987S tire sizes for R-compund

[HEDDOK]

I'm looking for an R-compound DOT tire. I'd like a dedicated set for Auto-X and track. The 987S uses 235/40-18 fr and 265/40-18 rear street tires. So the front and rear are different diameters. The PSM + ABS are set for these differing rotational speeds, 790 revs/mile and 819 revs/mile. This is a 3.57% delta. I can get a set of Yoko A048 for a great price... size 225/40-18 fr and 265/35-18 rear. Revs are 829 and 822 respectively.

Will this work without the TC, PSM or ABS getting pissed at me?

Secondly, will the increased understeer with the 225's up front give more problems? I'm a track novice.

All input appreciated

Brad

Edited June 22, 2005 by HEDDOK

[ar38070]

You cannot go by the marked numbers on the tires. A 235/40 from one manufacturer is not necessarily the same diameter as from another. You should measure the tires that you have now and see if there is really a 3.57% delta. What I really mean is the a 234/40 calculates to x diamter, a 265/35 calculates to y diameter. x and y may not be the real diameters due to a manufacturer's liberal interpretation of the spec.

Having said that there may well be that much of a delta and assuming that the PSM and ABS are designed for that delta (what is you source for that statement?) then you should try to maintain that delta with any tires that you get in the future.

986 systems were calibrated for the same size tires front and rear. The allowed delta front to rear was 4%. People have reported that putting on tires with that much delta upset the ABS system. A delta of 2% seemed okay.

Porsche (whether consciously or not) screwed people over with the new cars and their choice of tire sizes. You may have to wait until the tire manufacturers catch up with the new sizes.

While the reduced front tire size will cause the car to understeer more as a novice there will be other more imporatant things to concern yourself with.

[HEDDOK]

ar38070

Thanks for your quick response and info. The data for my street tires(PS2's) is from the Michelin website and the A048's from the Tire Rack specs. The one Porsche Tech I talked to confirmed that the stock setup is configured for diff dia front and rear but is unable to tell me what I may expect with off-spec tire sizes.

I agree that understeer issues are the least of my concerns, getting good quality instruction at DE's is most important. My only reason for dedicated track tires is that I can chew them up through inexperience rather then the much more expensive street tires.

I never realized it when I bought the car but they really have made things difficult with these oddball sizes. Any ideas????

Brad

[ar38070]

Measure the real circumference and calculate the real diameters to see what your true starting point is. Then I would try to stay within 2% of that. If the fronts are 3% smaller then I would try to find front tires that are 1-5% smaller.

It also may not be that critical. If you are not driving at the ABS or PSM threshold then the point is moot. The activation point may be a little early or late but it is not the end of the world.

Some people subscribe to the belief that when you first start doing DE's and Autox that you should learn on street tires as they give more warning. You could try this out but instead of using expensive street tires use cheap ones like Kumhos.

[HEDDOK]

My instructors said the same thing, stick to street tires at first. I'll shop for Kumho prices.

Brad