RE: ur-quattro suspension changes (was Wheel rate v. spring rate)

[Dave Eaton <Dave.Eaton@clear.net.nz>]

hang on a minute.  audi's suspension setup data has nothing to do with 
things such as ride height or roll-centres.  basically the data that 
*didn't* change (i.e. that published) is simply toe-in, camber, and caster. 
 and as stated, my information (admittedly limited about the early cars), 
has no change in these values.  but i could setup my tractor with the 
ur-quattro values for toe, camber and caster.  yes, my tractor has the 
ability to change all these values.  even caster.  and let me tell you, my 
tractor has a very limited cornering ability!  and the rear wheels are 
considerably bigger and heavier than the fronts!

put this another way, the wheel alignment data published by audi makes no 
mention of ride height, roll centres or any other such thing, even wheel 
sizes.  it assumes that you have a ur-quattro.  however, we know that the 
ride height changed between the series 1 (fuchs) and series 2 (ronal) 
ur-quattros.  audi's major revisions to the suspension (as detailed in my 
last email) indicates a awareness of the seriousness of the changes they 
were making to the car by changing the wheels, tyres and the ride height.

also a heavier wheel/tyre combination will *not* offer better transient 
response than the oem fitting, unless you adjust spring rates and dampers, 
that is.  how could it?  as for tyre stiffness, a 215 tyre on an 8" rim as 
fitted to my ur-quattro has a stiffer sidewall (it's vertical) than the 245 
tyre on a *7.5"* rim on my rs2.  think about it.  hence the better 
transient responses on my ur-quattro than on my rs2.  amongst other things.

anyway, i don't want to drag this out....my original satatement was based 
on the comments of a very experienced rally pilot who stated that most hot 
subaru's he prepares and modifies go better on 15" wheels than on 17" ones. 
 he also said that most owners however won't be seen dead in 15" wheels. 
 clearly you are one of these.  me? i love the look of the rs2 wheels/tyres 
but would trade them in an instant for 15" ones if that made the rs2 handle 
as well as my ur-quattro does on it's 15" rims.

now, lemme me see.  perhaps we should convince formula 1 to move away from 
13" rims.  :-)

dave
'95 rs2
'90 ur-q

-----Original Message-----
From:	JustaxPHX@aol.com [SMTP:JustaxPHX@aol.com]
Sent:	Saturday, 23 January 1999 16:07
To:	Dave.Eaton@clear.net.nz; quattro@coimbra.ans.net
Subject:	Re: ur-quattro suspesion changes (was Wheel rate v. spring rate)

>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...