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Discussion Starter #1
The standard size for 981 S is 235/265 45 19 but on the track you want to increase contact patch. Many of those I spoke with use 18" rims to save on tire cost but there are not many combinations out there. I was told of this interesting option. Use square set up of all 4 tires at 245, 255, or 265 with aspect ratio of 35 or 40 on an 18" rim, then put a 20-30MM spacer in the rear. Assume PSM off or on based on your skill level.

Thoughts?
 

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Never heard of this on a 981 - perhaps on a 987. Make sure you buy rims and tires from some place that will take them back if the combo doesn't fit.

Reasons IMHO for NOT doing this:
1) PSM assumes a certain stagger in front to rear rolling diameter. PSM can cut your throttle on turns unexpectedly if you alter this ratio.
2) PSM button on console does not turn everything off. Sport Plus will change kick in settings
3) Not all 18" rims fit over the S or GTS front calipers. If you have 18" rims forget a big brake upgrade
4) 30mm spacers on rear are going to put strain on the rear wheel bearings. Lug bolts are going to be expensive and a stud conversion may be a better option.
5) The likely reason for rear spacers is to clear the rear track rod end, which nearly hits a 19" rim with wide tires.
6) With 265 rear tires and 30 mm spacers, the tire may stick out of the wheel well, beyond the fender. You made no mention of adding negative camber, which would be the only way the tires may fit under the fender lip. Or you go to add-on fender flares after cutting the fenders.
7) I really doubt a 265 tire can fit in the front on an unmodified 981 due to fender liner and strut spring contact. Due to the angle of the strut, a larger diameter rim will clear the springs better. I tried 255 (19') on my Cayman and had rubbing at extreme steering angles. 245 x 19 is the widest I can get to fit the front safely and I was using 15 mm spacers there
8) The stock front rims are 8 and/or 8.5 inches wide depending on rim model. 8 inches is fine for 235 rubber, 8.5 inches fit 245 tire well, and 255 just barely fits a 8.5" wide rim, with a large bow tie cross section when mounted. A 265 tire is going to require a 9.5 or 10 inch wide rim. Do you really want to go narrower in the rear than stock? Especially if you have engine mods to increase HP? With narrower tread width, the rear tires are going to break loose more often when accelerating out of a turn. If this is such a great idea, why did Porsche not do it on their GT cars?
9) Going to 18" doesn't mean that you have to have 40 profile tires. Larger sidewalls will flex more and make the car feel like it is going to slide. PSM doesn't care about the profile as long as the ratio of the front to rear rolling diameters are the same. I currently run Michelin Sport cup 2 245/35-19 on the front and 295/30-19 on the rear (11" wide rims, 50 mm offset with 7 mm spacer (43 mm effective offset)). These tires have a 1:1 ratio and I had the front tires shaved to get the 1.03:1 ratio that PSM requires. My speedometer is off 3 percent, but the car drives just fine.

The best diameter rims are 19" as tires are cheaper (and available) than 20" rims. You can get them to fit with any good aftermarket rim.

One other note: 'increasing contact patch' as you described can significantly alter over/under steer and sway bar changes will likely be required.
Now, 981s have understeer built-in by Porsche and people increase front tire tread and track to reduce understeer. If you add rear tire tread width, then that increases understeer. Another technique commonly used to reduce understeer is to soften the front sway bar and stiffen the rear sway bar. In my car, I am running GT3 adjustible front bar at the softest setting and a Tarret 5 position rear bar at next to stiffest position.

V6
 

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Discussion Starter #3
I appreciate all the points you made and all make perfect sense.

a) I've invested in a couple sets of 18" rims over the last last 5 years with Cayman III rims at 8x18 and 9.5x18 and two years back bought a set of OG Racing at 8.5x18 and 10x18. Based on your recommendations it would appear, Michelin Sport Cup 2 245/35-18 and 285/30-18 combination will fit on either set of wheels with option to go to 295/30-18 on the 10." After 35+ tech inspections later, no shop has failed me yet for caliper clearances even with a GIRODISK upgrade to 350MM fronts.

b) Never requested tire shaving before, do you specify how much or is it a standard amount? And if they shave, is it right to assume if they took off 30%, you will get 30% less life of tire and do you change all tires when fronts wear out?

c) I am running with -3.1 negative camber in the front with the GT3 LCA and use a 7MM spacer to clear the potential rub with PSS10 shock tower spring. Any other consideration on spacers front or rear?



 

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I run 18" Apex wheels and Pirelli slicks at VIR on my 2014 981 S. Fronts are 265/645/18 and rears 305/680/18. -2.7 camber in front and -2.2 in rear. All systems off. The cars handles extremely well with no apparent issues. I love it!
 

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Running square on a Cayman is a bad idea. The weight distribution calls for a bigger rear tire. We had a guy try it with hoosiers on his 987.1 and he was way off the pace of other CS in the red DE run group back in the day with NNJR PCA.
 

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Tire width selection is as much a factor in car setup as wheel width, track width, spring rate, bars, and alignment. They are a complete system and should be modified as such. The ABS/PSM is also a part of this system and it looks for a 0.5"-1.0" shorter front tire diameter. I'd very surprised if you could just "go square" without numerous changes.

I ran this past year with 245s/8.5" in the front, 265s/10" out back, and -3.0 and -2.5 camber, respectively. The car is, perhaps, a touch too dull up front and a touch too stable under throttle. Much more grip and I think I'll be beyond the capabilities of PASM to control it. I'm going to start with a 9" front wheel and a 10" rear, then play with 245 and 255 in the front and 265 or 275 in back.

There are many 18" and 19" options that maintain a 20-30mm front to rear stagger and tire diameter.
 

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I would highly recommend 255/35/18's f and 275/35/18's rear. This combo will work especially with your camber. I do have adjustable Tarett swaybars, but currently they are set sort of soft, middle hole f and 3 hole from stiff in the rear. I am not sure how that would stack up to an OEM swaybar setup. The tires create a balanced roll rate, which is what I wanted. I have three sets of wheels and tires, all the tires are in that size. Michelin PS4's, RE71R's and Hoosier A7's. The last two sets are on 9 in wheels f and 10 in in the rear. The Michelins are on 8 in f and 9 in OEM wheels. I also have modest HP from my M97 S engine which relates to rear traction and tire patch to keep the car balanced and the slip angle less than 8% while still being able to rotate mid corner while running A7's.
 

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On my GT4 I'm running Hoosier 265/35 x 19 F and 295/30 x 19 R using CCW 9 x 19 51 offset F and 19 x 11 44 offset R. Added Rennline front camber plates to keep caster near OEM and solve a slight tire rub.
 

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On my GT4 I'm running Hoosier 265/35 x 19 F and 295/30 x 19 R using CCW 9 x 19 51 offset F and 19 x 11 44 offset R. Added Rennline front camber plates to keep caster near OEM and solve a slight tire rub.
Have you experienced any ABS/PSM issues with this set up?
 

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I'm running 255/35-18 25.0 dia. + 285/30-18 24.8 dia. no PSM or ABS issues
I love all the posts about how you can't run an unstaggered setup and all the computer problems you will have.
Been on the car for 4 years and 8600 miles, 50+ autocrosses most of which were with PSM on and it has never been a problem
 

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2016 Spyder is a 981
But 2 Porsche's ago I had a 987
What's the point ?
I believe the post is regarding 981's
 
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