Why are 987 S rear tires bigger diameter?

[SteveK-O]

I have an 05 987S. When I look for used winter or track wheels, I always run into the problem that 987 and 987S rear tires are bigger diameter than the front tires. Example:

My car

987S Front - 8J x 18 RO 57 with 235/40 ZR18, 12.7in tire radius

987S Rear - 9J x 18 RO 43 wth 265/40 ZR18, 13.17 in tire radius

Other cars:

986S Front - 7J x 17 RO 55 with 205/50 ZR17, 12.53 in tire radius

98S Rear - 8.5J x 17 RO 48 with 255/40 ZR17, 12.51 in tire radius

911 Front - 8.5J x 18 RO 47 with 225/40 R18, 12.54 in tire radius

911 Rear - 9.5J x 18 RO 47 with 265/35 R18, 12.65 in tire radius

Tire calculators say not to use these other rear tires as the radius difference is more than 3%. Why are the rear tires bigger diameter anyway? Does this mess up the ABS and PSM?

Edited February 23, 2008 by SteveK-O

[Loren]

From Porsche's Technik Technical Information Book (when the 987 was introduced)...

"A new tire generation with larger rolling circumferences has been developed for the new Boxster/S series to complement the new wheels. The primary aim in developing this new generation has been to achieve

a marked improvement in the tires’ performance, thereby lending the Boxster an even sportier character.

The tire circumference has been increased by 2.5% on the front axle and 5% on the rear axle. The larger rolling circumference increases the side wall height/tire width ratio in comparison with the previous Boxster

(986) model to guarantee an appropriate level of comfort despite the larger wheels. At the same time, the tire widths on the rear axle of the new Boxster/S have been increased from the predecessor’s 16" / 225 mm to 17" / 235 mm.

In the absence of the predecessor models’ 16/17" basic tires, the new 17/18" summer tires now serve as the comfort oriented tires. They offer high quality, sporty handling combined with good driving comfort. The 19" summer tire replaces the previous 18" tire when the focus is on sportiness and agility. It offers a substantial increase in performance.

The new tires have permitted a noticeable increase in the amount of force that can be transmitted both in the longitudinal and transverse direction. This results in a high level of agility and, at the same time, driving safety. A reduction in the braking distance is possible with the new tires, particularly the 19" tires. These improvements are made possible above all by the specially selected rubber compound for the tires and the new tread design."

[SteveK-O]

Does this mean they chose 265/40 R18 instead of 265/35 R18 (which would have match the front diameter) for the stock rear tires on the 987S for ride "comfort"?

[Loren]

Yes, that is the way I read it.

[SteveK-O]

Then the last thing to figure out is if there is any ABS or PSM trobule with having the rear tires smaller than stock. I will talk to some independent techs.

[renzop]

The height stagger really is baffling. I understand that staggered widths can aid in reducing oversteer and look a little cooler. But the height thing is weird.

Basically if you go to same height front and rear you've got some interesting issues:

1) The car is going to be a bit nose up (by 1/2" over the 95" wheelbase which is about 0.3 degrees) This might be slightly noticeable visually and it might make the front end a bit light at high speeds.

2) My guess is the ABS will not be affected as ABS tends to be most important for front wheel lockup and compares side to side rather than front to back.

3) PSM - can the 1" diameter difference effect PSM? That's 3.14" diff in circumference. I gues it depends on whether PSM uses front/rear diffs as part of its yaw calculation. If it sees the faster spinning rear wheels as the rear somehow getting ahead of the fronts or as wheelspin at the back it might be fooled into doing the wrong thing. Tough to say without trying.

One other data point - the snow tire recommendations have the stagger much less -5mm for 17s (that's 0.2") and 8mm for 18s (that's 0.3") My gut tells me its a comfort only thing but I sure would like to know.

I hope some one can get definitive answers on non-staggered heights as I think there is no choice when it comes to DOT R Hoosiers for autocross but to go same height.

Regards,

Alan

Edited February 28, 2008 by renzop

[berty987]

I think you are reading too much into it Alan. Tyre manufacturers like to standardise on tyre aspect ratios and set widths , therefore with the 987 having a wider tyre on the rear axle versus the front its almost impossible to get and exact match on rolling radius or circumference. The wider rear tyre was fitted to the 987 to give better grip and handling , whilst the marginal increase in front tyre width allows for more grip but a narrower track width than the rear , maintaining a sharp turn in. So long as the tyres are matched on the same axle , everything else should work itself out. I know of Caymans running track tyres which have different aspect ratios to road tyres without any issue.

[renzop]

Thanks Berty. Good to know.