986 Forum - The Community for Porsche Boxster & Cayman Owners - View Single Post - 18x8 ET 57 with 245/40/18 and 18x9 ET 43 with 285/35/18

steved0x · 01-30-2015, 05:10 AM

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.

01-30-2015, 05:10 AM	#12
steved0x Registered User Join Date: Dec 2012 Location: FL Posts: 4,144	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.