Re: GTV6 suspension geometry

[Scott Fisher <sefisher@xxxxxxxxx>]

Graham -- I haven't measured a 116's suspension, not having one to look
at. :-)  My explanation was meant as a starting point for visualizing
how suspensions work, and as a theoretical explanation for why a car
*can* understeer worse with lowered front springs.  (I've had such
experiences, but not with Alfas.)

> I feel it all depends on how your lowering has
> changed TWO aspects: not only the rate at which the suspension
> adds neg camber when compressed, but also the reduced body 
> roll moment, and hence roll angle. 

Precisely, and that will depend on a lot of factors, including the
spring rate you're using and the degree to which you lower the car.  (As
*well* as the actual camber curve, which as I say I don't know for the
116.)

> Oh, and there may be a third factor.... I gather 
> that the front lowering would reduce the roll-moment and hence
> reduce the weight transfer at the front, which I believe should
> tend to reduce the under-steer. 

That may well be the case; the question then becomes which one wins, the
increased positive camber or the reduced weight transfer.

> So apparently the lowering
> introduces two changes tending to REDUCE understeer, along 
> with Scott's camber change effect which tends to increase it, so
> Scott's conclusion may not always hold true.  

Certainly not for all cars, and there are things you can do to modify
the camber-curve effect such as fitting offset bushings, adjustable
arms, etc.  

In general -- and we all know how effective generalizations are -- there
is a lot to be gained, at least in race-track performance, by lowering
the car: various roll effects, an overall lower CG, possibly even
improved aerodynamics.  But it's not necessarily the only thing you need
to do.  My examples were meant more as a guide to visualizing what the
suspension as a whole can do, and to help get people thinking about what
may or may not be going on -- based, of course, on what their exact cars
actually look like.  There's still no substitute for crawling under the
car.

In fact, here's a technique I've helped a friend with: put the car up on
jackstands.  Remove the spring (or disconnect the torsion bar) on your
front suspension and mount a camber gauge (which can be as simple as a
protractor and a pointer) to your front tire.  Then, with a hydraulic
jack, raise and lower the front wheel through its full range of travel,
taking measurements every couple of centimeters.  The resulting plot of
those numbers is your car's camber curve; you'll know from this whether
you may want to adjust an initial setting, based on the ride height you
choose.

As a side effect, you may also want to "bump-steer" the car at the same
time.  That's done in much the same way, but with the protractor held
parallel to the ground and the wheel pointing straight ahead.  You're
looking for changes in the toe setting at different points in the
wheel's vertical travel.  It can be very spooky to have a car on which
the front wheels suddenly turn one way or another, even a fraction of an
inch, under compression.  And check Carroll Smith's books for the how-to
on adjustment, setting it up, etc.

All of this, of course, assumes that your bushings are sound (or that
you're using spherical rod ends/rose joints, if you're doing this on a
track racer).  

But yes... the real lesson from this is that suspensions are complex
systems, and if you set about changing them, it's best to do so
methodically and *as* a system.  Single changes (apart from simple ones,
such as adding a stiffer anti-roll bar) can often make matters worse
alone, but when combined with compensatory or complementary changes,
they can make things much, much sweeter.  And it's always best to have
an idea of what's going on before you start swapping parts.

- --Scott Fisher
  Sunnyvale, California USA

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