Wheel Offset Calculations (Correct Spacer?) -- Revisited

Hi gang, As winter and snow tire season is upon us, I figured I'd do one quick double check (measure twice, cut once). I know I asked some of these questions a long way back -- but I'm trying to see if my calculations are correct per my 987.2 2010 Boxster and ordering the correct wheel spacers for the rears. I think I figured it out, but it can't hurt to throw it out there before ordering. In short, I'd like to reuse my former 987.1 winter tire rims. These are 235/40/18 square, ET 54, 8Jx18. They worked great in the snow many years ago, but I don't remember the rear spacer size that was originally on there that came with the car (likely 5mm or 10mm). A narrower rear is great in the winter, as it allows for greater downforce and traction on the snow. Doing the math, to keep the outer rim positioned at essentially the same location as the summer OEM 265/40 18, ET43, 9Jx18, it appears that 20mm spacers would do the trick. Below is the diagram from the online calculator. What do you guys think? This seems to at least keep the wheel appearance per position visually close to stock (so the wheel wouldn't look sunken in and wouldn't potentially rub against anything). But would this significantly stress the suspension and mess up the handling by pushing things wider? I could instead get something like 10mm spacers, which would center the weight distribution right exactly where the OEMs were and yet would still prevent rubbing. I'm guessing it won't make any major difference, but figured I'd ask here as I'm certainly no wheel expert. Finally, as a weird bonus, the car strangely came with a single ECS 20mm wheel spacer with long bolts (per the aftermarket rims and tires, that I've since sold on) -- so I could save some cash and just order just a single wheel spacer with 5x bolts -- assuming 20mm would suffice. Thanks, everyone!

I've used 23 mm in the rear to get a flush look. That would go with 18 mm in front. However, I am compromised due to the Tarett Cup LCA's with their own spaces to kick up the camber, but at the same time also changes the track width. You just do not want your tire shoulders hitting the upper fender lips under compression. My camber is -2.9 f and -2.5 r so I'm not a stock OEM suspension platform. I can use the 987.1 OEM 8 " 57 offset f and 9" 43 offset rear with Michelin PS4Ss in 245/35/18 f and 275/35/18 r. This setup will be going on my car in a couple of weeks.

Thanks, @Apex1 - I'm not going to lie, but I'm still not educated per camber and track setups (yet!). My guess is that pe the above diagram, this would keep the rear rim faces positioned closest to where they sit per OEM rims (not even flush). It's just that it would appear to push the center of gravity on each tire just slightly wider (maybe 9mm further out), as these are 235s and not 265s on the rear and the rims drop from 9" to 8". I'm guessing this would be a negligible difference in terms of performance and wear on the bearings, etc.? Finally, I'm not even sure how to clear the test for what you suggested: Any thoughts, I'm all ears. I'm guessing if they're positioned as above and the tires are the same height, I'm guessing it shouldn't be an issue?

With a 34mm ride height drop, the fender lips are much closer to the tire shoulders. So a big compression could cause some marks on the tires shoulders. I have not experiences any rubs yet after 4 years with this configuration in the winter months. This is the way my car will look in a couple of weeks. It definitely is not a snow car.

Oh wow -- you've got a beauty there and your wheels look fantastic. To clarify, are you saying that we're both at 34mm (which I'm guessing is the stock ride height)? Or is this just the case in your situation? Just trying to figure out what I'd need to do.

My ride height is about 15mm lower than OEM. PASM lowers the car about 5mm and my Ohlins lower it about another 10 from that OEM. I had the car corner balanced as well so each corner is different than OEM as well, I also changed the rake, front to back a little so the rear is lower than OEM by a few more mm. This adds some weight over the rear to help get addition traction out of the corners.

I am also trying to figure out spacing to use 987.1 wheels and snow tires on my 2012 987.2 Cayman . The issue is clearance for the brake discs which I think is only an issue in the front. Specs: 987.1 wheels-snow tires 987.2 (2012 stock wheels/summer tires ) Front 6.5J x17 ET55-205/55 17 7Jx17 ET55 - 205/55 ZR17 Rear 8J x 17 ET40 - 235/50 17 8.5J x17 ET40 - 235/50 17 front Backspacing 5.92 in 6.17 in => difference is 0.25 in or 6.35mm rear backspacing 6.07 in 6.35 in => difference is 0.25 in or 6.35 mm I measure about 6mm of clearance from the stock 2012/ 987.2 wheels to the brake caliper. The 987.1 front wheels with snow tires just hit the brake caliper. I believe adding a 7mm spacer to the front and rear wheels. will compensate for the difference in backspacing and give clearance for the brake calipers in front. Inputs/thoughts welcome. A quick search located this spacer but I am looking for other sources for spacers. Amazon.com: StanceMagic - 7mm Hubcentric 5x130 Wheel Spacers (71.6mm Bore, No Lip) Compatible with Porsche Boxster 911 Carrera Cayman Cayenne Panarema Taycan - 2pcs : Automotive Buy StanceMagic - 7mm Hubcentric 5x130 Wheel Spacers (71.6mm Bore, No Lip) Compatible with Porsche Boxster 911 Carrera Cayman Cayenne Panarema Taycan - 2pcs: Wheel Adapters & Spacers - Amazon.com ✓ FREE DELIVERY possible on eligible purchases www.amazon.com Thanks in advance for constructive inputs.

I find that 17 inch wheels are a problem with front brake clearance. No way around this except for some aftermarket rim barrels that are completely flat. Your choice is to sell those rims and go with 18's, I recall a guy called CMOOSE on Pistonheads, that used very successfully 17 inch rims on his 06 Cayman S . Maybe in the 987.2 model the brake calipers are different requiring a larger wheel barrel. Not sure. But, according to many of his posts, the 17 inch wheels and his choice of tires gave him a very satisfactory ride over the English country side. Please send a picture of your project. Visual look certainly helps.

Anyone else want to weigh in?

It seems to me that the 20mm spacers would give you what you're looking for with the rear wheels, and yes, the wheels' center of mass would be maybe a half inch further from the hub than the OEMs, but I doubt you'd notice any difference in handling or that it would disturb the suspension much if at all. I'd get the extra 20mm spacer and give it a shot.

Apologies my chart of wheel/tire size in the original post didn't format well. It was actually legible in the original view on my computer. The issue I have with clearance is from the inside face of the front wheel to the caliper outside face. The picture shows the clearance with the stock 7J x 17 wheels that came on the 2012 Cayman . I misunderstood the backspacing dimension. A smaller backspacing number means more clearance from the inside of the wheel to the suspension components. https://www.autoanything.com/resources/wheel-offset-vs-wheel-backspacing-explained/ Link to a backspacing calculator. Wheel Offset Calculator Use our wheel offset calculator to visualize the new tire position when changing wheel and tire sizes. Convert wheel offset and backspacing to calculate the correct setup for your vehicle. tiresize.com I believe I can use the 987.1 narrower wheels on the 2012 987.2 with the addition of a 5mm or 7mm spacer for the front wheels only.

Thank you, @Kevin CS -- That's a huge help, especially given that returning a wheel spacer isn't an option. I generally like to "use what I already have" first, especially if it's invisible to the naked eye...and I have a perfectly good 20mm sitting here. This has been a question that has bugged me since I got the car, and I really appreciate your taking the time to answer -- as nobody has seemed to be able to weigh in much about the effects of slightly shifting the center of mass. Cheers and thanks!