The goal is to position the cranks over the front wheel in such a way so they there is no pedal or crank arm interference with the front wheel as it turns. If the crank arm were to hit the front wheel, the steering would lock up, resulting in a loss of control. So, basically, you want to position the bottom bracket as far ahead and as low as possible, while at the same time ensuring that the cranks do not interfere with the front wheel and so that they are placed the optimal distance from the seat for your leg length.
To find the best bottom bracket position, sit on your seat (with shoes on), and extend your leg while a helper holds the crank arm with pedal installed. Your helper can then take a measurement from the head tube to the bottom bracket or crank center so you know the correct distance from the head tube to install the bottom bracket. The lower front boom tube can then be cut and tack welded to the head tube as shown in the photo so that you can test the crank position for both your leg length and clearance over the front tire. You may need to make fine adjustments to meet both requirements, so take your time and get it right.
Once you have the lower front boom tube solidly tack welded in the correct position, you can measure the distance from the bottom bracket joint to the top of the head tube and cut the upper boom tube that will form a triangle between the bottom bracket and head tube. This tube is also fishmouthed to fit the bottom bracket joint and the head tube to create a good joint for welding. The resulting triangulated front boom will be extremely strong, able to withstand any amount of pedaling forces.
With the front boom installed, you can now install the cranks and rear derailleur in order to make up the long chain needed. You will need a chain about twice as long as a normal bicycle chain, but this is no problem, since you can simply join together two chains to make a new one.
For information on chain joining and sizing, see the tutorials section on our main web site. When sizing the new chain, set it on the largest front chain ring and on the middle rear chain ring and then make it long enough to pull the derailleur into the position shown in the photo, with the top idler wheel just over top of the lower idler wheel. You will also notice that the chain is in the way of the front wheel, but this will be fixed with the installation of the idler pulley.
An idler pulley designed for a 1/2 inch v-belt can be used to reroute the lower side of the chain over the front wheel as shown in the photo. There is never any tension on the lower (return side) of the chain, so it can be routed behind the top (drive side) of the chain, out of the way of the front wheel. Place the idler pulley on the lower front boom tube so that the lower chain will clear the front wheel and not rub on the top side of the chain. Any steel or nylon 2.5 inch to 5 inch diameter idler pulley that includes a ball bearing will work. You can purchase a suitable idler pulley at most yard and garden supply stores.
Once you have found the optimal place for your return chain guide pulley, weld the bolt to the boom tube as shown in the photo. Also shown is the completed welding on all front boom tubing joints.
Complete any leftover welding, and then clean up the welded areas. Your short wheelbase frame is now completed, and only needs a steering system to be installed for test riding.
Part 4 in tomorrow’s blog