Re: Tire sizes for urQ!? (longish)

[Richard Funnell <rfunnell@pobox.com>]

Eric,

You're lucky to have rolled fenders.  Who did the job on them?

See my comments below.  Thanks for the input.  Comments on the comments are
welcome.

Richard

At 06:01 PM 1/18/99 -0600, Fluhr wrote:
>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.
>
<snip>

>
>
>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. 

So far so good, though I think it's generally the tire width rather than
the wheel width that causes the problems.
 
>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.

Given that the tire on the A4 wheel was 205, like the stock urQ tire, you
would probably find it fits with just about the same clearance all around
as the stock tire.

>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. 

I'm assuming that the appropriate offset is determined by where the
clearance problems are, and what the tire size is.  From the spreadsheet I
did comparing known good and bad fits, it appears that any wheel/tire
combination that extends more than an extra 3 mm (of tire width) toward the
inside of the car causes problems.  urQ's with rolled fenders, on the other
hand, seem to tolerate at least an extra 27mm (of tire width) toward the
outside.

Assuming a 225 tire is 225 regardless of the wheel width (not an entirely
accurate assumption), an ET35 wheel places the inside tire edge exactly as
far in as the stock ET45 wheel places the edge of the 205 tire.  The
outside edge of the itre, however, sticks out an extra 20mm.  This
combination works well with the clearance problems.

>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. 

I would say a 225/50 16 ET35 is a great size, only (apparently) with the
rolled fenders.  Depending upon how you plan to use the car, you might find
a slightly narrower wheel would soften the ride a touch. 

>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.  

I'd look at the tire, not the wheel.  Again, I would expect a narrower
wheel to ride a bit softer which would be welcome using a 17" wheel.

>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 agree; maybe even a 7".

>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.

>From what I saw, you should use the ET35 with a 225 tire because the inside
clearance seems more restricted than the outside.

>Personally, I would like to run 16x7.5 ET35/38 rims with 225/50 
>tires

I'd pick ET35

>or 17x7.5 ET35/38 rims with 215/45 tires.  

ET 35 to 40 should be fine.

>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
>
>
>
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