Tricycle project notes

The tricycle project is in at least temporary abeyance, partly because of a period of madness, partly because all my spare money is going to people in Gaza so I can't afford to do it. But that doesn't stop me thinking about it; and it doesn't stop me gathering ideas. This note is just to record two ideas — one mine, one stolen — which I think will be useful if and when I take up the project again.

The disk brake motor

The generator of my wind turbine is an axial flux design; it's remarkably thin and flat. This is the core of the idea. Of course, the disk brake on a bicycle is even thinner; but because the spokes of a bicycle wheel form a cone, there's a certain amount of space (not very much) which it could expand into. An electric generator is essentially an electric motor run backwards; and, indeed, 'regenerative braking' with an electric motor is simply using that motor as a generator.

It's possible to use printed circuit boards to form the coils of an axial flux motor; there must be a practical upper limit to the power that can be achieved with this, and I don't know what that is. But the legal upper limit for an 'ebike' motor is 250 watts, which may be doable. A printed circuit board stator would help make the whole motor extremely thin.

Conceptually similar wheel mounted axial flux motors are being used in the current generation of Italian supercars, and generating 200Kw power in a package 100mm thick; so I don't think it should be beyond the wit of man to produce 250 watts in a package say 15mm thick.

I don't know whether it's necessary for the stator to have magnets on both sides of the rotor, but I believe that it is efficient to do so, and my turbine certainly does; one would intuit that having magnets on only one side would, at minimum, require a substantially stiffer structure to avoid warping.

So, the disk brake of a bicycle comprises a fixed part (the caliper) and a rotating part (the disk). There are standard mounts for both these parts. A motor has a rotating part (the rotor) and a fixed part (the stator). So if we can make a motor which can fit into the swept volume between a bicycle wheel and the fork which supports it, and whose stator can be bolted to the standard disk brake caliper mount, then that motor can be fitted to any cycle designed for disk brakes.

On the tricycle, one such motor could be fitted to each of the rear wheels.

Of course, regenerative braking has some limitations. It will not bring a vehicle to a complete halt; and when the battery is completely full, unless you have a mechanism to dump waste energy as heat, the brake won't work. But every legal bicycle must have two braking mechanisms, so if it has regenerative braking on one wheel and a conventional brake on another, that requirement is satisfied.

Also, regenerative braking may inevitably result in drag. Certainly motors with permanent magnets typically do drag, quite considerably, when not powered; I don't know whether electromagnet-based motors inevitably have such drag.

However, so far as I know, no one is yet making such a motor, and I think it is probably beyond the scope of this project to develop one.

The 3D printed armature

Matt Brown is an automotive engineer who got established (and comfortably rich?) working for Tesla. He now seems to spend most of his time working as a hobbyist on racing car and speed record car projects, and having a YouTube channel. He's a member of the 'Sendy Club', which is an amateur team which builds record breaking hill climb cars, and, as part of his work for that team, made a 3D printed armature for the bodywork of their latest car.

The armature was printed in quite small sections in an ordinary hobbyist 3D printer using a 'PAHT-CF' nylon/carbon fibre filament, although the 'HT' (high temperature) is only needed because the bodywork encloses very hot engine parts, and has to not deform when hot! The right adhesive, Matt says, to stick the nylon panels together is 3M ScotchWeld 8910, which is jolly expensive stuff — but he also says that they didn't use this; and given that once the armature is covered in carbon fibre it will be the fibre that holds it all together, using the 'right adhesive' probably isn't all that critical,

I don't know how light this sort of structure is, in terms of grams per square meter. It's probably not as light as nomex. But, it would reduce the amount of mould work very considerably, and would be particularly good for those parts of the structure — for example the shoulder pivots — which would otherwise require additional components.

I'm not completely sold on this idea, but it is worth considering.

Off the shelf aerofoil sections

In previous notes I've written about making aerofoil sections for the rear linkage by forming carbon fibre over hot-wire cut foam armatures. It isn't necessary to do this. Commercial off the shelf aerofoil section carbon tubes are available in suitable sizes. They're not cheap, but they should be consistent and would solve a problem. I would still need to make custom knuckles for them, though, and that was always going to be the hard part.