Wheels, Tyres, and why IRS is not better than live axle when upgrading

[Mark351]

Not sure if any of you are deeply aware of rules governing aftermarket wheels, but as far as the increase in wheel track in Vic goes, IRS-fitted cars are only allowed an addition 25mm increase. I think the maximum tyres width put on an AU XR was a 245 (on an 18"x8" wheel) so the max you could increase to would be 255 on each size. Those of us with older live axle setups, can legally go to 265mm of each side, as the max increase is 50mm.

With the rules also allowing only a 25mm increase in wheel width, I wonder how rigidly this is scrutinised, and if that is why 8.5" and 9.5" wheels are made available, when presumably those with an 8" with wheel could to to 9", except an inch is technically 25.4mm.

[Casper]

bwahahahhaaha.. laws.. bwahahahhahhaha..


Shhhhhhhhh, dont open my bonnet, I'll be stuffed.


Anyhow, good pick up and actually very good info to take note of .
While we are on "laws" though, apparently if we increase power more than 10% of factory fwkw we need an engineers certificate too!!! :

[XR6-VCT-2000]

only law i have ever hear about tyre width is so long as the tyres dont protrude out the guards ( the rears i mean , the front usually allways do a tad due to the nose of the car being narrower ) . The only thing i can see happening with irs and too wide a rim , is it would prolly cause a bad camber (riding the insides of the tyres more) , nothing a camber kit cant fix .Maybe without addition setup it might be illegal to go wider than stated on the IRS setup , but im sure there is a reason and a fix/mod u can do too to make it legal .

[T_Terror]

I dont believe thats correct about the 245 max width tyre for IRS Aus.

I remember the laws saying somehting like a certain amount of mm over stock width, didnt XRs come with 225 or 235 rubber?

No cop will ever pick you having tyres that are too wide, unless you are in a skyline with a loud BOV and a smartass P plater.

[XR6-VCT-2000]

If you open the front drivers door on the au xr (on most cars actually ) there is a sticker on the door stating maximum size tyres , it states 245's are max , allthough i think from how it looks on the chart/sticker that they are refering to is that is the largest width tyre you can put on 'THAT WIDTH RIM' that came 'WITH' the car ! ,

[tufLTD]

Originally Posted by XR6-VCT-2000

only law i have ever hear about tyre width is so long as the tyres dont protrude out the guards ( the rears i mean , the front usually allways do a tad due to the nose of the car being narrower ) . The only thing i can see happening with irs and too wide a rim , is it would prolly cause a bad camber (riding the insides of the tyres more) , nothing a camber kit cant fix .Maybe without addition setup it might be illegal to go wider than stated on the IRS setup , but im sure there is a reason and a fix/mod u can do too to make it legal .

Spot on. Wheel track is measured from centre of rim to centre of rim. Tyres won't affect this, only rim width will.
The reason for not allowing wider rims is the extra load it places on wheel bearings, although I don't know why IRS would be different to live axles?

[Mark351]

I don't write the rules!

T Terror, I said 255 would be max, given 245 was the max allowed by the manufacturer.

Also, beware of load rating for tyres. You are not allowed to fit tyres that are lower in load rating than those recommended.