This was an experiment to test the limits of electric bicycles – it’s an R&D project to one day enter the World Solar Challenge. I put together the most powerful electric bicycle hub, controller and battery components I could find. Selecting the bicycle took a long time. I needed something solid with a decent area within the frame for a battery but most importantly, a stable riding frame that could take the power of the hub motor. The KONA Unit was my choice because it has a plate that bolts to the frame that holds the axle (see here), a feature that would later become necessary. The Kona Unit is a Cromoly single speed 29er (see here for specs). Regular bicycles won’t be able to take the torque of high power and will strip the dropout. This happened to me after riding for 5 minutes with the 7mm aluminum factory plate – even with a regular torque bar installed. There’s no way a regular bicycle could take a few thousand Watts of power without being well reinforced. I made a 7mm stainless steel plate (pic here) for both sides that totally encloses the axle. This has held to date and will not fail. The next point of weakness is the frame itself around where the plate is bolted. The frame is built well so provided this isn’t under very harsh stress then it will also hold.
The motors can take more or less anything that you throw at them – or so I thought. My first effort was to get a rear motor with a 7 speed gear cluster. I powered this with a 72V battery and a 50A controller (theoretically 3600W of power). This went well until the rear dropouts gave way and the motor spun in the axle slot and severed the cables. Next motor, I opted for single speed as there was a higher rated option available and gears just became redundant anyway. With the same 50A controller and 72V battery the bike was extremely powerful and fast; quickly reaching a top speed of approximately 80km/h. It was only just possible to control the bike at this speed and definitely gets the adrenalin pumping but it was way too over-powered and you needed to be very gentle with the throttle. Eventually, when testing what would give first I took it onto the beach and up some sand dunes – really pushing all components hard. Finally, the wires between the motor and controller heated up so much the plastic coating melted and then something gave way within the motor itself – likely a Hall Sensor.
After blowing up another motor, a battery and a couple of controllers I believe the best solution for maximum sustained power with this sort of bicycle lies with a 72V 45A controller and 72V battery. The 50A controllers are a bit bigger and seem to give it extra grunt but the rest of the system doesn’t handle it securely – though this will improve soon. The 72V Li ion battery I’ve been testing it with successfully has a 15Ah capacity. This has allowed me to travel around 25km. With the slightly down-graded 45A controller I expect to get a bit over 30km. I couldn’t suggest using a lower capacity battery as the current drain will be too high and result in a short life or cut outs. I created a battery case from a wooden crate lying around work. Other mould options could be plastic, fibre glass and steel or aluminium. I like the wood though, just need a better carpenter. It should be possible to pack about 20% more battery in there to increase range.
The controller can sit well either below the frame for the larger 50A version or above the box & within the frame for the smaller 45A one. I tried controllers with sensors and ones without. Ones without the sensor are a bit more jumpy upon startup but more importantly they judder at high speed. For the extra potential problems with the sensors I think the performance outweighs their cons. I couldn’t test power outputs accurately because the system blew up my Cycle Analyst computer. Estimates are 3600W with the 50A controller and 3240W with the 45A controller (72V).
Future additions will be hydraulic brakes, suspension forks and a well crafted wooden box to hold the battery. Until you move to a country where laws allow you to ride something like this on the road then I suggest you find a good off road location to ride and get something properly designed from Stealth.
Matt – Solar Bike
