So I blew a rear tire at the track on Saturday, but was OK with it since I came prepared with my used compact spare from Ebay.

I was in the process of removing a front tire to use on the rear and planning to put the spare on the front. A guy whose opinion I usually trust came by and told me not to do that as running a front tire on the rear would burn up the differential because of the size difference. Diameter of the front tire is 25.5 in and rear is 26.2in. BTW - I had about a 125 mile drive home.

Is this correct? Car does not have a limited slip diff AFAIK.

I was able to get the rear tire patched and drove home very slowly. Will replace both rear tires ASAP, but want to know if the spare is really only usable in case of a front flat.

 

If that worried put BOTH fronts on the rear? I don't think that small difference is that big a deal. Some complication arises when you have a 4--but certainly not for a 2 wheel drive Carrera.

 

Could be a problem with AWD (C4, Turbo, Targa) but not with 2WD

 

Sprint and midget cars must go through a lot of rear-ends...well, if they had them anyway.

 

Wouldn't the effect be the same as going around a circular track for a couple hours in one direction, in other words, easily tolerated by the diff?

 

if that were a problem, then circling a track would also burn up the diff, because the outside tire does alot more miles than the inside tire. I think 125 miles would not be a problem. On my offroad vehicle, with undersized spare, I air down the bigger tire to bring the diameter down to that of the spare.

 

Thanks guys, I appreciate all the thoughtful responses. Pretty much confirmed my thinking (but then one always likes opinions that agree with one's own).

Forgot to mention that the car is a C2S. I'm familiar with the tire diameter restrictions on the 4s but didn't think they included 2WD cars.

I particularly liked the example of circling a track having the same effect, as that's what I spent most of the day doing before trashing the tire.

 

Is the inflatable spare the same diameter as the front, the rear, or something in between?

I think you're safe putting it on either axle.

 

Porsche delivers the compact spare with C2 and C4 variants in many parts of the world.
I would expect the diff can handle that just fine.. isn't that the point of the diff?
I would get the tire replaced asap, and not drive around on it for 800+ miles (arbitrary figure that seemed large enough for the point).

Can someone with knowledge of the diffs in a C4 talk to if it would be an issue with the AWD systems and if so, why?

 

I carried a 17" 996 spare in my 06 Cayman S. Never needed it but it was worth it for the peace of mind. Now that spare matched total diameter of 996's. The 987/997 has slightly taller diameter up front and good bit taller in the back. My plan was to only use the spare in the front and switch a front wheel to the back if I had a flat in the rear. Then both axles would have small mistakes, instead of one big mistake in the back.

Onto another question... I "knew" from collective info that it would clear the brakes. Does that 17" wheel clear the brakes of the 997.2? In my case, non-S.

 

I honestly don't think the OP had a problem, but I don't buy those reasons, Fin. I don't pretend to have time to do the math, but you pose an interesting question: how much farther does the outside tire travel in a lap of a track? Without the time to do it right, I'm going to be thinking about this all day, darn it. My first intuition is: two times 3.14159 times the track width. (That is, the distance between wheel centers on the given axle, not the racing surface.) Picture thirty feet for a rough image. That would take a lot of laps to add up to even one mile difference in the distance traveled.

More important, if we were to suppose the differential heated up in response to the difference in rotation rate from one side to the other, then the percent differential in a lap of a one-mile short track would be minuscule at 30/5280, that is one part in 176 or roughly 0.6%. The rear tire has a diameter of roughly 26" hence a circumference of a little under 82". To have no more effect than the on-track difference, that spare would have to be within half an inch of that circumference, or in other words a diameter of ... well, roughly 26". (I said I'm out of time.)

I don't say your spare needs to be that close to the original tire, just that the effect of going around a track isn't anything like as dramatic as a quite small difference in rolling circumference.

Gary, off to do stuff, but thinking about how we analytically determine that extra distance traveled by the outside tire

 

Okay, I'm sorry, it's obvious. But in my defense, I only got twenty feet down the hall before realizing the answer. Just had to quit typing and start thinking. Which... is probably good advice for many posts.

No time to explain but the arithmetic is the difference in distance traveled is 2*pi*R-2*pi*r which is to say: 2*pi(R-r) and since R always equals r+T, for the track width we have 2*pi(r+T-r) which becomes 2*pi*T. Jeez. Will think a breath longer next time. And will explain the why later if no one else has already.

Gary

 

All I know is that when I lock the diff. on pavement on my truck, it will chirp the tires at low speed in any kind of a turn.

 

Isn't the entire point of the differential to handle the different speeds of the two wheels? Otherwise we would all have locked differentials still in the cars.

I would think that you'd be fine limping the car home. At least I hope so as I equipped myself with the same spare setup for just this type of incident. Seems better than leaving a car on the side of the road hunting for a tow.

 

Yep. If you make a full circle, the outside rear tire needs to travel thirty feet farther than the inside rear tire and because you have locked the diff, they have to rotate the same number of times. It depends on things like the relative traction, but usually I'd expect the outside tire will squeal as it gets drug further than the rotation of its wheel wants it to go. Conversely, the inside tire may rotate without being able to move forward. One way or the other, you're going experience a total of thirty feet worth of tire slip between the two sides because the axle is essentially a rigid connection with the diff locked.

Actually, 2*pi times the track width of your truck, but that's going to be close to thirty feet, just like a C2S, unless you drive a true HMMWV. And the difference is going to be that same thirty feet (or so) for any full circle, whether it's in an especially wide driveway, a parking lot, or the big track at Willow Springs.

Gary