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      10-21-2012, 08:59 PM   #14
RPM90
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Drives: 335i Msport w/sport AT
Join Date: Mar 2006
Location: Chicago

Posts: 6,095
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Quote:
Originally Posted by rayusaFL View Post
Thanks for all the replies.
I just wonder why the dealer left 32 psi all around.

Am i going to notice any difference in handling when increasing the rear tires to 38 psi?
Thx.
Looking at the manual, tire pressures are different between 328i and 335i, due to the weight and weight distribution between the cars.

Tire PSI for up to 100mph:
328i with non staggered 18's:
32frt and 35rr.

328i with staggered 18's:
32frt and 32rr

335i with non staggered 18's:
32frt and 38rr

335i with staggered 18's:
32frt and 35rr

If you typically drive over 100mph often, for example you drive on the Autobahn daily, you should raise your cold tire pressures.

For a 335i with non staggered 18's, over 100mph pressures should be, and can go up to:
38frt and 46rr

335i with staggered 18's:
38frt and 41rr

I am currently using 37frt and 40rr.
I use these pressures, between the nominal and high speed, for better tire wear, and potentially better highway MPG.

I got these numbers from the manual, but it's usually best to use the sticker on the door jamb as some factor may have altered the psi to use on the particular tires your F30 came with.

Regarding what ride or handling characteristics you may feel, the easier to notcie aspect is that you may notice harder/harsher bump quality on sharper/larger hits when using higher pressures, as the added pressure will stiffen the sidewall, thus having less sidewall flex to help absorb some to the bump energy. OTOH, the stiffer sidewall can give some positive handling aspects resulting in sharper turn in and quicker transitional handling, as the sidewall flexes less it can result in quicker response.

RFT's in general are harsher with big bumps and hits than non RFT's.
By increasing the psi you may notice a bit harsher hit on those larger hits, but it's not crazy bad.
Handling difference can be harder to detect as it depends on your sensitivity and how fast you tend to drive in corners.

Even harder to detect is how psi changes between front and rear can alter under-steer or over-steer.
If you change the psi difference between front and rear, you may notice under/over steer changes, but again it depends on what you can notice to begin with.
For example, your nominal under 100mph pressures are 32frt and 38rr, a 6psi difference. If you have some noticeable under steer, you can play around with the pressures on just one end of the car, and then drive it to see if there is a positive change to handling.
I wouldn't recommend going below the nominal psi though
You can use the nominal and high speed psi range to play around witih.
It may not be night and day differences though, as it can be more subtle and it can be positive or negative based on what pressures you use.
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