Friday, 6 July 2018

Reducing Speed of Ebike

Reducing Speed of Ebike

There are many reasons why you might want to reduce speed of an ebike. 15mph is the max legal speed in the UK, so there is one reason. May be you would like to lower speed to match pedal gearing on a single speed bike. . . there is another. 

What I Did:

I attached a 1.2ohm resister to the signal wire of the throttle. There are three wires in a normal ebike throttle cable. They are nromally red, black and green. You need to solder a resistor inline with the green wire.

Addition of this resistor cut the top speed from ~25mph to ~10mph. Measured with GPS on flat road.

I added another 1.2ohm in parrallel to incrase speed, target being 15 mph. However a resistance of 0.6ohm ( 2 x 1.2ohm in parallel) put the speed right back up to 20mph+. 

Guess Work and Assumptions:

So from this we can assume that increasing resistatnce in the signal wire reduces speed. What resistance works on each bike is another question. Perhaps some bikes will require a higher resistivity. 

The perfect resistor for my bike would be between 0.6ohms and 1.2ohms.  But just 0.6ohms differnce can cause a 100% change in top speed, so it is a fine tuning process for sure. I have produced the below graph, which would indciate a resistance of 0.9 ohms would be about right for me.

The grapgh shows ohms required to give a percentage of original top speed. I know nothing about electronics. I would apprecaite pointers in comments if anyone knows something cool. 

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