[From Graham Butler April 4, 2011, with photo captions by Bill]
I have reworked the reverse pedal brake. I have found a spring that will offer enough torque to offer 1lb of correcting force at the end of the lever. This should be enough to get it back over center. I have reconfigured it to be a pull action not a push action. Added a mounting hole at the front for a light or other accessory. I think that this could be a really neat feature. This will be a nice place to include some machined aluminum light fittings or some chrome. I also through it might be nice to have a logo emblazoned on the front of the brake release.
I am also going to settle on 95 degrees for high rider, and 110 for low rider seating position.
Backpedal braking mechanism. Inside the red cam is a roller clutch. The cam does nothing when pedaling forward. But when you backpedal, the roller clutch engages, and then the cam rotates backwards. This image shows that the brake is "on". The cam follower (in black) is being displaced by the red cam, and is putting tension on the brake cable.
As the cam rotates counter-clockwise during backpedalling, the follower will approach the peak, and braking will increase. When the black follower goes over the "top" of the cam, it will fall to the rest position (the cradle part of the cam), and will remain there until you backpedal again. (Remember, when the crankshaft is rotating forward, the roller clutch does nothing. But when you reverse the direction of the shaft, the oblong needles in the roller cluctch jam instantly and grab the shaft with gusto)
While we've had a cam-based brake on the New England Handcycle for 30 years, we've never cracked the issue of what happens when you want to make the cycle go backwards. For example, rolling back out of an elevator. We did add a little release for cable tension, but this was very dangerous, as it was easy to forget to turn it back on, only to find that your main brake is gone when you need it most.
So, for this project the goal was a "failsafe" brake release. It was harder to figure out than you would think. At first, we were trying to force the brake back on after a release by having the pedals reactivate the brake. But one day, while walking around at Dogpatch Labs in Cambridge, I saw lots of Solidworks models on the screens of one company, and I got to talking. I asked them to review the brake release problems and one of the engineers said quickly "you need a deadman mechanism." If you hold it, the brake is released. If you let go, then the brakes will work.
Voila. We created a big handle (in black) that serves as a grab-bar for steering when you go backwards. And by pulling backwards on this handle, you rotate a silver cam, which moves out of the way of the brake cable, reducing brake tension, and allowing the bike to back up. (The pedals go backwards when you back up, and without the release, the brake would come on immediately.
Design Items to Think About (From Bill)
1. The red cam could be in any position when you attempt a release. The way that the release is "unreleased" when you let go is from the coil spring. That spring has to generate enough force to allow the silver "slacker cam " to re-energize the brake, essentially by going from "off" or slack cable, to full on, or fully taught cable.
2. The black cam follower moves in the machined slot, and its pulling the brake cable on the other side. The brake cable force acts in one direction on once side of the follower, and the cam force acts in the opposite direction on the other side. This might torque the cam follower and make it jam in the slot.
3. The working brake is now moving over two friction points: the redirecting "pully" the top, and the "slacker cam" below it.
4. Nice clean mechanism, looks like it will be simple and reliable.