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I just purchased a Cayman with 18 inch wheels for the track. I heard people use Nitto 245X40 front and 275/40 rear. My question are there any other modifications required so the tires do not rub.
 

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Tread width difference is only 1 cm, so not too much to be concerned about. Tire diameter is nearly identical.
 

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Two items to look at. 1. The front offset of 57 may have issues with the much wider Nitto tire hitting the base of the spring perch. A 5 or 7 mm spacer maybe required. Josh runs Ohlins coilovers which have a smaller diameter spring and spring base, so you have more area to work with. I would favor fender rub over spring rub, so check out the front spring distance to the Nitto's tire sidewall on lock to lock.

2. Please add a deep sump/x51 baffle with a windage plate (LN) to prevent the oil flowing up the inner side of the sump away from the oil pick up on high G corners. Many Cayman owners have blown up due to oil starvation, especially rod bearing #6. Get some heavy wt oil that can stand high temperature and has minimal reduction of the viscosity/oil pressure during the final 15 minutes of the run. An oil pressure/temp gauge would be handy. LN suggests Driven XP-9 for track day use. Its expensive and can only be used for 600 miles. DT-40 is also advised. Check out the web site RAT/540 for oil options where heat/wear/pressure scaring is a concern.
 

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The Ohlins actually make the problem worse, not better. The stock strut provides MORE clearance, not less. Keep in mind your stock spring is above the tire rather the beside it.

Two items to look at. 1 offset 57 f may have issues with the tire hitting the base of the spring perch. A 5 or 7 mm spacer maybe required. Josh runs Ohlins coilovers which have a smaller diameter spring and spring base, so you have more area to work with. 2. Please add a deep sump with windage plate to prevent the oil flowing up the inner side of the sump away from the oil pick up. Many Cayman owners have blown up due to oil starvation, especially rod bearing #6. Get some heavy wt oil that can stand high temperature and does not reduce the viscosity/oil pressure during the final 15 minutes of the run, an oil pressure/temp gauge would be handy. LN suggests Driven XP-9 for track day use. Its expensive and can only be used for 600 miles. DT-40 is also advised. Check out the web site RAT/540 for oil options where heat/wear/pressure scaring is a concern.
 

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Please check out the spring perch at 4:16 of this video. That perch sticks out and the top of the wide sidewalls may hit it during lock turns with brakes applied. This video is about adding a wheel spacer which will put distance between the inner tire sidewall and the spring perch. I would recommend ECS for their spacers, which lock onto the rotors with two screws, then the appropriate wheel lug. The also have a centering flange to help center the wheel on the spacer.
 

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I stick by what I said, this is what we do every day Apex :)

Please check out the spring perch at 4:16 of this video. That perch sticks out and the top of the wide sidewalls may hit it. This is about adding a wheel spacer which will put distance between the inner sidewall and the spring perch.
 

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Josh-you are the pro, so let's leave the perch spacing alone.
 

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Josh- I did find my source of info re the front spring perch, came from the Boxster forum 986, from J.fro; 03-02-2017 post #3.
"ran 245/40/18 NT01 tires on the front & 275/35/18 NT01 on the rear with 18x8 18x10 Carrera Lightweight reproductions for a few years. On the front I used 7mm spacers, 14 on the rear. This combo was lighter than my stock 18" turbo twists with inexpensive all season tires and acceleration was noticeably quicker. The nittos rubbed a bit at full lock in both directions, but that didn't bother me. The car NEVER had any understeer. Turn in was spot on. As the rears wore, or on cold days, I would get oversteer. I removed the rear sway bar and it cleaned this up very nicely."

I cannot say the tire to the base of the spring perch is exactly the same on the Boxster as the Cayman. Maybe the sidewall ratio is the issue with the spring perch as Ryan Clarks car had RE71R's 265/35/18 f and 285/30/18 and won the SCCA's Solo B/S class in his. SCCA says you can use up to 5 mm spacers. His was equipped with PASM and Sports Chrono. SCCA made all contestants take off the left front wheel showing the suspension to see if there was any rubbing of tires, which is a no no. His passed. Very tight 3 day event, with 54 drivers in his class. He won by .011 of a second. Coverage is listed in P9 Porsche wheels and tires that worked, pg 13.
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