Martin S.

In haste to get to the track and try out some new tires, I installed front and rear tires with Revolutions Per Mile calculation differential greater that 7% (I used a 205/40/17 887 Revs Per Mile, with a 255/40/17 828 Revs Per Mile on the rear), it seems that this %age delta front to rear is sufficient to confuse the ABS brain and trigger the ABS and Anti-Lock idiot light as well as well as turning off the ABS, not a good thing on the track. The brakes will work, BUT, front wheel lock up can be expected, also not a good thing, as your stopping is severely compromised and tire flat spotting will probably result.

I have heard that anything greater than a 6% Revs Per Mile delta front to rear will trigger the ABS light. If anyone knows the exact % delta that would be good to know when one shops for tires. BY the way, a very sweet tire combination appears to be the 225/40/18 with 275/35/18 with a delta of approximately 1.8%. Going to a 335/30/18 increases the delta to 2.9%, but I don't think a 335/30/18 will bit into the fender well of a 993 narrow body car without rubbing. If anyone has successfully stuffed a 335 in to the rear wheel well of a narrow body 993, that would be good to know too.

Since it was postulated by Steve Alarcon at Johnson's Alignment in Torrance, CA that this was the problem, I changed back to stock sizes to see if the problem went away. It did, on the track and on the street as well. I then wanted to know how Revs per mile were calculated...hence the data below from http://www.maximum-Cars.com and the article by Carl Lynn.


Understanding Tire Sizes
By Carl Lynn

If you don't have data pages or need dimensional information for a tire size, you can calculate nominal dimensions by using the following calculations. Individual tires may vary slightly from these dimensions.
For example, a tire with size 235/60 HR15:235 is the section with in mm. This tire is 235mm wide.60 is the aspect ratio. 60% of 235 is the height of the tire.HR is the speed rating.15 is the size of the rim in inches.

235 To convert to inches: Formula: Section Width in millimetres / 25.4 = Section Width in inches.Example:235 / 25.4 = 9.25" Nominal Section width.

60 To calculate Tire Section Height:FORMULA: Section Width x Aspect Ratio Percentage = Section Height.EXAMPLE: 9.25" x 60% = 5.55" Nominal Section Height.

15 To calculate Nominal Overall Tire Diameter:FORMULA: (Section Height x 2) + Wheel Diameter =Overall Tire Diameter. EXAMPLE: (5.55 x 2) + 15 = 26.1 " Nominal Overall Diameter.

To estimate Tire Revolutions Per Mile: FORMULA: 20,800 / Overall Tire Diameter =Tire Revolutions Per Mile EXAMPLE: 20,800 / 26.1 " = 796.9 Tire Revolutions Per Mile.

This section width is correct; assuming the tire is mounted on a wheel width that is 70% of the section width as specified by the Tire and Rim Association for measurement. In this case the rim would be 6.48" wide.

Since a tire's section width changes 0.2" for every 0.5" change in rim width, this change must be added or subtracted from the section width when alternate rim widths are being considered.

The tire's section width of 9.25" would be virtually unchanged when mounted on a 6.5" wheel, would increase to 9.45" when mounted on a 7.0" wide wheel, and would further increase to 9.85" when mounted on an 8.0" wide wheel.
Rating Max Speed (mph):Q 99
S 112
T 118
H 130
V 149
W 168
ZR over 149

Happy holidays,

Jack Ennuste

Manual says clearly on page 25 (ABS) that difference can't exceed 6%. Or ABS light will come on and ABS de-activates.

ca993twin

To further complicate this tire-size issue, cars with AWD must also keep the revs per mile within a very narrow range in order for the viscous coupling to work properly and not wear out prematurely. I don't know the spec for the AWD, it may be less than the 6% required for proper ABS function.

Tire Rack has the revs per mile posted for various tires. Interesting to see the considerable difference in the manufacturers specs for tires of the exact same *size*. If that data is true, than using the calculated method could introduce quite a significant error. I don't know how valid the tire rack data is, however.

TheOtherEric

Martin, I'm not sure why you say that. You aren't suggesting that it's better for your rears to lock up first, are you? That creates an unstable situation, causing the rear to come around (left or right...you can't control it). Non-ABS brakes are typically designed to have the fronts lock up first since it's presumably better to lose steering than to have the rear come around.

As for tire sizes, 225/40/18 with 275/35/18 is indeed ok, but 225/40/18 with 280/30/18 is even better (1.4% diff). See the tire calculator at http://www.mindspring.com/~cinque/test/tirecalc.html

225 and 280 (280 is max for narrow body) are standard S body sizes, and although I'm not fond of it (increases understeer) I'm running it now in my narrow body. The big question to me is "where can I find 235/35 or 245/35 for my fronts, to match my oversized rears?" To my knowledge, such sizes are not available. :-( I don't know how others have handled this, other than to just stick with stock 265 rears. yuk.

BTW, I'd REALLY like to know how much % difference is ok before hurting the viscous coupling in a C4, if anyone knows.

JohnM

Eric,

There is a 245/35 18 Pzero Rosso, that's what I have on the front now (on Carrera 8x18 rims) together with 285/30 18 rears. Feels good, but I won't get to the track with this combination until March.

Martin S.

Theoretically....if the front tire is too large compared to the rear, you could see some oversteer...the front gets too big of a bite compared to the rear....On the inverse side, I ran the 205s and the 255s, the totally wrong front size and the car handled like a dream, turn in was incredible. Just think of how good it will be with the correct tires on the car. ...also note, the sway bars were on the middle setting front and rear on the TRG sway adjustable bars...Just keep the rolling radius delta to factor specs or better and you will be just fine with your ABS, a lesson I have learned: The hard way again....