DDS (Double-Dip Suspension )

DISCLAIMER--Pay no 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" bellcrank.


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.

Jack 1/12/07

*The 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