Tolerable Diameter Difference for C4 - widebody
 

Good day, fellow Rennlisters.

I am the current owner of a 964 C4 widebody and couldn't be happier. The only issue I am having is with regard to the front tires rubbing against the inner fenders when I hit bumps on the road and when making full turns - the tires rub on the inside as well. The car is set up with lowering springs and H & R coilovers and currently has 245/45/17 and 315/35/17 (no problems with the rear).

I am considering changing the tire set up to 235/40/17 and 315/30/17 in hopes the smaller tires in front would be the solution to my rubbing issues. Problem is, there is a diameter difference of 0.17% vs my current set up (245/45/17 - 315/35/17) which has 0.0%. I have done some reading and with Carrera 4's, apparently there shouldn't be any diameter difference between the front and rear tires, otherwise the viscous coupler will get affected because of the mismatched reading between the front vs rear.

If this indeed is the case, are there any official tolerable limits for the diameter difference where the viscous coupler won't be affected so it won't detect any changes or it's 0.0% or nothing?

My other option is to go have custom inner fenders made which hopefully solves most of my rubbing issues.

Many thanks in advance for any advice given.

964 has a planetary gear centre diff. It's the 993 that has a viscous diff.

The max difference in rolling radius is given as 6%. This is the maximum the PDAS will automatically adjust for in its program. That's purely an electronic restriction, in that PDAS will shut down if it detects more than 6%. There is also a mechanical concern in that you don't want too much difference as that may damage the centre diff (maybe by over heating the planetary gear bearings I guess).

If you have a bit of a search you may find sone suggestions on tyres and wheel offsets for a widebody. It may be you just need a shim on the front to reduce the offset a fraction.

PS hello and welcome to rennlist :)

As mentioned above, a 5mm or wider wheel spacer may fix your problem.

Alexjc4 and Newsboy, thanks for the info and definitely a big help! Appreciate the welcome to Rennlist as well.