I’ve been thinking on and off about building some sort of hybrid pedal / electric powered vehicle for years. I saw a nice trike with bicycle wheels and a yellow bullet-shaped shell the other day while I was driving through Central London, which set me pondering again. I’m just setting my ideas down, as much for my own benefit as anybody else’s. The diagram above is my attempt at a steering rack and front suspension design. I’d like to pick up some decent speed (the law limits these things to 15mph though..) and ride over potholes without shaking the thing to bits, so front and back suspension is essential. I imagine that would be the hardest bit of the build, especially since bicycle wheels are designed to be secured at both ends of the axle – the axle is thin in proportion to the wheel diameter and is liable to bend if not held at both ends. This may be one reason why most I’ve seen whilst googling have quite small front wheels, to reduce the force multiplier acting on those axles.
THE LAW (UNITED KINGDOM)
The attraction of building an electric bike, tandem or trike is that if it meets certain requirements, it doesn’t need to be registered, taxed or insured. I wouldn’t mind taxing and insuring it, but registering a self-built vehicle means going through ‘type approval’, in this case the Individual Vehicle Approval test, which would cost £200 for a small vehicle like this. See the Gov.uk page. IVA is cheaper than I expected though, and could be worth considering if I ever do this project.
Here’s my full design brief:
- Three wheels, for stability
- Of those, two should be at the front. This has better stability on corners than putting two wheels at the back, and since I’m putting a pedal drive on this too, it’s simpler just to keep the back wheel assembly more or less like that on a bike, rather than building a chain drive or differential to the rear axle.
- Steering – after a bit more Googling I’ve seen a vastly simpler steering system than the first one I drew, where the front wheels are pivoted via handles at the top of the pivot bearings and aren’t linked by a steering rack. I suspect the handling of the designs I’ve seen could be improved by angling each wheel so the top is closer to the centre line than the bottom. This should make the wheels seek the ‘forwards’ position at all times, which should make up for the fact there’s no steering rack. The advantage of that simple steering mechanism is that it’s possible to support the axles at both ends, using standard bike front wheel frames.
- Enclosed in a lightweight shell
- This’ll reduce drag and keep me dry. I want to use this thing every day, and I know myself well enough to know I’ll jump in my car instead if the other choice is getting soaked in the rain on my trike.
- Seat should be low to the ground
- Reduces drag compared to a standard cycling position
- Low centre of gravity makes for good stability
- Pedal drive train
- The pedals will be at the very front of the frame, and the rear driven wheel will be right at the back. That means a long chain needs to pass to the joint of the rear suspension, then a shorter one goes from the rear suspension joint to the rear wheel hub. I’ve seen several designs where the longer chain passes through a pair of pipes underneath the seat. I assume some idlers on good bearings will be necessary at each end of the pipe to guide it into the pipe and reduce drag and wear.
- Gears mounted on rear wheel like normal bike
- Electric drive
- I intend to make this as simple as possible.. I was planning to use an electric hub motor like this 36V 800W one (roughly 1hp).. The main problem with that initial plan was that electric trikes can’t have more than 250W of power in the UK unless they go through the IVA test. The practical problem with using a hub motor means.. no gears! Also, that motor should in theory be more than enough to get me to the maximum legal speed (15mph), but what about hills? I live in North London, so there are some serious hills to deal with if I want this trike to be useful day – to – day. So I’ll probably attempt to use a frame – mounted motor passing through the same gears as my pedal power. This means transferring the electric power to the cog on the pivot of the rear wheel frame, probably via a toothed rubber drive belt – so I’d mount the motor behind the seat. In addition to the freewheel on the rear wheel (as in a normal bike) there should be a freewheel between the pedal drive train and the electric drive train. That means I can pedal without having to turn the motor or drive belt.
- Batteries – I want to use Li-ion of course. Best power to weight ratio. The biggest battery pack I’ve found (in one quick Google) is 36V 12Ah – that makes 36 x 12 = 432Wh, that is, it will power an 800W motor at full throttle for just over an hour. That’s probably OK for London journeys, which, when stuck in car traffic, often take about 1.5 hours.. driving this trike, I assume I’ll be able to avoid some traffic and cut the journey time, and I can be provide some pedal power to save the battery.
- I’d like to drive this in traffic safely, but it’ll be so low to the ground I risk being even less visible than cyclists. I’ll paint the shell a bright colour and add a flag on a tall pole, like some recumbent bikes I’ve seen.
- Lights and indicators – there’s no legal requirement for an electric bike to have indicators or car – style front, rear and brake lights, but I’d like all that anyway. I’m used to having indicators because I drive a car, and at least I want to signal clearly to compensate for being less visible than when I drive my Volvo Estate.
- Mirrors – I want to see other vehicles! I’ll add wing mirrors, probably mounted to the steering handles.
- Disc brakes on all three wheels. I could probably get away with just braking the rear wheel but it’ll handle a lot better with braking on the steering wheels too.
- If I’m going use this every day, I need to be able to carry my laptop bag and a bit of shopping without it getting wet.
- Frame construction
- I don’t know much about joining steel tube, but the simplest way seems to be fillet brazing. That means that I wouldn’t have to make super – accurate mitre joints on all the tubing. However it looks like it’s worth buying or making a super – accurate clamp to hold two pieces of tubing in place whilst you’re building up the fillet.
Simplified steering design:
Pedal drive train: