The Gas Strut Challenge - Single Large Gas Shock

The hard part about this new design is the gas strut.

Last time we did this, we had an inkling of the strut that we were likely to use as we were designing the frame and morphing geometry, which made it relatively easy to fit.

The challenge now is a little more complicated:

 1. We are traveling through a greater range of morphing motion, so
    the range of motion of the strut (Stroke length) has to be much
    greater unless we are to overload the frame.

 2. The geometry now lends itself nicely to a single strut.  This,
    however means that we will need twice the force out of a single
    piston.  It also means that you only need to purchase one per
    handcycle.

So the result is that we need a substantially longer shock that is almost twice as stiff.

Figuring this out was something of an iterative process:

I know that we will potentially need to lift at least 250lbs in the seat of the handcycle, and that the geometry of the strut with respect to the linkage arms in low rider mode would affect the mechanical advantage and thus the strength of the spring that was needed.   The spring could only fit a few ways, so I found the strongest spring available off the shelf, and then designed the geometry to provide the appropriate force.

This spring has 562lbs of force.  The barrel is 1.1 inches in diameter, and a stroke of 11.81 inches.  It is also a reducible force gas shock so it can be tuned down to lower forces.

Based on this I calculated where the pivots needed to be to generate 250lbs of lift.  It turns out that this point is almost half way along the upper linkage.  Then to fit the shock in I had to create an extension linkage to create some extra room for the longer shock.

This sounds very simple but fitting it into the geometry was really tricky.  Check out the attached screen shots.  Pay attention to clearance issues that limit the positioning of the shock.

Single large shock design.

Low rider. Note large shock retainer to give room for long single shock.

Mid rider 1

Mid rider 2

Almost all the way up.

Morphed up to high rider mode.

Closeup of single shock and lower link arm in high rider

Single shock rear oblique view.

Single shock side view.

Closeup of ball joint morphing joint.

Morphing Handcycle Design Meeting in Cambridge - Morph 2.5; A Cool Wooden Model; Bungee Lift System

Rory, Alan and Bill met to discuss next steps for Move With Freedom and the Morphing Handcycle and the Morphing Wheelchair. Alan brought along an important breakthrough: A low cost 1/8th scale model of the latest handcycle design. This working (it morphs!) model was made using a CNC router, cutting out thin plywood. Alan's friend did it for $100! Wow, that is amazing, and so useful. This model shows a new version of the design that includes automatic adjustment of the seat bottom and the seat back.

I'm holding up the single sheet of plywood that supplied all the parts for the model. It's really amazing that a fully working, accurate scale model can be made so quickly, easily, and cheaply.

This is what we're calling "Morph 2.5" Morph 2 weighed almost 60 pounds, while Morph 3 was about 40. But Morph 2 worked better, and had better steering. At the recent MassTLC Innovation 2000 unConference, Rory wanted to come with an improved vehicle. I a period of just a week, George Reynolds and Rory combined Morph 2 and Morph 3. They took the aluminum front end from Morph 3, and fixed the footrest design, and put that on Morph 2. Then George removed the vestigial angle adjust mechanism that didn't work out on Morph 2. That also removed one morphing joint. Voila. We cut some 13 pounds from the weight! (Background has been hastily Photoshopped to remove clutter.)

Here's Morph II as stood during our marathon measuring session last summer. Notice that the struts are gone, one morphing joint is gone, the steel footrests are gone, the steel front end is gone.

The main tube is crumpling. Oops. Too much force, all piling up in one spot. We'll have to fix that. Rory was noticing that the pedals seemed to be getting closer to him as he rode.

Closeup. The tube had too much force on its surface. Can't do that!

The steel "test rig" set up with a bungee lift system. No gas springs. Also note the seat has the self-adjusting mechanism. so when you morph, the seat automatically corrects for added tilt. It works so well you don't know its there.

Note the big black shock cord bundle. It's helping me morph up.

Now I'm morphed down. Note the path of the cord. It goes between the two steel members. The bungee lift system works better with a clear center span, and with flanking support members. This frame design can also accommodate a single gas spring.