[73tiiDavidPA]

Hello,

I need some advice on tire pressures for an upcoming auto-cross.

I'm running my first ever auto-cross. I have run HPDE with my 1973 2002tii and my 2002 325cic (e46 vert) and my 2016 228i. The auto-cross will be in the 2016 228i

The 228i is totally stock (36k miles) with a six speed manual. M Sports Package, Track Package, running 18" rims with All Season Michelin 225/40/ZR-18 at all corners.

I'm not buying tires or changing rims. If I get all excited and start running 3-6 auto-crosses or HPDE a year, then I will consider the investment. For the 1-3 events I will probably make, the investment is not worth it. Running decent all season tires still provides a lot of enjoyment.

The door jam sticker says 32 lbs. front - 38 lbs. rear for non-square tires when the rears are 245/35/18.

My gut says to up the pressures a bit, perhaps 35 front, 40 rear. I understand the lowering the pressure will put down a larger contact patch the theoretically more grip, though the sidewall rolling over will produce more sway.

Any thoughts?

[aerobod]

I would go with the same pressure all around and start with about 35PSI cold. Put chalk on the tyre sidewalls and see how far they are rolling, then measure hot pressure immediately after a run. If the runs are close together and the tyres don't fully cool, you may need to let some air out.

A hot pressure around 40 PSI for a summer tyre is about right for the correct amount of tyre roll, but it may be a lower pressure for an all-season that will have less roll due to lower grip. You may also find you need to run a higher pressure in the front than back, depending on the specific tyre and trade off from tyre heating due to cornering vs acceleration and braking and the effect of relatively low negative camber on the front.

[smchuck]

Originally Posted by 73tiiDavidPA

Hello,

I need some advice on tire pressures for an upcoming auto-cross.

I'm running my first ever auto-cross. I have run HPDE with my 1973 2002tii and my 2002 325cic (e46 vert) and my 2016 228i. The auto-cross will be in the 2016 228i

The 228i is totally stock (36k miles) with a six speed manual. M Sports Package, Track Package, running 18" rims with All Season Michelin 225/40/ZR-18 at all corners.

I'm not buying tires or changing rims. If I get all excited and start running 3-6 auto-crosses or HPDE a year, then I will consider the investment. For the 1-3 events I will probably make, the investment is not worth it. Running decent all season tires still provides a lot of enjoyment.

The door jam sticker says 32 lbs. front - 38 lbs. rear for non-square tires when the rears are 245/35/18.

My gut says to up the pressures a bit, perhaps 35 front, 40 rear. I understand the lowering the pressure will put down a larger contact patch the theoretically more grip, though the sidewall rolling over will produce more sway.

Any thoughts?

The 32/38 is to create understeer for normal road driving. I'd keep all 4 corners at same pressure. 40 hot sounds like a good start. without negative camber you might find you need higher than expected pressures to keep off the sidewalls.

[73tiiDavidPA]

Originally Posted by aerobod

I would go with the same pressure all around and start with about 35PSI cold. Put chalk on the tyre sidewalls and see how far they are rolling, then measure hot pressure immediately after a run. If the runs are close together and the tyres don't fully cool, you may need to let some air out.

A hot pressure around 40 PSI for a summer tyre is about right for the correct amount of tyre roll, but it may be a lower pressure for an all-season that will have less roll due to lower grip. You may also find you need to run a higher pressure in the front than back, depending on the specific tyre and trade off from tyre heating due to cornering vs acceleration and braking and the effect of relatively low negative camber on the front.

Thanks, I'll probably start at 35 cold at all four corners and adjust from there. Being my first auto-cross, and doing this for only pleasure, I don't intend to be overly serious. When my son was karting, half pound / quarter pound, sometimes different for all four tires was the norm - not looking to get that serious.... (at least yet, lol).

[230iZTR]

I autocrossed my first year on the supersports (before camber plates) I needed to inflate the fronts to around 37 to stop it from rolling over on turn in. I'm sure the all-seasons would probably behave similarly.

I recommend keeping a spray bottle handy. The first run was always slow with cold tires, the next 2-3 were great, and the started to fall off after that. So temp management is important.

[MarcoZandrini]

I would start with 35psi on all four tires. I'd buy a digital infrared thermometer to see the temps across the tread of each tire.