--- Nolan Penney <npenney@mde.state.md.us> wrote:
> Ah, now I understand.
> Not sure what one would gain going this route.
>
> The overall geometry would be essentially unchanged.
> Both setups
> result in a very short lower suspension arm. The
> upper arm stays the
> same, the leaf spring. The stock GT6 has a very
> short lower wishbone as
> the lower suspension arm. Using a double u-jointed
> driveshaft as the
> lower suspension arm would result in about the same
> length lower
> wishbone. It would be mounted a little further
> inboard (at the
> differential), but end shorter as well, at the
> rotoflex joint, instead
> of further outboard under the hub. So you would
> still get the positive
> camber as the suspension moves either up or down.
>From a geometry standpoint, this could work. Using the
halfshaft as one of the suspension linkages is not a
new idea, IRS Corvettes used it, maybe still do.
But most of these utilize the halfshaft as the upper
link, not the lower one, and for a very practical
reason.
If you analyze the forces involved, you'll see that
the lower link takes most of the side forces from the
tire contact patch. The closer the outer lower pivot
is to the ground, the lower the leverage forces will
be on both the upper and lower links.
In the case you have proposed, the leverage would
cause perhaps double the contact patch loads into the
halfshaft, whereas in the Jag and Corvette, the loads
are around 1/5 the contact patch loading.
For example, a 2000 lb Spitfire with 50/50 weight
distribution would carry 500 lb static on each tire.
Cornering at 1.0G, that would be 500 lb of cornering
force at the contact patch. Using a Jag/Corvette
setup, with a 24" dia tire, and the lower pivot 2"
above the ground, you would expect +600 lb into the
lower link, and -100 lb at the halfshaft.
-500 +600 -100
| | |
| | |
V V V
| 2" | 10" |
---+----+------+
ground link halfshaft
Using the setup you described, with a 24" dia tire,
and the spring eye (upper link pivot point) at 20",
you would see +1250 lb at the halfshaft, and -750 lb
at the spring eye.
-500 +1250 -750
| | |
| | |
V V V
| 12" | 8" |
---+---------+-----+
ground halfshaft eye
As far as camber change goes, it's gets a bit more
complex, but with all else equal, reducing the
vertical distance between the upper and lower pivots
on the upright generally makes for more camber change,
not less.
If you look at the rear suspension on formula cars,
they often try to place the pivots on the uprights as
far apart as possible. This helps to both reduce the
loading into any given link, and also to control
camber, toe, and castor changes as the suspension
deflects. As a side benefit, the effects of any play
that develops in the Heim (Rose) joints is minimized.
Having said all that, you could keep the lower
wishbone on a GT6+ as the lower link, and utilize a
solid (without sliding joints, etc.) halfshaft as the
upper link. The spring attachement to the upper end of
the upright would have to change, perhaps with a
shackle arrangment such as RWD cars using leaf springs
have. Or you could try to fit adjustable coilover
shock/springs, though space is very tight with the
GT6+ uprights. Maybe a push/pull rod suspension setup,
with the shock/springs mounted horizontally in the
space where the leaf spring used to go.
Not sure what advantage you would gain, as any play in
the U-joints would cause play in the suspension itself
as well as slop in the driveline. Unsprung weight
might go down a bit.
But it would be very cool.
Can you tell, I've also been wondering 'what if'
myself for a while?
Carter
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