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Discussion Starter · #1 · (Edited)
I want to put the most rubber I can get on a new Cayman GTS for autocross. Has anyone tried 315/30/18s in the rear or 285/30/18s in the front?
 

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Whether they fit will vary from tire to rim combination.
The Tirerack website can be helpful for checking fitment.
 

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you're not gonna be getting much oversteer with that kind of setup! haha, it'll be an animal to get around parking lots
 
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Presumably u are increasing rim size also, because those tire sizes are too big to be very effective with stock rims. Just saying...
 

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Discussion Starter · #5 · (Edited)
I am using 8 /12 Fronts and 10" rear wheels with 5mm Porsche spacers all around. These rims are smaller than Tire Rack recommends and do pinch in the tires somewhat but still allow for the most rubber on the road.

I know with the 986 Caymans, autocrossers were using the 285s all around a few years ago and were nationally competitive. The 987 hubs were changed and the 285s rubbed slightly on the front struts. I just don't know anyone who has tried them on the 981s nor the 315s in the rear. Anyone's experience is welcome.
 

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Has anyone tried 245 front and 285 rear on 8.5 and 10 inch rims. Was looking at getting pirelli trofeo R tires for track days on a set of 19 inch OZ racing rims. The wheels weigh 25 lbs less and the tires are only a few pounds heavier so it would be about a 20 lbs loss in unsprung weight. That's a pretty significant difference. I have seen 285s on 10 inch rims on a panamera so they fit well, but do they rub on a 981 cayman? Also wondering if the extra 10mm in the rear will cause some under steer or just make is easier to put the power down coming out of the corner.

The car is very neutral with track alignment now, and the back rotates easily with additional steering input, throttle lift ,and or throttle power oversteer. So much fun to modulate the car's position with just tiny inputs from the steering wheel and or gas pedal. I don't want to upset that balance with proportionately larger tires in the rear if that is a risk. Seems to me this should be great for the track and if the balance is slightly shifted to understeer then a stiffer X73 rear bar could be added to balance things out again. Any thoughts on this guys?
 

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Looks to me like someone (OP) forgot to STFF and see what the F/R size relationship would do to the electronics.
 

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Discussion Starter · #8 · (Edited)
To answer the previous two posts. I couldn't find the specs for that tire on Tire Rack, however, I have run Hooiser A6s and ZIIs in 285/30/18 on the rears (not 19s) on both my 987 and the new 981 GTS on 10" wide rims with no problems. I agree with Amdeutch using the 285 and 245 F could set up an electronics problem so when I have and do run the 285/30/18 rears I use a 255/35/18 front on an 8.5 in wide rim with no electronics problem as both sizes are approximately the same diameter and the balance of the car is near perfect. My original post here was to explore wider sizes and find out if anyone had used 285 fronts and 315 rears.
 

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To answer the previous two posts. I couldn't find the specs for that tire on Tire Rack, however, I have run Hooiser A6s and ZIIs in 285/30/18 on the rears (not 19s) on both my 987 and the new 981 GTS on 10" wide rims with no problems. I agree with Amdeutch using the 285 and 245 F could set up an electronics problem so when I have and do run the 285/30/18 rears I use a 255/35/18 front on an 8.5 in wide rim with no electronics problem as both sizes are approximately the same diameter and the balance of the car is near perfect. My original post here was to explore wider sizes and find out if anyone had used 285 fronts and 315 rears.
You should reread my post and note the "(OP)" reference, and not apply it to the post directly in front of mine. :hilarious:
 

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Discussion Starter · #10 ·
Amdeutch. I read that and the front diameter of 24.9 and rear diameter of 25.6 would not present a problem with everything I have read and tried over the years in fairly extensive testing. Everything I have read says that the rear diameter has to be equal or greater than the fronts.
 

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I want to put the most rubber I can get on a new Cayman GTS for autocross. Has anyone tried 315/30/18s in the rear or 285/35/18s in the front?
Amdeutch. I read that and the front diameter of 24.9 and rear diameter of 25.6 would not present a problem with everything I have read and tried over the years in fairly extensive testing. Everything I have read says that the rear diameter has to be equal or greater than the fronts.
315/30 = 94.5 sidewall and 285/35 = 99.75 sidewall; for the same wheel diameter
 

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Discussion Starter · #12 · (Edited)
That should be 285/30

The rears need to be taller than the fronts. I have never had an issue using this approach.

From Tire Rack
Hoosier A6 for front 285/30/18=24.9" diameter
for rear 315/30/18= 25.6" diameter


P285/30ZR18 SL 40 C A 1168 lbs. 44 psi 4/32" 24 lbs. 10-11" 10" 11.5" 10.9" 24.9
P315/30ZR18 LL 40 C A 1356 lbs. 44 psi 4/32" 27 lbs. 11-12" 11" 12.5" 11.8" 25.6"
 

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No one seems to mention this, but there's a tab with screw that intrudes into the rear wheel well on the 981 that wasn't there on the 987. Due to the new joint that runs horizontally from the door handle to the rear wheel well on the 981, there's a tab that sticks into the well as part of the fitting of the rocker panel/scoop panel to the bodywork. I am running 15mm spacers with stock rims and tires (Goodyear Feeble F1s in 265/35/20) and the tires almost hit this metal tab on compression turns. (At least one other poster reported this as well.) I'm almost certain that running a 10 or 11" wheel with appropriate tire width - 285, 295, or larger) would cause the tire to hit this tab under compression. What are people doing about this? Bending the tab is both difficult and risks bending the bodywork on the outside, hence visible damage.
Just curious.
 

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Discussion Starter · #14 ·
Dr. Phil,
Thanks for mentioning that. I hadn't noticed it. I think with wider tires with 20" rims and the wide spacer this could be a problem as you say. For competition I have used 285/30/18 rears and 5mm spacers. These tires are 2" shorter than the stock tires in diameter. I use the 18s because of the availability of sizes and the shorter tire changes the gearing to make the car faster. This may not be the "look" you want on a street tire set up but for competition it has worked well. I just measured the distance from the tire to this protrusion and it is 2 3/8" away. I don't think that there is any way that this will interfere even on compression. I went around a corner fairly hard and did 1.29g on the meter the other day with no issues. Going to a short and wider tire which stuck out beyond the fender to some degree, the tire would hit the fender before the tab/screw that you reference because it is inside and higher than the fender lip. Does this address your issue?
 

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Has anyone tried 245 front and 285 rear on 8.5 and 10 inch rims.
Not 285 on the rear, we run 275/35R-19 and 245/40R-19 as our competition tires. I'm pretty sure there's 5mm spare in there, I need to check sometime. The wheels have about 5mm more offset than standard.
 

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BIC:
Your post is very relevant, thanks. For competitors running 18s, I agree that the tab may not be a problem. As the rear suspension compresses, the camber increases, causing the wheel to "fold in and under" the fender lip. I guess I was more concerned with a street setup (mainly for appearances of course) where people run wider rims and tires and lower suspension setups. My 987 had PSS9 suspension set quite low and I was running 19x11" rims at the back. With the MPSS 295s I put on them, they would certainly have rubbed the 981 tabs. The new suspension on the GT4 will obviously alleviate any problems with wider wheels as I have no doubt it will be set by the factory not to rub. (Actually I am looking forward to seeing if the GT4 has the fender tab in the same configuration as the regular S or GTS, since it will be running an additional 10mm lower than the GTS with X73, and the wider wheels--but different suspension and hubs.)
Anyway, I know this thread is about competition, so I won't pollute it any longer. Thanks,
 

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Thanks for all the cool info. I know that the rears have to be bigger than the fronts or the PSM may not function properly, but can they be the same size for sure? Standard tires in 20" at 235/265 = 26.5/27.3 or a ratio of 97%. I have Sport Techno rims that are a half inch wider, so when I replace the standard tires I will upgrade to MPSS in 20" that are 10 mm wider at 245/275 = 26.8/27.6 or a ratio of 97%. These will better cover the wider rims.

Pirelli Trofeo R tires in 19" at 245/285 = 25.7/26.9 or a ratio of 95.6%. Only a one percent difference, so I would expect the PSM not be affected by such a change. All aspect ratios are 35 so I believe either set up should work with the PSM. There is unfortunately no 275/35-19 in the Trofeo R. A set of 235/40-18 and 265/40-18 = 25.4/26.6 or a ratio of 95.5% which sounds reasonable.
 

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Discussion Starter · #18 · (Edited)
lovetoturn,
I re-read your post and get that you are figuring the ratios of the diameters. You are correct in that as you said "rears have to be bigger than the fronts or the PSM may not function properly". I have read that this is also true for the ABS to function.

I have used the Tire Rack web site as a reference to get diameters. I run 255/35/18 and 285/30/18 Hoosiers which have respective diameters of 24.9" in the rear and 24.8 in the front. This is a .1" difference (99.6%) and still had no problem.

I also run Dunlop Direzzas in the same sizes which have diameters of 25" in the front and 24.8" rear and still have not had any problems. That is .2" smaller on the rear tires. Since this is the case and it worked on both my 987 and 981 I guess there is a little leeway in the "rear must be larger" rule.
 

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Thanks for the detailed update BIC. I am leaning at the 245/285 pirelli Trofeo R mounted on the 19 inch rims. Maybe also upgrading the rear bar to an x73 to help reduce any possible understeer and balance out the car with that extra rubber in the back. This will be a little closer to what is on the GT4 as long as the 285 will fit in the rear. There is that thing that sticks out in the wheel well according to someone else, but hopefully with a 40mm offset on the rims, which acts like a 10 mm spacer, and 5 mm spacers if needed, they should fit.
 

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Discussion Starter · #20 ·
lovetoturn,
I had my car up on a rack today and checked the "thing sticking out in the wheel well". I don't think it will be an issue with wider tires as the fender is lower and it looks like you would hit the fender first.

On another note and the intent of this thread, I was at my race tire shop and they had some used tires that I could try on my car. We mounted 285/30/18s in the front on 8.5" rims and 315/30/18s on the rear with 10" rims. All rims were 18" Volks and standard offsets. I use 5mm Porsche spacers. There was plenty of room on the inside but it looked like if there was a lot of body roll they might hit the fenders. I took the car out and ran it in a circle like a tight skid pad and checked on the g-meter and it showed 1.2+ with no problem of the tires hitting the fenders or anything else. Lots of grip with a bit of understeer.
 
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