Thursday, July 12, 2012

Build a simple short wheel base recumbent bike - Part 2

The angle of the seat is completely up to you. Anything from 25 degrees to 35 degrees (taken from the horizontal ground) will work.  If you lean your seat back too much, you will have to hold up your head, and if you set your seat to far forward, your knees will have to bend a lot while pedaling. Try a 30 degree angle and make only tack welds on the seat support tubing until you can sit on the frame and see if you like the angle. On a project like this, it is easy to make adjustments, and since your frame parts will differ from the ones I used, you will need to experiment.

To support the seat, the 12 inch long seat tube is welded to the top of the cut-off seat tube and then braces with any small tubing such as a seat of seat stays cut from another old frame.  When you are testing your seat angle, you can get away without the bracing as long as you don't put any hard pressure against the back of the seat. For riding though, you will definitely need the seat tube bracing.

The seat will be held to the frame by a set of tabs that have holes drilled for woodscrews. Any 1 inch wide 1/8 inch or similar flatbar can be used to make the seat mounting tabs, and they are made 6 inches long so that there is about 3 inches of material on each side of the frame tubing to mount the seat. Weld the seat mounting tabs to the frame so that the woodscrews will fasten each part of the seat in the approximate center of each board.

 The seat mounting tabs are shown here welded to the frame so that each seat board will be held to the frame by a set of wood screws.  If you are not yet sure of your final seat angle, then leave out the seat back support tubing until you have had a chance to sit on the frame and see if your seat angle is comfortable. You could also make an adjustable seat back by hinging the seat back support tube and installing some kind of clamp that will allow the seat back support tubing to be moved along the frame and then locked into position.

The seat is made of 3/4 inch thick plywood and foam as shown here.  Feel free to make your seat any width and height you like, but for a reference, I like to make my seat base  10 inches wide at the back, 8 inches wide at the front, and 10 inches in length. I usually make the seat back 10 inches wide at the back, 8 inches wide at the top, and 12 inches in length. To further enhance the seat, another top section can be added to support your upper back, but for now, this basic seat will get you started. For seat padding, I use a firm 1.5 inch thick foam that has been cut to fit onto the plywood seat boards.  You will need to install your seat padding before moving on with the frame design.

Fasten the two seat boards together where they meet using a pair of bent L-shaped shelf brackets so that your seat becomes one single assembly. You can then glue the foam to the plywood using some spray adhesive. The shelf brackets will be strong enough to hold your seat boards together yet still allow some give if you want to force the seat into another angle later as you make modifications to your bike. Once your seat is made, fasten it to the frame by using the four wood screws through the seat mounting tabs installed on the frame.

The main difference between this bike and a regular upright bicycle is the fact that the cranks are placed ahead of the front wheel rather than between the wheels. This recumbent position lets you push your full energy into the cranks and offers both a comfort and aerodynamic advantage over a regular bicycle seating position.

To get your cranks in front of the frame, you will need a set of tubes to create a triangulated boom and a bottom bracket to weld to the end of these boom tubes. Unfortunately, you can't easily remove the unused bottom bracket from your frame and transplant it here since it is holding all of your frame tubes together, so you will have to salvage one from another frame.

If you have to choose between a bottom bracket for a single piece cranks set and a 3-piece crankset, then choose the threaded bottom bracket for a 3-piece crankset as these will offer the most selection when choosing a cranks set (see our crank and bottom bracket tutorials on the main page for more information).

Clean up your bottom bracket shell with a grinder and then make a fishmouth cut on one of your boom tubes to conform to the bottom bracket as shown here. Before cutting the boom tube length, read ahead to see how the length of the boom tubing will be determined.

Part 3 in tomorrow's blog.