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Discussion Starter · #1 · (Edited)
I have manage to bent my two front Carrera S 20” wheels and damage the Pirellis P-zeros in less that 800 miles. This is not acceptable specially since none were damaged by significant potholes. The second tire when hit the pothole sounded like it collapsed hitting the rim even before the shock absorber did anything. To me it sounded like the sidewalls on the Pirrelis are not strong enough to deal with the weight of the tire assembly. The rear tires and rims are perfect, no damage at all. After studing the situation I realized that the Pirellis P-zeros are not extra load(xl) rated like the other alternative, the Michelin Sport 4S. The load index for the Pirellis is 88 versus the Michelin’s 92. I had a couple of turbo S before with a front tire size 245/35/20 and a load index of 95(the same load index as the undamaged rear 265/35/20 and no damage from the same roads.

I have two possible scenarios and need input from someone knowledgeable in this specific situations. Not guessing please.

#1. Switch to Michelin 4S N0 extra load in the oem size with a load index of 92 vs 88 for the front and 98 vs 95 for the rear and hope this increase is enough in the front.

#2. Switch to Michelins 4S extra load but in 245/35/20 front with a 95 index load and 275/35/20 in the rear with a 99 index load. I feel this is my safer option, specially in the front with the same tires size as the turbo S’s. But there are two unknowns with this option that hopefully someone can help me out:

First the front tires sizes comes on the N0 Porsche designation but not the rear ones(On the Tire Rack site the 265/35/20 Michelin’s 4S are only offered with no designation, made in the US or the BMW star designation made in Fr) I will have to buy the set as a non N0 set made completely in the US or mix the front NO (made in Fr) with the rear ones either with no designation (US) or the Michelin 4S BMW star designation made in Fr.

The second unknown I have is if the increased width with the resulting increase in overall diameter of the tires will affect any of the systems in the car even though I maintaining the same relation between the front and rear of 3% as oem.

So in summary I need answers to my unknowns In scenario #2 to see if it is even a feasible alternative to compare to scenario #1. Any educated info is appreciated since these are both very expensive scenarios to do on faith alone.
 

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Ummm... the sidewalls do not support any weight, that is done by the air pressure in the tyre. "Comfort pressure" gives less support.

20" wheels on the 718 have very small sidewalls and hence the tyre tread is close to the wheel, facilitating easier wheel damage by small impacts. That's why I specified the 18" wheels.

As far as the specific questions go I am unable to help. I hope you can find a tyre-based solution that's cheaper than changing wheels as well.
 

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The sidewalls may not support any weight, but they do make the tire stiff, and thus when you hit a pothole an XL rated tire will help avoid a bent rim.
 

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@zagaone , a higher load rating will help somewhat for reasons that are a combo of what @Greg W and @JakiChan both allude to.

While it's air pressure that ultimately supports the car, one simply can't increase pressure to prevent bent rims. The relative load and stress strength of the tire's sidewall construction -- including the bead, sidewall profile, and belt-ply construction architecture, among other things -- keep the tire 'together' and functioning when a sharp impact is encountered. A lower load rating is basically a clue that the tire construction factors above are of a lower consideration in the tire's overall design than things such as cornering pliability and road 'feel'.

Thing is, 20-inch rims are not ultimately made for handling. They're made for looks. It's been proven again and again that beyond a certain point, larger-diameter rims with lower-profile tires actually adversely affect handling, to the point where to survive on less-than-perfect pavement, the tire that fits on such a rim has to be designed to handle less well to still be viable for consumer use.

Even so, design can only do so much. For example: tires with an over stiff sidewall can bend rims, too. This is a serial issue with BMWs, many of which come with RFTs (run-flat tires) as standard, even though those tires never perform nearly as well in key ways as traditional tires. So simply stiffening the sidewall isn't an ideal solution, either.

Assuming you'll keep the 20-inchers, I feel your best solution is threefold:
- Find a set with as high a load rating as possible,
- Maintain tire pressures on the higher end of the tire's suggestions, and
- Ignore N0 spec as a prerequisite. Porsche has basically said that the only key difference between an N0-spec tire and a standard-issue tire is wet-weather traction at speeds over 100 MPH.

If the problem persists -- as much a function of driving environment as anything -- I would strongly encourage that you either downsize to 19-inch rims or upgrade to forged wheels.

Hope this helps.
 

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Discussion Starter · #5 ·
Thanks Viffermike for your input. I do have a set of 19” wheels that I will be using with Michelin’s Cups 2 during “special” days.

As far as the 20” oem wheels I think I can get away with the Michelin’s ps 4s in 245/35/20(f) and 275/35/20(r) which have a xtra load index of 95 and 102 respectively but the front and rears are not N0 spec. This is a huge improvement over the stock Pirellis that are standard load of 88 & 95 and better than the Michelin’s ps 4S in the stock oem sizes of 235/35/20 & 265/35/20 which have xtra load index of 92 & 99 and are N0 spec.

Do you know if the 10mm wider wheels(f & r) with the same aspect ratio of 35 and the resulting .3” larger diameter is significant enough to have any negative effect on the different Porsche systems, even though the relative ratio front to rear stays the same at -3% for both sets of 20” wheels? This difference in sizes is similar to new thread at 9.5mm vs used tires with less than 2mm thread. I don’t thing this is enough of an increase in diameter, specially being the same front and rear, to cause any ill effects on the car behavior, but will like to here your opinion.

My last result will be to go oem sizes on the Michelin’s but it might not be enough of an improvement in load index to be reasonably safe, specially considering the acquisition price of over $1.5k.
 

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Thanks Viffermike for your input. I do have a set of 19” wheels that I will be using with Michelin’s Cups 2 during “special” days.

As far as the 20” oem wheels I think I can get away with the Michelin’s ps 4s in 245/35/20(f) and 275/35/20(r) which have a xtra load index of 95 and 102 respectively but the front and rears are not N0 spec. This is a huge improvement over the stock Pirellis that are standard load of 88 & 95 and better than the Michelin’s ps 4S in the stock oem sizes of 235/35/20 & 265/35/20 which have xtra load index of 92 & 99 and are N0 spec.

Do you know if the 10mm wider wheels(f & r) with the same aspect ratio of 35 and the resulting .3” larger diameter is significant enough to have any negative effect on the different Porsche systems, even though the relative ratio front to rear stays the same at -3% for both sets of 20” wheels? This difference in sizes is similar to new thread at 9.5mm vs used tires with less than 2mm thread. I don’t thing this is enough of an increase in diameter, specially being the same front and rear, to cause any ill effects on the car behavior, but will like to here your opinion.

My last result will be to go oem sizes on the Michelin’s but it might not be enough of an improvement in load index to be reasonably safe, specially considering the acquisition price of over $1.5k.
I think you'll be fine running 10 extra points on the aspect ratio on each axle. Depending on the design of the sidewall, you may or may not run into rubbing issues with the fender and/or spring assembly, so I'd keep an eye on that. Otherwise, 3 percent shouldn't be a huge deal and shouldn't adversely affect any of the nannies present -- It's when that's done on one axle only that issues with ABS, traction control, etc. can happen.

(IIRC, it is common on 981s to do exactly what you're suggesting with winter tires because there are many more tire models available in 245/275 than 235/265.)

If anyone disagrees, or knows of any adverse effects that increasing the rolling diameter on all four wheels by 3 percent would have, please contribute and let us know!
 
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