Speed limiter

A speed limiter is a governor used to limit the top speed of a vehicle. For some classes of vehicle and in some jurisdictions they are a statutory requirement, for some other vehicles the manufacturer provides a non-statutory system which may be fixed or programmable by the driver.

The legal definition of a moped in the United Kingdom was revised in 1977 to include a maximum design speed of 30 mph (48 km/h). This was further revised to 50 km/h (31 mph) in the 1990s, then 45 km/h (28 mph) in the late 2000s to fall in line with unified European Union licensing regulations.

To comply with this, mopeds typically include some method of onboard speed restriction to prevent the machine exceeding the prescribed speed (on a flat road, in still air, with a rider of standard height and weight). Older models such as the Honda C50 used a simple centrifugal governor as part of the transmission, which progressively and severely advanced the ignition as speed rose past a set point, causing engine power to fall off rapidly at higher rpm and road speed, but maintaining the low- and moderate-speed hill climbing ability of the unrestricted version. Other systems achieved a similar result with simple restrictor flaps in the air intake, much like those used to restrict the power output of full-size motorcycles. Modern mopeds use electronic systems with speed sensors that can cut the ignition spark (and, where fitted, interrupt fuel injection) once measured speed reaches or exceeds the set point, maintaining full power right up to the limited speed.

Early restriction methods could be defeated by simple physical modifications (e.g. cutting out the restriction plate). Modern electronic limiters at the very least require replacing the friction rollers in a scooter's CVT transmission, or even changing wheel size and/or reprogramming the engine management system, all in an effort to fool the sensors into detecting a lower than actual road speed.

Public service vehicles often have a legislated top speed. Scheduled coach services in the United kingdom (and also bus services) are limited to either 65 mph or 100 km/h (62 mph) depending on their age (newer coaches have the lower speed version installed, in line with harmonised EU regulations), though for city buses the use of limiters is to satisfy regulatory requirements, as many city buses cannot achieve these speeds even on an open roadway.

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