Backspace question, early Fuchs

[jameshtaylor]

The backspace for early 6, 7 and 8 inch Fuchs wheels is said to be 112mm on a nice Rennline chart widely circulated, and on what looks like a page from an early factory manual as well. Both have appeared dozens of times over the years here and elsewhere. But when I measure my 6's and 7's, I get 121mm. Are we dealing here with a typo endlessly repeated, or is there something about this I don't understand?

[M_deJong]

Measured from the bead seat?

[andyjboy]

I think that whoever compiled the Rennlist table (Sherwood Lee ?) calculated the backspace figures using the Factory's offset (ET) figures & (incorrectly) the rim widths - rather than the overall width of the wheels.

The formula used to determine a wheel's offset is:

ET = backspace - 1/2 overall width of the wheel


So, for example it's known that the offset for a 6x15" Fuchs is 36mm & using the formula above (but incorrectly using half the rim width) we get:

36 = x - 76 (half the rim width in mm)

Thus arriving at a backspace of 112mm


I don't have a 6in Fuchs to hand but if we say the overall width of the wheel is 6.75in (approx 172mm) - using the formula correctly we get:


36 = x - 86 (half the overall width in mm)

- backspace (x) is now 122mm

[jameshtaylor]

When I measure (pad up to plane of wheel edge) I get 121. But 122 suits me just fine. Glad to know that most of the published charts are wrong.

[Harvey Weidman]

That chart has been a problem since it was made. I think Andy explains it well how the numbers were misrepresented..
The back space is about 4 3/4 on most of the models. It does vary by the thickness of the edge as that thickness varies by up to .080 ~2mm depending on what series wheel that you are measuring.
The exception that I work with all the time is the 9&11s. Their back space is a little bigger, usually about 4 7/8. And the 7Rs are about 5 3/4.
Of course the 944 16s are a whole other bag.
Learning back space, front space and how to figure offset is the basis for finding wheels that fit.
H

[KenJ]

thanks for bringing it up James.... I'd done the same as you and did actual measurement with a straight edge and it had me scratching my head over the chart.

[jameshtaylor]

Thanks all. Backspace and offset are confusing enough without throwing some bad data in!