Step 5: Fuss with Drivetrain
I built a crank mast that allows for sliding the assembly fore and aft for leg length issues. I prefer to keep the seat near the rear tire, for good braking and traction reasons. The bike car is great on those points, by the way. The tube section added on top makes place for a derailleur as well as a bottle cage.
Although large-loop chain tensioners exist that release and take up a great deal of slack, I'm using a simple short tensioner back near the jackshaft. This takes up slack when I go from large to small front chainring. I welded up a mount point onto the large jackshaft bearing mount. Seems to work well, although it won't take up enough slack to allow me to alter the crank mast position without also changing the physical length of the chain. I don't mind doing that, and I rarely need to alter the cockpit for anyone else. It seems quieter and less draggy than the larger units. and I'm running less chain with saves weight. (But with this bike car, unlike my racing bikes, it has not been an obsessive goal to be nuts about weight.)
I'm using a fixed cog on the 5/8" keyed jackshaft to drive the chain from the rear 8 speed internal hub. Then a freewheel cog from the jackshaft to the chainwheels. I can play with the sizes of these to fine tune the gear output.