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Motorsports & Track Forum
From Auto-X to Trackday to Racing and Professional Motorsports – this is the place to discuss making BMWs fast

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Old 02-12-2004, 11:49 AM   #1
mfino
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Tire pressures for Autocross

What tire pressures would you recommend running for an autocross event? I'm going to start this year, and I'll be running on 225/50/16's(probably Kumho Ecsta Max) . Any other helpful tips would be greatly appreciated. Thanks
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Old 02-12-2004, 12:03 PM   #2
AndyZHP
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A good rule of thumb and generally the baseline I start out with on my first run is max PSI in the front and 2 to 3 less PSI in the rear.

I just bought a set of 265/35/18 Kumho V700s, I have not autocrossed with them yet, however from my initial testing 40 PSI up front and 37 in the rear seemed to work really well. I will use this as my baseline at my first event.

Less Understeer = increase front tire presure / decrease rear tire pressure
Less Oversteer = increase rear tire presure / decrease front tire pressure

The greater the tire pressure, the greater the grip (to a point anyway)
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Last edited by AndyZHP; 02-12-2004 at 12:08 PM.
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Old 02-12-2004, 12:22 PM   #3
mfino
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Thanks Andy. i have another question for you. I'm currently running a stock sport suspension, if I were to begin doing some modifications, is there anything that you might recommend. For example, are strut bars a must when autocrossing?

Thanks again

Quote:
Originally Posted by AndyZHP
A good rule of thumb and generally the baseline I start out with on my first run is max PSI in the front and 2 to 3 less PSI in the rear.

I just bought a set of 265/35/18 Kumho V700s, I have not autocrossed with them yet, however from my initial testing 40 PSI up front and 37 in the rear seemed to work really well. I will use this as my baseline at my first event.

Less Understeer = increase front tire presure / decrease rear tire pressure
Less Oversteer = increase rear tire presure / decrease front tire pressure

The greater the tire pressure, the greater the grip (to a point anyway)
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Old 02-12-2004, 12:52 PM   #4
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Quote:
Originally Posted by mfino
Thanks Andy. i have another question for you. I'm currently running a stock sport suspension, if I were to begin doing some modifications, is there anything that you might recommend. For example, are strut bars a must when autocrossing?

Thanks again
IMHO, you won't really see a difference in your auto-x times with strut bars. The only thing they are good for is to help stiffen up the chassis. They certainly won’t hurt, but I would spend my money elsewhere first.

Here is a list of what I would consider the best mods in order of importance (where you will see the biggest results).

1. Seat time. I would look into attending an Evolution Autocross School.

2. R compound tires. Either the Hoosier A3S03s or the Kumho V700s (which is what I have) tend to me the most popular. R compound tires are by far the best mod you can make to your car to improve your time. You can easily shave 1 to 2 seconds of your time (on a 60 second course).

3. A custom alignment. Max negative camber all the way around. 0" toe-in at the front and 3/16" toe-in at the rear. This is what I am running which is a good street/track compromise.

4. Stiffer sway bars. I run the stock class so I am only allowed to swap out the front sway bar. I'm running Racing Dynamics' 27 mm front sway bar set to full soft.

5. Suspension. Koni single or double adjustable shocks and struts (or a complete coilover kit). Since I am running the stock class I am only allowed to swap out the shocks & struts (no springs, coilovers or camber plates). I'm running TC Kline's Koni double adjustable shocks and struts set to near full stiff. With a set of Konis at this setting you get zero (to little) body roll in the corners.

Good luck and most importantly have fun!!
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Old 02-12-2004, 03:39 PM   #5
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Quote:
Originally Posted by mfino
What tire pressures would you recommend running for an autocross event? I'm going to start this year, and I'll be running on 225/50/16's(probably Kumho Ecsta Max) . Any other helpful tips would be greatly appreciated. Thanks
For that size tire on a 3000lb+ car with a fully set up suspension, divide the weight of the car by 100 and add 2 to 3.

3300lbs/100lbs = 33
33 + 3 = 36psi

Since your car has streetable suspension instead of autocross stuff, I suggest adding about 5 psi to that number. Adjust pressures to minimize tire rollover.


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Old 02-12-2004, 04:12 PM   #6
mfino
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Thanks. I will try 38-40 and see how that goes. I'm guessing the autocross is going to be extremely addicting. We'll see how the first run goes.

Quote:
Originally Posted by teamdfl
For that size tire on a 3000lb+ car with a fully set up suspension, divide the weight of the car by 100 and add 2 to 3.

3300lbs/100lbs = 33
33 + 3 = 36psi

Since your car has streetable suspension instead of autocross stuff, I suggest adding about 5 psi to that number. Adjust pressures to minimize tire rollover.


Ed
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Old 02-12-2004, 04:33 PM   #7
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Quote:
Originally Posted by teamdfl
For that size tire on a 3000lb+ car with a fully set up suspension, divide the weight of the car by 100 and add 2 to 3.

3300lbs/100lbs = 33
33 + 3 = 36psi

Since your car has streetable suspension instead of autocross stuff, I suggest adding about 5 psi to that number. Adjust pressures to minimize tire rollover.
I'm curious how you came up with that formula to determine optimum tire pressures. What would the formula be for my car with 225/40 18s be?
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Old 02-12-2004, 04:58 PM   #8
teamdfl
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Quote:
Originally Posted by sfzBimmer2
I'm curious how you came up with that formula to determine optimum tire pressures. What would the formula be for my car with 225/40 18s be?

That's an old school pressure recommendation from a Kumho rep when the V700s were new. I was running 225/50/16s on a 3200lb car. Since you are reducing the sidewall area by running an 18, bump your pressures up even higher by a couple more psi.



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Old 02-12-2004, 07:29 PM   #9
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On street tires i found that 38 in the front and 40 in the back worked well to reduce understeer on stock sport suspension. Now that i am running R compound V700's i run 40 all around. It seemed to work quite well...

Make sure you take a good tire pressure gauge with you...2-3 pounds off
can make a big difference.
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Old 02-15-2004, 12:19 AM   #10
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Make sure to check ur tire pressure before each run. Pressure can climb rapidly because of the increased heat in tire. 1st time I autocrossed I started with 40 all around and after 3 runs I was sliding all over the place. Check my tire pressure 40psi became 52 psi after 3 runs in 60 degree weather. After that I always check before each run
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