[A4] Wheels?

[gt40]

From: Brian J White <brian at bjwhite.net>
Date: Fri, 21 Apr 2006 08:24:22 -0700

>Rocky's blanket statement below depends on many factors.   He's 
>assuming that the larger wheel/smaller tire combo will be heavier 
>than the 16".  Which in a lot of cases is true, but what exactly 
does 
>"faster driving" mean.  Yes, heavier wheels can affect handling 
and 
>acceleration, but we're talking Audi A4s here, not club racing 
>Alfas.   You would hardly feel the difference in a couple-three 
>pounds per wheel in a heavy car like the A4.   That being said, 
35 
>pound wheels are never a good idea.
>
>To me, I like the stiffness of the smaller sidewalls on a 17" 
wheel 
>on the A4.   Slightly wider tread pattern does turn in a bit 
crisper 
>but also looks darn good compared with 16x7 wheels.
>
>And let's face it.  The A4 isn't a track whore...it's a daily 
driver.

And yet it does pretty well on the track, for a heavy, 
underpowered 4-door sedan.

I race cars with the National Auto Sport Association, an on 
occasion, I've taken my A4 TQMS on the track (Hallett Motor Racing 
Circuit outside of Tulsa, and Motorsports Ranch outside of Fort 
Worth, TX.)  For as heavy as the car is, I was impressed with its 
handling.  Pull out 400 lbs and give it another 200HP and it would 
make a fine track car.

As for a different sized wheel affecting handling, the effect will 
be minor, based on the wheel & tire size alone.  A larger effect 
would come from using different brands or models of tire.

But, all things being equal, a tire with a shorter sidewall, will 
give you crisper turn-in and more feedback through the steering 
wheel (as the front tires loose grip, you can actually feel it 
through the steering wheel.)

BUT -- the shorter sidewall tire may actually have less ultimate 
grip, depending on the road surface.  The tire's sidewalls act as 
an undamped spring.  Tires with a taller sidewall have a lower 
effective spring rate in the sidewall than do tires with shorter 
sidewalls.  If you're cornering on a bumpy road, the tire with the 
shorter sidewall will have more of a tendancy to skip over the 
bumps, and as it's skipping, it's not generating as much lateral 
force.

On my race car, I'm 0.3 seconds faster at Hallett running a 50-
series tire compared to a 40-series tire simply because two 
important corners are fairly rough.  With the 40-series tire, I 
have to go through the corner a good deal slower to control 
understeer.  The taller tire handles the bumps better and allows 
for a faster cornering speed.

As in everything, it all comes down to compromises.  On the 
street, I'd run a taller sidewall because the tires are usually 
cheaper, and they tend to protect the rim a little better from 
curb-rash.  Performance?  Feh -- save that for the track where it 
counts. 

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