A motorcycle engine is an engine that powers a motorcycle. Motorcycle engines are typically two-stroke or four-stroke internal combustion engines, but other engine types (such as Wankels and electric motors) have been used in small numbers. Although some mopeds had friction drive to the front tyre, a motorcycle engine invariably drives the rear wheel, power being sent to the driven wheel by belt, chain or shaft . Most engines have a gearbox with up to six ratios. Reverse gear is occasionally found on heavy tourers and sidecar motorcycles. Gear changes are normally effected using a foot-pedal and manual clutch, but early models had hand-levers. Some bikes have automatic or semi-automatic gearboxes.