The Fool on the Hill: Thoughts on bent cycles

I'm getting old.

I've decided to give up my car, mainly for environmental and political reasons, but also for economy, so these days I'm largely dependent on bicycles for transport. In an environment with a lot of wet and windy weather especially in winter, I find that bad weather is a deterrent to cycling. Also, as I age, my balance is getting noticeably worse — I regularly fall. Thus far, it doesn't seem to affect me on a moving bicycle, but I particularly often fall when getting on and off bicycles.

So I need a vehicle for my old age. Why then am I looking at machines designed for racing and for speed records?

Well, I'm mad.

But.

The geometry of efficient light vehicles

Bicycles are very efficient machines, but it's patently obvious that they're much less good machines than they easily could be. Aerodynamic drag scales with both the square of the speed relative to the air, and the the frontal area exposed to the air; an upright rider on a bicycle exposes a lot of not particularly aerodynamic frontal area to the air. If you can lie the rider down you can greatly decrease the frontal area, and if you can enclose the rider in a smooth aerodynamic envelope you can greatly reduce the coefficient of drag.

But there's a mechanical problem. If the rider is facing forward, but reclined, their feet, where they're doing the pedalling, are close to the front wheel. Getting the drive from the front of the machine to the back of the machine, when you have the rider in the way and you're trying to keep the frontal area and the mechanical drag as low as possible, is not easy.

The obvious solution is to go for front wheel drive, but unless you adopt rear steering — and dynamically stable rear-steered bicycles are exceptionally difficult to design — this means that you have a driven wheel which must also steer, which is in itself mechanically complex.

Graeme Obree — a man I much admire — built a bike in which he lay prone on his front, so that his feet were at the back, over the rear wheel. This gave him a short, relatively direct, rear drive train, and reasonably simple front steering. But he is also mad, and he's a great deal tougher as well as being a vastly better athlete than me; and even he could not make that bike successful. As far as I know, no one else has even tried this configuration. I think trying to achieve good cardio-vascular performance when lying on your front is not easy.

So what I'm going to concentrate on, in thinking about the design of a vehicle for my old age (and, strictly, more: a practical one person light vehicle) is three slightly different machines which all take a front wheel drive, front wheel steer approach. Each has a teardrop-shaped shell in which the rider sits or lies, almost prone, with the crankshaft in front of the front wheel, and a front wheel which is approximately 500mm in diameter over the tyre, which sits between the user's knees and which is driven (which means that all of the transmission gubbins is also between the user's legs).

Terminology

'Shell'? 'User'?

These machines challenge vocabulary. You sit on a conventional bicycle; it feels normal to say that you 'ride' it, and are thus a 'rider'. Further, conventional bicycles are skeletal machines of which the most prominent feature is the visible wheels; which is why the common name for them is 'bicycle' (or abbreviations thereof), and consequently of the rider is a (bi)cyclist.

But what we're talking about here are things you sit in; fairly tight capsules you sit in, but in nonetheless; and the wheels are generally enclosed, and not visible. So while 'fuselage' or 'nacelle' would make equal sense, in what follows I'm going to call the monocoque the 'hull', and the user the 'pilot', since these words seem to me to fit better.

The Inspirations

I'm going to start with two fairly similar two-wheelers — literally bicycles, although neither of them looking much like anything you'd recognise as a bicycle.

Beano

Beano is a remarkably small vehicle. Its builder, Miles Kingsbury, apparently couldn't fit comfortably in it, so he sold it to Stephen Slade (known as 'Slash'), who has won five world human powered vehicle championships in it. Kingsbury discusses the design (and a proposed successor) here, and calculates that to maintain 35 miles per hour Slade needs to expend only 169 watts. Beano is claimed to weigh 17 Kg, which is a lot for a bicycle, but really quite modest for a fully faired machine.

Woodstock

Snoopy, Seventy-seven, and Woodstock are a succession of vehicles designed, built and campaigned by Russell Bridge. Seventy-seven is an outlier, being another Battle Mountain special, but the core elements of the design are shared by all three and I will concentrate on Woodstock as being the current (and most practical) iteration. Woodstock is considerably bigger than Beano but still extremely compact; Bridge, who is a surprisingly chunky guy for such a good athlete, only barely fits. He manages to average 35 miles per hour in three hour races, expending a bit above 200 watts on average; but he's still very slightly slower than Stephen Slade in Beano.

Mosquito

Mosquito is a fascinating vehicle, and will I think be my primary inspiration. It's been developed by an eccentric French engineer, who calls himself just Nicholas. Rather than being a racing vehicle it is conceived as a fast tourer, for use on public roads. It is technically front steer, but by an interesting use of geometry. It is a tricycle with, like a traditional tricycle (and unlike most recumbent tricycles) one wheel at the front and two at the back. Only the front wheel is driven; but the rear wheels are mounted on a four-bar linkage system which creates a virtual pivot point — a virtual steering head — which is considerably above the vehicle and considerably in front of the rear axle. To steer, the rear of the hull is moved towards the rear wheel on the outer side of the curve, tilting the hull into the curve in the way a normal bicycle tilts, and thus turning the front wheel relative to the rear wheels. The consequence is that the hull, pilot and transmission turn with the front wheel, and there's no actual steering head to make things complicated.

Alone among the vehicles I'm seriously considering, Mosquito is not a monocoque, but a body-on-chassis design. While I understand and sympathise with the designer's reasons for this, I think if I were building a Mosquito-inspired vehicle, it would be a monocoque.

Being a tricycle, Mosquito obviously has more aerodynamic drag than either Beano or Woodstock, and its body is not as streamlined as theirs. So for any given speed, Mosquito will need a higher power input.

Transmission

What's interesting (to me) about Beano is the drive train. The chainring drives a primary chain on the right hand side of the bottom bracket, which drives a conventional 10 speed cassette mounted above the steering head, but, crucially, uses only the higher eight ratios of it, because the ninth sprocket is used to drive a secondary chain which runs down the left hand side of the front fork to a single sprocket attached by a ratchet to the front wheel. Braking is by drum brakes, which I think is a consequence of the age of the machine — bicycle disk brakes weren't widely available when it was built.

Beano's drive train and steering system is built on a small, apparently steel, subframe, which is bolted into the monocoque.

The benefit of this design is that the transmission can be relatively narrow; the downside is that it is tall, and must intrude into the pilot's vision to some degree. It's interesting to note that Seventy-seven, Bridge's Battle Mountain special, has a transmission much more like Beano's than like Woodstock's, but Seventy-seven has no windows at all and the pilot 'sees' via a video screen, so forward vision is not an issue.

Woodstock is simpler. It has a mostly conventional derailleur setup on the right hand side of the transmission, with only the addition of an idler sprocket at the steering head to prevent the pull of the chain affecting the steering. The transmission and steering system is built on a mostly-carbon fibre structure which appears to be bonded into the monocoque. Braking is by one cable-operated disk brake, on the front wheel; there is no rear brake.

Mosquito is even simpler, because, not having to allow the front wheel to steer relative to the transmission, it can use an entirely conventional bicycle drivetrain, and, in fact, any conventional bicycle transmission would work. This means, of course, that an epicyclic would be possible, which could reduce the overall width of the transmission, at some cost to mechanical efficiency. You couldn't combine an epicyclic with a hub motor, however, unless you put the epicyclic above the wheel, much like Beano.

Mosquito's transmission, like Beano's, is built on a small steel subframe, which is bolted onto the (wooden) chassis. The steering is (obviously) separate, which makes things simpler.

Structure

Beano's hull is moulded in composite with an outer layer of carbon/kevlar fabric. I haven't seen any discussion of the layup, but looking at the video it looks as though the shell is about 6mm thick, and I guess it's two layers of the carbon/kevlar twill sandwiching a filler layer which may be foam. This looks extremely good and is obviously extremely durable — the vehicle has been frequently crashed in over fourteen years of racing including at least seven world championships, and it still looks extremely good.

Woodstock is again a moulded composite, laid up (as I understand it) from the inside 200gsm carbon fibre twill, 100gsm glass fibre (I'm guessing also twill), 3mm nomex core, 200 gsm carbon. There's no aramid/kevlar anti-abrasion layer, which slightly surprises me as Woodstock, like Beano, is a racing two wheeler which can be expected to fall over and slide in crashes.

Mosquito has a very different structure. It has a wooden chassis supporting a bodywork of woven wooden laths covered in doped fabric, like an early twentieth century aeroplane. The reason is a concern — which I share — about the environmental cost of composites. However, given the relatively small amount of composites in a vehicle of this scale, I think for this application I would still choose composites.

All three of these vehicles have the hull made as a lower half and a separate, removable, upper half; the upper half being removable for access and for maintenance. None has any roll-over protection (although Bridge's Battle Mountain special, Seventy-seven, has a once-piece monocoque shell with a small top entry hatch, a front crash bulkhead, and two roll-over hoops).

Vision

Mosquito has no windows, but a hole through which the pilot's head emerges, again like an early aeroplane. Beano has essentially a wrap around window of simple curvature clear sheet, I'm guessing polycarbonate — you must need a fairly large sheet of polycarbonate to cut that out of! Woodstock's window is similar in concept to Beano's, and made of 0.75 mm Lexan. It's smaller than Beano's, however, extending only as far back on each side as the back of the pilot's seat. Videos from September 2024 show this window quite scratched following crash damage, and it also suffers badly from reflections from within the vehicle; but these may be inevitable on a vehicle of this sort.

Stability

I am thinking about a Mosquito-like tricycle, rather than a bicycle. I am, as I say, old; my balance is not good, and my strength is not what it once was. I live at the top of a big hill. There is already a problem for me maintaining speed on a bicycle up the hill, and in a fully faired two wheeler, once speed has dropped too low for stability, you've got problems. Both Beano and Woodstock have retractable landing wheels, but, fun though the idea of such a two wheeler undoubtedly is, I don't think this is enough to make them practical vehicles for me.

Collision safety

So my vehicle will have static stability. Nevertheless, I think a roll over hoop is important, and that a four point seat belt is at least worth considering. In a collision, I think it would be safer to remain in the hull than to be thrown out of it. The roll over hoop would also be useful to transfer and spread load from the upper knuckles of the four bar linkage to the rest of the hull.

Access

So I think that, rather than having a separate bottom shell and top shell, I would prefer to have a one-piece monocoque; and my thinking at this stage is that I would have a Woodstock-like wrap around windshield, but attached to the hull only on the right hand side of the vehicle, so that the left hand side can be released to flap open, and then, once the pilot is seated, closed and tensioned using an over-centre clip by the pilot's left shoulder. It would probably also be nice to be able to remove the windshield entirely in good weather.

This method of access definitely needs to be prototyped before building! I need to be able to keep on getting in and out of this vehicle for at least ten years, and my mobility could well deteriorate over that time. Woodstock has deliberately designed flats on the gunwales on both sides of the lower shell outboard of the seat, on which to place your hands when getting in and out. That solution would not work if the windscreen was permanently attached on the right.

Cargo

Cargo stowage would be behind the seat; there might be a (small) separate cargo hatch, or cargo might be accessed by folding or removing the seat. Given my lifestyle, it needs to be able handle a 15kg butane gas bottle as cargo, with a tare of 40kg; so the cargo bay needs to be both big enough and sturdy enough, with an opening wide enough, to accommodate this (or else I need to design the vehicle to pull a trailer, which is certainly possible).

Ergonomics

Controls

All three vehicles under consideration use handlebars located below the pilot's legs, with ends bending upward to near-vertical for the grips. The position of these grips, outside the pilot's thighs, obviously works ergonomically. Gear control using a time-trial style bar-end shifter, as used on both Beano and Woodstock, seems sensible.

When I rode a windcheetah I found the joystick steering entirely intuitive, but it has rarely been copied and the windcheetah didn't have a front wheel between the rider's legs. I suspect there would not be room for a joystick on this machine.

For a vehicle used on the public road, two separate braking systems are essential, and on a tricycle balance of braking between the two wheels on the same axle is also important. So I think hydraulic disk brakes on all wheels, with the two rear wheels being on the same hydraulic line; with a conventional flat-bar style lever on each handlebar, front brake to the right, because that's the convention I'm used to.

Rear vision

Rear vision is problematic. On both Beano and Mosquito, the rider can in theory turn their head and look behind, but I think that in the semi-reclined position, at least for me, that wouldn't really work.

Woodstock has wing mirrors positioned on either side inside the windshield, so I doubt the pilot can see anything directly behind; but they're good enough, I think, for most purposes. However, it would be nice to have some sort of electronic display, for data display, navigation, and so on, and to fit a rear-pointing camera in the tail of the vehicle and be able to show a rear view on that display would not be too difficult.

Hydration and nutrition

Woodstock has bidon holders on the back of the seat, but they're obviously not handy to reach while moving, and in video of racing I've seen Bridge using a camelbak-style hose. Certainly being able to stow a camelbak bladder probably behind the seat would be a good plan. There's also nowhere handy in any of these vehicles to stow snacks for eating while moving; some sort of pocket accessible from the seat would be useful.

Ventilation

You'd think ventilation wouldn't be much of an issue for Mosquito since the pilot's head is out in the air, but the designer has cut more and more holes in the bodywork of the prototype over the years for cooling. However, Mosquito is often used in southern France.

By contrast, Beano — which has been raced very hard over the years — has one large hole at the base of the windscreen, probably 120mm by 70mm, and air will come in through the front wheel opening, but apart from that it's pretty sealed; and Woodstock has one hose about 60mm diameter bringing air from the nose to just in front of the pilot's face, another taking air from the nose to a vent at the base of the windscreen, a really very small opening for the front wheel, and nothing else. Both Beano and Woodstock are raced — hard — mainly in England.

Because a Mosquito style vehicle does not need to have steering integrated with the front wheel, there's a lot less going on in the top of the transmission subframe assembly, which makes ducting air through it even more practical.

Woodstock's ventilation scheme seems to me worth copying; it could, in fact, be improved on, since on Woodstock the ventilation hose is attached to the top of the carbon fibre structure supporting the transmission, whereas it could very easily have been made integral to it. You also need apertures for air to flow out at the rear of the vehicle; Woodstock's windows are permanently slightly open at their trailing edge.

On my proposed design, the upper four bar linkage knuckles are inevitably going to cause turbulence, and a good position for an outlet ventilator might be just behind them.

Information display / instrumentation

On Snoopy, Woodstock's predecessor, the information display panel is positioned centrally on top of the transmission subframe; on Woodstock it can't be, because the air hose is there, and so it's displaced to the left where it seems often worryingly close to the pilot's knee. I think integrating the ventilation duct into the structure and putting the data display back on top of it would be an improvement. An alternative would be to position it on the right, where Woodstock's right hand wing mirror is; and I think this would be particularly suitable if a rear camera/simulated mirror were integrated into the display.

It seems sensible to use a small Android tablet as an information display, even if it means writing a custom app, since information from a number of sources will need to be integrated. The alternative is to integrate a Raspberry Pi (or similar) into the vehicle, handling all data logging, motor control, and other electronic functions; in which case it could also integrate picture from a rear facing camera.

Night time use

The vehicle obviously needs to be able to be used at night, so it needs to have an integrated headlight and tail light, and probably ought to have integrated turn signal lights since the pilot could not easily make hand signals! In addition, it should probably have large red retro-reflective patches on the rear and white on the front, since it is not a vehicle other road users will be used to seeing and it needs to be seen. An internal switchable 'map light' would also be useful.

Suspension

Suspension would be nice but easily gets complex and expensive. In a later section on front wheel servicing I suggest a pivoting front fork resting on elastomer stops, which could provide a small amount of front suspension at reasonable cost. I've also thought that torsion bars in the rear axle assembly could provide some degree of suspension for the rear wheels. However, these are nice-to-haves which might not appear in the initial build of the vehicle.

Noise

Sitting in a small closed composite shell is going to be noisy. I really don't see any way round this, except perhaps noise-cancelling headphones.

Electric assistance

I am be no means confident that I can ride this proposed vehicle up the hill to home, especially when tired after a long trip, without assistance. I'm by no means confident of it even now, and it's inevitable that my strength will decline.

So a legal 250 watt electric assist system is a must, as far as I'm concerned. I'm reasonably satisfied with the eBikeMotion x35 system I have on the Topstone, although the app for it is very poor. Cheaper systems are available from Chinese suppliers, such as Bafang. It seems to me to be sensible to go for a hub motor of the 'rear' (i.e. driven, and with a freehub to mount a cassette) wheel type, mounted in the front wheel.

I've not yet thought where would be a good place to fit the battery, but it can certainly go (at cost of long cable routing) behind the seat.

Servicing

Front wheel, brake, transmission, and motor

The front wheel (indeed, all the wheels) will be either ISO 406 or ISO 451. The final choice depends on what I can fit around and what rims and tyres I can get, since all the wheels will have custom hubs and consequently I'll have to build them all myself. A wheel in a well with a closed top, inside a vehicle, is awfully awkward to fix a puncture on. There has to be some convenient way of getting the wheel out of the well, either out of the top or out of the bottom.

The best solution I've thought of so far is for the vehicle to have a front fork (which it doesn't actually need except for servicing, since dropouts could be built into the sides of the well) which pivots around the bottom bracket axis. If you lift the front of the vehicle, the fork will naturally pivot down, bringing the wheel with it. There needs to be some simple mechanism to lock it in the down position.

The wheel can then be removed and worked on like a conventional bike wheel, and when it's reinstalled, the fork can be unlocked and kicked backwards, and the front of the vehicle lowered onto it. It probably needs some sort of elastomer stop to rest on when in the working position. Other parts of the transmission can also be serviced with the fork in this position.

There probably needs also to be a hinged fairing to streamline the front wheel opening when the wheel is in its working position, connected to the fork by a strut, so that it opens when the fork is dropped and closes again as the fork returns to its working position.

Pedals, cranks and chainring

If the vehicle is going to be built as a one piece monocoque, servicing the bottom bracket, chainring and crank attachments is going to be awkward. I don't see any way around this, while retaining reasonable roll-over integrity.

It should be possible to reasonably easily detach and service pedals when they are in the rearmost position in their cycle.

Rear wheels and brakes

On a Mosquito-like design, the rear wheels are out in free air and very easy to work on, thanks be!

Overall design / aerodynamics

Aerodynamic drag is the main cost to be tackled in designing a low power vehicle, so it's important to get the drag coefficient as low as possible. That means, really, as near to a teardrop shape as possible, and with as small a nose as possible.

In the nose of a feet first recumbent is the crank assembly, with the pedals, and your feet going round them; which overall describes an ovoid shape with a long axis the length of your foot (obviously, in your cycling shoes, but my Dromartis fit pretty closely) plus twice the length of your cranks, and a short axis the height of your foot plus twice the length of your cranks. Unfortunately, if you're more or less lying down, the long axis is vertical. The width of the swept area is the width of your two feet together, plus the 'Q' measurement — the distance laterally between your feet when they're on the pedals.

My feet are about 250mm long, and I can't change that. I'm used to standard cranks, which are 172mm long, so the long axis on the setup I'm used to is almost 600mm. My feet together are 185mm wide. Standard Q measurement is about 150mm; possibly 142 for a 'one by' transmission, which I'd prefer to use. So we're looking at about 230mm. It might be possible to shave a little off the Q, but not much. So that's a frontal area at the nose of 0.138 square metres... which doesn't sound so bad when you put it like that, but really every little helps here.

A lot of people playing with these sorts of vehicles are using short cranks; Bridge, for example, uses 140mm cranks in Woodstock (he's also using unicycle cranks to reduce the Q slightly). With 140mm cranks I'm down to 530mm x 230mm = 0.1219 square metres, which is a 12% reduction. Again, I need to test at a pre-prototype stage whether I can pedal efficiently with 140mm cranks while lying almost prone, because if I can't and I've made the mould to just fit them, that will be a hugely expensive mistake.

So what will this design look like?

The hull will look very much indeed like Woodstock's, but about 200mm longer behind the seat to add cargo volume. The floor of the rear of the hull, behind the seat, will also be lifted slightly, and flattened, to allow for the rear axle assembly to move under it. On either side, behind the seat, close to the rear corner of the windshield, will be a knuckle where the four-bar linkage leg attaches. From this, on either side, the four bar linkage leg will run down to the rear axle assembly.

The rear axle itself will not be straight but will be bowed downward in the middle to allow more clearance for the hull above it. At each end of the axle will be an exposed wheel, with a hydraulic disk brake. These wheels will be either ISO 406 or ISO 451, same as the front wheel, so that one spare inner tube will fit any.

A system of wires (probably) or small hydraulic rams (possibly) will link the floor of the rear of the hull to the axle assembly, to control steering.

Materials and rough costs

Composites

I'm pricing for a hull layup with one laminate of 200 gramme carbon, one of 3mm nomex, one of 200 gramme carbon, and a final outer layer of 200 gramme carbon/kevlar for abrasion resistance. I think this is probably stronger than needed but will consult with folk who know better.

On the mould, I've priced only for chipboard to make bulkheads; timber to skin the mould we can cut here. I'm planning a male mould (plug); this means that the outer surface of the final moulding will not be perfect, but I don't think I'm very bothered; if I am I can paint it later.

I'm not currently planning either on vacuum bagging the layup, or on using peel ply; I know both of these things could improve the final product, but it adds complexity and I don't have experience; and do not want to get it wrong on a project as big and expensive as this.

Other parts discussion

I think I may have the chainring I need 'in stock' from a previous project. Some other cycle parts could probably come out of the parts bin, making savings.

Rear axle assembly and four bar linkage

The custom engineering parts I need are the upper and lower knuckles for the four bar linkage, the stub axles, and the front fork. The uppers I could probably fabricate out of carbon, with bronze or PTFE bushings; and if that works, it can be done with spare carbon and epoxy left over from the hull moulding, because there's bound to be some. The lowers are going to have to be milled, probably out of aluminium; they will need to incorporate the mountings for the rear brake calipers. The four pivots of the linkage have to be exactly parallel, so this needs to be done right.

The stub axles can be done very nearly for free if Rob gets his lathe working again, and can probably be done at very low cost on Finn's lathe.

The rear axle, and the legs of the linkage, can be made of carbon, and again I anticipate there will be enough spare carbon from the hull moulding to make these. They could equally be made of timber, which I have essentially for free.

There are aspects of the steering system I don't have fully designed, even in my head; but the system used on Mosquito is very simple, and I could simply copy it.

Costed parts list

So this is looking like a roughly £3,000 project. Which is doable, over the course of a year or two.

Project plan

Steering prototype

I need to dummy up the four bar linkage to create a steering prototype, which I can adjust to get the steering geometry right. This doesn't have to be full size, and I think Technic Lego would be ideal.

Static ergonomics prototype

I need to establish how all the parts fit together around my body, and the minimum size of the necessary hull. Which means I need a structure in which I can sit, in the position I'll be in in the actual vehicle, and actually pedal against resistance. It doesn't have to actually move, so it doesn't matter how heavy it is and it doesn't need to be strong.

To get this far, I need

• The crankset with 140mm cranks;
• Something to drive to provide resistance;
• An old BMX wheel to serve as front wheel;
• A piece of bendable metal pipe to dummy up a handlebar -- copper plumbing pipe is easy to bend and would do fine at this stage;
• A wooden beam to build the whole thing on, which I have;
• Timber to fabricate a seat, which again I have.

Timber functional prototype

It would be possible to build a wooden functional prototype, comprising a chassis with the correct spacing of cranks, handlebar, front wheel, seat and four bar linkage, and rear wheels. But actually, this would be a lot of work and probably involve a lot of parts which would not subsequently be useful. So it could be quite expensive. I think it is not a substantial value add over the static prototype, and that consequently I should not do it.

There is just one 'gotcha' to this. The loading on the pivots of the four bar linkage is at an angle, neither parallel to the axis of rotation nor normal to it, but in so far as I can see very nearly at 45° — which is very unkind on them. They need to move relatively smoothly. The 'wider' the knuckle is, the better it will resist twist, but the worse the aerodynamics will be. A PTFE bushing may be enough, or actual bearings may be needed. The Lego prototype is not going to show us this. I'm guessing that I'd like at least 100mm between bearings — or to put it differently, a 100mm long bushing — onto a 10mm diameter stainless pivot. I may be over-engineering that ridiculously, but, worse still, I may not.

If I do a life-size model of the linkage, with life size loads on it, we can assess the extent of this problem. It doesn't have to be a moving prototype, or even to have actual wheels; it could be dummied onto the back of the static prototype.

If I don't get this adequately evaluated in prototype I'm going to have to build in 100mm x 100mm mounting pads for the knuckles at each 'shoulder' of the hull, in order that I can bolt on replaceable knuckles in case the first ones don't work well enough. And fairing that neatly will not be easy.

140mm cranks for one of my working bicycles

To get used to pedalling with short cranks (although pedalling with short cranks on an upright is going to be different to pedalling with short cranks when recumbent). If I can get a short-crank crankset which fits the Slate, and use that both on the Slate and on the static prototype (and, if successful, on the final vehicle), that would save a small amount of money.

CAD file of final vehicle

Not strictly necessary, once I have the Lego prototype for geometry and the static prototype for measurements I could just eyeball the rest. But the knuckles are certainly going to have to be properly designed and CAD files for them produced, and it would probably be a good idea to have a visualisation of the whole vehicle before spending a lot of time and money on building it!

Conclusion

At this stage of the design exercise, it looks as though this is potentially buildable with (mostly) the skills I have. The lower knuckles of the four bar linkage seem to me to need to be milled from aluminium, which I can't do, and don't have any friends who can do, but the rest is all doable. The price is also manageable — not in one lump, but it doesn't have to be spent in one lump.

I think I shall buy some Lego.

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