E-Mountain Bike

Friday morning, Tryphena, Great Barrier Island, the sun is shining, good music on the stereo, I’m having a look at the Twenty Fourteen WordPress theme thinking: will I bother opening up a can of worms on a ‘Muslim Friday’?, it’s going to be a good and interesting day, I’m going to take it easy, it’s going to rain, that’s the plan for today.


I was up all night yesterday, like most Thursdays, reading stuff on the internet. I like to read up on electric mountain bikes (e-bikes), batteries, electric motors as I have been planning to build an e-bike for a while. It doesn’t matter how you look at it, e-bikes are just the way to go. Especially when you live off the grid and fuel prices are shocking. (Shocking like mobile internet data prices.) I already have a dual suspension mountain bike with proper hydraulic breaks, so what I am interested in are conversion kits.

  • Power

Not sure how much I weigh, but I feel like 75ish kg and according to Wikipedia an elite athlete is able to produce 6 W/kg for an hour, then again, those doped tour de France cyclists are able to attain an instantaneous maximum output up 1700 W(atts). Just found a much better link about cyclist power-outputs.

Putting it into another perspective, a horsepower is the equivalent of 745.7 W. Therefore, a 1000 W electric motor produces 1.34 horsepower. Clearly 1 kW is plenty more than I can produce. The electric hub motor drives the wheel directly and losses attributed to motor efficiency are presumably comparable to those associated with pedalling. So 1 kW is plenty, plenty of power, and you can buy such a motor (48 V) for about 300 USD.

As I said, especially in off grid living situations e-bikes make sense. Using renewable energy to charge the battery means free energy and zero-emission, the engines are also very quiet compared to combustion engines.

  • Battery

This is going to be the expensive part. A 48 V LiFePO4 with a charge capacity of 30 Ah costs about 1000 USD and two 20 Ah ones 1300 USD. Expensive, but range should be a priority. Time for a naive example:

Going full tilt, e.g. 1000 W(atts) at 48 V(olt) using 20 A(mpere) for 1.5 h(ours) equals circa 30 Ah and 1.5 kWh. I’ve got a good feeling that I can reliably ride with two 20 Ah packs from Tryphena to Port FitzRoy and back draining about 50-80% of the battery capacity. Hmm, it’s about 100 km return and it is either uphill or downhill, but regenerative breaking systems are affordable, that’s for taking it easy, you’re on a push-bike after all, and not going at max. speed. Okay, maybe you need another 20 Ah pack, just thinking of riding up Okiwi hill….

If you are paying for electricity, you should know how much 1 kWh costs. 1 kW of photo voltaic panels cost about 1000 USD. Granted, there are more costs associated, there is a charge controller for the solar panels, an inverter or similar to recharge the batteries and a couple more things for the e-bike. However, living off grid means that you have or will install renewable systems anyway to power the house.

  • Front- vs. Rear-Wheel Installation

Having the motor in the front wheel means easier conversion, think also gears, dérailleur, break system and being able to assist actively, which gives 2WD, and that could be useful. Rear-wheel drive will be better going uphill, especially when slippery. A big argument for rear-wheel drive is, however, safety. Imagine going full tilt, enjoying the thrill, giving it heaps, just coming out of the corner and accelerating full tilt and the fork snaps. Especially aluminium forks, the torque that a 1 kW engine produces can make it snap.

  • What else

I read a bit about torque arms and they come in ranges, but that is just something that needs to be taken into account. Perhaps it is worthwhile to mention that there are a few things to consider, for instance, if a battery is overcharged drastically, it starts burning, and all components must be appropriately installed and matched to each other.

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