Wheel size question - 986 Forum - The Community for Porsche Boxster & Cayman Owners

I have a 2002 986 base model that has the sport classic wheels. I'm chasing down a steering wheel shake at 70mph and as part of my investigation I switched the front and rear wheels on both sides to see if it made a difference. The wheels are the same size, as are the tires ( 235 / 45R17 ), however when I swap the fronts with the rears the rear wheels now appear to be set further inside the wheel arches, and vice versa for the fronts ( they're further out in the arches ). I don't understand this as the wheels and tires are the same size and I don't see any spacers installed anywhere. I've tried to show it in the pictures below. Here's how the car looked BEFORE the swap: The only difference between the front and rear wheels is the model of tire ( albeit the same size ), surely this can't make a visual difference like this? The car absolutely handled better with the original set up ( that makes the steering wheel shake at 70mph ), but with the wheels switched that shake altered for sure, potentially lessened but overall handling was worse. Not sure what learnings to take away from this experiment. I feel like I'm missing something here... Other work I did prior to this to try to fix the steering wheel shake was: New front inner and outer tie rods New front sway bar drop links New rear sway bar drop links Alignment Wheels balanced ( twice ) New rear sway bar bushings

Are you sure the wheels are identical? I think the ET could be different between the front and rear wheels by your description after changing the wheels. Not much of any help. Shaking could be that one of your wheel could be damaged by example a pothole. Inner barrel of the wheel. But that should show up when you balanced the wheels I guess. It can also be the tires. Have someone experienced drive the car and see if they can give you a clue what needs to be fixed.

Something is very odd here, as your car started life with 7x17 wheels front and 8.5x17 wheels rear. So, what size is actually on the car? Given the tires you are running, it seems like the latter. As far as what you are seeing, at some point, you give it to a pro and pay for diagnosis. I think you’re at that point.

Quotes source post: 668150

"Wheel offset, often indicated by the "ET" value (short for the German "Einpresstiefe"), is the distance in millimeters between the wheel's mounting surface (where it attaches to the hub) and the wheel's centerline. It's a crucial factor that affects how the wheel sits within the wheel well and can impact handling, appearance, and even brake clearance. " The ET is normally marked on the interior of the wheels. That would explain your question about the appearance and the change in handling. Someone has messed with your car, as mentioned by tcoradeschi, the wheels should be staggered (different sizes front to rear). Your troubleshooting points in the direction of one of the front wheels or tires being damaged. Anyone in your area who would lend you a set of front wheels for a road test?

Quotes source post: 668152

I’ll agree. I have 18” carrea wheels on mine. Different width front and rear and different ET. Should be the same on 17”. I would check the part numbers on the wheels. They should be different front and rear. Also check that the tires are correct for the wheels. https://teile.com/en/porsche-tequipment-shop/model-boxster-986/5/wheels-rims-and-accessories/601c-wheel-rim-sets/218/17-inch-sport-classic-wheel-set-7j-x-17-et55-85j-x-17-et50/20772 “Front rims: Article number: 99336212451 Quantity: 2 Rim width: 7J Rim diameter (inch): 17" Offset (ET): 55” “Rear rims: Article number: 99336212655 Quantity: 2 Rim width: 8,5J Rim diameter (inch): 17" Offset (ET): 50”

Are you running spacers?

Vehicle came from factory with different size wheels and different size tires front and rear, a configuration known as staggered. Approved wheel/tire configurations are listed in owners manual. It's an error based on poor information or poor understanding of engineering to fit same size wheels and tires front and back on these vehicles. I'd think it difficult to solve a problem before correct parts are fitted.

https://imgur.com/a/z2yQeRW I copied the original configuration from Porsche, on 17" rims you can go with two sizes on the rears with the same rim and are allowed to have a 5mm spacer on each rim.

Apologies for the delay, it took a while before I had time to pull the wheels off and check them. From the looks of it, the rears are marked with 126 55, and the fronts are marked with 124 51, I'm assuming that's what gives the wheels the different sizes? I now have the wheels back in the original configuration, but now I'm wondering if this is actually a valid setup for this car in the first place ( previous owner put these wheels on ), and maybe this is the root of my steering wheel shake?

your fronts are marked 7J17et55 and your rears are 8.5J17et50 fronts == 7 inch wide wheel, 17 inch diameter, and offset of 55 rears == 8.5inch wide wheel 17 inch diameter and offset of 50 boxsters have a staggered setup, so this is normal, I don't see why it would create a steering wheel shake inherently. maybe a bent rim?

Quotes source post: 668245

So, you have correctly sized wheels - 7x17 front, 8.5x17 rear. You don’t have correctly sized tires on them. Should be something like 205/50x17 front and 255/40x17 rear. Not saying that has a damn thing to do with the vibration you’re dealing with, but it does explain what you saw when you swapped front for rear.

Quotes source post: 668254

This is actually super helpful. I was considering getting new tires anyway, just to rule them out as the cause of the vibration ( I think they're cheap ones ). I'll plan to get correctly sized tires and see if that changes anything, regardless, I'm sure it isn't helping the car with the current ones on it. Thanks again for spotting that. It has had 2 alignments, and 2 wheel balances, both at different places, and nobody noticed.

Quotes source post: 668254

205/ 50 R17 fronts and 255/40 R17 or 225/45 R17 on the rears

Quotes source post: 668266

Good catch! I corrected the typo.

I went with firehawk indy 500s (rebranded Bridgestones) from Costco in the stock sizes. Pretty happy with them so far

Quotes source post: 668266

Thanks so much, this is really helpful. For the rears, are the 2 different sizes just personal preference? What's the difference? Thanks again!

I am running the smaller ones as winter tires and the fat ones for summer. Just personal preference and sometimes it is difficult to get winter tores in the bigger dimension.