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Discussion Starter #1
330Ci Sport Package
UUC sways set to oversteer
17" M68
225(F) 255(R) GY GS-D3's

Im running an auto-x this weekend and what to why what the best tire pressure is for the above? 40(f) 37(r)?
 

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it depends on too many factors.

the best thing to do is get some chalk at home depot or other hardware stores. then before your runs make several lines from the outside of the tread to the side of the sidewall.

after your run check the chalk. if the chalk mark is gone pass where the thread ends, then your tires are rolling over and you need more pressure.

Start with about 36-38 and increase by 1 to 2 psi depending on how badly the tires is rolling over.

good luck.


EDIT: oops I just saw the date and I guess it's too late but you can still do it for other events :)
 

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Discussion Starter #4
Yea I experienced to much understeer, i think i needed more air all around but...mostly in the front.
 

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vaio76109 said:
Yea I experienced to much understeer, i think i needed more air all around but...mostly in the front.

use the chalk method. it's a lot better than guessing :) it's scientific guessing :D

however, with your tire setup, you wont be able to get away from understeer.
 

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MrMotorwerk said:
its your staggered setup.

The rears are 255 and the fronts are skinny 225.

Yep. 225s have less grip than the 255 so the 225s will always lose traction before the 255s in a given turn.


This is the exact reason why I run 245/45/17 on all 4 corners on the M Coupe in it's autox configuration.
 

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Der Meisterstück
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I run 255/35/18 on all four corners.

running the same size tire on all four corners basically makes the car very neutral. You want to maximize the contact patch on all four corners while keeping it predictable.

With a staggered setup you will never have enough grip up front. Especially with the over aggressive staggered setup you have. The stock "sport" staggered setup is pretty close to having the same size tire on all four corners.

Most autocrossers that have a set of autocrossing tires have the same size tire for all four corners. Unless you have a Porsche or E46 M3.
 

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vaio76109 said:
WOW, how do run 255's in the front? Do your fenders/wheels rub? What rim width are you running?
255 is about the max you can run on 8.5" wide wheels. after that the tires don't get the proper support and they don't work as well.

I'm cheap so I run E36 M3s rear wheels 17x 8.5" which are not the correct offset, so I have to use 5mm spacers.

even with playing tire pressure you wont be able to get rid of understeer. Just keep that in mind. If you find your self understeering through a corner then just brake a tad bid earlier and go slower. It will be faster than to understeer through the turn.
 

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Grüne Hölle Junkie
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I've been playing around with tire pressures too, since I have just started to track my car this year.

I found that the BMW recommended tires pressures for my car contribute alot to the understeer. I also have the factory M68's (still), 225f/245r, and the factory pressures were 32f/38r...

wellllllllll.... after I noticed the shoulders of my fronts and the center of my rears were wearing faster than the rest of the tire, I started to play with the pressures...

I finally settled on 35f/33r and now the car doesn't push through corners as much as it used to. (I also do what notE46 recommends- brake harder, sooner BEFORE the turn, and try to hold the turn just short of understeering)

The only mods I have done to the suspension (so far) is put some Bilstein PSS9's on. UUC sways are in the mail; when those are installed, I guess I will have to play some more with the pressures (next spring... when the track re-opens... if the weather's good... if I still remember how to get to the track... :p )
 

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even with same size tires all around (i'm running 255/40/17's on ssr comps on my 330ci) Without major suspension mods the cars still understeer like crazy... I'm still working on dialing it out... the car definetly can be a good fun auto-x car locally.. but nationally unless your an amazing driver they're a hard car to campaign... you need lots of negative camber up front.. i'm running 2.5 degrees of negative all around and i've realized i'm gonna have to go up to 3 in the front... adjustable sway bars.. full stiff in the rear.. full soft in the front.. and a good coil over setup.. then its all about lightening the car as much as you can legally... seats are a huge gain in that area.. plus then you wanna work on power output.. such as a light weight flywheel... pullies.. software.. intake... etc..

now this is all for campaigning the car in the scca DSP class..

just my $0.02 from the 2004 New York Region DSP Champion. :woot:

~T|/\/\
 
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