Mid-Drive Electric Bicycle

The finished bike features an underslung motor with a three stage reduction gearbox to the back wheel. A primary belt drive reduces noise from the high RPM while the last two stages use chains for higher torque capacity. In fact, the last stage is the bicycle's original derailleur drivetrain, effectively mimicking a sequential 7 speed gearbox. A 48v 20Ah battery provides a range of 30-60 miles depending on ride style.

Compared to a conventional hub motor, this configuration offers multiple benefits. Direct drive hub motors of comparible wattage offer low torque performance with a significant weight penalty of roughly 7kg. This high KV motor on the other hand, weighs only 3kg and offers nearly 4 times the torque due to the 22:1 drivetrain. The downsides however, are the added complexity and noise.

The image above shows two perpendicular sketch pictures being used to trace a three-dimensional model of the bicycle frame. Shown below, is the finished frame geometry with sketch pictures hidden for a clearer view.

The above video shows the initial point cloud results from the photogrammetry. Unfortunately, the final mesh was subpar and thus, unused. This was likely due to the thin bicycle frame having poor contrast against the gravel background.

Using the previous frame model as a guide, a mounting solution for the motor was developed. It clamps onto the down tube and consists of two 3D-printed halves. Added to the top of this stack was a jackshaft housing, also 3D-printed, accommodating two standard ball bearings and a 12mm steel shaft.

An exploded view shows more clearly the different components. In order of power transmission, there is an 18-tooth motor pulley, a 40-tooth jackshaft pulley, through to another 18-tooth pulley and finally, a large 120-tooth pulley affixed to the pedal chainrings. The press-fit ball bearings can also be clearly seen in this view, along with a c-shaped tensioner affixed to the rear of the motor.

A friend of mine helped by lending use of his angle grinder; we cut the chainrings off an old steel crank arm.

Next, we MIG welded a BMX freewheel onto the crank arm.

A set of chainrings was salvaged from another old crank arm with use of the angle grinder.

A large hacksaw and bastard file set was used to increase the inner diameter of the chainrings.

This allowed the chainrings to fit perfectly over the lip of the BMX freewheel, ready for SMAW welding.

Shown below is the completed crankset, with welds visible.

The mechanism can be disassembled and lubed by unscrewing a lock ring, which releases the ball bearings.

By only placing welds on one side of the crankset, the outward aesthetics are unchanged. The finished product can be seen below.

The pulley was printed using a large 1mm diameter nozzle to reduce print times. This has the added benefit of increasing part strength and layer adhesion. Printing with a large diameter nozzle was possible due to the lack of small features on the part.