Re: ur-quattro suspesion changes (was Wheel rate v. spring rate)

[JustaxPHX@aol.com]

>i thought my point was quite clear actually.  with a 17" rim and wide tyre,
you are adding many kilos of unsprung
>weight on each corner of the car, designed originally for the weight of the
15" time and tyre.  therefore, this will have a
>detrimental affect on handling and transient response (which you mention as a
benefit of the heavier wheels!??) of the >vehicle on the road.

My point was that a 17" 40-series tire generally offers better transient
response, amont other performance parameters, than a  15" 50 or 60-series
tire, despite the extra weight.  And in my experience -- including on my Ur-Q
-- the tradeoff between the extra weight and wider, stiffer tire isn't a bad
one, performance-wise ... someday, when you've got nothing better to do, you
might try swapping your RS2 wheels onto your Ur-Q and trying it for yourself.
It might be educational.

>last year "performance car" magazine did a series on wheel/tyre upgrades
which identified and covered this issue.  from >memory, in all cases they
found that changes in the spring and shock absorbers were required to improve
the feel and >handling of the car......

Well, since I've done that as well, it's probably not surprising that I prefer
the 17" setup for most driving.  For the track or autocrossing, though, I swap
back to the 8x15s ... although if decent race tires were available in
235/40-17, I'd be sorely tempted to run them instead.

>there is also fable (which you also mention) about the lack of change to the
ur-quattro suspension during the life of the >car.  while it is true that the
suspension *setup* information did not change, there were *many* revisions to
the design.

Sure, there were detail changes to various suspension parts over the years but
not to the suspension *geometry,* which is what dictates the relationship
between the tire and road at any given point in time.  The only exception is
the rear end of the post-'82 cars, when the a/r bar was deleted and the tie-
rod mounting points were moved, and perhaps some of the later 20v cars ...
even the Sport Q geometry was basically the same as the Ur-Q even if all of
the parts themselves were completely different.

The nominal diameter of the original 205/60-15 tire is 627mm and it's 596mm
for the later 215/50-15s ... if you go to the trouble of plotting the
suspension, then you'll see just how much a difference of 31mm makes in terms
of the position of the roll center, both statically and dynamically.  The
difference in offset (45mm v. 25mm) changes things quite a bit from the side
perspective as well.  Like I said, the suspension can't be designed to work
optimally with both of these setups; either one or the other had to be
compromised to some extent as a result of the change.

>so in summary, your point about audi not caring about the change in wheels
and tyres from 6x15" to 8x15" is patently >not the case.

I didn't say Audi didn't care about the change.  I suggested that Audi either
planned on using 8x15 wheels much, much earlier in the design process than has
been publicly acknowledged or they didn't and simply got lucky when they found
them to work well at a later date.  For various reasons, I suspect the former
more than the latter but you never know...