evokoto

Hello guys,

I just purchased a 2006 Cayman S last month and so far I am loving it! I currently have 19" Carrera wheels (stock tires - 235/35/19F & 265/35/19R) and OZ Ultraleggera HLT (18x8.5+50F & 18x10+37R this is with 5mm wheel spacer front and back) wheels. I am planning to use my OZ Ultraleggera as my autocross wheels as soon as my Bridgestone RE71Rs are worn on my stock 19s. When that happens, I would like to stuff as much rubber I can possibly do. Question - Can I run 265/35/18 and 285/30/18 without having any fitment issue and no error message? The reason why I am planning to use the size mentioned is that Bridgestone RE71Rs doesn't come with a lot of size option for 18s. I have heard that the ABS and PSM can act weird when putting a different size tires on the car. Do you guys know what causes these issues? Is it the ratio between the front and rear tires' diameter or is it the ratio between the tire that you want to run to the OEM size?

Thanks in advance!

PS: I tried researching about this but I can't find a clear answer. Thanks again!

86-951

I think the issue with PSM is if you get the difference of the tire diameters too far from that of the stock tire sizes, the difference between the front and rear wheel RPM will cause the PSM to think one end of the car is sliding. The lineal tire speed will be the same front to rear but the rotational speed will not. That said…

I just switched from Hankook RS-3s in the stock 18” sizes to RE-71Rs on the same size OZ wheels as yours. I went 255/35/18 f and 275/35/18 r. The diameter difference is 0.6” vs 0.8” for the stock sizes (rears are taller). No rubbing and no PSM issues in normal driving or in anger. The result with these sizes is an almost square setup as far as the usable tread width. The 255/35/18 also has a very stiff sidewall. The result has been transformational my last two autocrosses. I don’t know if it’s the ‘stones alone or the sizes too but I’m very pleased with this setup. 2011 Cayman S, BTW.

motoring

from previous experience, the car wants to see some stagger, rear tire slightly taller than the front. I ran an 06 S in the old stock class on 285 hoosiers on all four corners. and the car would go into ice mode on braking very easily.

I confirmed this on an ASP Boxster, running the same square 285 setup, I had Ice mode issues. Going to 285 front 305 rear removed all ABS problems.

Sprockett

I am currently running exactly as you describe...RE71R in 265/35 front and 285/30 rear. With SCCA legal offsets and 8.5/10" wheels there is no rub at either end. The car has been driven at interstate speeds on the street with no complaining from PSM at all and I've noticed nothing with braking either.

For what it's worth, the 285s come tantalizingly close to mounting on some 8.5" front wheels. We could not quite get them to seat a bead...but it seemed so close that we tried for hours. If the tires weren't brand new or you could leave them out in the sun for few hours...they may actually go on.

evokoto

Sweet, thanks for the info! I just don't want to spend $1k on tires and will have problems. Now, I kind of want to try out RS4s since they have more options to choose from.

evokoto

Thanks for the info. You think 295/40/18 will fit the rear?

evokoto

Really? This is really interesting... I heard a lot of people are saying that I will have a problem if I run on this setup. How long have you had the tires and how many miles have you driven on it? Also, what year is your car? Thanks for the reply!

Sprockett

People say a lot of things without first hand knowledge...I can only tell you what I have experienced. I've had no funky behavior on the limited street miles I've done with PSM or ABS.

And while I trust Keith (motoring) experienced what he says above I can say I've not encountered ice mode while autocrossing my car with this tire setup either. Last year, another local fellow was running the same setup all year on his daily driven street car (987.2 PDK Cayman S) and never mentioned anything like this. I shared his car for a pair of 2 day autocross events and never had an issue with ice mode.

Take this information for what is it worth...I offer it only for your consideration and not as a promise that you won't have problems.

evokoto

I understand and I am happy to know that setup works. Do you have a 987.1 S or 987.2 S?

Sprockett

2006 Cayman S

evokoto

How many miles have you driven on it in the street and how many runs have you driven it in autocross?

Jack Yeung

Anyone know if I can fit 275/35R18 and 295/30R18 in the car for BS

tacurl03

It all depends on the specific tires and the ratio between the front and rear tires. Subject to alignment settings, wheel sizes (and if you don't mind some mild rub at full lock), there is enough room to run tires that wide both front and rear. On my 987.2, for thousands of both highway and road course track miles, I have successfully run RE71 in 305/30R19 rear with 275/35R18 front (yes staggered rim sizes), A052 in 295/35R18 rear with 265/35R18 front and PS4S with 295/30R19 rear and 255/30R19 front.

Tim

Hal

Several years ago, member Bill Verberg posted on either the 993 or 997 forum a really good explanation that unless you also change the wheel width, stuffing bigger tires onto the same wheel does not enlarge the contact patch. It only changes it’s shape. You may like the way bigger tires look on your OZ wheels, but they are not going to add to your grip.

tacurl03

Yes, this is a great point. I moved up to a 9.5" wide front rim and an 11" rear when I went to the wider tires on mine. That gets in the neighborhood of having the tread width the same as the wheel width (some better/closer than others), though you could argue those tire sizes could use even a little more wheel.