e-bike speed limiting - how?

[crassius]

haven't worked on one in many years, and that one was limited by lack of power

see a lot of ads now saying 'top speed 20mph' and am wondering how the speed is limited to that number (in motor rpm, or in controller, or ??)

is it limited only in factory bikes or also in DIY kits?

 

[Lungcookie]

Voltage can limit top speed.
The rpm of the motor can limit top speed.
Tire size could too.
You can use a Cycle Analyst(or similar) to limit speed/amps/watts and throttle ramping settings.(so you don't do wheelies)

The 20mph/750w is due to control by your local governments, thank your nanny.