Does anybody here have an informed opinion as to whether substituting 265/35 ZR 18's for the rear instead of 265/40 ZR 18's will have any ill effect on the ABS function ? Michelin (at least up until now) does not make the Pilot Sport Cups in 35's for the rear. I spoke with Gil at Tire Rack , and he was not able to say one way or the other. Thanks in advance !
Rick,
Look at the tire's specs, and what you need to be concerned with is the tire's overall diameter... so look up the diameter of the 265/35-18 sport cups, and compare it to the diameter of the stock 265/40-18 PS2 that comes on the car as stock. The % difference between the sport cup tire and the stock tire shouldn't be more than 4-5% tops, otherwise the PSM and the ABS will not work correctly and will start to throw computer errors.
brad
Beez,
By my calculations, diameter of the 265-35's = 335 mm, while diameter of the 265-40's = 345 mm. The 10 mm difference is about 2.89 %. So you think that's OK, as it falls below the 4 % threshhold ?
I think that's fine, aren't the factory rolling diameters for 18' and 19' wheels and tires slightly different anyway? I'm sure they don't build ABS programming for what wheels you get from the factory.
Unless I'm out of my mind (no nasty comments please) you need to compare the Sport cups to the stock supplied 265/40 PS2s supplied with the car. When I looked on tirerack, it showed the Sport cups 265/35's as 25.2' and the stock PS2 265/40 at 26.3' a percentage difference of 4.36% by my calculations... so you'd be right on the edge.. it may or may not be a problem...
brad
K-man S,
The diameters of the 18' and 19' are almost the same because they use 40 series profiles tires for the 18s and 35 series profiles for the 19s.
brad
I believe it's the front to rear rolling circumference difference that is of concern. The computer doesn't care what tires are on the car, just that they are all rotating within the same (acceptable) speed range. ABS/PSM intervenes when a tire's rotational speed (among other factors) is out of range which I've always heard is about 6% for ABS. I've no idea about stability managment thresholds since mine has always been controlled by my hands & feet.
Fitting 265/35 will increase that ugly tire-fender gap. A concern because it's important to be both fast & fashionable on the track.
I'm also interested in fitting 18' MPSC, although I've deferred my Cayman to the summer to, hopefully, get good answers to this & other questions, such as lowering a PASM car.
Edited by - lizardS on 01/17/2006 11:53:09 PM
You got it right. It's front to rear ratio that matters for the ABS. The lower 35 ratio in the rear is better for the track (autoX too) as the gear ratios are already long on the Cayman S.
Ideally you should go with a 235/35 up front to compensate, but if not I don't think the 4% delta will make a big difference in the way the ABS works. I guess it could have a bigger impact on the PSM but I turn that off anyway at the track.
Edited by - cgomez on 01/18/2006 2:27:58 PM
http://gs.tolan-hoechst.com/tirecalc.htm
