attention to the welding (click the <Sage advice> button
for a pic of my welding duck), nor to the Scotch tape shock brackets.
This mockup was slapped together from castoff parts because, though
I was far from my cars, a string on <http://autos.groups.yahoo.com/group/Locost_North_America/>
(<Another front suspension> I think it was) inspired me
to share the idea even though it's early in the development program.
As the wheel comes
up, it pushes against the shock from both ends--the lower end
at its connection to the pushrod, the upper end via the "rocker"
On the production
model we'll probably make the brackets out of metal instead of
tape, and put nuts on the bolts, and things like that--hey, it
was late at night and I had a plane to catch the next morning.
But to get back to the subject, you can see above that for every
inch the pushrod moves, the shock compresses two inches. Also
note that the center pivot is slightly outboard (closer to the
bottom of the picture) than the pushrod and shock pivots, which
provides a rising springrate as the pushrod moves higher. By moving
these three pivot points relative to each other (done by replacing
the bellcrank) one can vary the suspension travel limits, the
spring rate, and the rate of springrate rise.
So why bother? Long
control arms either need to have their shock attachment bracket
moved toward the chassis (which makes the control arm either weak
or heavy--or both) or they need the shock to lay down at a shallow
angle, and a direct shock attachment at a shallow angle (the above
is at 30 degrees) puts a high load on the shock and reduces shock
travel relative to wheel travel. Since the DDS reduces shock load
in half and increases shock travel by two (in this example, though
different bellcranks can comfortably place those ratios anywhere
from 1.5/1 to 3/1) the shock and spring can be lighter...I'd guess
the unsprung weight is similar for both layouts, so that's not
much of a factor.
The benefit is, it
allows the shock to work harder for a given amount of wheel travel.
At 30 degrees, the DDS shock is loaded roughly like a direct attachment
shock at 60 degrees.
If I were making
just one car I'd probably do like Heikki is doing (message #39376
on the Locost_North_America forum, and the first post of the <Another
front suspension> string) and move the upper shock attachments
outward from the chassis. The standard shock mounting position
looks slick, but it's not well suited to the long control arms
of wide track Locosts--too shallow a shock angle and the lower
shock mount ends up in the middle third of the lower control arm
(measuring ball to ball), which is a bad place for a beam loading.
The pix above don't
show the design of the bellcrank very well, so here's a tracing
of the laser cutter toolpath for the bellcrank shown:
If you'll do a bit
of mental origami you can see how it's boxed in. The main part
gets two 90 degree bends along the straight horizontal lines,
a 1" tube is welded into the big holes (joining the top and
bottom) and the small strip wasbent to fit and welded across the
open side of the bellcrank. The two tangs remaning on the inboard
side of the bellcrank were bent to the shape of the ends and welded
into place, but that's too many parts and three difficult bends
and in general too much work, so here's what the current bellcrank
toolpath looks like:
Again, the part is
bent 90 degrees at the straight lines (there are four of them
now) and welded into a box. Only one part, no curved or complex
bends, and only a small weight penalty. If you print the drawings
you can cut them out, fold them, and play with them. If you want
to make your models full scale, the holes on the left are 1/2",
the holes on the right are 10mm, and the center holes are 1",
and the 1/2" and 10mm holes are 7" apart center-to-center.
The center pivot is made by reaming the 1" tube to 7/8"
and inserting two 3/4" (7/8" OD) bronze shoulder bushings
into the tube, and a ground, hardened and polished 3/4" steel
tube inside the bushings..
So there it is. This
is a completely* untested system, all we've done so far is jump
up and down on the front end of a test chassis and say "Cool!"
to ourselves as we watch the bellcranks rotate and the shocks
compress. Interesting so far, though, and we'll tell you more
as the project progresses.
concept is not untested, just this application of it. West
Racing Cars (formerly Stohr Racing Cars) has been using a similar
setup for years, with spectacular results--see their cars at http://www.westracecars.com
where you can catch a glimpse of their front suspension by following
their menus through Components > Parts > Suspension Components
> Control Arms