Why do RWD cars run more tyre pressure on the rear?

[Christian]

Hi

...well... why?

I always assumed pressure and weight on the wheel correspond to each other. More weight - more pressure.

Been running 2.3 front 2.2 rear for ages with good handling.

But chatting to a friend with a BMW yesterday I found out this is wrong, had a look at the sticker in the door and...
2 bar front 2.2 bar rear...
Wtf? I was running wrong pressure all those years? :whack:

So can somebody explain these standard settings and what are you guys running for fast road use?

Cheers
Christian

[Raddy]

correct tyre pressure cannot be set just by sticker information... correct tyre pressure is depending on the tyre type/construction, weight on it, road surface quality, drive style, ambient temp, susp. geometry.... as you can see, thats quite a lot of variables, in practice thats one way how to find correct tyre pressure that will suit all above at once=by measuring tyre thread temperature. Its easy, quite fast and can be done for different situation (fast road, track). Use laser/infrared thermometer and look for same temeperature at the outer parts compared to centre part. If all measure points show more or less same temp., your tyre have correct pressure. Higher outers=too low pressure, higher centre=too high pressure. Thats it...

[Christian]

Thank you for the answer.
Of course when modifying and setting up your vehicle you need to adapt tyre pressure accordingly.

But I don't understand why Nissan set it to 2 front, 2.2 rear on a standard RWD car.
And with the BMW M3 it's the same. More pressure to the rear.

The rear is lighter -> less load on the tire -> more pressure??? I don't get it....

[Barryjdart]

Tyre pressure can be used to adjust the over/understeer characteristics of the car. A rear wheel drive car will tend to be an oversteering car whilst a front wheel drive will tend to understeer with the same pressures front/rear.

In order to help counteract this effect many manufacturers of RWD will run the rears at a higher pressure than the front and FWD will run higher front pressures. See below:-

http://www.nyracer.com/overunder.htm

Hope this sheds some light on the subject.