BMW 3-Series (E90 E92) Forum > BMW E90/E92/E93 3-series General Forums > Regional Forums > UK > UK Technical Forum > Tyre pressures

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      04-21-2009, 03:23 PM   #1
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Tyre pressures

Ive now taken my runflats off whats the pressures meant to be now, I read that they should be inreased. The ride is much better but very soft when cornering.
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      04-21-2009, 04:05 PM   #2
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I'm running 3.1 bar rear and 2.9 front. Its working well for most normal driving. I just have to avoid pot holes
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      04-21-2009, 04:07 PM   #3
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I have kept mine the same which is 38 and 42 respectively on front and back. If you inflate the non run flats too much you will be prone to uneven tyre ware. My tyres are 18"x225x40 on front and 18"x255x35 back

The whole point about non run flats is to obtain a more compliant ride. Inflating to more than this will surely be counter productive.
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      04-22-2009, 04:05 AM   #4
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Originally Posted by wrighty View Post
Ive now taken my runflats off whats the pressures meant to be now, I read that they should be inreased. The ride is much better but very soft when cornering.
You've now lost a lot of the turn-in precision and handling capability of the chassis by radically altering the suspension spring and damping rates. To get it back, you're going to have remove sidewall compliance, which is achieved by increasing tyre pressure, which will harden up the ride and put you right back where you were with the RFTs, but without their added safety.

There's no free lunch here. Add undamped compliance through softer sidewalls and you'll get a plusher ride ala Mercedes but more body roll and less sporty steering ala Mercedes.

What you're asking for is a more compliant but equally sporty ride. Changing tyres will give you one or the other.

Suspension is a combination of springs to absorb the bumps and damping to control resonance. The tyres are part of the system in that they provide a spring (the air within the tyre) and damping (sidewall compliance).

Changing runflats for normal tyres adds spring and removes damping, which makes the entire suspension more compliant (softer ride) but less controlled (more body roll, less sharp turn-in).
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