steve_eo

I'm appologize as I know this topic has been covered, but I couldn't find a thread that answered my question.
Question: can I run the same tire diameter on a C4S without screwing up the VC? From reading the threads on this, I believe that a 6% difference is tolerable. Since 0% is less than 6% I should be safe? In the 0% case, both front and back tires will be rotating at the same speed....will this effect the front/back torque split and in turn affect how the car handles?
I've already replaced the VC (unknown why it broke-maybe from PO) so I don't really want to end up with another broken one!

Thank You for any insight.
-Steve

Jeff 993TT

Steve, you want the rear to be slightly smaller diameter than the front. This keeps the VC thinking that the rear tires are slipping a bit, which will put a little bit of power to the front.

If you are switching tires/diameters, just make sure that the ratio of your new tires is similar to that of the stock diameters.

steve_eo

Jeff, thanks for replying. So your recommendation is to ensure that the front still gets some torque. I wonder if I'd be damaging anything by using the same diameter tires but knowing that I'd be lacking front torque.

I think I can answer my own question now: I'd be within spec (less than 6% difference-so no damage should occur) but lacking in front torque by having the same tire diameter.
Does that sound correct to you guys?
-Steve

TheOtherEric

Jeff-
I'm curious as to your basis for that statement. I've just never heard it before. I was always under the impression that you simply want the tire diameters to be as close as possible.

Steve-
Dude you're being paranoid! But I understand where you're coming from. You'll be ok.

Jeff 993TT

No real factual basis, just heard it a few times over in the 993 TT board and it seems to make intuitive sense.

Also, the stock diameters are slightly smaller in the rear than in the front. With the stock sizes of 225/40-18 and 285/30-18, the revs per mile are 829.3 and 841 respectively. For a difference of 11.7 revs per mile. That means the rear turns 11.7 revolutions more than the front for each mile. ( smaller diameter means more revs )

For Normally aspirated cars using the 225/40-18 and 265/35-18 tires, the diff in revs is only 7.

If you want to use larger tires on your 4wd car, then many people use the
235/40-18 and 295/30-18 sizes. The revs per mile are 818.8 and 833.3 respectively. For a difference of 14.5. That means the rear turns 14.5 revs more than the front per mile.

So the larger sizes are probably just fine on 4wd cars. Of course, I'm not sure how tire wear would affect these numbers. ie, i sometimes run through 2 sets of rear tires for each set of front tires.

Viken

If that is the case, then Porsche made a mistake by installing a 265/35-18 tire on the rear of many narrow body C4's. That tire is roughly 1/4" bigger than a front 225/40-18.

Viken

Jeff, I am curious as to why you think you should post something non-factual. All it does is confuse the already confused board users.

Viken

Steve, try to keep the front to rear diameter difference to no more than 1/2", if possible. Stock tire diameters work best, but no problem deviating a bit from those.

Jeff 993TT

That's a good question... So there's evidence for both ways. Would be intresting to hear the real answer.

Jeff 993TT

Sorry to all to whom I confused. Viken, I'll do more research in the future before I post.

JimBob Jumpback

bubbas,

yall iz way kronfuzed bout tireds so JimBob goin ta straytin yall out. da frunt and reer tireds hafta be egzaclee 3.45% da saym. Ifn dey be anee more differant, den yall be messin up yalls kars.

dunt dew it.
Yur Fren,
JimBob

Viken

Are you trying to confuse the issue even more, JimBob? How did you come up with that 3.45% figure? And, what do you mean by them to be exactly the same? I guess they don't call you The Guru for nothing.

JimBob Jumpback

why I bein gald yall dun axed JimBob dat qwestshun, bubba viking. see, yall nowz dat tireds be round likey pi. now hang on hear, cuz JimBob gonna git teknikal cuz I iz an enganear. so pi, which be a littel bit mor dan 3, divideded by 911 fer the kind o kar, den multiplikated by 1000.00000 to konvert it inta percentagers be given yall 3.45.

now dem kalkulatshuns is a bit ruff, but ol JimBob is pert suure it iz at leest as valid as da idear of needin ta be keepin da reer tireds smaller dan da frunts. now all yall udder bubbas kin tell peeple ta keep em da same sized, pluse or minuz 3.45 percant, and yall kin point ta JimBob's sienterrific post rite hear.

Yur Fren,
JimBob