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

In a message dated 1/22/99 11:39:12 PM US Mountain Standard Time,
Dave.Eaton@clear.net.nz writes:

>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.

Agreed but Audi's published alignment specs aren't my only data source...

>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.

Which, in a roundabout way, tends to confirm my point since this would only be
appropriate if the underlying suspension geometry is the same for all three
generations of the car.

>however, we know that the ride height changed between the series 1 (fuchs)
and series 2 (ronal) ur-quattros.

Yes, but why?  Simply installing 215/50-15s in place of 205/60-15s will lower
the car by 15mm due to the difference in the overall diameter of the tires.
Isn't it interesting that the claimed difference in ride height between the
later cars and early ones was -- wait for it -- 15mm?  Hmmm...

>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.

I disagree ... changes in ride height have no effect whatsoever upon the car's
suspension geometry.  Neither do changes in the shape of control arms, ball
joints, a/r bar mounting points or the stiffness of rubber bushings ... unless
you change the location of the mounting points of the various suspension
components, all you're achieving by changing ride height is to reset the
suspension's static equilibrium to a different point along the same curve.

>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?

All other things being equal, the shorter a tire's sidewall, the stiffer it is
and the less it deflects, which means the quicker it will respond to steering
inputs.  A 235/40-17's sidewall is nominally 94mm long and a 215/50-15
sidewall is 107.5mm, which works out to a difference of almost 14 percent. 

However, I will grant you that unless the rest of the suspension is designed
to take advantage of this, changes in spring rates and shock damping will
probably be necessary on many cars to realize this potential improvement in
performance ... unfortunately, making a direct comparison is often difficult
because it's so hard to isolate the potential variables.  My personal
experience with running 17s on an Ur-Q has convinced me that, overall, it's a
plus despite the drawbacks you have pointed out.  (Plus, it also means I have
room inside the wheel for those Porsche brake calipers I have sitting on a
shelf in the garage!)

>as for tyre stiffnes, 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.

Sure ... but what if Audi had put the same 245/40 on a proper 9x17 wheel
instead of the almost criminally skinny 7x17 they used instead?  For optimal
performance, the general rule of thumb is that the wheel should be at least as
wide as the section width of the tire.

>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.

I don't disagree ... Paula's always telling me that bigger isn't always better
and I'm sure it's as true for wheels as it is for anything else (ahem).  But
before you automatically dismiss the use of 17" wheels on an Ur-Q, it might be
wise to drive one that's so equipped.  I've done so and am reasonably
confident you won't be as disappointed as you suspect.

>he also said that most owners however won't be seen dead in 15" wheels.
clearly you are one of these.

On the contrary.  If, overall, I found the 17s degraded the performance of my
Ur-Q, then I wouldn't use them.  There are a few drawbacks, to be sure, but
there are also many benefits, appearance being one of them.  I assure you that
I'm not a slave to automotive fashion ... not only are my exhaust pipe tips
stock but I don't even own a baseball cap let alone wear one on my head
backwards!  :^)

>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.

Apples to oranges ... they're two entirely different cars and were designed to
serve two entirely different purposes.  From what I understand, Audi never
intended the RS2 to drive like an Ur-Q or even to be compared to it.  You're
lucky that you have one of each and can appreciate them for what they are
without having to choose between them.
>now, lemme me see.  perhaps we should convince formula 1 to move away from
13" rims.  :-)

Can't ... for reasons unknown to me, 13" wheels are required by the rules.
BTW, speaking of F1, the city of Phoenix is just putting the finishing touches
on a multi-story parking garage that has been built on the site where the
garages were for the Phoenix F1 races.  Fortunately, I was able to snag a
cinderblock from what had been the Ferrari garage that has some grafitti
written on it in Italian ... haven't got a clue what it says -- it's probaby
something obscene! -- but I display it proudly on a bookshelf regardless.  :^)