[msm335]

I've done a little bit of autocrossing with my local BMW clubs. Being a beginner, I haven't messed with my tire pressures. But my 335xi really seems to understeer badly. Which admittedly is probably partly the nature of the beast. But I've been told adjusting tire pressure could help. I have the OEM sport package with 18" wheels and staggered tire sizes (225 40s on front, 255 35s on rear). Does anyone have experience with this set-up (either on 335i or 335xi) and what tire pressures do you typically run? Thanks.

[Spydeputy]

i also would like to hear about this topic too.

[The HACK]

First, describe what sort of understeer you're experiencing. Is it corner entry understeer, corner exit understeer, or steady-state cornering understeer? Identifying WHY your car is understeering and what sort of understeer it has is the very first step in solving your problem.

Second, assuming that telling you "it's the driver" isn't going to be any helpful to you, I would suggest getting some CHALK and marking your tire surface all the way up the sidewall. After each run, check the chalk markings and see how far up the sidewall you are wearing. If the chalk disappears long before the round edges of the shoulder, you're over-inflated. If the chalk disappears long PAST the round edges of the shoulder, you're under-inflated. Inflate to a pressure that yield the maximum contact patch first before you worry about understeer. Once you've established maximum contact patch pressure for front and rear tires, you can increase/decrease pressure from the end with more grip to alleviate understeer issues.

And the result will be DIFFERENT for each driver. Pressures that works for my driving style will not necessarily suit yours.

[nicknaz]

I've been using cold pressure of 33psi F 34psi R on my 328 with stock sport package tires. this yields hot pressure of 40psi front and rear which I feel maximizes the contact patches.

Also, FWIW, the cheap fix I've found for corner entry understeer is slow the car down more before turning in and for corner exit understeer it is roll on the throttle a bit less aggressively

I've been told that the ideal way to dial in tire pressure is to use a probe type pyrometer to check the inner, middle and outer temperature of your tires immediately after each session and use the tire pressure that yields the most consistent temperatures across the tire (ie this is the quantitative way to validate the "maximum contact patch pressure" that hack discussed in his post") but this was too expensive for me so i eyeball how much i scrub the tread and the sidewall instead (i try to scrub the entire tread without going over into the sidewall)

[Ramos]

In my opinion, no amount of pressure adjustment will help you...
You should invest in a square setup ... (same size tires all around)

[chaz58]

Yeah, the XI cars are going to understeer more than the RWD cars. This is partially weight distribution and partially torque distribution. I find that the RWD e90s are easier to recover from understeer than the AWD models (which tend to be more terminal).

There is a lot of good advice above. In your case the best thing to do is to avoid the terminal AWD understeer in the first place. Slower turn entry speeds are the key. I have videos of me pulling away from 335 cars on the straights (Track) because I wasn’t fighting understeer – I entered the turn slower than them, and exited faster. Their horsepower couldn’t make up for the lost momentum.

So, slower into the corners, and don’t get on the gas until the chassis has settled and you have hit the apex (and don’t apex too early).

Specifically for your question – some FWD cars put an extra 20PSI in the rear tires to:
1) Increase the spring rate (and oversteer)
2) Make the car more loose
I have done this on AWD cars and it can be effective in helping the car rotate in an autocross situation (especially since you have staggered tires).

Some people even mark the car’s alignment settings, and increase toe out for an autocross, setting it back to stock when the event is over. That may be a little more than you want though.

[nicknaz]

Originally Posted by Ramos

In my opinion, no amount of pressure adjustment will help you...
You should invest in a square setup ... (same size tires all around)

i think that would put the OP in a modified class since he is running a non factory wheel width in front. may not be something he wants to do.

i think pressure adjustment is helpful to get the most out of the setup (just like more seat time is helpful)

[200Duece]

Our local autoX only looks for R compounds to bump into a dif class. But if you're gunna buy new wheels and tires, the stickier the better

[nicknaz]

Originally Posted by 200Duece

Our local autoX only looks for R compounds to bump into a dif class. But if you're gunna buy new wheels and tires, the stickier the better

guess it depends on club (or region) rules. SCCA stock class allows r comp tires but only on stock width wheels

[chaz58]

Good point, but a square setup is stock on the AWD BMWs.

Still, it is important to know the rules in your region. We have about 1/2 doxen auto-X clubs here. Some clubs/areas have more fudge factor than others. Spending $500 for tires/wheels on craigs list can be a lot cheaper than tearing up his 18" your OEM tires, and allow him to upgrade rubber later.

Originally Posted by Nick Nazareno

i think that would put the OP in a modified class since he is running a non factory wheel width in front. may not be something he wants to do.

i think pressure adjustment is helpful to get the most out of the setup (just like more seat time is helpful)

[200Duece]

Originally Posted by Nick Nazareno

guess it depends on club (or region) rules. SCCA stock class allows r comp tires but only on stock width wheels

True , and technically if you are running 4 fronts or 4 rears, they are all "stock wheel" sizes right

[nicknaz]

Originally Posted by 200Duece

True , and technically if you are running 4 fronts or 4 rears, they are all "stock wheel" sizes right

haha yea, if they let it slide running 4 rear wheels to get a 255 width square setup with would totally be the way to go

[Ramos]

I don't even think the OP is competing.. He said he was a beginner doing a few events with his local BMWCCA...

[Chowbow]

Best way is to test like the HACK mentioned. On my stock staggered setup I was getting some understeer but not horrible. A lot of times, it's just going into the corner too fast. When I first started autoXing, every car understeered for me because I was going way hot into all the corners. Sometimes you need to go slow to get fast. I still autoX on a staggered setup right now, and don't get too much understeer. I do run considerably higher pressures up front though.

[satakal]

Originally Posted by Chowbow

Best way is to test like the HACK mentioned. On my stock staggered setup I was getting some understeer but not horrible. A lot of times, it's just going into the corner too fast. When I first started autoXing, every car understeered for me because I was going way hot into all the corners. Sometimes you need to go slow to get fast. I still autoX on a staggered setup right now, and don't get too much understeer. I do run considerably higher pressures up front though.

Blasphemy!

Learning to drive is for poor people. The only fast german cars are then ones with sweet mods.

Remember: you can't buy talent, but you can buy speed.

[200Duece]

+10000000000000000

you can always make up little mistakes in the corners on the big straights with big HP

[Ramos]

Originally Posted by Chowbow

Best way is to test like the HACK mentioned. On my stock staggered setup I was getting some understeer but not horrible. A lot of times, it's just going into the corner too fast. When I first started autoXing, every car understeered for me because I was going way hot into all the corners. Sometimes you need to go slow to get fast. I still autoX on a staggered setup right now, and don't get too much understeer. I do run considerably higher pressures up front though.

You cure understeer by running more pressure UP FRONT??

[Ramos]

Originally Posted by satakal

Blasphemy!

Learning to drive is for poor people. The only fast german cars are then ones with sweet mods.

Remember: you can't buy talent, but you can buy speed.

Listen to this guy, he has a supercharger and NOS on his 335 !!

[The HACK]

Originally Posted by Ramos

You cure understeer by running more pressure UP FRONT??

Pressure and grip is a voodoo science. Sometimes adding pressure up front may actually gain you more traction, sometimes it will take away traction. The only thing I learned, is to never assume doing something to the suspension will yield desired result unless you test it.

[Chowbow]

Originally Posted by Ramos

You cure understeer by running more pressure UP FRONT??

Up to a point, it works. Try it. Street pressures on the stock staggered 18s is something like 32F/36R. Go up to 43F/42R and the car is more neutral. At least on my setup, for me, with my driving style, it is.

On my 19s, I run 44F/41R at the autoX (hot pressures) and it works best for me. I'm on Eibach ProKit springs with Bilstein HD shocks, 19" CSLs with 235/265 tire setup. This is on regular old street tires though, things change a lot with R-compounds and slicks.

[satakal]

Originally Posted by The HACK

Pressure and grip is a voodoo science. Sometimes adding pressure up front may actually gain you more traction, sometimes it will take away traction. The only thing I learned, is to never assume doing something to the suspension will yield desired result unless you test it.

that's so true. I observed a similar thing over the weekend. I was driving a Ford GT at the High Speed Ring and the thing would just swap ends with the tinyest bit of lift mid corner. I was like... WTF, the car won't let me throttle steer.

So, I turned the traction control all the way off (a counterintuitive stab in the dark) and it worked out great.

[200Duece]

that Moded Vet I was in, is the exact same way the more you back the nanny off the faster it goes.


When does that damn game get released anyway?!!