Wednesday, September 06, 2006

Recumbent Chain Gear

I found an easy solution to a small problem in the design of my recumbent. The top side (tension side) of the chain was too close to the botom of the seat when the slack was taken out of it. Some recumbents use a rubber skate wheel here, but that seems to me to be a poor choice, due to the amount of tension on the chain causing rapid wear in the wheel. Others use a small idler pulley from a rear derailleur, but that seems to me like it would produce a chattering feeling under strenuous pedaling due to it's small diameter (and resulting small number of teeth in contact with the chain).

So, my solution was to remove the rear gear from a small child's bike (that I bought at a garage sale - geeeze, give me a break). It was about 3" in diameter, with a nice 1.25" hole in the center. By chance I had a roller bearing with a 1.25" outdside diameter, which I had removed from a fitness trainer (that I turned into a bike maintenance stand, but that's another blog story for another day). The gear fit perfectly over the bearing, so I tack welded it on. Digging through a tackle box of old hardware that I keep under my workbench, I found a shouldered bolt that fits the inside diameter of the bearing perfectly, and is just the right length to allow me to add a chain keeper under the head of the bolt.

The gear is large enough to handle the chain smoothly, even when very tight, and hard enough to not wear out quickly. Now all I have to do is weld a nut onto a standoff where I want the pulley to be located, and the chain will be pulled down to clear the seat bottom. I think that location will be just aft of the steering tube along the top tube (see the picture below). Any further forward, and I would run the risk of sucking brake and shifter cables into the gear when turning sharply.

My only concern is that the outer race might have overheated during mig welding. I removed the grease seals first, kept the welds quick, allowed some cooldown time between welds, and oiled it well before reinstalling the seals. It feels ok now, but time will tell. If it fails, I might need to heist another kids bike - I have a spare bearing.




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