38 PSI for back tires?

[Dave1027]

That's what my door jam sticker says. Why is it supposed to be so much higher than the fronts? I thought a 50/50 weight balance should mandate even pressures front to back.

[brettbimmer]

The idea with this higher rear figure is to make sure that the car will always understeer, which is really redundant as our rear tires are wider then our fronts anyway. I believe it is a CYA move by the BMW Brass in case you are ever in an accident and try to sue them. I run 32 PSI all around, with 38 PSI all around as my hot temperature on the track.

[M0nk3y]

Brett nailed it on the head. Overload the front with grip and it will understeer.

I run 38 hot front and 36 hot rear for autox.


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[Mtnman]

I used to run 39.5 rear and 38 front on the zhp. I had perfectly even tire wear as well. the 135 is the same way, with a bmw suggested 3-4 psi increase in rear over front.

[Crickett]

Edit: shoulda figured I'd get ninja'd by the track rats! :ninja

Y'know, I scratched my head about this topic & dove into the world of tire sizes, load indicies, & vehicle weights & ended up with a shrug & "I guess the BMW engineers had a reason, probably mostly because the rear is a bigger tire".

Longer version: I'm sure the final recommended tire sizes & pressures were chosen with balancing performance (exciting, neutral sports car handling) & predictability (less exciting, for-the-masses understeer tendencies) in mind. More grip in the rear (or less up front) induces understeer at the limit of traction; a wider tire with higher PSI has more traction available (to a point). So wider & more inflated rear tires will hold on more than the narrower, less inflated fronts will at a given cornering load.

To reduce understeer / increase oversteer, drop rear PSI / increase front, decrease rear tire width / increase front. That's why you'll find a great many enthusiasts swapping the stock staggered wheels for a square setup.

And that's my (grossly oversimplified) $0.02.

[danewilson77]

Running 36/38 F/R

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[Mike V]

Good info here...

http://www.zhpmafia.com/forums/showthread.php?1579-What-is-the-recommended-tire-air-pressure-for-the-E46-BMW-330-ZHP&highlight=tire+pressure

In that thread, we learned that our recommended pressures varied. The door jam on my 05 indicated 46 in the rear.

I run 33 all around on most of my cars, including the zhp when I owned it.

[derbo]

35 all around is my choice.

Hot is 38-40 on the track depending on tire selection. Dunlop Star specs don't like 40..but RE11 do... at least on my car..

[echo46]

I'm running 38/39. I've experimented with 36/40, 35/40, 35/38 etc. found this to be the best for my car.

[nike001]

Running 36/38 F/R

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Yep same here.

[johnrando]

35F, 37R. No idea why I run that. I'm soon going square so it doesn't really matter.

[Stu]

I've been noticing more center tire wear with 38 psi in the rear, but 32 in the front seems perfect. I dropped the rear down to 34 cold psi. What do you all think?

[Dave1027]

I dropped my pressures down to 34 psi front and rear. I'm afraid to go lower because I don't want to crack a rim if I hit a pot hole.

[328ioc]

I am currently running 34f and 38r with no problems.

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[Washburn]

so no one runs the 32F / 38R OEM spec with OEM wheels? why not?

[328ioc]

so no one runs the 32F / 38R OEM spec with OEM wheels? why not?

Personally I like the ride with a few more psi in the front.

That's why I run them a little over inflated.

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[Washburn]

I see - thanks; I might try that myself next time I check air pressure...