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[Jonjt]

Ladies and gents,
The general consensus around here is that a square setup seems to be the fastest setup for track duty. This advice seems pervasive but, I cannot yet find a technical explanation as to why the E9x is faster with the same size tires all around. Can anyone provide this?

Thanks

[jpirelli]

Not necessarily faster... depends on driver but without getting too technical you get less understeer with a square setup. Can rotate car better with throttle and so on...

[Jonjt]

I was looking for something very technical or, highly empirical (testing) actually.

I can understand how a square setup would promote neutral steer for a car with perfect weight balance, at the limit. But, as we all know, actually driving a real car far different from the bicycle model many use to explain rudimentary chassis dynamics.

[CombatNinja]

The e90, like virtually every car on sale, is set up to provide mild, progressive understeer as the cornering limits are approached. This is "safe" because 99.999% of drivers out there will naturally lift off of the accelerator and/or brake as this begins to become an issue. This will cause the front end to regain some lost grip by transferring weight forward on the chassis. Since understeer is simply a loss of grip at the front wheels ahead of the back, everything is fine and the (compromised) car and (inexperienced) driver are in agreement. This is almost never the fastest setup for any chassis and will serve to annoy anyone above a beginner level on track. The quickest, dirtiest way to remedy understeer is to give the chassis relatively more front grip with more rubber. A bone stock e9x with a square setup will still understeer but it will be less so than on cars with staggered setups. Getting the car completely neutral involves tweaking bushings, swaybars and ride height but for a quick fix you can't beat squaring up the rubber. As a bonus, you can rotate and greatly extend the life of your tires.

[Jonjt]

Originally Posted by CombatNinja

The e90, like virtually every car on sale, is set up to provide mild, progressive understeer as the cornering limits are approached. This is "safe" because 99.999% of drivers out there will naturally lift off of the accelerator and/or brake as this begins to become an issue. This will cause the front end to regain some lost grip by transferring weight forward on the chassis. Since understeer is simply a loss of grip at the front wheels ahead of the back, everything is fine and the (compromised) car and (inexperienced) driver are in agreement. This is almost never the fastest setup for any chassis and will serve to annoy anyone above a beginner level on track. The quickest, dirtiest way to remedy understeer is to give the chassis relatively more front grip with more rubber. A bone stock e9x with a square setup will still understeer but it will be less so than on cars with staggered setups. Getting the car completely neutral involves tweaking bushings, swaybars and ride height but for a quick fix you can't beat squaring up the rubber. As a bonus, you can rotate and greatly extend the life of your tires.

I agree, most cars are designed to understeer and yes, lift-off will transfer weight to the front wheels, given transient cornering.

I'm just surprised that no one talks about all the things you mentioned, in conjunction with wheel sizes. Anti-roll, for example, has a significant effect on which end of the car breaks "static" friction first. Adding a stiffer rear sway, for example, could allow for a far more neutral steering car, while still allowing fatter rubber in the back. Considering that the widest 18 I could fit in the front is about 245, without rolling, it may be very beneficial to dig deeply into alternate suspension setups. 245 in the rear (making for a square setup), might make for serious traction deficiency (especially during corner exit), considering how much power a FBO N54 can put to the ground.

[NiVeDh]

Originally Posted by Jonjt

Considering that the widest 18 I could fit in the front is about 245, without rolling

I beg to differ: http://www.e90post.com/forums/showthread.php?t=657280

[Jonjt]

Any rubbing? Are you lowered? Can you go full lock without any problems?

I know the rear fenders on the E90s allow for wider tires than the coupes but, I do believe the front fenders are the same, right? I'm surprised you can fit a tire that wide up front.

[NiVeDh]

Originally Posted by Jonjt

Any rubbing? Are you lowered? Can you go full lock without any problems?

I know the rear fenders on the E90s allow for wider tires than the coupes but, I do believe the front fenders are the same, right? I'm surprised you can fit a tire that wide up front.

Car is lowered. No excessive negative camber (-0.8° front). No rubbing whatsoever, even under full lock.

[Jonjt]

Originally Posted by NiVeDh

Car is lowered. No excessive negative camber (-0.8° front). No rubbing whatsoever, even under full lock.

Great to hear. Thanks man.