North American Turbocoupe Organization



Wheels backspacing and offsets
85Original Offline
Member
#1
I want to put 275/40/17’s with 9” wheels on the rear & 255/40/17’s with 8” wheels on the front of my 85.5 Turbocoupe. I don’t want the outside edge of the wheel to move much further out than it is with the stock 7” 10 Holes, and the Quad shocks are removed.

Here’s the problem: I’ve read every applicable post on NATO that I can find, and a lot of numbers don’t seem to jive. Using my stock wheel as an example, tell me where I’m wrong. 7” wheel w/0.88” (22.35mm) offset. If the centerline is 7” divided by 2, I get 3.5 + the 0.88 positive offset equals 4.38 (I read numerous times that the backspacing is 5”, but my math doesn’t tell me this). This should be my backspacing. Correct? If this is correct and I subtract 4.38” from the total wheel width of 7”, I have a remainder of 2.62”, which is the distance from the wheel mount to the outside edge of the wheel.

Lets say I want to put 275/40/17’s on the rear using a 9” wheel, w/ 2.7” from the wheel mount to the outside edge of the wheel. Subtracting 2.7” from 9” leaves 6.3” for back-spacing. Wheel centerline would be 4.5”, subtracted from 6.3” gives me an offset of 1.8” or 45.7mm +offset. Yet I keep seeing numbers like 15mm offsets to 20mm, even though the stock offset was 22.35mm on a narrower wheel.

What am I missing here? Are most people moving the outside edge of the wheel a lot further outboard than I am? Do I have to move more outboard to find any wheels that will have an offset that works? Do my numbers make any sense?
R. Libby
85.5 Med. Regatta Blue TC
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Robert Camp Offline
Posting Freak
#2
It's the defintions of backspace and offset that are upsetting your calculations.


The 7" width refers to the bead width, not the overall width of the wheel. There is the thickness of the rim at the bead (inside and outside) to add to the overall width.

Offset is the distance from the center line of the bead width to the mounting surface, measured in millimeters. Example, the centerline 7/2 = 3.5 from inside of bead.

Backspace is the distance from the outside edge of the inner side of the rim (including the bead width) to the mounting surface, measured in inches.

Offset and backspace are not referenced from the same point or a directly related point. So you can't add offset and backspace to obtain useful info. But you can derive some useful equations if you have both the backspace and offset.

Also, the outside bead will sometimes be a little thicker than the inside bead, affecting the frontspace. That's usually minor though and can be ignored.

By the way, I measured the backspace as 4 15/16, not 5 and have seen 22mm listed as the offset.

You can calculate the overall width (assuming inside and outside beads are the same thickness)

OW = 2*(backspace - (offset/25.4))
Frontspace = OW - backspace
Robert Camp
'86 Medium Regatta Blue TC, 5-speed, original owner.
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