Mini chopper
Mini choppers are scaled-down versions of custom-built motorcycles known as choppers and are generally constructed from 1" steel tubing or 3/4" steel black pipe. The tube or pipe is bent and then welded together to get the desired angles and shapes of the frame, which is usually custom made.
Engines and transmission
Mini choppers use a variety of engines although the most typical would be a Briggs & Stratton or Tecumseh horizontal shaft industrial engine with a non-tapered shaft. Other engines include small motorcycle engines and power sport engines such as ATV and dirt bike engines. If an industrial engine is used,some sort of transmission also needs to be used to apply power to the rear wheel. Centrifugal clutches, which allow the engine to idle without moving the chopper, are also used however they result in a fixed gear ratio for both low and higher speed operation. However, some people opt to make a jackshaft, which allows the builder to change the gear ratio to his liking or to change which side the axle sprocket is on.
There is also a transmission called a torque converter, or CVT (Continuously Variable Transmission). The torque converter is essentially an automatic transmission that changes gear ratios according to the speed of the engine. When the engine is at low RPMs, the gear ratio is at 1:x, depending on the ratio the torque converter has (This allows for more lower end torque). As the speed rises, the gear ratio changes to 1:1. As the higher end RPMs are reached, the gear ratio reaches X:1, which allows for a higher top speed.
Wheels
Mini choppers often use bicycle or moped rims and tires on front and large pneumatic tires in the rear. Commonly golf cart wheels or 205/50-10 style tires are used in the back. The rear also needs something to let the wheel spin, and that's where the axle comes in. There are two different types of axle setups used; a live and a dead axle.
Axles
A live axle is probably the most common axle setup used on mini choppers. The "live" term indicates that the axle spins along with the wheel, sprocket, and other pieces anchored to the axle. Since the axle spins, bearings are mounted to the swing arm frame. This is a very simple axle setup, and components are relatively easy to change. Each piece, such as wheel hub, disc brake rotor, sprocket, etc., are slid onto the axle and are held in place by set screws or locking collars. The components can also be separated by bushings that are cut to the desired length. Since the axle spins, the wheel, brake, and sprocket components must be locked to the axle. This is accomplished by using a square key that fits into a groove on the axle and the components.
A dead axle (or hub and bearings) is much less common than the live axle setup used on most mini choppers. As opposed to a "live" axle, where the axle spins along with the wheel, this axle does not spin. The wheel or hub has built-in bearings to allow it to spin. Depending on the setup, all spinning components are attached to the hub or to the rim itself. The bearings and hub spin on the axle. This is the type of setup that is used on most motorcycles. Often, the axle is inserted into slotted dropouts and the chain is adjusted by moving the wheel forward or backward within the slots. This allows the engine to be mounted which makes the bike's drive train much more rigid and reliable. There are also other advantages to this type of axle, including the possibility of a cleaner and more attractive rear axle area, but this setup is often more expensive than a live axle and custom parts are much more difficult to locate.
Custom parts
Custom parts are what make choppers art, whether it's coming up with a new frame design or thinking out of the box and using a car rim for the rear. Many builders build their own gas tanks that are similar to full-size motorcycle gas tanks, but scaled down to fit the bikes. Sheet metal parts such as laser cut gussets are also used as accent pieces on the bike. The desire is to produce an individualized piece of mechanical art.