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Old 04-20-2017, 01:19 PM   #1
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Offsets ... ?

Hi I have a 986 S with 18" wheels and I'm at a loss trying to understand wheel offsets, etc. All I want to do is push the wheels out so the tire edge is closer in line with the fender and the car looks more aggressive in stance. I don't want to compromise the handling or the wear and safety of it's original engineering. How do you determine what is used ( a spacer?) and how much and how does the offset play into that?

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Old 04-20-2017, 01:59 PM   #2
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do you know the offsets of your current wheels? they will be stamped on your wheels. or, if oem, what wheels are they? what tire widths/sidewalls are you running?ride height? if we know that them we could advise the max spacer width that would work for you.

otherwise, offset is the distance the mounting face of the wheel is offset (+ or -, often called 'et'; ie an offset of 65ET) from the centre line of the wheel. OEM boxster wheel will have an offset of about -50 mm front and -50 mm rear. often the '-' is left out. a larger offset wheel (ie, a 996 narrow body wheel has rears in the -65 mm offset range) will move the wheel further inboard and risk contact with struts, parking brake cables, trailing arms, etc. a smaller offset (ie, a 964 wheel at -35 mm or so) risks contact with the fender. spacers can move a wheel out if it is too far in, but can't bring a wheel in that is too far out. the math is easy; wheel offset + spacer width = new offset. so, a 20 mm spacer on a 996 narrow body rear wheel at 65ET is -65 + 20 = -45. limits for a boxster is max -40 front and -40 rear. of course, tire width and height, aggressive camber, and ride height are all variables that can affect fit. if you are on oem wheels then you are looking for spacers in the 7 to 15 mm range.
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Old 04-20-2017, 02:20 PM   #3
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I run 20 mm front and 15 mm rear spacers with factory wheels and offsets on my 04 S

be sure to get longer lug bolts or studs if you will be using spacers
for studs, you need 90 mm with a 20 mm spacer. Might be able to get away with a 75 mm with a 15mm spacer, depends on the stud. Most of the longer ones have an unthreaded space of around 15 mm at the end. For safety, an open end lug nut should have at least 15 mm of thread showing past the not
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Old 04-20-2017, 03:11 PM   #4
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Lower number offsets (eg. 40) will push the wheel out. So a 50 offset with a 10 mm spacer will be like a 40 offset.
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Old 04-20-2017, 03:27 PM   #5
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I'm running the stock upgrade 18" turbo twist wheels and tires.
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Old 04-20-2017, 03:38 PM   #6
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Just to add to your confusion...
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Old 04-20-2017, 03:44 PM   #7
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Okay. So the max spacer for the rear would be 10mm? Would doing that (adding a spacer) require an alignment as well? Would 10mm spacers have that aggressive stance?
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Old 04-20-2017, 03:52 PM   #8
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sorry, oem boxster wheels are 'around' 50ET; some are 53, 57, etc., hence the variety of spacers available to push you out to 'around' 40. you should really check your wheels as the turbro twists can be 996 or 993 wheels and you may already have spacers on the rear and not know it.
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Old 04-20-2017, 04:35 PM   #9
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I run 7x17 et55, 7.5 x 18 et50, and 8 x 17 et50 fronts with 20 mm spacers and 8.5 x17 et48, 9 x 17 et52 and 9 x 18 et52 rears with 15 mm spacers with out any clearance problems. I have also put the 20mm spacers on the rear without any issues.
Tire size fronts are 225/45-17, 235/45-17 and 225/40-18 with 255/40-17 and 265/35-18 rears

Front camber is -2.2 and rear is -2.5 on M030 ROW springs so its lowered around 15-20 mm

Don't know where RK got 40 offset as a limit. Wheel width factors in with offset as well

there are a bunch of wheel/offset calculators on the web you can compare dimensions with

Adding spacers pushes the outside edge out and the only issue could be fender clearance. . With my setup, its is not a problem
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Old 04-20-2017, 04:56 PM   #10
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Online Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit will show you the effect of offsets.
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Old 04-20-2017, 05:37 PM   #11
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i'm basing my -40 max on this ...

wheel fitment question

with max wheel width and max tire width (first post is a pic of a 275 tire on a 10" wheel at -34 offset). bit more room on the front (i am at -38 currently on an 8.5 w 235's) but really camber and ride height dependent.

edit to add that it becomes part of the math - if your wheel is a 9" (ie, an inch narrower than mine) then you can add 1/2" = 12 or 15 mm to my max offset. if you are on a 265, then you are 10 mm narrower than me, so you can add 5 mm to the offset.

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Old 04-21-2017, 02:01 AM   #12
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I run a 9" wide rear with a 20mm spacer bringing the ET down to around 35, sits dead flush with the arch.
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Old 04-21-2017, 03:35 AM   #13
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Does the ET value tell you the width of the wheel? Obviously, I'm not removing the tire to measure and I'm getting the impression that there are different sizes of 18" Turbo Twist stock wheels?
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Old 04-21-2017, 03:53 AM   #14
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Quote:
Originally Posted by pidj View Post
Does the ET value tell you the width of the wheel? Obviously, I'm not removing the tire to measure and I'm getting the impression that there are different sizes of 18" Turbo Twist stock wheels?
ET is the offset of the wheel, width is different. It's the combination of the 2 which determine how far a wheel sticks out.

"The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel."

Last edited by That986; 04-21-2017 at 03:57 AM.
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