Re: 105 suspension speculations

[Simon Favre <simon@xxxxxxxxxx>]

Ben Dixon wrote:

>I've been talking to a few people recently, and begun to wonder about
>putting a spacer between the chassis and the lower 'A' arm pivot plate
>(if you think of the 'A' arm as a triangle, hinged at the base, then
>when I say 'pivot plate', I mean the base of the triangle)

This does NOT sound like a good idea for 2 reasons.  As mentioned on
previous digests, racing Alfas were often fitted with "drop spindles" or
"knuckle risers" (Oh no!  Not THEM again!!).  These goodies were well
researched mods for racing whose purpose was to get the geometry into a
region where the camber change under compression would be more suitable
for HARD cornering.  The drop spindle moved the lower ball joint down
WRT the wheel center.  This lowered the car without changing the angle
of the lower arm.  If the ride height was brought back up, you could
make the camber get aggressively negative under cornering because the
arm would be pointed slightly downward at the end.  What you are
suggesting would tend to point the lower arm UPWARD at the end, because
you lowered the inboard pivot.  This is exactly the opposite effect of
the drop spindle.  The knuckle riser did the same thing at the opposite
end. By moving the upper ball joint upwards, the upper arm would be
tilted upward at the end for a more rapid transition to negative camber
under hard cornering.  Pointing the end of the lower arm upwards makes
the wheel go towards positive camber under cornering, OR whatever the
upper arm does to make negative camber will be somewhat negated by the
motion of the lower arm.

The second reason this is probably a bad idea is from a stress point of
view.  Moving the lower A-arm pivot down with spacers will put more
stress on the securing bolt, and the surrounding sheet metal. This is
one area you do NOT want to tear out.

>a friend mentioned that they were going to try this to get some -ve
>camber (being much cheaper than adjustable upper arms)

If cheap is all you want, just lower the car. ;=)

>my first thought was that this could possibly do very nasty things to
>the suspension geometry - you are moving the lower pivot point down and
>out.  Ive been trying to work out what effect it would have on the
>camber vs travel plot.  I'm going to try and build a model and work it
>out.  Maybe I'll try and model it on my computer....

There are programs you can buy for this. You need to know the EXACT 
dimensions of the suspension components, the relative positions of the
suspension pickup (mounting) points, etc.  These may not be so easy to
come by.  Every approximation you make to the measurements will degrade
the model.  Since this is a dynamic problem, I would tend to trust a
specialized suspension geometry program rather than a generic 2D or 3D
CAD program.

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