pstoppani

So, all the r-compound rear tires seem to be 4 to 5% shorter than the stock rears (265/35x18 or 265/30x19 are roughly 26.3" tall). Toyo RA1 and Hoosier come in 275/35 which are only 3% shorter. I've always read that you should not go over 3% in tire height/circumference in order to not mess up the ABS system. Does anyone know if the Cayman (or Boxster) can handle a 5% reduction in the rear tire height? It sure would be nice to have more choices (not that RA1's or Hoosiers are bad choices but I'm kind of fond of the Michelin Cup tires but both the 265/35 and 285/30 18's are almost 5% shorter than stock).

Thanks.

LastGT3

I think the key factor for ABS functioning is the differential between circumference of the tires on the car as OEM installed - meaning - as long as the front rear OEM difference in circumference is maintained, I do not see why that would trigger the ABS - ie fooled that the tire is slipping and therefore = increased circumference. If that can be maintained with the avail R-compound, then I would guess you are OK.

STLPCA

AFAIK, no ABS problems if the front to rear difference is below 6%.

mooty

pete

"stock rears (265/35x18 or 265/30x19)"
__________
NO, they are 265/40/18 or 265/35/19

pstoppani

Mooty, yes, you are right, wrote down the track tires sizes accidently. However, I believe I got the percentages of the tires size differences correct.

Based on the responses so far, it seems that the track tires available should be fine since typically (for the sizes I've looked at (235/265, 235/285, and 245/275) the front is typically 1% shorter than stock and the rear is about 4 to 5% shorter.

I'm about to pull the trigger on the 245/40x18 and 275/35x18 Toyo RA1s.

FTS

I am debating pretty much the same thing. However, I think I will be ordering 255/35/18 fronts and 295/35/18 rears. The rears won't fit on 9" wheel however, so ordered 10" ones.

Kumho Ectsa MX has those sizes or Goodyear Eagle F1 GS-D3. Obviously they are not R-comps, but should be enough for my DE needs.

Jim Michaels

I posed the rear tire size question on the PCA Tech Q&A under Cayman and got an answer from Scott Slauson today. My interpretation of his answer is that the front and rear tire diameters should not vary by more than 5%. Note that this interpretation is different from the notion that the tire sizes should not differ from the OE front to rear stagger, which is .9". He said to compute the 5% limit from the smaller tire (the front 235/40/18 in this case), which means (again in this case) not to go over 1.27" larger or smaller than the diameter of the front tire, which is 25.4". Anyone can check out his answer at the PCA Tech Q&A site (under Cayman) to see if they agree with my interpretation of his response. Bottom line is that we shouldn't have any problem using the 265/35/18 rear size with the 235/40/18 front size. The same for the R-compound rear size switch using 19" rims; the 265/30/19's should work okay.

pstoppani

Thanks Jim. I interpret the same as you. I thought the limit was 3% but at 5% there are lots of options.

mooty

i read the tech QA, however, i dont really understand it.
pete and jim, you seem to know what needs to be done.
can you list the sizes that are ACCEPTABLE and AVAILABLE from mpsc and ra1, pretty please ;-)
i have both 2 sets of 18" and 1 set of 19" wheels for cayman, one of the 18" set is 8.5" and 10".

thanks a bunch.

pstoppani

In 18", these all work:
Michelin PSC 235/40 265/35 or 285/30
Yokohama 048R 235/40 265/35
Toyo RA1 245/40 275/35

There are two equations that you need.

First, take the smaller tire's diameter and add/subtract 5% of its diameter: d +/- d * .05. This gives you the range of the other tire's size so that it is within 5% of the smaller size.

So, for a 18" MPSC, the smaller between front and rear is the rear 265/35 we find it has a diameter of 25.2. 5% of that is .05 * 25.2 = 1.26. Hence, the other tire must be in the range 25.2 - 1.26 and 25.2 + 1.26, or 23.94 to 26.46. The front 235/40 has a diameter of 25.4 so it is okay. The other rear size of 285/30 has a diameter of 25.1, so it needs the front to be 23.845 to 26.355. Again, the 235/40 is in that range.

The second equation says each tires should be within 5% of the original sizes so that you don't throw off your speedo too much. So, you do the same 5% calculation for each of the original tires and make sure the tires you pick are within that range. For the above 3 tire sizes, we know the fronts are the same size, so just need to calculate the percent difference of the 265/35 and 285/30 compared to the stock 265/40. They are -4.55% and -4.92% respectively. So the 285/30 is right at the limit. The 285s may cause a bit more undesteer when matched to 235's and you'd add more grip to the rear; 255's would be a better match in the front, but I didn't find any. Personally, if I were going with MPSCs and 235s in the front, I'd go with the 265s to maintain the traction balance.

The Toyo RA1s listed above are the closest to stock diameters. The fronts are exactly the same and the rears are only 3.03% smaller than the stock rears. The 245/275 combination should provide equal amounts of extra rubber front/rear.

For 19" these work:
Michelin PSC 235/35 265/30

GT3 Nut

Which tires do you guys think will work the best on the Cayman? Which size combo? The RA1 combo is wider than the MPSC. Is that always better? Why?
I was thinking of going with the MPSC, because I have 2 235/40-18 from the front of my GT3 (Which I never did use much - maybe 4 or 5 laps). I never really gave the MPSC a chance. I currently use Pirelli Slicks on my GT3, but I would like to try something with the Cayman. I will probably take it next month to Summit Point, or maybe to the Zone 2 DE at VIR, or both.

pstoppani

Matt, I editted my previous post with some notes on the good tire combos for the Cayman. The MPSCs are superb tires; I use them on my Turbo and love them. I've used the RA1s many times and they are also great tires; have them shaved if using them in the dry though. Since you already have the MPSC 235s, I'd just get a set of the 265/35s for the rear.

GT3 Nut

Thanks Pete. I belive that is what I will do.

64wrdgerz

What wheels do you guys recommend for track tires?
When ordering a new car do I stick with the standard 18" Cayman S wheels for track tires and buy something else for streets - or vice versa - or go with light forged for track tires in 18" which seems to have the most choices of tires.
Are track tires legal in Autocross?
Thanks

Jim Michaels

In SCCA autocrossing R-compounds are legal in stock classes, but there has been considerable discussion about whether to change that for 2007. I plan to order another set of Cayman S 18" wheels with Michelin Cups mounted for autocrossing and track. Someone here reported weights for those wheels at 21.5# front and 23.5# rear. Aftermarket forged wheels would be lighter (18-20# maybe?), but also more expensive. The other attraction to aftermarket forged 3-piece wheels is that you can order the widths you want. The restriction for SCCA autocrossing, however, is that one can't run any different size wheels in stock class than Porsche offered as an OE option. Thus, one could run the Porsche Carrera Sport 8.5 & 10X19" wheels (a bit on the heavy side), or a lighter 19" forged aftermarket set with the same widths. Those widths on an 18" rim would be attractive, but that wouldn't be acceptable for SCCA stock class.