Cold PSI for Michelin PSS Tires?

[BMW135pls]

I know this topic has been discussed at length, and I've read a handful of threads on the topic. What I've read said that the tires are great for traction, but the sidewalls are fairly soft, but that improves greatly with higher tire pressures. When I got the tires installed, the tires read about 28psi cold by my meters (both of which gave different readings, and probably aren't accurate either of them) I read some threads about tire pressure for these models, and thought that I would up the cold pressure to pretty close to 40, the rear tires being a bit higher than the fronts. I used an Excel air tower to fill the tires (which I actually repair and maintain as part of my dayjob, but I don't work for Excel) and the results are a bit peculiar if not disappointing.

I got the tires nice and hot before stopping at the station and filling to about 43 hot for the rears and 40 hot for the fronts. By my estimation, hot pressure multiplied by .82 should give the estimated cold pressure. The following morning I metered my tires and found that the cold pressure was equal to if not exceeding the pressure ratings given by the air tower the day before. I metered almost 50psi on the rear tires. Needless to say I drained the pressure down to about 38psi for the rears and 35 psi for the fronts, both cold. Still up from what they once were, at less than 30 cold for all four tires.

This confuses me quite a bit, for two reasons. One, I thought that pressure drops when tires rest, so either the Excel towers have some software which assumes that the person doing the filling doesn't know what the hell they're doing, and calculates the difference between cold and hot pressures, assuming the tire is hot when filling, and that the pressure set on the machine is the desired cold pressure after the tire has cooled down. First of all this angers me, as I could have caused potential harm to my vehicle as a cause of this, and because the set pressure simply wasn't accurate. Second, because I actually think the performance of my car might have dropped since the pressure increase.

Before the pressure increase (when the tires were filled to a measly 27-28psi cold) I was able to clock as low as a 4.5s 0-60 time. And after the pressure increase to 35F and 38R, I haven't formally clocked my time, but I have noticed an increased tendency of the car to sit still and spin its tires without going anywhere on launch. Either we are dealing with chaos theory and perhaps micro changes in driving practices, or the higher pressure ratings on these tires may increase handling, but actually decrease straight line performance?

Anyway, that's enough brain diarrhea for one post. Anyone have any input?

[AyJay87]

Generally, lower tire pressures increases traction. The lower pressure allows the tires to flex a little more, so as you accelerate and the weight transfers onto the rear tires, this applies more force and a larger contact patch makes contact with the pavement.

Obviously there are other factors such as tire construction, compound, pavement temp, etc.

Almost, forgot, I run 33F and 37R.

[Adam135]

I've been running 32 front/back

[Crazycarl]

I run 36 cold which stays at 36 cold in winter, Denver, and 40 summer when hot.

[Suhb]

i believe AyJay is right. Depending on the road surface The lower PSI will accelerate you quicker. Remember asphalt isn't a perfectly smooth surface. Lower PSI will let the tire flex and squirm more to dig into and grip the asphalt better, essentially increasing contact patch. I wouldn't calibrate my tire's PSI for acceleration though.

Here's a good source: http://www.tirerack.com/tires/tirete....jsp?techid=58

[JimD]

I fill to the pressure level on the sticker on the drivers side door jamb. My fronts are the same as backs so they are oversize but I still use the stock pressure. Actually I dial it up a couple psi on the general theory that they may fall a little before I check again and a little higher helps gas mileage.

I use AcuTire pressure gauges after reading a review on line where they were the most accurate. I fill with a 12V Viair 70p compressor unless I am at home and decide to drag the hose out for the 120V compressor.

[Bullitt]

I run 38psi cold. On long road trips 40psi cold. No issues with grip with these higher pressures. I also run -2.4 camber on the street with 0.04 toe-in and shoulder wear is right where it should be.