More microcar stuff

microcar/Electric micro-car thoughts.md

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+# Electric micro-car thoughts
+
+You may have noticed that the tricycle project has gone quiet. Part of the reason that the tricycle project has gone quiet is because all my spare money, which I had hoped could be devoted this year to making the vehicle, is now going to people in Gaza; but part is because I was never fully confident that the very radical ideas in the tricycle project would actually make a practical vehicle for my old age. Also, the tricycle would absolutely not be able to carry a passenger, and being able to carry a passenger is pretty useful.
+
+The subject of this essay is a sort of alternative to the tricycle project; it's a (relatively) low cost, ultra lightweight, four wheel vehicle like a very small two seater sports car. The idea is not that it should be capable of being propelled by pedals.
+
+Note that the crisis in Gaza has not gone away, and consequently I don't expect to ever actually build this vehicle; it's probably that I shall be supporting people in Gaza, and therefore won't have much spare money, for the whole of the rest of my life.
+
+Legally, the micro-car would need to fit the ['light quadricycle' regulations](https://assets.publishing.service.gov.uk/media/68be9d97de0987fe84e0dcf7/motorcycle-single-vehicle-approval-inspection-manual.pdf?page=12), specifically:
+
+> * with a maximum design speed of 45km/h (28mph)
+> *  fitted with a spark ignition internal combustion engine having a cylinder
+>   capacity not exceeding 50cc, or
+> *  fitted with any other internal combustion engine which has a maximum net
+>   power output of 4kW, or
+> * fitted with an electric motor with a maximum continuous power not exceeding
+>   4kW
+> * not exceeding 425kg unladen (not including the mass of traction batteries in
+>   the case of electric vehicles)
+> * if designed to carry goods a payload not exceeding 200kg
+
+28 miles per hour is a little slower than I'd like, but it would be adequate for what I need to do.
+
+Four kilowatts is probably enough to get a two hundred kilogram vehicle (plus passenger and some cargo) up the hills I need to get up, but possibly quite slowly!
+
+Note that '[microcar](https://www.national-microcar-rally.co.uk/whats-a-microcar.html)' is a designation used in some motorsports events, where it describes vehicles which are mostly significantly larger, heavier and more powerful than I'm describing here.
+
+## Digression: cyclekarts
+
+[Cyclekarts](https://en.wikipedia.org/wiki/Cyclekart) are small home-built racing motor vehicles, built to a rule which specifies a Honda GX200 (or clone) engine: single cylinder, air cooled, 200cc, with a power output of 4.8 kW; usually in conjunction with a continuously variable transmission which will not be 100% efficient. The maximum allowable weight is quoted as 113kg. This means that they're fairly similar to (slightly higher than) the power/weight ratio that I'm considering here. Cyclecarts conforming to this standard appear to race at speeds up to very nearly 50 mph. They run on 17 inch moped wheels, with tyres that are designed for mopeds. They have relatively primitive suspension on the front wheels, and usually none at all on the rear; they also do not normally have a differential.
+
+On dirt tracks and [on wet tarmac](https://www.youtube.com/watch?v=0ilgpRQhw2A) they appear to slide and drift a little more than would be desirable on the public road. I don't know to what extent this is due to poor suspension. On dry tarmac they appear to track very predictably.
+
+## Donor car
+
+This project doesn't really need a donor car, but a good windscreen is pretty hard to make from scratch. So I think that cutting from a scrap car the windscreen with its surrounding metalwork — the cowl, the 'A' pillars and the front structure of the roof — would be pretty beneficial.
+
+Because the windscreen surround should ideally form part of the rollover protection, it needs to be very solidly integrated with the tub, and consequently it would probably be a good idea to buy the donor vehicle before the design of the tub is completely finalised. The width of the tub  should be the exact width across the 'A' pillars of the donor, and the top of the rollover bulkhead should be level with the top of the windscreen. 
+
+The same vehicle could be a donor for the wheels, and possibly some other components, such as steering column, steering rack and possibly switches. 
+
+Because I only want the area around the windshield, a car which is an MOT failure, or badly rusted underneath, or heavily accident damaged elsewhere, would be perfectly possible.
+
+### [Citröen C1](), Peugeot 107, Toyota Aygo
+
+I think the best candidates would be the Citröen C1, Peugeot 107, or Toyota Aygo, closely related small cars. The Citröen variant was available with a large fabric sunroof, and one with a sunroof would make a particularly useful donor. 
+
+The Peugeot and post 2014 Toyota variants have headlamp units which are not utterly horrible, but I doubt that they are LED. The wheels for these cars have 14" rims, and the recommended tyres are 155/65R14, low rolling resistance. That doesn't sound impossible for a low power electric vehicle; it's the same as what the new Citröen Ami runs.
+
+All vehicles are between 1,620 and 1,630 mm wide. There is some tumblehome, but it looks from photographs as though they are at least 1,450 mm over the 'A' pillars. The weight is between 780 and 956 kg, depending on model
+
+### [Fiat Panda](https://en.wikipedia.org/wiki/Fiat_Panda)
+
+An alternate donor car would be the Fiat Panda, either second (1,578 mm wide) or third (1,643 mm) generation. Overall weight is 840–1,115 kg, depending on model. The wheels are bigger than on the Citröen/Peugeot/Toyota cars. The bigger wheels would I think look better, but they would be heavier, and weight is an issue.
+
+### [Smart Fortwo](https://en.wikipedia.org/wiki/Smart_Fortwo)
+
+The Smart Fortwo is also worth considering, although I think its wheels may be too small to be useful. The first generation is 1,510 mm wide, and appears to be at least 1,400 wide over the 'A' pillars; however, these will not be rare. Second generation, 2006-2014, is 50 mm wider; the third generation, 2014-current, is 1,663 mm wide, which is possibly too wide for what I'm considering. Weight is 660-880kg, depending on year. Again, all generations were available with a canvas roof, which would be particularly easy to adapt.
+
+### Incorporating elements from an actual car into the CAD model
+
+Really quite good 3D models of most modern cars are available online in formats that can be imported into [OpenSCAD](https://openscad.org/) or [Blender](https://www.blender.org/); however [the ones I've found are neither free nor cheap](https://3dmodels.org/3d-models/fiat-panda-4x4-2012/#360view), so until I have got an actual donor in mind this isn't worth buying.
+
+## Overall design concept
+
+### Drive and braking
+
+I like the idea of having a one kilowatt hub motor in each wheel, giving four wheel drive and potentially traction control and thrust vectoring. This also means no differential, no prop shafts, no universal or constant velocity joints.
+
+There's a Canadian made brand of [suitable hub motors, 'Grin'](https://ebikes.ca/product-info/grin-products/all-axle-hub-motor.html), which will take a 15mm axle, which should be strong enough. They aren't cheap; equivalent Chinese ones cost from under £170 each. The Grin motors are supported for four wheel/single sided use, however, and also support regenerative braking. Using bicycle hubs means I can easily use bicycle brakes, and if I use 'downhill' style brakes they're going to be more than adequate — especially as I anticipate that regenerative braking will do most of the work (actually, given regenerative braking, it's unlikely that downhill style brakes would be needed).
+
+I want to do double wishbone suspension all round, because of course I do. I'd love to use air suspension units, but that might be too expensive. I'd even more love to use hydropneumatic suspension, but I'm going to rule that out as a bridge too far.
+
+### The tub
+
+My current idea is to have a tub which might essentially be a marine plywood 'egg-box' structure,  probably mainly 8mm thickness, It would have outward sloping end bulkheads; the front and rear subframes, to which the suspension would mount, would be anchored to the tub end bulkheads at the top, but so that, in impact, they will rotate back under the tub, thus lifting the tub and vectoring impact under the occupants. That basically means that the top mount is fixed but can rotate, while the bottom is braced by struts which are designed to collapse progressively when overloaded.
+
+The intention is that the tub should be a passenger safety cell, which should as far as possible be designed to survive a road-speed collision — allowing that this is a very light vehicle without significant kinetic energy of its own, which is likely to be tossed aside by a road-speed collision with a larger vehicle, provided it can be prevented from going under the other vehicle.
+
+The rear bulkhead of the tub would be reinforced with a box section which would provide the primary rollover protection. This would probably also be a plywood structure, possibly reinforced with some structural timber such as ash.
+
+The current structure I envisage is a punt about two metres overall length and 1.5 metres in width, with a central longitudinal box member 250mm internal width and about 270mm tall whose sides are bonded into both front and rear bulkheads and to the floor; this box as well as contributing to the strength of the tub will also contain the batteries. A transverse box girder will run across the floor just under the front edge of the seats. Along each side of the tub will be a deep box section sill, about 270mm tall at the rear but significantly lower at the front of the door opening, 140mm wide at the top and at least 70mm wide at the bottom. From the 'A pillar' location forward to the front of the tub, the sill box extends upward to meet the apron which extends back from the top of the front bulkhead to support the lower edge of the windscreen.
+
+The whole of this plywood structure should be sheathed, at least on the outside, ideally with a carbon/kevlar twill to protect against splintering or penetration.
+
+### Roof
+
+I retain my dislike of closed cars. The roof needs to be removable, either of fabric or else, more probably, a light timber structure. It should be possible to store the removed roof in the vehicle.
+
+### Doors
+
+Doors, and particularly hinges, are problematic. I don't have any current plans for how they could be made. Having windows which retract into the doors is almost certainly not possible, because the sills must be high for strength, and because the height of the door windows is determined by the height of the windscreen, which is expected to be taken from a car designed to have both a greater cowl height and much lower sills.
+
+### Suspension
+
+Suspension will be mounted to two subframes, which I'm currently thinking will be welded metal, probably aluminium. I think that the two subframes will be substantially identical, and that all the lower wishbones will be substantially identical, and that all the upper wishbones will be substantially identical. That makes manufacture, and also having spare parts, easier. It would be great if all the kingpins could also be identical, but I'm not confident of that.
+
+Of course it would also make four wheel steering possible. It's not currently my intention to actually implement four wheel steer.
+
+There needs to be an inward tilt ('camber') of the kingpins, so that the axis of the kingpin intersects the road in the track of the centre of the contact patch. This is most easily achieved by making the lower wishbones longer. Having longer lower wishbones also helps with keeping the contact patch flat in cornering. For the front axle, there also needs to be a rearward tilt of the kingpins, in order that the axis of the kingpin intersects the road forward of the contact patch. That could be achieved by rotating the whole of the front subframe backwards by a few degrees, although doing this will make dive under braking worse.
+
+A suspension/steering geometry which both resists brake dive and provides castor requires [ball joints](https://www.igus.eu/igubalConf/Igubal/Step1) at the ends of the kingpins, which is quite common on cars but which I don't yet understand how to make strong.
+
+[This site](https://suspensionsecrets.co.uk/) appears to be very informative and useful.
+
+### Wheels and tyres
+
+Bicycles (and mopeds) lean into corners. Consequently their wheels and tyres are almost always radially, not axially, loaded. Cars don't; the axial loads on a car's wheels and tyres can be high. The weight of the microcar, however, by car standards, is very low — I'm aiming for less than 25% of the weight of even a very small and light four-seat car.
+
+Thus, car-style wheels and tyres would be a large, possibly excessive, component of the weight of the vehicle, and wheels, of course, are entirely unsprung weight. Furthermore, attaching a car wheel to the sorts of hub motors I'm considering would be very tricky and would need some interesting custom engineering. Also, car tyres at standard pressure might not be sufficiently loaded to give adequate grip, and if depressured to give an adequate contact patch, would certainly have undesirable rolling resistance.
+
+The axial loading on the wheels of the microcar would not be significantly greater than on a tricycle tandem, or on a conventional electrically assisted velomobile, and they all use bicycle wheel componentry without problems.
+
+Cyclekarts, which have pretty much the same weight and performance envelope as what I'm considering, race on moped wheels and tyres, and this seems to work satisfactorily. Lacing moped rims onto hubs designed for bicycles would not be hard. 
+
+However, moped rims, and the tyres and spokes for them, would have to be bought; car wheels would come almost for free if a donor car was used. Moped wheels are also typically not designed for single sided support, so some modification would be required; but not as much as integrating car wheels with bicycle style hub motors. Probably the much simpler integration would mean that moped-style wheels would end up cheaper than using the car wheels.
+
+Finally, bicycle wheels would probably work and would be both lighter and simpler to build up than moped wheels, but the vehicle would be putting loads on bicycle tyres that they're definitely not designed for.
+
+### Seats
+
+The seats should form part of the structure of the tub and should not be adjustable, both for lightness and for space. The pedal box should therefore be the adjustable part which allows the vehicle to fit drivers of different sizes.
+
+### Pedals
+
+This is not a vehicle designed to be pedalled like a pedal cycle. Rather, it should have pedals like an automatic car: an accelerator pedal, and a brake pedal. They should be built onto a tray — the 'pedal box' — whose position in the tub can be fairly simply adjusted, to allow for different sized drivers, but once adjusted is firmly in position and will resist the pressure of urgent braking.
+
+The accelerator pedal obviously sends a signal — probably a variable voltage — to the motor controller; and that's fairly simple. The brake pedal is two stage. The first stage is similar to the accelerator: it sends a signal to the motor controller to engage regenerative braking. The signal from the brake pedal must completely override the signal from the accelerator. At about 75% of travel, the brake pedal also engages the hydraulic brakes, progressively. This means that if regenerative braking fails for some reason, the brake pedal still works to halt the vehicle; and that the brake pedal can be used to bring the vehicle to a complete stop. 
+
+Whether the handbrake lever engages the same hydraulic calipers as the footbrake is a detail I haven't decided. It seems to me that having both regenerative and hydraulic braking provides the 'two independent brakes' the law requires, so the handbrake does not have to be separate from the foot brake.
+
+### Clamshells
+
+There needs to be bodywork both to the front and to the rear of the tub, to cover the wheels, provide mounting points for lights and number plates, and smooth the airflow. I'm referring to these units as clamshells. These could be:
+
+1. **Glassfibre**: cheap and nasty, toxic materials;
+2. **Carbon-kevlar sandwich**: more expansive, toxic materials, very light, potentially very good looking;
+3. **Flax-balsa sandwich**: as expensive as carbon, few toxic materials, quite light;
+4. **Red cedar strip**: *much* more expensive, some toxic materials, quite light, extremely good looking; 
+5. **[Aircraft ply](https://www.aircraftplywoodandtimber.co.uk/birch-plywood.html)**: very light, not too expensive, much simpler moulds, but no compound curves possible;
+6. **Aluminium**: out of my skillset; potentially cheapest, but I really don't have the skills even to approach this.
+
+Having seen video of cedar strip bodywork recently, I would really love to do it, it is astonishingly beautiful; but it would be at least £5,000 for the bodywork alone, which pretty much doubles the cost of the project and is far beyond what I can afford.
+
+All of the options except aircraft ply and aluminium would need perfect female moulds to be built. That's possible, but it's a lot of  extra material and cost. Aircraft ply would come in at around £1,000, possibly less, for the ply, but it would need epoxy and would be stronger with a light glassfibre or carbon sheathing. The moulds required would be enormously simpler and cheaper than for other options except aluminium. 
+
+Aluminium would be less than £400 for the materials, but making good welds in thin aluminium sheet it said to be quite difficult, and there would be a *lot* of welding needed. Compound curves would be possible, but only if I bought and learned to use an English wheel. Generally it is the skills that would need to be learned that make me cautious of attempting aluminium. I think I can probably learn enough welding to do wishbones and subframes; attempting bodywork feels like a bridge too far.