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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Wheels and Tires Forum Sponsored by The Tire Rack > Understeer: Balance your car with tire pressure



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      04-01-2012, 05:22 PM   #1
Doyle
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Understeer: Balance your car with tire pressure

I have gotten quite a few PMs and have seen a few discussions also centered around the following question:

How do I mitigate understeer?

Well, the answer is--it depends. Everyone has different driving styles, different tracks/backroads, and different suspension setups. The cheapest way to answer the question is to look at the tires.

For this, we'll need:

-1 tire pressure gauge (the better the quality, the better the results)
-Some sidewalk chalk
-warm tires
-course, track, backroad

1) Get a base reading of your current tire pressure.

2) With your chalk, draw a line on the sidewall of your tire, perpendicular to your rim, and that traces from your rim to the tread.

3) Some tires have a small arrow that dictates where the optimal end point for the contact patch is. Make sure your line goes beyond this point. If your tires do not have this, than take note of where this point is; just shy of where the tread ends.

4) Get another reading of your tire pressure, just in case the pressure dropped.

5) Go and drive the course or backroad like you normally would. Come back.

6) Check your chalk mark. If the chalk mark still extends beyond the triangle (or where the triangle would be) and onto the tread, you have too much pressure in your tire.

If the chalk line has been rubbed off below where the triangle is (or where the triangle would be), then your tire pressure is too low.

Your goal is to get the chalk line dead even with the triangle. Titrate the tire pressure up or down depending on your results. Now you have your "hot" tire pressure.

7) Give the car a good cool down, and let the air in the tires cool. After a few hours, go and take your new tire pressure. Now, you have your "cold" tire pressure.

Now, note: This is a very top level, and general guide/suggestion to managing tire pressures. Based on ambient temp, course, tire wear, etc all of this could change.
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      04-02-2012, 12:17 PM   #2
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Thanks for the information and great writeup Will be trying this with my PSS.

Edit: Surprised no one commented on this yet.
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      04-05-2012, 11:30 PM   #3
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thank you Doyle
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      04-06-2012, 01:47 AM   #4
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this should be a sticky,
... so simple
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      04-06-2012, 09:37 PM   #5
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Quote:
Originally Posted by Javier View Post
this should be a sticky,
... so simple
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      04-07-2012, 07:25 PM   #6
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Very simple, easy to incorporate into air/tire management. Cool
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      04-08-2012, 08:49 AM   #7
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Great post, we see this a lot at autocross with the chalk lines. My rears are always "over-inflated" because I need the extra pressure to help loosen up the rear end. My fronts are right on the edge of being perfect and almost slightly under-inflated. This is how I am able to balance out my car, basically prevent the understear as much as possible. I an running KW V1 coilovers and my car is about as low as it can go without causing the suspension components to "bottom out" against their limits. I lowered the front slightly more than the rear and that helped a little. By the end of autocross the other day, I had my rear tires near 40 psi (39 or 39.5 warm) and my fronts were down around 34 psi. With my last dyno around 387hp and 423tq at the wheels, this would sound like a rear end that spins out but I still have more rear grip than front grip... at least at the lower auto-x speeds.

I did install the BMW carbon fiber lip/wedge to the rear deck and with my front angled setup the car makes awesome downforce on the track so at faster speeds the car handles much better/balanced. I use RE-11 tires on stock 189 rims btw (I think they are the 189 rims lol, E92 coupe with sports package 2008).

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      04-09-2012, 11:38 AM   #8
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Not much in racing is free, but air is one of them.

Zeph has the perfect example of how you can use this tool to your advantage.
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      04-27-2012, 11:29 PM   #9
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Thanks for this Doyle.

I went to Big lots and got chalk for $1.50

Now for the bad news. It seems I've been over inflating my tires because after driving home, spiritedly, my chalk marks looked the ones in the first pic! It looks they've worn a lot too...I'm dropping them back down to the door jam rating of 32psi front and rear.
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      11-24-2015, 12:51 AM   #10
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Great write up, as always, thank you.

Forgive my ignorance, but wouldn't you want to over-inflate the fronts and under-inflate the rears to mitigate understeer and not the other way around as Zeph seems to be saying?
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      11-24-2015, 06:30 AM   #11
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Quote:
Originally Posted by 330XD EVO View Post
Forgive my ignorance, but wouldn't you want to over-inflate the fronts and under-inflate the rears to mitigate understeer and not the other way around as Zeph seems to be saying?
Yes. But, like most things, it's not that simple. Changing tire pressure changes several things, effective spring rate, size of tread patch on road surface, sidewall flex. etc. Some of the effects offset others, so it's really a trial and error process to find what works for you. Unless you go way overboard on your pressures, it should be considered only a very fine tuning technique. I believe it makes sense to generally follow the procedure outlined above to find your proper tire pressure, which should give you the tire's maximum grip, and then look to other suspension tuning to change the car's balance.
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