18x8 ET 57 with 245/40/18 and 18x9 ET 43 with 285/35/18 - will I need spacers?
 

I have some 18" Porsche wheels inbound for my 2000 Boxster S in the following sizes:

Front: 18x8 ET 57 with 245/40/18 mounted
Back: 18x9 ET 43 with 285/35/18 mounted

According to this site: 944racing

These look like OEM sizes for some of the Boxster 18" wheels as well as some of the Cayman wheels.

Looking at some of the threads where folks on here have had the Cayman S wheels installed on their Boxsters, some ran spacers and some didn't.

Mine are coming with some tires already installed, that are slightly wider than the OEM sizes:
Backs: OEM size is 265 (20 mm narrower than the 285 that I will have)
Fronts: OEM size is 225 (20 mm narrower than the 245 that I will have)

With these slightly wider tires, would I need to run some spacers? If so what size? Any recommended vendors?

Thank you!

Steve

The front would need a 13mm spacer, the back will fit fine, you will actually have 9mm more room to the inside on the back wheels. 245 tire on a 8" rim is the max recommended, but for the 9" rear 265 is the max recommended size tire.

I got may spacers from here. 2003 Porsche Boxster Roadster S H6 3.2L - ECS Tuning

Have you already bought the tires? I hear 245 are a bad size for the front of the boxster for street use, they tend to follow road grooves really bad.

A better daily driver setup would be 235/40ZR18 275/35ZR18

this website came in handy, when i was prepareing my Fuchsfelgen project...
1010TIRES.COM - Wheel Offset Calculator

In the end several factors determine the spacers you need...

wheel size
tire size / brand
suspension type

As far as i heard, a tire size smaller than recommended for the wheel is kinda "okay"... a size bigger than recommended "troublesome".

Mine:
Front 8J x 18 ET52 --> 5mm spacers = ET47 (within oem range) --> Conti 225/40ZR18
Rear 10J x 18 ET65 --> 18mm spacers = ET47 (within oem range) --> Conti 265/35ZR18

I see what you guys are saying, I just looked on tirerack for the 285/35/18 and the range of widths is 9.5-11. I am 1/2" below that. For the fronts I am within the range.

These tires are for track and autocross only.

The wheels are coming with the tires on them already, Hankook RS-3 with a lot of tread and life left, January 2014 production. I was going to need a new set of them soon (currently running on 17s) and I figured this would be a good chance to get some 18s.

I will research into this some more, thank you for the suggestions and info.

Steve

Factory 18's are 7.5J x 18 ET50 and 9J x 18 ET 52 with 225/40 front and 265/35 rear tires

This is what I based your clearance on. Using the same tool BRAN posted.

1010TIRES.COM - Wheel Offset Calculator

If the tires are already on the wheels then I guess 285 will fit the 9" wheel.

This site here gives you a list recommended tire sizes for wheel widths.

Tyre Size for Rim Size/Width Calculator - What's the best tyre size for my rim?

The seller ran the 2014 season on these tires in his Cayman (50 runs) before selling the car at the end of the season. He placed mid pack in A Street at National Championships. I am getting more details from him on why he went 285 instead of 275 or 265 (both of which are approved for 9). Since these will be mostly for autocross and DE I am hoping they will work for me.

I am using these exact same wheel sizes on my 986S, and mine are Cayman wheels. The ZII's I have mounted on them are:

Front: 255/35-18
Rear: 285/30-18

Mine fit without any spacers at either end, but I normally run about a 7 mm spacer on the front to gain front track width. ECS Tuning have the best spacers by far.

Your results may vary depending on coil overs, camber setting, and even caster setting at the front can effect the front tire potential.

I know, I know, these tires are wider than recommended for these wheels, but it works fine with ZII's which have relatively flexible sidewalls. I believe RS-3's are similar.

IMHO, the 245/40 & 285/35 combo should be good for track days, but the O.D. is too large to be optimum for autocross when you consider the very tall gearing these things have.

Good luck

Not true, I put 245/40's on my 987 and didn't notice any increased "tramlining" than with the 235's.

I should have been more specific. I've been told this about the 986 model, your 987 has electric steering.

From looking at tires the 987 series seems to run a slightly taller OEM tire than the 986, so brining a set on 987 mounted tires down to a 986 you would be running slightly taller than optimum. I am not that competitive in Autocross so I probably wouldn't be able to tell the difference anyway :)

Where I run autocross is on a closed runway on an old airport and the tarmac isn't the best; it is pretty rough. I may just stick with my old 17s for Autox and save the "newer" 18s for track days.

Offests - I think I am finally getting it. Last night I looked up the OEM wheel widths and offsets for the 986 and compared them with the Cayman wheels offsets. Here is how I did it. I took the wheel width in inches, converted to MM, and divided by 2. I added the offset to the back side, and subtracted it from the front side. I used this to see how much of the wheel was inboard of the mounting point (flush to the rotor) and how much was outboard. This does not take into account any changes in the tire width or diameter. In my example since I am going to be running a slightly taller tire but I think it will not be significant with regard to possible contact.

Here is an example:
OEM 986 wheel sizes:
17s
Front: 17x7 ET 55 - 7" = 177.8mm/2=~89mm. Outboard: 89-55=34mm, Inboard 89+55=144mm
Backs: 17x8.5 ET 48 or 50 8.5" = 215.9/2=~108. Outboard: 108-50=58mm, Inboard 108+50=158mm
18s
Front: 18x7.5 ET 50 = 7.5"=190.5/2=95. Outboard 95-50=45, Inboard 95+50=145
Front: 18x8 ET 50 - 8"=203mm/2=101.5. Outboard: 101.5-50=51.5mm, Inboard 101.5+50 = 151.5mm
18x9 ET 52 - 9"=228.6mm/2=114.3mm. Outboard 114.3-52=62.3mm, Inboard=114.3+52=166.3mm

The 18" Cayman wheels are
Front: 18x8 ET 57 - 8"=203mm/2=101.5. Outboard: 101.5-57=44.7, Inboard: 101.5+57=158.5
Back: 18x9 ET 43 - 9"=228.6mm/2=114.3mm. Outboard=114.3-43=71.3, Inboard=114.3+43=157.3

OEM Inboard ranges for front are 144mm and 151mm. The Cayman wheel is 158.5. I can see this is where the 7mm spacer comes in for track width. When I did this last night there was one other OEM front width/offset that was around 160 but I can't find that configuration today. (I will edit tonight when I get home and see if I was crazy or if there was another OEM size from the Bentley) That led me to believe I could run with no spacers in the front.

For the backs OEM inboard ranges are 158 -166. The Cayman is at 157 so it is within those same ranges.

I think I will order some 7mm spacers, and I am going to convert to these studs:
Competition Wheel Stud Conversion Kit-Rennline, Inc.

It sounds like the 45mm stud with the ball seats lug nuts is what I am going to be going with. Does anyone have any feedback? I am headed to do a search right now :)

Thank you everyone for the great feedback and taking this thread where it is going. I have (and am still) learning a lot.

Nope, 987's have the same old Hydraulic steering as the 986.

My mistake, thought I read it did.

The person that told me this was Johnny Danger when he gave me his number to call him about wheel and tire setup for my car. He has run a number of combos and he said not to go 245 in front because of the "tramlining".

We where talking the 986 though. Maybe Porsche changed something in the 987 that reduces "tramlining".

Even with my current 225's in the front I get some "tramlining", will 235's I'm getting be worse, guess I'll find out.

The 981 has electric steering.

I think tramlining is dependent on the tire itself- ie the strength of the corner where the sidewall and the contact patch meet.
Regarding rim widths what it comes down to is support of the tire. Where you are in the approved rim width range effects the shape of the contact patch which effects how the tire performs and communicates. If you never get to the limits of adhesion this matters less but an oversize tire will tend to feel numb- like wearing oversized winter mitts. The car will also feel heavier.
Then there's scrub radius. That's the arc the wheel passes through as you rotate the steering. Offset effects this and can change how the car steers. I ran a 17mm spacer on the front of my car with stock 18*7.5*50 and the steering was abrupt and too sharp- it looked perfect but it wasn't worth the loss of precision to me.
Just saying there's a lot to consider when we stray from how Porsche set things up and we often discover unexpected consequences. But then that's how we learn isn't it? Play on ...

To update this thread, the wheels fit just fine on the car. The mounted tires are another story. I bought this set of wheels/tires from a person that had done one season of autocross on them, in a Cayman. The Cayman (and presumably 987 Boxster) run a slightly taller tire, about 1/4" taller. The set I bought had slightly larger tires, around 1/2" taller. These taller tires fit in the back, but it was close. In the front, the taller tires would not fit, the top of the tire was firmly pushed against the "cup" on the strut that holds the bottom of the spring. I don't believe it was a spacer issue, as the stock tire goes in about as deep.

Maybe stock sizes of Cayman tires would fit on a 986 Boxster, but I bet it would be very close in the front:

Stock 986 Boxster front - 205/50/17 - diameter 25.1" (from Miata.net tire size calculator)
Stock 987 Cayman front - 235/40-18 - diameter 25.4"
My larger front size - 245/40-18 - diameter 25.7"

The backs looked so good when I put them on :) But it was a no go in the fronts.

I may try and take the tires off and sell them separately as I have always wanted a set of the 18" Cayman S wheels, or I may try and sell them as a set for someone that tracks a Cayman or 987Boxster.

After all my calculations about offsets, etc... I never considered tire diameter as being an issue. So I learned something yesterday.

Steve

PS I never did let the car down off the jackstands so maybe it would have been the case that once loaded these tires would have fit. But the way it was pressing against the bottom of the cup that holds the bottom of the spring, it seemed like being loaded would have no effect or possibly make it worse...

You need a 13mm spacer on the front. Running those wheels without doing that is suboptimal.

I had a 5mm spacer but in order to get clear of that strut bottom cup I would have needed to come out quite a bit (originally I thought over an inch), I don't think another 8 mm would have done it. The top of the tire was up on that that strut bottom hard. Dang because it did look really "tough" :)

http://986forum.com/forums/uploads01...1429205202.jpg

Ok I uploaded this picture and am able to see it zoomed in for the first time. Maybe another 8mm would do it... Hmmm... When I was peering up under there last night it sure looked like there was no way but this picture seems to tell a different story...

No matter what tire you run, you need to use at least a 13mm spacer. If you don't the front wheels will be tucked in too much compared to the rear wheels, which will be suboptimal.

Looking at ECS website they don't have a 13mm spacer kit, So I would go with the 15mm spacer kit.