I am currently running all stock 19" wheels and tires (PS2s) on my CS. I want to go to an 18" setup for DE and Auto-X use to preserve the 19" wheels and tires for road use. I am going to go with the Volks Racing TE37 wheels which come in a 10" wide rim with a 42mm offset for the rear.

I wanted to make sure that this setup would clear the shock tower which is the closest component to the inboard side of the tire. According to the Tire Rack spec sheet the section width of my current Michelin Pilot PS2s (265/35R19) is 10.7" when mounted on the stock 9.5 x 19 wheels with a 46mm offset. According to my "precise" calculations, this works out to a "tire" backspace of 181.89mm. I assume that the tire is symmetrical on the rim so I just took 1/2 of the section width (10.7" = 271.78mm/2 = 135.89mm) and added the offset (46mm) to arrive at this number. I know that ordinarily the backspace of the wheel is what is calculated but looking at my current setup it will be the tire that makes first contact with the shock tower as there is only about a 5/8" gap.

So, I performed the same calculation for the new 265/40R18 tire which has a listed section width of 10.7" on a 9.5" rim. This section width should increase about 0.2" when mounted on the 10" rim (source: Tire Rack). Taking 1/2 of the 10.9" (276.86mm/2 = 138.43) and adding the 42mm offset results in a "tire" backspace of 180.43mm. This all looks good except the 18" wheel will drop the point of maximum tire width about 1/2" and since the shock tower is at a slight outward angle will move the widest point closer.

Last winter I ran the TE37 in the 8.5" width (49mm offset) at the rear with the Pilot Alpin 255/40R18. Using the same methodology (reducing the listed 10.2" section width to 10.0" because of the 1/2" reduction in rim width) I arrived at a "tire" backspace of 176mm. This caused no problems and is only slightly (4.43mm) different than the calculation for the 10" rim with the 265 tire.

Are these calculations correct? Are my assumptions proper? Am I over analyzing the situation?

I still think it will be okay without spacers but would like to know if anyone else has used this setup in the rear and whether they needed spacers or not.

There is no issue with the front, as the TE37 is 8.5" wide which I know will work as I have already used this size with my winter tires.

Sorry for the boring details but being an engineer I can't control myself whenever there is math involved.

I did not check your math, but offer up two suggestions. First, if you have not used it, this offset calculator is good: offset .

This is just looking at the wheel, not the tire, but can save some calculation time.

Next, I am an engineer too, but am a bit jaded - to borrow a quote from House, "Everybody lies".

I am working on some wheel and tire projects also, and for fitment I always start by driving my car up on ramps. This way I can get under it while there is still full weight on the wheels. You can then measure the REAL clearance, as opposed to calculating the theoretical one. You can also see if there are other obstacles, like wires/tubes/hangers which affect the clearance. I find "surprises" about half the time I have a project going on, so I tend to go to empirical measurements as early as I can, before getting too far down the road.

Along with a camera to record what I see, I also usually take some light cardboard, scissors and a pencil. For difficult to reach areas, you can quickly cut a template to fit odd clearances, and mark it up with arrows at the critical points. You can then measure it out from under the car with a ruler.

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....its even SAFETY YELLOW! What could be more practical???

I would love to be able to directly measure different combinations of wheels and tires to confirm fitment but don't have the time or money that would allow me to. So I settle for the first step in any design work which is to make some assumptions, perform some calculations and eliminate what doesn't work before doing any actual testing. This usually keeps you from heading down any dead ends.

Just subscribing to this thread since I'm in the same boat and am looking at a set of 18s w/ 10" rears and 47mm offset.

Would prefer not to use spacers.

Zoomee, still did not check the math, but I did finally read the details of what you were putting on: 265/40/18 tire on an 18x10 rim with 42mm offset (right?).

I bought a nice set of Carrera 3 takeoffs. They are 265/40/18 Bridgestones on an 18x10 rim with offset 58.

A 7mm spacer is required to clear the strut tower (inside clearance) equivalent to offset 51.

A 23mm spacer is what I am using, gets close (but not super close) to the fender (outer clearance). Equivalent offset is 35. Side view here.

Will be putting these back on this weekend; if you need additional info, let me know.

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....its even SAFETY YELLOW! What could be more practical???

No math check here either. Not enough coffee yet to even think of it. However, did you try calling Tire Rack? I've always found them helpful even if there isn't a sale in it. They might not have the data on that wheel or tire but it's real simple to find out.

A real fit check on something like this is worth all the calculations in the world when it finally comes to plunkin money down. It's rather easy to miss something when working in a complex 3D environment. You may interfere with something not the least obvious.

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Les

Please STOP........... My Brain Hurts..................

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Cheers Karel
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'Power is Nothing without Control..........'

[
I bought a nice set of Carrera 3 takeoffs. They are 265/40/18 Bridgestones on an 18x10 rim with offset 58.

A 7mm spacer is required to clear the strut tower (inside clearance) equivalent to offset 51.

[/quote]


Pardon my innumeracy, but would I be correct in assuming that a 51 or lower offset works w/o spacers on an 18" wheel with 265 width?

That's what I read, which would mean that the wheel I'm hoping for 18" x 10" w/ ET of 47mm should work w/o spacers, but I'd like confirmation....

Well, I put the 18" x 10" x 42mm on the rears with 265/40R18 Goodyear Eagle F1 Asymmetric tires and the fit was perfect. With the car on the ground there was about 1/2" of clearance between the inboard edge of the rim and the strut tower. After an Autocross on Sunday and a DE on Monday (yesterday) I looked and saw no signs of contact. So, no spacers needed for this wheel and tire combination. YMMV with a larger tire.

Okay, how about this one:

I have 17" stock Cayman wheels and I recently purchased weedos 19" Carrera S wheels. I put them on one side to check the fit and the rears appear to sit farther inboard than the stock wheels--which is at the very least aesthetically odd looking.

Here is the tire/wheel/offset sizing:

Stock:

Front 6.5" w/55mm offset with 205/55/17 tires
Rear 8" w/40mm offset with 235/50/17 tires

19" Carreras:

Front 8" w/57mm offset with 235/35/19 tires
Rear 9.5" w/46mm offset with 265/35/19 tires

I know the Carrera set up is available from the factory, but I'm assuming at least a 5mm spacer is utilized on the rears. I think the front (stock versus the 19" Carreras) will fit without adjustment.

Anyone care to comment on this set-up? TIA

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Boxer_ben

This is the setup I'm currently running. No spacers and all works well.
I'm just trying to find out about the 18x10 question above

Theoretically, with the 47mm offset your inboard edge of the rim will be 5mm closer to the shock tower than my 42mm offset on the 18x10 wheel. Since I measured 1/2" clearance to the shock tower with my setup you should end up with about 0.3" clearance. The size of your tire may come into play if it is much wider than the 265 which doesn't extend beyond the inboard edge of my rim. Of course the max width of the tire occurs at a larger diameter than the rim and the shock tower is angling away from the wheel the further out you go from the center so you may be able to get away with a wider tire.

Zoomee

Thanks,
yea calculating it out from my current 9.5" 46ET to the 10" 47ET rim I"m losing 7mm inside and extending the rim 5mm.

I wont get back to my car for another week so I cant check myself, but I feel as though I should be fine. I dont need any more than a 265 tire. Seems as though they should work, I just wanted confirmation before I purchase the rims, incase someone has placed this sized rim on their CS / boxster. Looking on the porsche website they list 19x11.5 sport Designs as optional upgrades on the Boxster, but it doesnt state the offset or if spacers are required, but I would guess that our rims should clear