Tire pressure vs rolling diameter

[Tapani]

I played around with tire pressures..... and measured the height of the rim top edge from the garage floor. There's quite a bit of difference between the front and the rear.

If the 4WD system is sensitive to uneven tire wear the pressure has a bigger effect.... several mm in the radius.

I had to increase the front to 40 psi and decrease the rear to 26 psi and there was still maybe 2mm difference in the radius.

Any comments?

225/45-17 Nokian Hakka Zs (good for 300km/h) on STi wheels.

Tapani

[svxcess]

Someone did a similar post along those linesa long time ago. As, I recall, they wanted each tire to share the same load equally and have the same rolling diameter..

I think they started by taking the overall weight of the SVX and, using the front/rear weight ratio, calculated how much weight was on the front tires and the rear tires. They played around with tire pressures to achieve the same amount of load on each. It was a really weird set of pressures.

As recall, They took a SVX weight of 3549 lbs and, using the weight distribution of 59/41, calculated there was a load of about 2094 lbs. on the front wheels and 1455 lbs. on the rear wheels.

Divide that by 2 and each front tire supported 1047 lbs. and each rear tire supported 727 lbs.

A final tire pressure figure of 46 PSI front and 33 PSI rear, This gave a weight of around 22 lbs. for every PSI.


Now add in a driver, or a driver and passenger (and their weights), or a transmission swap, and things change. Really was a worthless post, plus the car drove badly when I used these figures.

.

[Tapani]

Found it

http://www.subaru-svx.net/forum/show...e+ratio&page=4

Is the OEM pressure recommendation the same for ACT and VTD cars?

T

[jetboy]

Originally Posted by Tapani

Found it

http://www.subaru-svx.net/forum/show...e+ratio&page=4

Is the OEM pressure recommendation the same for ACT and VTD cars?

T

They were nice enough to put a sticker in my door which says 33 in front, 29 rear. VTD. I assume they put the right sticker in...

[stevek]

Obviously tyres with the same rolling radius are good news for our cars. Only time I have seen this take priority over even tyre pressures though is on a race kart, where driven wheels are on a solid axle. And even then within limits.

[kjoyes]

would using this information could you figure out the optimal tire size for both the front and rear like 2 different size for both the front and the back or does this not apply to tire size just air presure?

[LetItSnow]

Originally Posted by svxcess

Now add in a driver, or a driver and passenger (and their weights), or a transmission swap, and things change. Really was a worthless post, plus the car drove badly when I used these figures.

Lots of static measurements for conditions that are only pertinent when the car is in motion...

[ensteele]

Tire pressure is one thing, and tire size is another. I would adjust pressure, but I would NEVER put two different tire sizes on.

[Chuckls]

Originally Posted by ensteele

Tire pressure is one thing, and tire size is another. I would adjust pressure, but I would NEVER put two different tire sizes on.

Unless.....RWD....or VW

[david_12121]

the recommended tire pressure was calculated based on the weight on each tire, and if you wanted it to be exact, (down to 2 mm!) you have to increase the driver side compared to the passenger side
then again, you'd have to change it based on the weight of your passengers, as a 160lb person can easily result in a 2mm difference(guess)
and in a corner it obviously changes again due to weight transfer and body roll
I'm no expert but my guess is that it can only be harmful if the difference is a huge amount...you know, like one tire is full and the other has a leak of some sort