The Biodegradable Tricycle

One of the design goals of my tricycle is as a prototype for sustainable transport. To be sustainable, it has to be fully recyclable, and the simplest way to make anything fully recyclable is to make it biodegradable: nature will do the recycling for you.

Metals, of course, aren't, in the strictest sense, biodegradable, although all except gold will eventually degrade in the environment, typically by oxidation, and the metals I'm likely to use oxidise into not especially toxic oxides; but in any case metals are valuable, and pretty easy to collect and recycle. I'm not guilting about using metals.

Epoxy doesn't strictly biodegrade, either. It is degraded by exposure to ultraviolet light (sunlight), and very gradually breaks down. As it does so, it exposes the materials encapsulated within it to environmental degradation. But I don't think carbon fibre does degrade, except mechanically, by abrasion; or else be burning. When abraded it breaks down into small particles which are very strong, sharp, and therefore, mechanically toxic. Making the tricycle primarily out of carbon composite, then, leaves a hard-to-recycle, potentially toxic, piece of waste at the end of its life.

Weight

The weight of a mostly-human-powered vehicle is very significant. In hilly country — and I live in hilly country — getting an even slightly heavier bicycle up a hill is much harder work, and as I age, there's an increased risk that that work will become beyond me. After my first experience of COVID, I could no longer cycle on an unassisted bike up the two miles from the village to home. If I had not had access to electrically assisted bikes, I'd have given up. Fortunately, having rebuilt my fitness with electrically assisted bikes, I can once again cycle an unassisted bike up that hill.

But I'm very much aware of the cost of weight. If the tricycle is too heavy, it won't be pleasant to ride; if it isn't pleasant to ride, I won't ride it.

A biodegradable tricycle is possible

As I wrote last month, biodegradable composite materials are available. It would be possible to build all the composite components out of them. It would be heavier and less strong — balsa, as a core material, is three times more dense than Nomex, and flax fibres are one fifth the strength of carbon — but would it be strong enough and light enough? I don't know. It might be.

Similarly, the four bar linkage, which in my current design is made of carbon fibre bars and 'forged carbon' knuckles, could be made with aircraft-quality spruce bars and aluminium knuckles. It would be a bit heavier. How much heavier? I don't know.

The perfect, and the enemy of the good

The quest for perfect purity, however, may prevent us achieving sub-optimal but good-enough solutions. As I wrote of my house, back in 2013

So yes, it isn't perfect. This is not the platonic eco-house. It would have been better had I used sheep-wool as insulation, and better still if I had come up with some more bio-degradable solution to glazing. But for all that, this house is light on the land; this house will leave little trace. And I'm proud of that.

When my house rots, as it will, it will leave behind glass, glass-fibre insulation, its steel bathtub, and now, its new steel roof. It isn't perfectly biodegradable. But it's a lot more biodegradable than most modern houses, and it provides me a home I can live in. It's good enough. It's sufficient, I think, if the tricycle is good enough.

Downsides of carbon composite

For convenience here I'm using 'carbon composite' to mean composites including carbon, aramid and polystyrene, as opposed to flax, balsa and possibly spruce. Both mixes would include epoxy, and as there don't seem to be significant technical or cost differences, that will be 'bio-epoxy' as opposed to conventional petro-chemical based epoxy.

What I'm trying to think about here is the downsides of carbon composites other than their environmental cost, which is the primary objection to using them.

1. Carbon fibre structures tend to shatter on severe impacts, resulting in sharp edged, irregular shards that can cause severe injury. Flax, being softer, won't do this.
2. Carbon fibre surfaces reflect a lot of sound and absorb very little; the inside of carbon fibre structures tend to be very noisy spaces. Flax absorbs sound much better.
3. Carbon fibre blunts tools, making completed structures difficult to drill or reshape. Flax won't do this.

None of this gets away from the fact that carbon is five times stronger than flax. But is it stronger than I need?

The actual weight difference

I've been trying to derive an actual weight difference, based on the bill of materials I've estimated to date. There are problems with this: it's hard to compare like with like. I can't get balsa core sheets in the thickness I can get Nomex sheets in; and while a sandwich material with a woven layer with strength in tension on each side, and a core material with strength in compression between, is what gives me strength, I don't know how much much strength I need in the woven layers.

The performance of the balsa compared with the Nomex is sufficiently similar that it isn't an anxiety here; but the performance of the flax will be much less good than the performance of the carbon. Fitting balsa to the compound curvature is likely to be more work than fitting Nomex, but it won't be impossible.

However, assuming I can get away with a single layer of 200 GSM flax on each side of the core, the comparison doesn't look so bad.

I had been planning for one carbon/kevlar layer in the carbon layup in order to protect against shards; with flax, the shard protection isn't necessary, so I can drop it, which saves a layer of cloth and also saves (although I haven't computed it) some thickness of epoxy. The increase in weight is in the balsa; not merely because balsa is inherently five times heavier than Nomex, but also because the minimum available sheet thickness is twice as much. But my estimate for weight of materials for a carbon/Nomex hull is about 9.75 Kg vs a flax/balsa hull at about 12.25 Kg. That's a 2.5 Kg difference, or a couple of litres of milk in glass bottles.

But would that be strong enough?

Finding out

I think the best way to evaluate this would be to make a mould for a small panel with approximately the same compound curvature as the tricycle hull, and lay up a carbon/Nomex test panel, and a flax/balsa test panel, on it; and then compare the strength of the two panels. This will cost, of course; the materials used in the test panels will be wasted, and can't be reused. The cost (I've just bought the materials) is £128. That's affordable, for a test. It will also allow me to test how smooth I can get the outer surface using a male mould, which will also be valuable knowledge.

If a flax/balsa tricycle will be nearly as good as a carbon/Nomex one, then of course I should build one!