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Old 12-31-2017, 03:28 AM
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Cloud9...68
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Default Porsche wheel offset accuracy?

I'm not sure which would be the best forum for this question, but this one is frequented by people with a lot of technical knowledge, so I'll start here. I have a set of 17 x 9" Twists on all four corners of my 1992 968. The wheels are stamped with "ET55" stamped into their backs. I assumed this means they have an offset of 55 mm. Given this, I would expect a backspacing of 55 + ((9.5/2)*25.4) = 169.3 mm. But when I actually measure the backspacing, I get a measurement of 183 mm. This implies that the offset of my wheels is actually 68.7 mm, which obviously is very different from what is stamped on them. Am I missing something, or doing something incorrectly? Thanks.
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Old 12-31-2017, 03:50 AM
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Edit: never mind
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Old 12-31-2017, 08:58 AM
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You didn't burn an inch did you?
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Old 12-31-2017, 09:32 AM
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Originally Posted by Cloud9...68 View Post
I'm not sure which would be the best forum for this question, but this one is frequented by people with a lot of technical knowledge, so I'll start here. I have a set of 17 x 9" Twists on all four corners of my 1992 968. The wheels are stamped with "ET55" stamped into their backs. I assumed this means they have an offset of 55 mm. Given this, I would expect a backspacing of 55 + ((9.5/2)*25.4) = 169.3 mm. But when I actually measure the backspacing, I get a measurement of 183 mm. This implies that the offset of my wheels is actually 68.7 mm, which obviously is very different from what is stamped on them. Am I missing something, or doing something incorrectly? Thanks.
The formula is (55 + ((9/2)+0.5))*25.4 = 182mm, you cannot add the 1/2in to the wheel width before you’ve halved the width. Another thing that will affect the result is that actual wheel widths are rounded up or down to the nearest 1/2in so your wheels may not be exactly 9in wide. Best thing is to measure the actual width between the inside rim edges.

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Old 12-31-2017, 10:40 AM
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Whoops - I had a typo in my formula. I meant 55 + ((9/2)*25.4), which comes out to the 169.3 mm I had in my post. This is the formula in "High Performance Handling Handbook, by Don Alexander. Where does the 1/2 inch in your formula come from? Thanks.

And here's how I did the measurements:



Backspace measurement method


Measurement

I'm running pretty narrow tires (225/45-17's - a whole 'nother discussion), which don't protrude out beyond the rim, so the straightedge only contacted the rim when I was measuring my backspacing.
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Old 12-31-2017, 11:51 AM
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Originally Posted by Cloud9...68 View Post
Whoops - I had a typo in my formula. I meant 55 + ((9/2)*25.4), which comes out to the 169.3 mm I had in my post. This is the formula in "High Performance Handling Handbook, by Don Alexander. Where does the 1/2 inch in your formula come from? Thanks...
Good question about 1/2in, in honesty I donít know, these are just formulas I have that Iíve used and that work. Maybe Iím wrong or someone with more knowledge can chirp in and confirm. It does seem odd now that you mension it. What I have are:

half width + offset + 0.5 = 4.5 + 2.16 + 0.5 = 7.16in or 182mm; or

half (width +1) + offset = 0.5*10 + 2.16 = 7.16in or 182mm

EDIT: After reading up on this further the reason for the extra 1/2in when calculating backspacing from offset is because backspacing is measured to the outer rim and not to the inner rim where the width is measured. The difference in the rim lip is commonly taken as 1/2in per side. This graphic might explain it better-




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Old 12-31-2017, 01:56 PM
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Mike,

Thanks for taking the time to research this little puzzle - you seem to have found the answer.

For me, the relevant measurement is the distance between the hub mounting surface and the actual edge of the wheel (the 183 mm I measured), because I'm working on putting together a set of 10.5" wide 17" BBS E26 3-piece wheels through Bob Woodman Tires. I've measured a gap of 19 mm between the inner edge of my current wheels and the front coil spring with the weight of the car on the ground. This gives me as healthy 202 mm between the face of my hub and the spring. I'm hoping this will allow me to fit a wide enough inner barrel that I can use a thin enough outer lip to avoid having to roll my fenders.
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Old 12-31-2017, 10:49 PM
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In case you don't already know of it, 'Will they fit' is an extremely helpful tool in visualizing any sort of wheel you are thinking of changing too. Just measure the clearance around your current wheels...then go to http://www.willtheyfit.com/ and plug in your current specs, and the specs of the wheel you want to run, and you will instantly see whether it will fit or not.
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Old 01-01-2018, 01:49 PM
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Thanks - that is a useful site. I have a similar calculator in Excel that I've used in the past. However, I'm afraid it won't help with my particular situation, because I'm trying to configure a set of 3-piece wheels, using used componenets from Bob Woodman Tires. I don't know of a way to translate the width of the inner barrel and the outer lip of a 3-piece wheel to an offset and a backspacing, especially since the width of these components also depends on where the mounting surface of the wheel center is relative to the lip of the inner barrel. I think the only way to do it is to supply them the space I have between my hub mounting surface and the coil spring (202 mm in my case, which fives 19 mm of clearance betwen the inside of my current wheel and the spring), and specify a spacing I would be comfortable with. In my case, I think 10 mm between the wheel and the spring is reasonable, which would give a "backspacing" as I've measured it in the images above of 192 mm. Then, I'll have to have Bob Woodman Tires mock up the centers they have with a couple different width inner barrels, and measure the backspacing the same way, until they find one that comes closest to the target of 192 mm. Then they'd add an outer lip that give my target total width of 10.5". If anyone knows of a better way, please let me know.

By the way, does anyone have any experience with Bob Woodman Tires? They appear to be the only game in town for configuring 3-piece wheels with used components, which keeps this undertaking to a reasonable cost. And they say they specialize in Porsches, which is useful, given our cars' oddball bolt pattern and offsets. They were initially rsponsive to my initial inquiry back in August, but since then, they never respond to my voicemails or emails, or to messages I leave with their account person, who is the only one who ever answers their phones. When I first contacted them in August, I told them I wasn't in a rush, as I had just purchased a new set of tires, and it was just getting into their busy racing season, but it's going on five months now, and the crickets are deafening. Any experieces out there with these guys? On the few occasions I've talked to them, they've been pleasant and helpful-sounding, but I'd like to get these wheels sometime before I'm eligible for Medicare...
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