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Top speed

Top speed

According to UK and EU law, completed eBikes must be limited to 25kph on roads.

In line with this, we offer our 250W kits configurable to a 25kph speed limit on request. However it is the eBike builder’s discretion to choose whether they want a legal on-road bike or an off-road bike.

Many people are interested in having a bit more than 25kph speed limit on their bike and choose a kit or controller from us that is not restricted. They are then surprised when they can only go up to 32 kph or so…

A de-restricted controller doesn’t mean you have NO limit on how fast you can go. There is an inherent limit introduced by the maximum motor rpm.

For a given battery voltage, there is an inherent limit to how fast the motor can spin. This is because of a physical effect called “Back EMF” (this will be the subject of another blog).

As the motor spins faster, eventually the “Back EMF” voltage will reach the battery voltage, and at this point you cannot go any faster.

There is a relationship between this “Back EMF” and the motor speed. It is called the motor constant.

The torque (acceleration) you get out of the motor is also linked to the motor constant.

You don’t need to worry about the details of the physics behind this. The important thing is that increasing the motor constant (by adding more copper windings) will increase the torque, but decrease the top speed.

Increasing the top speed of a motor means decreasing the torque (acceleration)

So – there is a trade-off between top speed, and torque.

We find a happy compromise, our motors are wound for a top speed of 32kph at full battery. Meaning as the battery discharges and the voltage drops it will reduce down to 25kph.

If you want to limit it to 25kph all the time then you need a suitably configured LED display, or to set this limit manually with the LCD display

Final point – there is one way to “over volt” the motor and increase top speed. For instance use a 48V battery and controller on a 36V motor. This is usually a bad idea though (subject for another blog)

Hopefully that makes sense! Let us know if not 🙂