February 16, 2007
Starting work on the drive leg
Today I started work on the drive leg for Within. The crank and chain ring is my SRM small single ring unit. The SRM bottom bracket cartridge fits into this super slick item I purchased from Nova Cycle:
It's an Eccentric Bottom Bracket. The internally expanding part has both positive expansion and positive contraction. You slip it into a shell and rotate the unit in the shell to take up any chain slack. Then you tighten the expansion bolt with a alan key which secures the device into the shell. This type of bottom bracket can be used with a single speed bike where there are no chain guides to take up slack in the chain. It works VERY WELL. I am impressed. No need for a chain guide roller. An objective with the drive leg is to make it with as few moving parts as possible. Simple, robust, water tight and easy to service is the goal.
I tack welded a 1" square stainless tube to the bb shell so that it was flush to the end of the shell. I have 1/2" stainless round tubes that I can weld onto this square tube that will house the chain on both sides.
AT the other end of the square stainless tube, I welded on two shorter 3/4" square stainless tubes and bolted the right angle gear box to it. The chain runs straight from the SRM chain ring down to my 11 tooth cog mounted on the gear box axle.
Next, I will weld the two half inch round stainless chain tubes where the chain runs and build a stainless housing for the small cog. This will keep water out of the chain and the cog and will bolt to the angle gearbox. It should be water tight all the way up to the bottom of the chain ring.
In the drawing I did, I don't show the 1" square tube running all the way down to the gear box. I thought that I could support the gear box with the small 1/2" round chain tubes if I added plenty of cross bracing between them. It would make for a very thin hydrodynamic fin, but I'm not sure it would be strong enough. I can decide later because I need the full length of this square tube to hold everything square and straight until I can weld on the chain guide tubes. They have to be in exactly the correct position, or the chain will rub as it slides through the small tubes.