I've been told that you can not run different size rim widths on an AWD car. Is this true? I think my buddy said that it has something to do with not being able to match up the tire diameter and that it will kill the center diff. I wanted to run a wider rear rim then the front (8" on the front and 8.5" or 9" on the rear). Just want to make sure this is right before I order rims. Thanks!

Dave

 

You can do it.

 

The difference is wieght and resistance. The car depends on whats is programed for. I've seen different tires make a difference. If there is a problem, the car will toss you a code. Thats how I found out about the tires.

Mitch www.cp-racing.com

 

If you look in the Oct issue of Sport Compact Car issue where there is a black JDM Subaru on the cover there is a story about the car running in Japan on the Wangon with an EVO8. It's mainly a Cusco deal. Anyway, the STi is set up with different size rims. 17x9 with 53mm offset (F) and 17x8.5 with 48mm offset (R) and then running 235/45-17's. Same set up on the EVO but with 32mm offset for (F&R).

They say "The resulting rear contact patch is slightly less square, making the onset of oversteer less abrupt, and overall balance just a touch sweeter".

So give it a try. I'd do it but I have the wagon with two sets of rims but the tires are different and I don't think 235/40's will work on 7.5" rims

 

Yeah I saw that article. I know that even though they are running the same size tires that the overall diameter will still be different because on the 17x9" rim the tire will strech so it will give it a smaller diameter then the rear. I think I'll try to contact cusco and see if running a slightly different overall diameter tire will mess up the center diff.

Dave

 

The diameter of the tire is determined by the tire, not the wheel. You can run different widths as long as they are reasonable and you use the same tires front and rear.

 

But if you put the same size tire on both rims and one rim is wider wouldn't the tire be a little streched on the wider rim. In turn making the overall diameter smaller?

Dave

 

No, not really. What the smaller wheel rim does is curl the sidewall in more. A wider wheel would move the sidewall to a more squared off form. The actually diamerter of thew tire would change insignificantly. Different size wheels just move the sidewalls in and out. The diameter of the tire isn't going to change if you go from a 7.5" to an 8" rim width. The sidewalls just would round over as much.

 

*In Theory* there would be a diffrence in height between lets say a 235/45/17 mounted on a 9" wheel vs. a 7" wheel. IIRC a .5" difference in wheel width will have a .2" in contact patch. It is only logical that although the 235 on the 9" tire would be wider, it must also be shorter than that 235 on the 7" wheel.











In theory.



I think.

 

Thats what I was thinking. I think I may go with an 18x8.5 on the front and 18x9 on the rear with a 235x40x18 tire. I'll just run an offset on the rear to give me a deeper lip on the rear rim. Thats the best I can think of to compromise.

Dave

 

FYI: according to SCC the STi and the EVO both put the wider rims up front.

 

Yes, wider rims in the front. Reverse stagger of sorts. Anwyays, the difference in diam is not that big of a deal. I mean we are talking mm there. Even when you have same rims and tires front and rear and you pull 3 people in the back seat, you don't think rear overall diam would be less than front? of course it would. Same difference would be if you put 1/2 wider rim in the front (or the rear if you wish) and use the same size tire. These small differences are not going to do anything bad. Remember, the reason for that is that if the diameters differ too much, you center diff would overwork itself and overheat. So technically it is designed to take some differences.

I personally always wanted to try 17x7.5 w 225/45/17 in the front and 17x8.5 255/45/17 in the rear. I wonder how the turn in would be.

 

...then you must have the hook up on some new Cusco Active Center Diff Controller - those sizes are about 1" difference in OD!

On a slightly side note, a few weeks ago I e-mailed SCC w/ a tech Q: Why do so many AWD euro tuners (A4/S4/RS4, etc) stagger their tire sizes. I am still waiting for a reply.

 

Cool, keep us updated. Looks like I will just have to play with the rear offset to get that staggered lip (rear lip wider then the front lip) look I am looking for.

Dave

 

Oops! I meant to say 255/40/17. Not 45. The difference in sidewall thickness is less than 1mm.

I don't even think they make 255/45. That was a typo.

Right now I am running 255/40/17 all around.