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Discussion Starter · #1 ·
The 2014 CS spec calls for front tires of 235/40/19. Hoosier, Toyo et all do not seem to make tires of this exact size. Toyo does make a 245/35/19. Will this slightly wider tire rub the fender on turns?

Hoosier makes the same 235/35/19 for the front, but it mates with a 295/35/19 in the rear compared with Porsche spec of 265/40/19. Are the Hoosiers too wide for a Cayman S?

Complicating things is that I have H&R lowering springs. Would any of thee R-compound tires lower the car any more, which would introduce it to even more gravel?

TIA
 

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The 245/35-19s will likely fit, at most requiring a spacer and longer lug bolts, if you haven't converted to studs.

However, the big issue is PSM, the Porsche Stability Management system. With stock tire diameters, the rear tires are about 1.03 larger in rolling distance per revolution than the fronts. The PSM is tuned to this ratio. If the front tires slow more than 2 percent, then the PSM can kick in and cut throttle, thinking a tire is slipping, trying to save you. Even though there is a PSM on/off button on the center console switch array, there are at least 5 subsystems that compromise PSM and the button only turns off two or three of them. That is why you cannot run on a rear wheel-only dyno. A PIWIS II can turn all 5 systems off (called Rolling Road mode) to allow the use of a two wheel dyno, but brakes, especially ABS, is affected and the car in Rolling Road mode is not very safe for the street. I've done it to/from the dyno shop and the car gets twitchy over bumps even on dry pavement with Rolling Road mode enabled (PSM totally disabled).

I had the throttle cutback happen several times with stock sized tires at a drag strip, when I over inflated the 235/40-19 front tires and under inflated the rear 265/40-19 tires. This made the front tires bigger and the rear tires smaller and exceeded the two pecent tolerance. The common drag theory is the fronts need to be more solid to reduce rolling resistance and the rears deflated to get more grip/tread on the ground for launch. I am currently running shaved 245/35-19 Michelin Sport Cup 2 front tires, and non-shaved 295/30-19 rear tires (also MSPC2), and have yet to stress them on track. Unshaved, they both have the same diameter/rolling distance per revolution and would cause PSM issues on a track. Last year I ran 235/35-19 and 295/30-19 MPSC2s (1.02 rear/front ratio) and had no PSM issues as long as front and rear tires were equally pressurized, even though they both were about 1 inch smaller in diameter than stock. Note GT4s have a 1:1 front/rear ratio programmed into PSM as they come with 245/35-20 and 295/30-20 tires.

Putting 35 series on the front means you need new rears of the correct diameter to keep PSM happy.

Note any change to the tire width can alter the under/over steer characteristics of the car. Wider rear tires tend to increase understeer on the track and wider front tires reduce understeer or increase oversteer. I went with the shaved 245/35-19s in the front as the car has moderate understeer induced by the 295/30-19 rear tires and 245s were as wide a tire as I could fit in the wheel well with any size spacer (I tried and failed to fit 255/35-19s due to rubbing the springs at extreme steering lock). I am also adding a stiffer rear anti-sway bar to help the oversteer problem.

Hope this helps.

V6
 

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If I was going to do this, I'd go with a setup like this. They're both 25.5" tall.

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V6, I also have a 2014 981CS, but with stock 20" wheels. I plan to switch to gt4 size wheels (8.5x20 & 11x20) or spyder (8.5x20 & 10.5x20) and MSC2 tires 245/35/20 & 295/30/20. Is our rear wheel tub same size as the gt4? I do have some suspension mods (coilovers, adj endlinks, etc). Will that change the fitment issues?
 

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Discussion Starter · #5 ·
Thanks for all replies.

Complicating things for me are not just the lowering springs and suspension mods, but my own technical ineptitude. My best alternative may be Pirelli Trofeo Rs, which fit my current Champion Motorsports 20" wheels, then later get some used 19" Porsche wheels and decent all-season tires for regular use. That also would give me the 19" wheels to use if a better idea (like Hoosiers or Toyos) comes along.

Three seconds a lap would not have saved me from getting smoked Tuesday: Two GT4s, two Radical race cars and three Porsche Cup cars ensured that my left arm got a workout from giving point-bys over some 200+ track miles. :)
 

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V6, I also have a 2014 981CS, but with stock 20" wheels. I plan to switch to gt4 size wheels (8.5x20 & 11x20) or spyder (8.5x20 & 10.5x20) and MSC2 tires 245/35/20 & 295/30/20. Is our rear wheel tub same size as the gt4? I do have some suspension mods (coilovers, adj endlinks, etc). Will that change the fitment issues?
Once converted to coilover springs, the GT4 front suspension is similar to the 981 front suspension, but offset may be different. GT4 rear suspension is a new design, not similar to regular 981s.

In some ways 20" rims work better than 19" rims. In the rear, 11x19 rims comes very close to the rear track rod end (Not the tire, the rim). A 20 inch rim would put the rod end 1/2" inside the outside bead edge of the rim and give a few more mm in clearance. In my case, I have about 5mm clearance from the end to the outside of the rim. The spacer had to be carefully chosen to keep the tire inside the fender well and not hit the springs or track rod ends. In my case 19x11" rims with 295/30-19 tires, I had to buy a 50mm offset rim and add a 7mm spacer to optimize the position. This resulted in a 43mm offset. Another reason 20" rims may be better is clearance for the rotors and calipers for a bigger than stock diameter brakes. Stones are less likely get caught up between the rim and caliper, gouging the rim.

Coilover springs generally are less in diameter than normal springs and give extra clearance.

Need to check the allowable rim diameter vs the 295 tire. If too narrow, could result in a bow tie profile where the sidewalls stick out further than desirable and could result in more chance of sidewall damage when rubbing against a curb, etc. A bow tie profile could help protect the edge of the rim from damage, but could hurt lateral support in a track turn.

One last point, if choosing an aftermarket rim, in most cases, do not choose the rim with less native offset than needed. In my case, the OZ Ultraleggera 11" rear rims were available in 40 and 50mm offset. I figured that I needed between 40 and 50mm offset, so I chose the 50mm offset as the mounting surface being closer to the outside of the rim, it enabled the use of spacers to push the rim outwards off the hub. Too little offset and the rim will stick out too far and the only solution is to machine the inside mounting surface, which isn't normally done due to cost, complexity, and potentially weakening the center of the rim. If I had purchased the 40mm rims, it would have fit and stuck the tire outwards 3mm more than what I settled on, which might have been OK.

So, don't try the rear rims for a wide-body 991 as the 70mm offset would likely require a 25mm spacer to fit and it would put a lot more stress on the lug bolts and possibly the wheel bearings.

V6
 
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