Quote:
Originally Posted by E89Tardis
I've finally joined the ranks of the Non RFT community with the 18" PSS.
I'm interested in what tire pressure is recommended for this car with these tires. Given the RFT's have such a stiff sidewall, I'm guessing BMW spec'd the nominal tire pressures with this in mind.
Does anyone know of any BMW spec's for the non RFT with a softer sidewalls and radically different roll geometry in our cars? Has there been a consensus of the ideal PSS pressure or some way calculating it based on tire size and load?
Also I'd be interested in any suggestions about the value of the original tires. All four look excellent to me with lots of deep tread. Should I put them in the forum classifieds or craig's list and what do you think would be a fair price range.
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There is no correct answer. The car mfg put a sticker on the car to cover their assets/asses over the most extreme possibilities of speed and weight for liability reasons. Remember the Firestone tires on the Ford Exploders (not a typo) - substand tire design + low tire pressure + bad vehicle geometry = blowouts, rollovers, and possible death.
Normal use
I know this is a jeep with big tires but the same principals apply for any tire outside the mfg specs.
http://www.youtube.com/watch?v=ojGsavK0Uwc
Auto cross
Put in extra 4-6 psi in your tires over mfg ratings. The reason for this is to keep your tires from rolling under during hard cornering. But how much is too much? Put chalk on the edges of your tire, in three places around the diameter, and you can see how far over the tire was going during your runs. Bleed out a little if the chalk is still showing on the tread, or add a little more if the chalk has been worn off down the sidewall. The line of worn chalk to remaining chalk should be right at the corner of the tread and sidewall. Keep notes on how many psi you ran, and where the chalk line was, for your next event.
Search of "chalking tires autocross" on the web = Lots of different ideas.
If you want the easy, or lazy, answer go with the mfg spec tag inside your door no matter what tire you put on.
Otherwise you have to experiment depending on your type of driving.
You stepped outside the box and bought non RFT's - the journey is not over. Go have fun and experiment. Personally I run 34/38 psi F/R on my 235/40x19 & 265/35x19 F/R PSS Non RFT's for normal around town and short single person trips. I bump them up by 2-4 psi depending on max load (2 people fully loaded for a trip out of town - packing my Conti Kit and Stop n Go Plug kit) or 6 psi for hard driving/auto cross.