Gweedo

I'm planning on getting some 18" wheels and tires for my '95 C4 and I am trying to decide on tire sizes and aspect ratios. Is there a maximum difference in overall tire diameter discrepency between the fronts and the rears on a C4? No more than .5"?

Thanks!
Matt

Joe S.

Yes, no more than a half inch difference between front and rear is what was recommended from what I found from my search in the achieves.

ca993twin

I've also been trying to find the max allowable difference, but also the optimum difference... my suspicion is that the optimum is to have the rears turning a teensie bit faster than the fronts to keep the viscous coupler warm and ready for instant use, but not so hot as to wreck it. I've found it very difficult to find really definitive answers to both max allowable and optimal. BTW... 1/2" difference in overall diameter sounds like quite a bit... but clearly I don't know.

Joe S.

Here was probably the best answer...

https://rennlist.com/forums/showpost...68&postcount=8

ca993twin

Joe,

While Viken often had the correct data, his answers were a bit terse. He says : 1/2" max tolerable, but doesn't say which end should have the larger/smaller tires... assuming it matters. And we still don't know what the optimal delta should be. I dunno know... somebody MUST have more information than was shared by Viken.

Joe S.

Steve,

There was another thread I was looking for that was debating this issue as well. I'll see if I can find it.

Joe S.

Some more reading on the issue...
https://rennlist.com/forums/showthre...diameter+tires

ca993twin

Joe,

Thanks for doing all the leg work and digging up the old threads. The interesting factoid here is that the narrow body C4 came with stock tires where the rears were very, very slightly taller than the fronts, where as the wide body & turbos came with tires were the rears were substantially smaller (but well within the 1/2" max) than the fronts. Perhaps it simply doesn't matter much which end is larger, and perhaps the "optimal" delta is not an issue. But I'm an engineer... and love to know this stuff anyway.

Joe S.

No problem Steve. I'd love to know the right answer too. I was surprised that I didn't find any information on this in the manuel. As an aside, I know that when I've taken the GF's Audi S4 in with all wheel drive for new tires they have been very **** about the diameter delta between the front and rears.

Red rooster

The front tires must be slightly larger than the rears .That means the fronts turn slightly slower so the viscous coupling can work.
Get the rears larger and you have a C2 !!

Geoff

ca993twin

Geoff,

Your explanation matches my understanding... except... why the stock wheels on the NB C4 had the rears slightly larger than the fronts. Is it possible that VC can warm up so fast that it doesn't really matter? Can you cite any reference material? I'd love to see some damn data, being that I'm an engineer.

RallyJon

I think a lot of people would. Curious that you can choose the spec of your center diff/viscous coupling from among several for a plebian Subaru, but for Porsches no one seems to even know what the stock spec is.

https://rennlist.com/forums/993-forum/221046-993-c4-is-the-rear-diff-an-lsd-too.html

The theory that the viscous center diff was designed to run with preload from different diameter tires was debunked, however.

Red rooster

Steve,
A viscous coupling has to have the power input speed higher than the output to function. Think of it as a slipping clutch ! The shear forces in the silicon allow the output speed to get close to the input speed , dependent on power being transmitted.
If the output is already being driven faster than the input , no power can be transmitted.
Because of the non linearity of power transmission versus in-out speed differential it is generally a good idea to have a speed pre-load to ensure some kind of operation.Hence slower turning front wheels.
The 964 C4 has an epicylic drive to arrange torque split irrespective of wheel size , within reason.
Interesting to see that the 997 turbo will have a 959 style ,controlled slipping clutch pack for power delivery to the front. Again , wheel diameters, front -rear will be a factor !

What tire sizes are you looking at for the NB 994 C4 ?

Geoff

Phil

Hey Steve,
How is the new Ruf doing?...........hey I found this in a search, but will try to tone it down for you.......OK?

Steve, Please , try to keep the front to rear diameter difference to no more than 1/2", if possible please. Stock tire diameters work best, but no problem deviating a bit from those. other than that......remember the front tires were smaller than the rear tires.....not in diameter, but rather in width. Most 993's have matching 18 inch rims, or 17's for that fact, hell I have even heard of a few 993's having 19 inch rims

Of course I understand you may not want to take Vikens advice. Have you tried googling your question?
Me personally.....I prefer terse and correct rather than PC and wrong.........good luck with your quest for the knowledge presented in a "terse" manner, and your quest for a wrong answer presented in a 'frosting of chocolate, and icecream" pie
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ScottMellor

Um, Phil, besides the fact that we all know Steve is a Tire Heritic, I don't think the question about which end of the car should be larger was addressed by the very sorely missed Viken.
You didn't use to be so testy!