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      10-05-2019, 05:04 PM   #1
Desertnate
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Autocross specific tire pressure advice for run flats?

This isn't a question regarding proper tire pressure for street use. I can get that off the door jam sticker.

I've run a few autocross events now and I'm still trying to get a handle on proper tire pressures. While I'm sure I'm still carrying too much speed into the corners, I still feel like something isn't right at the front of the car and I'm not getting the grip I thought I would on turn in and experiencing almost more oversteer than my GTI which was FWD. Some might be my fault and some might be the horrible 19" Bridgestone S001 runflat tires too I'm sure.

At the SCCA novice school, one of the instructors who is also an EVO School instructor suggested running the tires at max PSI up front and max -5 in the rear in heavier cars like my BMW to keep the tires from rolling onto the sidewall in the turns and help keep the handling neutral.

So far, I've done two events using the instructors guidance and I didn't come even close to the sidewall wear. On my next event, I left the rears the same but dialed back 2 PSI in the front. I didn't really notice any handling difference, but did notice I was using the entire tire tread, but was just shy of the sidewall.

After hunting around I found a chart on tire rack that mentions running 35~40 front/30~40 rear. Since the door jam sticker on my car states 35/38 for my 19" wheels, this recommendation isn't much different from street use.

Yes, I know RFT's are part of the problem and I'll solve that issue when they wear out. At this time, I'm not heavily enough into the sport to invest in a dedicated set of wheels and tires. I'm dong this for my own improvement and entertainment.

Any advice on set up for the tires which will give me a good platform while I work on improving my skills?
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      10-09-2019, 09:56 PM   #2
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Quote:
Originally Posted by Desertnate View Post
This isn't a question regarding proper tire pressure for street use. I can get that off the door jam sticker.

I've run a few autocross events now and I'm still trying to get a handle on proper tire pressures. While I'm sure I'm still carrying too much speed into the corners, I still feel like something isn't right at the front of the car and I'm not getting the grip I thought I would on turn in and experiencing almost more oversteer than my GTI which was FWD. Some might be my fault and some might be the horrible 19" Bridgestone S001 runflat tires too I'm sure.

At the SCCA novice school, one of the instructors who is also an EVO School instructor suggested running the tires at max PSI up front and max -5 in the rear in heavier cars like my BMW to keep the tires from rolling onto the sidewall in the turns and help keep the handling neutral.

So far, I've done two events using the instructors guidance and I didn't come even close to the sidewall wear. On my next event, I left the rears the same but dialed back 2 PSI in the front. I didn't really notice any handling difference, but did notice I was using the entire tire tread, but was just shy of the sidewall.

After hunting around I found a chart on tire rack that mentions running 35~40 front/30~40 rear. Since the door jam sticker on my car states 35/38 for my 19" wheels, this recommendation isn't much different from street use.

Yes, I know RFT's are part of the problem and I'll solve that issue when they wear out. At this time, I'm not heavily enough into the sport to invest in a dedicated set of wheels and tires. I'm dong this for my own improvement and entertainment.

Any advice on set up for the tires which will give me a good platform while I work on improving my skills?
I'm only an intermediate but here's my take:

If you are encountering oversteer than lowering the rear pressure and increasing the front is correct. However, there is only so much tire pressure can correct for. Perhaps you are simply going too fast for the tires or you are on the throttle too soon. Personally, I find oversteer to be preferable in a tighter technical autocross. What I'm constantly fighting for is grip on throttle for corner exit which can be helped by a later apex, better rubber, and better throttle modulation.
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      10-10-2019, 02:15 PM   #3
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You're M3 is going to be a different animal than my 435, but do you increase with air pressure at all to prevent sidewall wear issues, or do you run the normal street setings and then adjust as they heat up?

I'll continue to keep the fronts a little higher to balance things out. At this point I'm just wondering what a good starting point might be.
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      12-02-2019, 12:20 PM   #4
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I overheated about 1/2 inch up the sidewalls on my RFTs in two autocrosses at around 30-32 PSI all around. I should have increased the tire pressures before I went out but I was in a rush just to get the numbers on both times. I'd definitely make sure you don't go too low as they'll roll over, I'd probably stay >35lbs. Admittedly I am not the smoothest driver as I was having way too much fun sliding around so I am at least partially to blame.
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      12-04-2019, 03:01 PM   #5
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Ive been road course tracking my f30 on run flats. I usually like them around 30 psi hot but autoX is different world.
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      12-06-2019, 02:34 AM   #6
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Camber, you need more of it , try the M3 LCA, it'll give you a bit of camber up front which might make a big difference while solid state cornering and keeping the front end in line better during max gload. You might give up a smidge of turn in immediacy, but that's more than compensated for mid corner.
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      12-13-2019, 09:58 AM   #7
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Just make sure you don't do the M3 LCAs if you have xDrive. There is a video on youtube of a guy that tried this and killed both front axles (They're $1200 each apparently). It makes sense as it is pulling on the joint since the LCA is longer. Looks like camber plates are the only option for us xDrive folks as there is no way to notch the strut tower and move the mount. It would be cool if someone could make an upper strut mount that just adds 1 degree of negative camber since the camber plates available are $$$ and add additional NVH.
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      02-20-2020, 03:13 PM   #8
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Quote:
Originally Posted by Desertnate View Post
This isn't a question regarding proper tire pressure for street use. I can get that off the door jam sticker.

I've run a few autocross events now and I'm still trying to get a handle on proper tire pressures. While I'm sure I'm still carrying too much speed into the corners, I still feel like something isn't right at the front of the car and I'm not getting the grip I thought I would on turn in and experiencing almost more oversteer than my GTI which was FWD. Some might be my fault and some might be the horrible 19" Bridgestone S001 runflat tires too I'm sure.

At the SCCA novice school, one of the instructors who is also an EVO School instructor suggested running the tires at max PSI up front and max -5 in the rear in heavier cars like my BMW to keep the tires from rolling onto the sidewall in the turns and help keep the handling neutral.

So far, I've done two events using the instructors guidance and I didn't come even close to the sidewall wear. On my next event, I left the rears the same but dialed back 2 PSI in the front. I didn't really notice any handling difference, but did notice I was using the entire tire tread, but was just shy of the sidewall.

After hunting around I found a chart on tire rack that mentions running 35~40 front/30~40 rear. Since the door jam sticker on my car states 35/38 for my 19" wheels, this recommendation isn't much different from street use.

Yes, I know RFT's are part of the problem and I'll solve that issue when they wear out. At this time, I'm not heavily enough into the sport to invest in a dedicated set of wheels and tires. I'm dong this for my own improvement and entertainment.

Any advice on set up for the tires which will give me a good platform while I work on improving my skills?
in terms of training this is not necessarily a bad thing however it is tough to work with only tire pressures when you're on 19s...there's little to no give in the sidewall so if you are planning on wearing those tires out you will have to drive around the "problem".

the smallest mistake anyone can make in autocross is braking too early so you can start by doing that to settle the car before corner entry. give yourself plenty of space and gradually develop a "feel" for how much distance the car needs to make it through the element.

i think you're rear pressures are ok but your fronts might be a bit high. I understand that lowering them will cause some sidewall roll but it will increase your contact patch so you could potentially get some more heat in the fronts which is needed to grip...challenging to do all this on a runflat but you'll be a better driver for it.
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