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ChosenGSR
05-22-2006, 10:04 AM
The more people I talk to the more opinions I get on this subject. I am still using the stock setup 225F 255R Micheline PS. Some tell me to run equal pressure F/R others tell me to run more upfront to counterpoint the under-steer bias. I usually try to maintain about 39-40PSI at the track, which means bleeding out some air as the day progresses. Thoughts? :icon_conf

Barry
05-22-2006, 05:52 PM
The more people I talk to the more opinions I get on this subject. I am still using the stock setup 225F 255R Micheline PS. Some tell me to run equal pressure F/R others tell me to run more upfront to counterpoint the under-steer bias. I usually try to maintain about 39-40PSI at the track, which means bleeding out some air as the day progresses. Thoughts? :icon_conf


I had a 330Ci before the M3 and I ran it at the track.
I set the cold psi's at 35 in front and 42 in the rear and that worked very well.
Barry

jhm5
05-22-2006, 07:35 PM
And another view - equal pressure on street - 35 psi, which is an average of what bmw recommends. But on track 38 front, 36 rear cold. Autox - 38 front and 34 rear, remove spare tire to reduce weight - autox - haven't tried that on track.
So - clearly you'll have to experiment.
My guess is 42 rear, previous poster's recommendation, is so high that it reduces traction, just as 34-36 rear does relative to front... so either setup might work to reduce the car's natural understeer.
Maintaining 40 psi hot is probably ok for front, but you'll want something different at the rear - i say 2 psi or so less.

Pinecone
05-22-2006, 07:53 PM
You are either running too much pressure on the track or too little for autocross.

If you run 38 cold and get 40 hot, you are not driving very hard. :)

True street tires need a lot more pressure than you think. As do Hoosier A tires. I run 51 front (bleed to maintain) and about 44 rear on the LTW for autocross with A3S04s.

On the other hand, the Michelin Pilot Sport Cups on the M3 on the track, I look for about 34 - 36 hot pressure. But they are known to run lower pressures.

The Gooyear race tires on the SRF we shoot for 31 psi all around when hot off the track. The Yoke A048s ran a big higher IIRC.

jhm5
05-22-2006, 09:53 PM
now chosen gsr is really confused...

clarifying my post -- run 2 to 4 pounds less in rear, compared to front.
i run 38 psi cold front (which gains 4-8 psi hot track), 2 pounds less rear track (4-5 psi less rear autox)

if the original poster is trying to maintain 40 hot, that probably is a bit low, but my bottom line message is, run less rear, not more than the front, to help a relatively stock bmw turn in.

dderr
05-23-2006, 09:36 AM
I'll second John's suggestion (although you need to change your moniker to jhxm5). I speak from my experience (I've driven a couple of tracks and autocrossed a couple of times so I'm an expert!)
For virtually all track and autocross applications, most, if not all, BMWs should run more pressure in the front tires, generally four to six psi. Why? Well, for starters, the fronts are seeing a much, much higher loading than the rears--ignore that 50-50 weight distribution stuff--especially when you're trying to make the car go against Newtonian physics just because you want to hit the apex at turn one. You have a few hundred pounds of iron up there, the whole cars weight is most likely still biased to the front from all of the braking you just--hopefully--did, and now you want to make this right (or left) turn. The back tires (for most students under normal conditions) are just following along and aren't getting half the load, probably. Why does BMWs legal department set the recommended pressures so that there's more pressure in the back? So that the car will understeer (or puuush, as they say from VA on down) Maybe the same rationale for the wider tires and wheels in back. But you've had high quality driver education, you know better than some suit and tie...
Keep in mind that the purpose of the added air is to stiffen up the tires' sidewalls and thus keep the tread in better contact with the pavement. But also bear in mind that there can be too much of a good thing--too much air can reduce your tire's contact patch by causing the center of the tread to bow out (centrifugal force and all) and reducing grip on the sides.
So, go with this: For the street, two pounds more in front (I run 33F/31R) the range depending on road conditions--it'd be dumb to run 36/34 on DC streets, you'd shake the car apart. For the track, there are of course some variables, but at least 4-6 psi more in the fronts is appropriate and the primary variable being track layout. A tight-turned track like Jefferson or Shenandoah (Turns 5-8, Turn 14), or an autocross loads the front tires a lot more so add 6-7 psi (40F/34R) as opposed to a less severe track like Watkins Glen or Road Atlanta where 38F/34R would/might be more appropriate. These are all COLD pressures. I generally ignore hot pressure readings as they can be misleading. High (hot) readings up front usually are indicative of tire overload. Try slowing a little more before turn in or turn in more progressively--turning slightly earlier but more softly and progeressively increasing the turn-in as you go deeper into the turn. Or both. Accelerating at turn-in, which is the proper and correct procedure, also greatly reduces the loading on the front tires and distributes it more evenly between front and back (remember that from the classroom?). Ride with your Instructor and have her/him point it out.
Other variables to consider are tire and wheel width (a wide tire on a narrow rim needs more air to stiffen up a more bowed sidewall. Conversely, a narrow(er) tire on a wide rim has relatively straighter, and stiffer, sidewalls and needs less stiffening), tire construction (the old BFG R1s had very stiff sidewalls and thus called for much lower pressures whereas the Toyo RA1 has relatively soft sidewalls and therefore you need more air (relative to the R1)), suspension set-up (a neutral steering set-up would reduce the load on the front (loaded) tire at turn-in), and ambient temperature (if it's gonna be a hundred degrees that day you'll find that you'll need a pound or two (or three or four) less. And don't forget the aforementioned driving technique--throwing a car into a turn really eats up tires.

All said, air pressure is the cheapest (free or at most 25 cents--50 cents if you are slow) and maybe most noticable performance upgrade available...

ChosenGSR
05-23-2006, 07:48 PM
So looks like the consensus is to have more pressure in the fronts to reduce under-steer. Although from the post above I am not entirely sure why one wouldn't worry about the hot temperature and only the cold. I was under the impression that I should be bleeding the pressure between runs to try and keep equal pressure left to right etc. Maybe I simply misunderstood that point. I am not sure why I would "adjust" my driving style on the track to not overheat the fronts or certain tires. From what I was told it's absolutely normal for certain tires to experience more loads due to the layout of the track. For example a track with a lot of hard left turns will increase the pressure unproportionally on the front right for example. Correct me if I am wrong.

Scotty
05-23-2006, 09:08 PM
Why don't you just measure your tire temperatures after the first few runs and see where they're at? Then you something measurable to base your changes on.

ChosenGSR
05-23-2006, 09:41 PM
Why don't you just measure your tire temperatures after the first few runs and see where they're at? Then you something measurable to base your changes on.

That's exactly what I do, I check/bleed after each run. It's just confusing because there isn't much consensus on this subject. I suppose I'll keep on running front pressure bias, keeping the fronts at about 42 degrees hot and perhaps 37 or so on the rears.

jhm5
05-24-2006, 08:08 AM
For purposes of a driving school or autox, i agree with dderr it's easiest to use cold pressures. Keep it simple. Try 38 or 40psi front and 2 or 4 psi less in rear as a baseline. See how the car feels. If the tail is too happy, perhaps add a couple psi to rear. If it understeers too much, drop 2 psi rear (and/or improve your turn in style).
Hot pressures will vary depending on the track. Summit Point is mostly right turns. The left front gets most of the stress and it will be the hottest tire, therefore the highest hot pressure. Dealing with hot pressures is more complicated.
But by all means, check pressures periodically during an HPDE, to make sure you're not getting a flat, which makes the driving experience very interesting.

Rafgar
05-24-2006, 09:26 AM
Alex,

John and Dwight are essentially correct. Keep the fronts at least 2 psi higher than the rears.

Keep in mind that during an autox run, your tires will not get very hot (as compared to driving on the track). This means that you start with higher pressures than you would use at the track.

On a stock 330 with street tires and in nice dry conditions, I would start with 42 front and 40 rear and then bleed down if necessary. Remember that you need to brake for the turns (don't over drive the car) and that you need to be as smooth as possible with your inputs).

ChosenGSR
05-24-2006, 10:51 AM
Thanks everyone. :icon_wave Rafael, if I start with 42 upfront (essentially cold) it almost seems like I might be approaching 50s by the end of the first 20-30 min session. Is that safe on street tires?

dderr
05-24-2006, 12:16 PM
42psi front would be acceptible at a tight auto-x and maybe at the Jefferson circuit but I still think 39psi (40psi tops) is more appropriate for front cold pressures. At the rear, 40psi is way too much, in my experience, even for an auto-x; 34 at the track, maybe 36 at an auto-x, split the defference for the Jeff. Look at the front tires after a track session and determine how far the wear goes up the sidewall. You can look at the rears also but you won't see any wear beyond the tread corners (you wouldn't see any wear even if the pressures were only 25psi!). You could blow away your money on a pyrometer but those readings might only be indicative of the last turn or two before you dove into the pits and before you frantically rushed to get out and make the tread temp measurements--your cool down laps will cool down the tires, too, you know. I, personally, am not too concerned about hot temp readings of >50psi (I think once I measured a hot tire reading of 65psi, dropped that pressure three psi and the next session the car turned in like a pig--even though it is a pig--and I wiped the corner off of the tire almost to the cord). Your tires can handle the pressure. And if indeed you are overloading the front tires by trying to muscle the car into a turn then your driving technique needs to be tweaked--you'll wind up going faster and saving money spent on tires.

Rafgar
05-25-2006, 10:40 AM
Thanks everyone. :icon_wave Rafael, if I start with 42 upfront (essentially cold) it almost seems like I might be approaching 50s by the end of the first 20-30 min session. Is that safe on street tires?

Alex,

Yes, you are correct. My comment on starting with 42 was for an autox.

For track sessions, as mentioned by Dwight, the starting pressure depends on the track and conditions. You probably want to start around 40 front and at most 38 rear - then check the pressures after each session to balance the car (side to side as well as front to back).

The comments on using cold pressures as your starting point was to make it easier to gauge.

The best way to make sure that you have optimum tire pressures is to use a temperature gauge not a pressure gauge.

Barry
05-26-2006, 04:31 PM
Alex,
For track sessions, as mentioned by Dwight, the starting pressure depends on the track and conditions. You probably want to start around 40 front and at most 38 rear - then check the pressures after each session to balance the car (side to side as well as front to back).
The comments on using cold pressures as your starting point was to make it easier to gauge.
The best way to make sure that you have optimum tire pressures is to use a temperature gauge not a pressure gauge.

The set up for Autocross is different than the set up for HPDE's. When I autocrossed I ran more psi in front because you just needed to stiffen up those sidewalls. But in my 330Ci, for the track, I experimented and found that the 35F and 42R worked really well on the Summit Point main track.
Barry

Pinecone
05-26-2006, 07:32 PM
For track days I have a basic cold setting that I modify once the tires are hot. I go for HOT pressures on the track. Cold to hot on the track can be a good bit of pressure and be very different front and rear.

At FATT sessions autocrossers tend to have WAY to high starting pressures that makes life fun. The track generates much higher pressure gains.

As for the quesiton about 50+ psi hot on a street tire, no problem. The tire is rated for a maximum COLD pressure (on most performance passenger car tires I run, that is 44 psi) that will go a good bit higher in high speed summer driving.

In autocross I set the pressures cold to what I want, then bleed each run to keep the same pressure. I get 2 - 3 psi gain first run (normal day) and then 1 - 2 psi for a couple of runs, then pretty much no change.

I disagree about the value of a pyrometer. Yes the tires cool down fairly quickly but not THAT quickly. But you do need a person to read tire temps as you roll into the pits, not after you slowly drive back to your paddock, unstrap, take off your helmet, climb out, THEN take temps.

And if you are serious, come in off a hot lap and have someone take temps, then go back out. No cool down lap.

As for price, OG had a basic pyrometer at about $100. Not too expensive, even better with a couple of people going in together.

A pyrometer is very important when you start to set camber when you upgrade the suspension.

With a pyrometer you shoot for even temps across the tread. With stock suspension you will likely see higher outside edge temps, but go for an even gradient across the tread. Center hot, lower pressure. Edges hot increase pressure.

dderr
05-26-2006, 07:42 PM
Barry--
If you ran/run the same tire pressure scheme in your M3 (asyou ran in the 330i) that explains (to me, anyway) why you were so slow and tentative in the turns at the Shenandoah circuit last weekend. Just as at an autocross, the front tires absolutely need the extra pressure up front to stiffen the tires' sidewalls in order to provide some responsiveness at turn in. And, in my experience driving lots of different BMWs at the track and in autocrosses, 42psi rear is excessive (by at least 6psi)--it might be an appropriate figure for a hi-power rear-engined car like a 930 turbo where the driver likes to power oversteer, but in a BMW, maybe not the best way to go...