Bob M

I am considering going in a new direction for track tires. Right now I am using MSC's and want to go to a slick. The brand I would like to try is Dunlop but the sizes that I would like to use have slightly different diameters. Can anyone tell me just how much of a difference between front and rear diameters would be safe to use on a twin turbo, if any at all. As much as I would like to try these tires, I don't want to damage my front differential. Thanks for any feedback on this issue!

Bob

Bob M

Thanks Chad.

Based on your info, it looks like Dunlop does indeed have some choices available for TT's. I am looking at the GT series tires. Anyone running these tires? They also have a Super Sport Race tire but looks as though the largest size available is only a 285/30ZR18.

Joe Weinstein

What are the diameters (revs/mile)? Note that even if you are within any guideline about
differing diameters, you will get very different handling at the limit with different diameter
tires. If your rears are slightly larger, they will be rotating slower than the fronts, and the
viscous drive will actually be trying to slow the fronts down a tad. This means you will
get more understeer/push. Alternately, if the fronts are a little taller, you'll get sharper
turn-in, and a looser rear end.
The thermal/frictional stress you put on the viscous differential will depend linearly
on the difference in diameter and the speed (mph) you drive (gearing doesn't matter).
Joe

Iloveheat

What kind of front differential is the one in the 911 turbo (996TT)? Open, LSD, visco...?
Thank you
Paul

rbcsaver

Not sure I fully understand Bob M's question. Are you saying that you want to put new tires only on the rear, and their diameter's are different ? Or that the 4 Dunlop "slicks" have a fifferent diameter than the stock tires? It is never a good idea to mix tire brands, even with the same size/diameters because of the different tire characteristics. I can't understand how the same brand/model tire would vary in it's diameters, if that's what you mean from front to rear. As long as they are all the same brand/model/diameter it shouldn't matter what size (as long as they fit and don't rub). The caveat is that your speedo may not be accurate, because of rotational speed. All the non-OEM wheel and tire setups, from 16 to 19, or more that ppl are using certainly have different outer diameters than the stock setup.
regards,
Steve

Joe Weinstein

He's simply discussing his desire to run with a set of Dunlop slicks. His concern is that
they aren't available with fronts and rears in the same rolling diameter.
Another issue I need to mention is that your ABS will effectively drop your braking
ability by the percentage amount of difference of the front and rear tires. During hard
braking, the system will react as if the smaller-diameter tire was turning faster (which
it is), and treat that as if those wheels have less traction. It will lighten up on the
other tires, in the faulty assumption that they are locking up.

Bob M

Steve -- Joe summed up my situation pretty well.

Joe --
Taking Chad's figure of 3% as an acceptable difference in tire diameter front to rear, at what point would the difference become either noticeable or adangerous safety issue with the ABS system?

The tire size I am considering for the front is a 235/640R18, which is the metric equivalent to a 248/37R18 and has a 25.2-inch diameter (same as the MPSC's). Rear size is 280/650R18, which is the metric equivalent to a 301/31R18 and has a diameter of 25.4 inches. Applying Chad's figure of 3% to a 25.2-inch diameter front tire would make it acceptable to go to a rear tire with a diameter up to 25.956 inches. Therefore my proposed tire sizes "seem" to fall within the safe zone of diameter difference. Do you all concur?

Bob

Bob Rouleau

Local tire gurus say that a 0.25 inch differential in rolling diameter is fine for the ABS and AWD system of the 911. It worked fine for me on my C4S a few years ago. Note OEM tires approved for the TT can be up to 0.25 inches different front to rear so I'd say you are very safe with a 0.2 inch difference.

Rgds,

Joe Weinstein

Hi Bob.
A big question is whether you get your diameters from calculating based on the xxx/yyyR18
designations, or alternately if you are getting them from the manufacturer's printed tire spec data.
Any tire FAQ should include the fact that the xxx/yyyR18 designations *are only approximate*!
All online tire fitment calculators are based on these, which have a putative measurment meaning,
but all these calculators are also therefore only approximate. You need to look at the spec sheet
to get the real truth, especially for competition tires. I have seen 225/45/16 tires with wider tread
than some 245/45/16 tires. I have seen two 245/45/16 tires differ by over a quarter inch in diameter,
and these two tires were from the *same manufacturer*! (Just different model tires).
If you run a slightly smaller tire in the front, you'll push a little, and your rear will come around
some more on trail braking. I am agnostic about what is the safe range of difference, because I do
not know if it is based on street speeds or on track-typical speeds. ANy difference in diameter
means the viscous power transfer unit will be caring a constant load all the time. This constant
load will be greater linearly with the speed and with the diameter difference. I have not heard
directly of anyone burning out their viscous unit, but I have been told by experts that that is
possible...
Joe

Bob M

Joe and Bob,
Thanks for the experienced opinions. The tire specs I quoted were from Dunlop, so I assume that they are accurate. This may be a leap of faith, but is probably better than most. It looks like I can't attach an Exel file, or I would show you exactly what information I have in hand.

Bob