Everyone who has ever hacked apart a bicycle has also likely felt the urge to turn it into a chopper. That tinkerer fantasy has become reality. My good friend Victor found a children’s ride along bike in a dumpster somewhere. I dropped it off at Ben’s. Mix with a steering tube I spent way too long hacksawing off a trashed bike in Davis, a spare wheel, an oxy-aceteline torch, scrap square steel tubing, and a sawzall and you have yourself a chopper.
Little thought went into the design until the basic fork was built. Then it became obvious something was wrong.
Bicycle steering is not as simple a matter as throwing some scrap metal together in whatever configuration looks coolest. Click here to read about the design of forks. The design of the fork and the steering tube angle have a great impact on the rideability of the bike. With the common nearly verticle steering tube, the slight bend in the fork that is seen on most bikes is enough to make the bicycle stable and responsive with some understeer.
With a nearly 45 degree angle in the chopper and a similar bend in the fork, the chopper suffered from an unforseen problem. Here is an illustration of the fork as it was originally. When the wheel is turned, the height of the bike drops drastically. The bike’s own weight keeps it from easily centering again! This oversteer makes riding the bike in a straight line difficult.
The solution was to offset the fork from the steering tube +2 inches. Here is an illustration of a fork with an offset. The fork was cut near the steering tube and 2 inch long pieces of steel were welded to move the fork forward. Now when the bike is turned, it rises slightly. When the rider lets go of the handlebars, the weight of the bike+rider forces the fork to center (although not much). This solved the oversteer problem. Now it has very slight understeer.
The large front wheel was replaced with a smaller one later on. However, both the small wheel and the large wheel were eventually used on the recumbent bicycle we built next. This, combined with the ridiculous (for an adult) riding position consigned the chopper bike to the rust heap.
May 2008 update
The chopper is back from the grave! I threw on a fresh coat of paint and some styling and turned the chopper into a pedal power generator using an old Zap DX friction drive conversion and the rear end of an old bike. The rear end of the bike keeps the wheel off the ground so the Zap DX motor can power an old Acura fuel pump. I stuck the outlet from the pump into a Walgreen’s garden gnome… and now he pees! Of course! The kids at the Maker Faire loved it.
One of the adults who seemed to enjoy making the gnome pee.
It would take an entire six pack to make me go like this gnome. Go gnome go.
This guy totally gave me two taloned thumbs up after seeing the gnomes. I guess I’ll have something to fall back on when the alien Overlords arrive.