DIY eBike Conversion

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This is a recap of a project to convert my ageing mountain bike into a Pedal Electric Assist bike or "eBike". There are plenty of places that will sell you a whole new eBike, but conversion of an existing bike seems to be a less commonly explored option. However since my bike rode nicely and suited my needs well, it seemed like an interesting option. To keep the process relatively simple I started with a conversion kit purchased from Woosh Bikes.

What is an eBike?

An eBike is bike fitted with an electric motor, a battery and, a control system. They typical work by detecting when you peddle, and then provide some additional "assistance" to reduce the rider's effort. The level of assistance usually being selectable from a number of levels. Some systems may also include a throttle facility to allow the system to power the bike when not peddling.

Road legal eBikes on the UK have motors with power limit of 250W, and must have a maximum assist speed of 15 mph (you can ride faster than that, but after 15mph is reached you are on your own, the motor will not assist further). If you exceed these limits then your bike will then be classed as a motor bike, and will be subject to the same requirements; i.e. will need to be taxed and insured, and the rider appropriately licensed.

Types of Motor

There are a number of ways that a motor can be added to a bike, but notably hub motors, and drive train motors. Hub motors, as their name suggests, are built into the hub of a special wheel. You can get appropriate wheels with motors to replace either the front or back wheel. Drive train motors tend to be mounted on the main frame of the bike, and nearer to the pedals. These provide their power input into the shaft driven by the pedals, so that the motive power is taken to the rear wheel via the sprocket and chain drive system along with the manual input from the rider.

For this project we are going with a geared hub motor mounted in a new rear wheel. This will leave the existing 21 speed drive train largely untouched. Geared motors have the advantage of being able to provide more torque, and the expense of slightly more noise when running.

The conversion process

Getting started

To do a conversion like this you will need a few specialist bike tools for taking apart the pedals and crank set, removing the bottom bracket bearing, and for dismantling the rear derailleur cassette. A proper bike stand is also highly recommended unless you like working in awkward positions and have several pairs of hands!

Stage 1 - fitting the crank rotation sensor

To be able to sense the rotation of the pedals, you need a crank rotation sensor. Most of these consist of a disc that one fits to the crank shaft, and a sensor module that is either mounted to the bottom bracket itself, or fixed to the seat tube on the frame. The disc typically contains a ring of small magnets, and the sensor a couple of hall effect detectors to sense the movement of the magnets as they pass the sensor. Sensor sets are made for fitting to either left or right hand sides of the crank, and there are also some that fit to the inside of the chain wheel rather than onto the crank shaft itself. A few are available that fit to a spoke on the wheel (although these are far less desirable since you will then need full wheel rotations before the sensor can "see" the movement).

I opted for a sensor designed to mount behind the flange on the bottom bracket, and a magnet disk that mounts to the crank shaft behind the chain wheel.

To fit this, you need to:

  • Remove the crank and chain wheels on the right hand side
  • Remove the bottom bracket
  • Fit the sensor over the bracket and refit to the frame
  • Fit the magnet disc on the crank shaft
  • Refit the crank and chain wheels.

Now here is where I met the first few interesting challenges. My bike dates from (probably) the 90's (and was second hand when I bought it in the early 2000s), and uses a square drive tapered crank bottom bearing. On this setup the crank set, is retained onto the shaft by a single screw that draws the crank onto the square drive shaft. Since the crankshaft is tapered, there is a strong interference fit between the crank and the shaft. Removing the retaining screw is easy, however this does not actually free the crank from the shaft. To do this you will need a special crank puller tool.