg i would suspect.

So here is my question can i use 33-35psi front 28-32psi rear safely???

I will say you auto-Xers are on to something. This stuff is a BLAST!!!
I might just give up drag racing for a year or two!!!

thanks for any help you can give me!!!


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post #2 of 8 Old 10-02-2010, 11:56 PM
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Short version: yes. IME, you can auto-x safely with air pressure in the low 20s, but not that you'd want to. 36 front, 32 rear is a common starting point, but I've done 32 front and 28 rear before. With R comps, I think I have done 26 front and 22 rear. Their warning probably pertains to cars with tires that have 60+ series sidewalls.

One of the experts like Strano can correct me if I am wrong, but typically you raise front pressures and lower rears to fight understeer. I've known people to auto-x foxbodies on street tires before with 40ish psi front starting pressures. But with -3 camber, you shouldn't need that much. You may just be understeering due to coming into corners way too hot or taking funny lines since you are new. Try being a little more patient and see if you can't get someone to take your tire tems (inner, middle, outer) after your runs. I like to tell my students to pretend that the cones are children. SCARE THE CHILDREN BUT DO NOT HIT THEM! This usually makes them drive better, and thus faster.

Also, per John Ames (multi time national champ badass and fox body god), the fastest way to get a mustang around a course is if it is just barely understeering around the corners.

Glad you are having fun!


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post #3 of 8 Old 10-03-2010, 12:11 AM Thread Starter
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THANK YOU!! I guess i should give you a little more info. The car is a 2000 mustang gt 5 speed. The car is supercharged and has a lots griggs parts on it. The car is still very new to me (less than 1500 miles). The tire's are 265/35/18 BFGoodrich g-Force Sport. They are new 300 miles.

thanks jon

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post #4 of 8 Old 10-03-2010, 09:09 AM
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One approach is to draw chalk lines at intervals around the tires, from the tread onto the sidewall. After a run, take a look at how much chalk has been scrubbed off. Ideally, so the theory goes, the chalk should be rubbed off all of the tread but still there on the sidewall. If the chalk is rubbed off the sidewall, you need more pressure. If there is still chalk on the edges of the tread, you can use less pressure.

Be careful going lower if you are already getting a lot of outside edge wear.

As the previous poster mentioned, going into turns too fast and getting massive understeer is a very common issue. Sometimes it is hard to notice in the heat of a run. It wasn't until my first in-car video that I was able to see this problem in my own driving. Watching myself after the event made it obvious how badly I was over driving the slow turns A more experienced person told me that, on a street tire, a little squealing is good, but shrieking is bad.


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post #5 of 8 Old 10-07-2010, 01:05 AM
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It's been an aiful long time ince I drove on street tires... so I do not remember what tire pressures I used to run...

I use DOT R comps (Kuhmo V710's now) and generally run 30 or so in the front and 26-30 in the rear depending on temps/sun vs shade, ect. I think I am going to try a set of Hoosier A6's next season though so it will be yet another learning curve.

Typically lower pressures yield more grip with more "roll over" and higher pressures reduce "roll over" but yield less grip. You need to find a balance between the two to become competative. keep in mind that while lower pressures yield more grip, this only occurs until you start to "roll over" the tires and grip is lost due to loss of available contact patch.

I agree with the above statements of look to the driver first because rookie drivers commonly overdrive the cars... but I disagree with higher front pressures reducing under steer.

understeer = turn wheel, car plows through turns (or tries to go straight)
oversteer = turn wheel, car rear end tries to come around (or fish tails)

slight oversteer is where I like my car... every driver is going to like a car setup to be slightly different.

another common rookie mistake is that when the car starts to understeer, drivers tend to turn the wheel even more to try to compensate for the car not responding to the driver input. unfortunately this action simply makes the correction harder to accomplish once the car slows and grip is regained... the proper action to take if the car understeers is to straighten the wheels and attempt to make the turn again.

Tip to remember: Your tires only have 100% to give... If you are sliding or spinning your tires, you are asking more than 100% from them. further more if you are at the edge of traction limits in braking and then you turn the wheel, either you are going to lock up the tires or you are going to under steer because if you are asking 90% from your tires in braking and then ask another 50% in turning your are asking 140% from them and it will not happen.

I'm gonna stop there because I know it is a lot to digest, but keep digging and you will get it.

BTW, I like the advice about using chalk to measure tred wear... the moment I started doing it I improved a lot because I was able to better understand what was happening to my tires when I drove.

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post #6 of 8 Old 10-07-2010, 09:37 PM
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The tires max load rating and pressure should be in mind. Compair it to the weight of the car. My old 84 stang had about 800 lbs rear possibly 2200 on the front. Often tires on the front might have a different weaker load rating than the rear. This would require more air in the front than the rear. One might even have a tires on the front that are too low of a rating for the weight. Also concider the weight transfer cornering at 1G. If your front tires are rated at 1300 lbs at max inflation is that enough? Possibly not a good load rating for a car that weighs 2200 lbs in the front at 1G or better.
Check many of the race DOT tires many might be too low of a load for you.

How about the wheels? Do you know there load rating? My car eventually bent my IMSA approved rims. Drove me nuts figuring it out.
My next rims were Crnterline semi custom. I think they were rated at 1600 lbs each with 33" tires mounted on them. I ask about the rating with smaller dia tires. The Sales rep at Centerline kinda danced around the subject. Later found out the smaller dia the tire the more load on the wheel.

Its also hard to find low aspect ratio tires with a high enough load rating. The lower the aspect ratio the lower the load rating. Just some stuff to mull over.

Back to air pressure. The chaulk on tires is a good idea at the track how about at home? A good way to figure a starting pressue is to find a flat area such as a garage floor. Pull in to a good flat area place white paped in front of each tire and pull forward past the paper and compar the width and pattern transfer to the paper for full contact. Do it with the tires hot and cold. Note the pressure giving the best contact.
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post #7 of 8 Old 10-08-2010, 11:30 AM
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Quote:
Originally Posted by aurdraco View Post
Short version: yes. IME, you can auto-x safely with air pressure in the low 20s, but not that you'd want to. 36 front, 32 rear is a common starting point, but I've done 32 front and 28 rear before. With R comps, I think I have done 26 front and 22 rear. Their warning probably pertains to cars with tires that have 60+ series sidewalls.

One of the experts like Strano can correct me if I am wrong, but typically you raise front pressures and lower rears to fight understeer. I've known people to auto-x foxbodies on street tires before with 40ish psi front starting pressures. But with -3 camber, you shouldn't need that much. You may just be understeering due to coming into corners way too hot or taking funny lines since you are new. Try being a little more patient and see if you can't get someone to take your tire tems (inner, middle, outer) after your runs. I like to tell my students to pretend that the cones are children. SCARE THE CHILDREN BUT DO NOT HIT THEM! This usually makes them drive better, and thus faster.

Also, per John Ames (multi time national champ badass and fox body god), the fastest way to get a mustang around a course is if it is just barely understeering around the corners.

Glad you are having fun!
I think you got that air pressure stuff backwards.

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post #8 of 8 Old 10-09-2010, 02:51 PM
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Air pressure has a lot to do with the weight on the tire and its rating. Rear wheel drive heavy in the front light in the rear. Most of the time requires more air in the front than the rear.
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