# Re: ur-quattro differences (continued)

```>Isn't track width measured from the center line of the wheel?

Yes, it is ... as a practical matter, though, it's usually measured as the
distance from the outside of each wheel to the inside of the opposite wheel,
then averaged.  This takes into account the difference that could be caused by
alignment settings...

> for the 8" wheels the center is only moved 4.4 mm per side for a total 8.8
mm increase
> in track. My understanding is offset should change to compensate for
> different width wheels as Audi did when changing from the 6" to " wheels.

Well, let's run some numbers and see:

6x15 wheel w/45mm offset

Width: 152.4mm	(6" x 25.4mm)
Center: 76.2mm		(152.4mm / 2)
Inset: 121.2mm		(76.2mm + 45mm)
Outset: 31.2mm		(76.2mm - 45mm)

8x15 wheel w/24mm offset

Width: 203.2mm	(8" x 25.4mm)
Center: 101.6mm	(203.2mm / 2)
Inset: 125.6mm		(101.6mm + 24mm)
Outset: 77.6mm		(101.6 - 24mm)

As you can see, when Audi swapped the 6x15s for 8x15s, the inner edge of each
wheel moved inward by 4.4mm per side and the outer edge moved outward by
46.4mm; the difference between these two figures is 42mm, which is then
divided by 2 to determine the change in offset, which is 21mm.  The difference
in track width is determined by adding together the change in offset for each
wheel, which works out to 42mm.  This is pretty close to the 40mm that Dave's
quoting from the Autocar specs and also the reason why you must roll the
fender lips on the early cars in order to fit 8x15s without rubbing...

Now, for fun, let's try an 8x15 wheel w/45mm offset

Width: 203.2mm
Center: 101.6mm
Inset: 146.6mm
Outset: 56.6mm

In this case, the inside edge of the wheel moved inward by the same amount the
outside edge moved outward; as such, the center of the wheel is still 45mm
away from the mounting flange so there's no change in track width.

Did I clarify things or further add to the confusion?  It may help to make a
semi-scale drawing and plot the relationships between the various numbers.  I
would've done so for you but ASCII doesn't really lend itself to technical
drawings...

JG

```