Wheel Offset Example/Question

[John Karasaki <johkar@teleport.com>]

Okay all you wheel gurus out there.  I finally got tired of saying, "The 15
x 8 Ronals (OEM) with 215 tires rubbed terribly on the rear of my TQC, but
the 16 x 8 BBS wheels with 225 tires fit no problem.  Isn't that amazing?"
without really knowing why.  Well I finally calculated the backspacing and
what I'm calling "overhang" (in ignorance of the proper terminology) for
both the BBS RF wheels and the OEM Ronal 16 spoke wheels found on the later
TQCs.  "Overhang" is  the distance from the hub face to the bead of the rim,
a measurement of how far the rim will stick outward from the car (and if it
will rub the fender when you consider the section width of the tire).

The 15" x 8" Ronal has an offset of 24 mm (If I read the wheel right; these
are the numbers after an "E" cast into the face of the wheel near the bolt
holes).  So this wheel has a backspace dimension of 4.945" and an overhang
of 3.055".  The BBS wheel has a backspace dimension of 5.378" and an
overhang of 2.622".  No wonder the 215 tires rub on my '82 TQC (without
rolled fenders) with Ronals and don't rub at all with the 225 tires on the
BBS wheels.  The Ronal's offset is +0.433" greater than the BBS wheels!  Rub
city!

Is my math right?  Ben Howell?  Eric?  Eliot?  Anyone?


P.S.  I know that if the wheels are the same width, only the offset is
needed to calculate how much more one wheel will stick out than another.  I
needed "overhang" because I was comparing several wheels of different
widths: Fuchs, BBS (OEM), BBS RF, Ronal (OEM, V8), Ronal (OEM, TQC), etc.
Regards,

John Karasaki

The Karasaki's, proud owners of AUDI automobiles

1981 Coupe
1982 TQC
1984 5000S Wagon
1990 V8