Current anti-lock braking systems have such low tolerances for front to rear tire diameter differences. An example - the 350z system can only work properly with less than a 3% difference between original front/rear tire diameters. So if you decide to replace the rear's 245/45-18 (approx 26.8 o/d) with a 275/35-20 (approx 27.7 o/d), and do a correct "+2" on the front, say 265/30-20 (@ 26.3 o/d), you could have possibly rendered the system enactive.

I'm wondering if the RX-7s braking system has the same type of issues as its successors. With so many FD owners on this site going with a "staggered" (for lack of a better word) tire set up, Im wondering if the ones' doing so are decreasing their safty margins. Maybe some of the ole' "swamis" can chime in on this one for me. TIA

~Mike

 

I've run 275/40/17 (25.7" diameter) rears with 235/45/17 fronts (25.4" diameter) for years on street and track. The stock diamerter on the OE 225/50/16's was 25.0". I've had no issues with the ABS, and speedo error is on the order of 2-3 mph tops.

 

PTR, thanks for the reply. I think I need to clarify a bit. I was wondering if anyone has a 3% outside diameter difference FRONT to REAR with their new set-up (EX: front tire 26.3 o.d. and the rear 27.7, about 3% difference). The o.d. you had discussed is close to being 3% bigger than the original equipment o.d., but that wouldn't affect the ABS system as your front-to-rear stagger is still within 3%.

My thinking is that the front tires are spinning at a faster rate than the rears hindering the ABS system's ability to accurately read the contact patch slippage. This could ultimately lead to one corner not getting full ABS effect thus increasing stopping distances. Thanks for helping out though.

~M

 

A friend of mine has used A032s on track, with the 275/40/17 and the 235/40/17 front size. He had to because the "32" isn't available in either 245/40/17 or 235/45/17, and his front wheel is only a 8.5" and too narrow for the 255/40/17. He's even further biased the equation with a M2 6-piston front kit vs. the stock rear, and as far as I know has never had problems.

 

I see. I'm asking because I will be able to get my hands on some slightly used Sport Cups and as you may know, they are specifically sized for the Porsche. I may be able to get my hands on 4 235/40-18s but I may have to settle on whatever's avail. Thanks once again, PTR.


~M

 

In general the only cars that are really picky about tire circumference are those with traction or stability control systems which the RX-7 does not have.

For ABS only the system is merely looking to see if a tire is locking or not, it really doesn't care what they are doing in relation to eachother speed wise (especially on an FD because it's only 3 channel anyway; it's not capable of controlling the rears independently).

Traction control and stability control systems on the other hand are constantly comparing wheel speeds to eachother to figure out what the car is doing and using that info to clamp individual calipers even though the driver isn't touching the brakes. Varying the tire circumference on these can trick the system into thinking the car is either more or less out of shape than it actually is. If there were a way to reprogram the traction control computer for the new tire sizes that could be easily solved though...

An FD isn't going to care about a 3% difference front to rear.