RE: big wheels on Audi

[David Eaton <David.Eaton@COMPUTERLAND.CO.NZ>]

some ruminations on the wheels/tyres thing; a few factors to take into
account when putting larger wheel/tyre combinations on your audi.  i
welcome discussion on any point below, or things which i haven't thought
about...

1) unsprung weight.  you should try and ensure that the weight of the
wheels & tyres is as close as possible to that of the old combinations.
higher unsprung weight will affect spring & damper performance, and so,
handling.  these things can, of course, be changed, but not without a
lot of experimentation to get them right.  larger wheel/tyre
combinations can easily add many kgs to the unsprung weight of the car
and put a much heavier load on the springs and (particularly) the
dampers, with the result being the ride shot to peices.  to cope with
this you need better dampers.

for example, here in new zealand, there are any number of honda crx's
with 17" wheels, 35 section tyres with lowered springs, and so much bump
steer and bad damping that they have major problems going around
corners.  this is a classic example of the consequences of paying no
attention to damping.

2) rolling diameter of the wheel.  increasing the rolling diameter of
the wheel will have a number of undesirable effects.
	- speedo inaccuracy
	- raising the roll radius and ride height of the car, which again will
affect turn-in and cornering.
	- clearance to the guards/fenders under lock/full bump

where possible, keep the diameter of the wheels much the same.  a good
handling car can be ruined by a relatively small increase in ride
height.

3) adhesion and handling.  larger tyres do not necessarily a good
handling car make.  there are a number of factors to take into account.

- the rim width will affect tyre sidewall flex.  generally, the wider
the rim, the straighter the sidewall, the better the turn-in and the
firmer the handling.  the ur-q is a good example of this with it's wide
rims on 215/50's which result in vertical sidewalls.  on the opposite
end of the scale is some old avants with major sidewall flex on 60
section tyres (for comfort).

- contact area of the tyre will change with different tyre profiles.
generally speaking the more the contact patch changes longitudinally
(ie. larger diameter, smaller width), the better the braking and
traction, the more the contact patch changes horizontally (ie. greater
width, smaller diameter), the better the turn-in and cornering.

- the larger the contact area the better the dry weather adhesion, and
the worse the wet weather adhesion.

as an example of this, the ur-quattro has oem ronal 15x8j wheels and
215/50 tyres.  due to the width of the rims and the relative narrowness
of tyres, the sidewalls are vertical which is partly responsible for the
quattro's sharp turn-in and "strapped to the road" feel.  simply putting
235/45 tyres on the car, which will result in negligible change in ride
height, will introduce more sidewall flex due to the greater width of
the tyre, will *reduce* wet weather performance and braking, but will
improve dry grip and braking, but reduce turn-in. a good idea? you
decide.

hth,
dave.
'95 rs2
'90 ur-q
>------------------------------
>
>"Marcus Oesterwinter" <marcuso@u.washington.edu> wrote:
>> 
>> As a followup to my previous message about wanting to know if I could put
>> big wheels on my Audi (it was suggested that it could not be done.)  I
>> wonder, is it possible to just get longer shocks/struts that would allow
>> more clearance?  If so, I am guessing that I would have to use ones made
>>for
>> a diffent type of car.  Are there any that would do the job?
>
>