Small Bike (part II)

Small Bike (part I) covered most of the front end and the dropouts.
Next I worked on the front and rear forks, which were very similar. Both are essentially 2 dimensional shapes. So I cut them on a waterjet, and then used a boring head on a mill the clean up the tube clamps. For the rear fork I milled a 2″ slot to fit the 2″ main body tubing. The slot is milled slightly at an angle so that the rear fork is not perpendicular to the body tube. The angle was set by bolting an angled block to the fork cutout using the 4 screw holes shown below. The angle on that block was eye-balled for whatever looked good, and cut on the mill. I don’t think this slot actually had to be cut, but I think overall it looks better.

partially assembled rear fork clamp

Here it is on the 2″ body tube. You can see that it is angled backwards just slightly. The screw with the nut is just because I ran out of screws that were the right length.

rear fork clamped onto the body tube



Next I worked on the seat post. I CADed a truss-like structure and cut it out on a waterjet. There is lots of room for creativity here, but it would have worked just as well without cutouts. If you do this without cutouts, you can probably get away with thinner material, too (I used 1/4″ Al). The tubing is stock. The clamps for the seat tube that are bolted to the truss were bored out on a mill and were cut so that with the tube clamped inside, the inside spacing from truss to truss is 2″. This structure has to clamp on that same 2″ tubing. Then I made a seatpost out of aluminum. I did it mostly because the inside diameter of the seat tube was not standard for bikes, but the homemade seatpost also helps keep with the general theme.

disassembled pieces of the seat post

here is the seat post stuff assembled

For the bottom bracket I needed a way to get bottom bracket threads, and I didn’t want to have to buy a set of taps for it. Luckily I had an old mountain bike frame, that had been pretty beat up. But the bottom bracket was in good shape. I cut it out of the frame and turned it down in the lathe. You can still see the holes on it where it was once welded to the other tubes in the frame. Again, I used the waterjet to make the rough cutouts, and used a boring head to clean up the clamp for the bottom bracket. The cranks, sprocket, and pedals are all just regular bike parts.

bottom bracket assembly

another view of bottom bracket

Here’s starting to put things together:

For fun I made my own handle bars. I drilled 0.75″ holes at a slight angle into a 1″ tube. Again, just a bunch of eyeballing here, no tolerances really to hold to. It helps a lot to drill one side, insert the 0.75″ tube, and then eyeball the other side. Next time I will play with welding, too.

homemade handle bars


coming together

For the drive side rear dropout tube I had to go back and mill out a chunk for chain clearance.

chain clearance

Having the bottom bracket in a clamp allows me to make side to side adjustments, so I can get the chain line really straight. The fact that the rear wheel and bottom bracket modules are clamp-on provides a chain tensioning function. The position of the seat can also be adjusted for comfort. Although these wheels are definitely not built for an adult sized person to be riding on them, so its not good to have the seat all the way in the back because it puts a lot of stress on the rear wheel.

After a year of pretty consistent riding (rushing to class everyday, around campus, and to the grocery store), the wheels have definitely worn out the most. They tend to bulge out at some places and make the outside diameter of the wheel off center from the axle (this may also have to do with the fact that I sort of over inflate them, they are only rated for 40psi).

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