A sidecar motorcycle is a three wheel vehicle with the sidewheel not directly aligned with the rear motorcycle wheel, and is usually powered by the rear wheel only. This is different from motorcycle trike, where both rear wheels are powered and share a common axle.
The sidecar consists of a frame (which is rigidly fixed to the motorcycle's frame on one side and is supported by the sidecar's wheel on the other side) and of a body. The body typically provides one passenger seat and a small trunk compartment behind. In some case the sidecar has a removable soft top. In some modifications the sidecar's body is used for carrying cargo only (like a truck's platform).
Before the 1950s, sidecars were quite popular, providing a cheap alternative to passenger cars; they have also been used by the army and police. During WWII, German troops used many BMW and Zundapp sidecar motorcycles. On German and Soviet military sidecars, the side wheel was sometimes also driven using a differential gear), to improve the vehicle's all-terrain ability.
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