Analyzing A Deceptively Simple Centering Spring for the Morph

This post will go over the centering spring design we'll be using on the Morph. This design has been used on the New England Handcycle for over 30 years with great success, and it was carried over to the Travel Bike (shown below) that was built for me by One-Off Titanium (Mike Augspurger). The Travel bike and the Morph share many design elements, including the 16" front wheel, the coupler to allow breaking the main frame into

Here is the centering spring as installed on my Travel Bike. Note the silver coupler in the main tube. Because the bike comes apart here, there two carabeners below allow the centering spring to be dismantled when the bike is taken apart.

Closeup of the centering spring. I can't say it's pretty, but it works perfectly, as the explanations below will outline. This same mechanism will be incorporated on the Morph, but on the front end, rather than under the main tube. And, we'll make it look much nicer.

This series of three photos shows how one chain becomes slack and the other stays tight when the rider starts turning the wheel.

This is a bottom view (well, the bike is flipped vertically) showing the size of the wings.

When the wheel is turned, the wings pull on one chain, thereby stretching the elastic.

As the wheel is turned, the moment starts decreasing, while the stretch on the elastic, (I call it the "gap"), increases. The lines in red on the left side show the relative sizes of the moment and the gap.

As you start to turn more sharply, moment starts to fall quickly, even though the elastic is still being stretched more. This creates exactly the characteristic you want: Good centering near center, and up to about 45 degrees, and then a reduction in centering as you turn even more. Because the handcycle can make extremely tight turns, even more than 90 degrees, its important the the centering spring not fight you when you're doing these tight maneuvers.

At 100 degrees, we see that that gap is big, but the moment has dropped to almost zero. Just perfect. You can steer as extremely as you want, but lets say you need to grab the rear wheels and push like a wheelchair -- the centering spring will make sure the wheel stays straight.

Below are the five slides in a picture viewer, so you can click through them without the explanations above.

Here is the Powerpoint in case you'd like to play with it.

The Morph will implement the same mechanism, but the wings will be fixed to the main tube, and the elastic will be on the front end.