Build a kid's long-wheelbase low racer recumbent bicycle

Step 3 Extend Top Tube and Chain Stay

Measurements are tough on this one, but you will need to add conduit to stretch the bike out. Base the measurement on the length of the rider sitting down with legs stretched out. The new tubes should allow for the measured length from the seat back (with padding, etc.) to the pedals when they are as far as they get from the rider.

I was able to jam the new top tube piece into the old top tube and weld. If I had more time, I would have smoothed this transition a little more with body filler. Weld the top tube about halfway down the seat tube.

The new chain stay piece will attach centered up where the old one did on the bottom bracket. you will have to make a little bracket for it to attach to both the bottom of the seat tube and the chain stays. If you attach the new tube to the bottom of the seat tube, it will be too high to weld to the old chain stays, so I cut the tube so that I can weld a piece of 1/8" stock to both the new and old chain stays. That bracket is shown below.

I do my best to keep everything straight and have had pretty good luck with eyeballing it. These long bike frames are long enough that you can see if something is off. If you aren't confident in your eyeballing abilities, clamp the frame to something you know to be straight when you weld to form a temporary jig.