Re: Tire sizes for urQ!? (longish)

[ejfluhr@austin.ibm.com (Fluhr)]

Richard,

Here are some of my thoughts on that issue.  I have a 1982 urquattro
with rolled rear fenders and I want to go to either 16" or 17" rims.  
I have been very concerned about tire fitment, and so have done some
calculations on it.

>Calculated good combinations (right at the edge) end up as: 
>
>- - 225/50 16, ET 35
> - same inside clearance as stock 206/60 15, ET 45
> - 20mm closer to the fender
> - 0.7% larger radius (in the noise)
>
>- - 225/45 17, ET 35
> - same inside clearance as stock 206/60 15, ET 45
> - 20mm closer to the fender
> - 1.2% larger radius (still in the noise)
>
>In both cases this fit will be so close that variations in tire design will
>make it or not, and rolled fenders may be needed to be sure.  Rolled
>fenders seem to allow at least an extra 6 or 7 mm, which seems logical
>looking at the lip.  In either case, going to a 215 tire and ET 40 wheel
>should be a slam dunk, since that reduces the fender side increase from
>20mm to 10mm.  


The factory wheel is 15x6", ET45, with a 205/60-15 tire.  This means
that the wheel extends 121.2mm into the car from the mounting face
of the wheel, and 31.2mm out towards the fender. 

To get an idea of fitment, I mounted the 5-spoke 16x7" ET45 wheel from
the A4 on the rear of my urquattro, and it appeared to fit fine with 
plenty of space with 205/55-16 tires.  Doing the calculations, this
shows the wheel to extend 133.9mm into the car and 43.9mm towards the 
fender.  Comparing it to the 15" rim, the 16" wheel extends 12.7mm
(1/2 of an inch) further towards the inside of the car.

So, now the question becomes, "What width 16 inch or 17 inch wheel
and tire will fit under the urquattro's fenders?"

Based on direct experience, 16x7" ET45 will fit with 205/55 tires.
Since I only saw the wheel with 205/55 tires on it, I do not know
if 225/50 tires will fit without rubbing on the inner suspension
components.

It seems commonly accepted that 16x7" ET35 and 16x7.5" ET35 will both
fit the urquattro.  The 16x7" sounds reasonable, since the wheel is
moving 22.7mm out toward the fender (and only 2.7mm inward), relative
to the stock 15x6 wheels.  The 7.5" wheel is a bit more extreme,
extending 29.05mm further towards the fender and 9.05mm further towards
the inside.  I might feel more comfortable with an offset of 38mm for 
a 7.5" wheel, since that would result in 133.25mm inner and 57.25mm 
outer rim edges. 

Now, the fellow who owned my urquattro before me used 16x8 wheels
with an offset of ET35 and 225/50-16 tires.  From what I understand,
this rubbed until he rolled the rear fenders.  Still, I would hesitate
to use a 16x8 wheel unless I could verify that the offset would work
with the tire I was using.  The 8" rim with a 35mm offset yields an
inner extension of 136.6mm and an outer extension of 66.6mm. 

As for 17" rims, I would expect them to fit similar to the 16" rims.
Hence, an ET35 for 17x7" and 17x7.5" should work fine.  Now, 17x8
may be a bit trickier.  Jeff Goggin and Ben Howell both seem to be
OK with 17x8 ET35/33, but they are only running 215/45-17 tires
rather than 225/45-17.  The 215 tire seems rather narrow for an 8"
width rim, and I would be more inclined to use a 7.5" rim.  I would
probably go with a greater offset (i.e. 38-40mm) if I planned on using
a 225/45-17 tire on a 17x8 rim, if there was enough clearance on the
inner suspension components.

Personally, I would like to run 16x7.5 ET35/38 rims with 225/50 
tires or 17x7.5 ET35/38 rims with 215/45 tires.  I know most TSW
rims come in 16x7.5 sizes, but I do not know if any rims are
available in 17x7.5 widths.  In that case, 17x7 may be the best
choice for 17" with 215 tires.

Later,
Eric
'85 CGT, '82 urq
---
Eric J. Fluhr                                Email:  ejfluhr@austin.ibm.com
630FP Logic/Circuit Design                   Phone:  (512) 838-7589
IBM Server Group                             Austin, TX