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[Fraggy]

Im curious if you guys can share what your optimal tire pressure is, especially if you still have on the factory RFTs and Sport Pack.

Currently, I am using the recommended 29/32 with 17's. Is anyone using a different pressure and what effects has it had?

Thanks guys!

[scubajag]

I'm using 35 in the front and 39 in the rear with RFT. Ride is fine at 10,000 miles.

[Fraggy]

Are you shittin me! The handling must be awesome if the ride is acceptable. Wow, cold pressure of 35 in the front, will try it tomorrow

[ToddL]

I'm running 35 front as well, although I run 32 in the rear. The problem I had with the 29 psi spec in the front was that the tires were rolling over in hard corners and generating understeer. 35 balances out the car much more nicely. The difference in ride quality between 29 and 35 I have found to be somewhat minimal. Good luck.

[txusa03]

I have 18 OEM RFT and 38 front and 45 rear. I am at 26,500 miles.

[Fraggy]

Originally Posted by todd325i

The problem I had with the 29 psi spec in the front was that the tires were rolling over in hard corners and generating understeer.

Thats the same issue im having, especially in long sweeping turns.

[ToddL]

Originally Posted by Fraggy

Thats the same issue im having, especially in long sweeping turns.

Same here re: long sweepers. I suspect this low front tire psi spec is to to make the car "safer" for the masses and soften the ride ... forget that. I stepped up the pressure in 2 psi increments until I found the level I needed. 31 was not enough, 33 was close but not quite there, and 35 was spot on. 37 and 39 didn't seem to generate any incremental improvement, just more harshness over the lovely upper Midwest roads I travel.

I've been thinking about the 255's in the rear recently, and I suspect that if one runs 225's all around (like the new Potenza RE-01R, which look evil) we wouldn't have to crank the fronts up quite so much to balance the car. I'll be putting the Dunlop snows (Winter Sport M3) on soon, sized 225 all around, so it will be interesting to see how they compare in terms of the psi required to mitigate the understeer problem. I realize it's not an apples to apples comparison, but it should at least be accurate directionally.

[Fraggy]

Before leaving the house this morning, I pumped them up to 32 front, 35 rear. No loss of ride confort at first. But when the day was hotter on the way home, the rears were really thumping over bumps. I lowered them 1 pound, and shit, it all went away. The back felt as smooth as the front. I may up the fronts tomorrow to 34, and have 34 all the way around.

Does anyone know why BMW recommends more pressure in the rears?

[xyz123]

Originally Posted by Fraggy

Before leaving the house this morning, I pumped them up to 32 front, 35 rear. No loss of ride confort at first. But when the day was hotter on the way home, the rears were really thumping over bumps. I lowered them 1 pound, and shit, it all went away. The back felt as smooth as the front. I may up the fronts tomorrow to 34, and have 34 all the way around.

Does anyone know why BMW recommends more pressure in the rears?

The tires and tire pressures are undoubtedly a factor in the suspension design so they would be chosen by BMW designers to proved neutral handling or understeer or oversteer as the case may be. The front and rear suspensions are totally different designs so it is not totally surprising that different tire pressures front and rear are specified. I find it interesting though that BMW doesn't specify different pressures for different tires (standard vs sport, 325i vs 330i, etc.) so I guess it can't be that crucial.

Racing drivers who drive cars near their limits would no doubt have the heightened senses to feel it but I would say that typical street drivers wouldn't notice any difference in handling from a few psi difference in tire pressures. If they could, there would be no need for them to check tire pressures with a gauge, they would know by how a car handles if a tire needs air. So a few psi off the specified values is unlikely to be dangerous but it will likely show up in increased long term tire wear. Another sign that sticking to specified tire pressures is not that critical is that BMW still hasn't corrected the huge discrepancy between the tire pressures specified in the manual and on the plate in the door frame.

[gos]

Originally Posted by Fraggy

Before leaving the house this morning, I pumped them up to 32 front, 35 rear. No loss of ride confort at first. But when the day was hotter on the way home, the rears were really thumping over bumps. I lowered them 1 pound, and shit, it all went away. The back felt as smooth as the front. I may up the fronts tomorrow to 34, and have 34 all the way around.

Does anyone know why BMW recommends more pressure in the rears?

Once the tire is warmed up, in most situations, higher pressure grips better. By keeping the pressure higher in the rear, it "induces" understeer because your fronts will slide first. Manufacturers feel (and surely have the data) saying that a slide into the guardrail due to understeer is safer than a spin back into the highway due to oversteer.