Re: Suspension Geometry

[drubie@xxxxxxxxxxxxxxxx (David Rubie)]

[excellent suspension stuff deleted]
> Graham's final comment is also important:
> 
> > how would his 15 degrees figure be altered if the car 
> > is lowered, with consequent changes to the wishbones' initial
> > angles?  
> 
> Precisely.  Lowering a car by inserting shorter springs means that the
> car retains the same camber curves dictated by the suspension arms --
> but you start *at a different point* on the camber curve.
> 
> Let's take the example of our car that provides increased negative
> camber for the first few degrees of body roll and then starts to provide
> increased positive camber.  With stock springs, this car will feel
> nimble and responsive as you enter a corner, and then will become more
> stable as roll progresses; finally, if you really overcook it, the car
> will simply understeer off course nose first.
> 
> Now you lower it, oh, let's say 1.5" is enough to do the trick.  You
> enter a corner and you're immediately in the understeer phase of the
> curve, so you dial in more steering and more steering and eventually you
> either scrub the outside of the front tires all the way off or you bitch
> about the spring manufacturer making junk. 
> 

[more deleted]
Sorry to include so much of the original text, but it was important.
I have a real world example of this now.  Recently, I tired of the
plough understeer that my poor old Alfetta exhibited, and sent it
off for a "money no object" spring replacement, with the express instruction
that the car have standard ride height, given all the things I have read
on the digest about the careful design of the front suspension of
the Alfetta.  Note, it didn't start out as money no object, just
ended up that way :-)

Anyway, the car had been lowered about (oh) two inches by the DPO, and
was itching to remove it's exhaust on every speed bump, so I had two
reasons to get it sorted out.

Arnoldo and Pino, in Hornsby, Sydney did the work and a fabulous
job it was.  They recommended the 24mm bars from a GTV6 (new,
very expensive at $400AUD each), some local coil springs that
were stiffer than the originals (very cheap, $190.00 the set of
two, not sure about the brand).  They also replaced all the front
suspension bushes and the ball joints.  They told me to keep
the yellow Koni shocks (but readjusted them) as they were still
very good. (bought second hand just after I recommissioned the car).

The result?  The onset of understeer is now light years away from
where it was.  I think this is mostly due to the ride height
being correct, rather than the general stiffening of the springs.
It's still no MX-5, but it's a bunch more stable at high speed
that that fidgety scamp of a car.  The Alfetta is now getting
used much more frequently, sometimes in preference to the Mazda
just for thrills alone, which says something I guess.

I'm not sure whether you can gain anything my lowering the cars for
the track, but for spirited road driving, I'd have to say that simply
lowering the car without making other changes will be largely
a backwards step.

dave.

------------------------------