Building a Human-Powered Vehicle, Part 1

 

As the name suggests, this series is about the design and building of a human-powered vehicle (HPV). In fact, one that’s powered by pedals. Now you might ask what such a series is doing in a high performance on-line magazine devoted to cars. It’s in here because with the exception of the motive power, much of the decisions were the same as taken when building a one-off car - perhaps a kit car or one designed for the track. For example, the decision to use either a monocoque or stressed tubular space-frame; the weight distribution; brakes; stiffness (in bending, torsion and roll); measuring and eliminating bump-steer; spring and damper rates; dynamic castor and camber changes; Ackermann steering, and so on. I’ve drawn primarily on automotive technology in design of the machine – in fact it’s been much more about ‘cars’ than ‘bicycles’. So if you want stuff on the fundamentals of vehicle design and construction, read on. Yep, even if this machine is powered by pedals...

My interest in building a human-powered road vehicle goes back a long way – but with a gap of many years. When I worked as a teacher, I helped a school team design and build a machine for the annual Australian Pedal Prix (see From the Editor for more on this event). While many of the vehicles (including the one I was involved with) were pretty simple, at the event itself some very professional vehicles could be seen. Full aerodynamics, carbon fibre, and ultra lightweight frames. With these vehicles you simply can’t fit a bigger motor, and so excellence in design and construction becomes paramount. (It’s a race class with a semi-fixed engine power but almost complete freedom of design!)

However, I didn’t do anything further about these vehicles until I recently stumbled across the Greenspeed recumbent three-wheelers. For me these trikes have revolutionised the whole concept of human-powered vehicles – making them viable for a huge range of people and activities (like commuting to work, for example) in a way that a traditional bicycle simply can’t emulate. In fact, I was so blown away by the machines, I bought one second-hand. It’s called a GTR and I’ve ridden it extensively. (See Driving Emotion.)

However, the Greenspeed GTR has some downsides – the primary negative being that it has no suspension. The suspension is supposed to be provided by the hammock-like seat and flex in the chrome-moly tubular frame but the reality is that nearly all of it comes from the tyres. To get a comfortable ride on the poor bitumen roads on which I mostly ride, I had to drop tyre pressures from a recommended 80+ psi to just 20 psi, resulting in an increase in rolling drag, a handling trade-off and a much greater likelihood of punctures.

However, the Greenspeed has some brilliant design characteristics, optimised in the long period over which the machines have been constructed. The relationship between the seats, forward-mounted pedals and the side-mounted steering arms is perfect. The weight distribution (a third on each corner) is the optimal compromise between rear wheel traction up steep hills (more weight wanted on back wheel), lateral cornering performance (more weight located between front wheels), and braking performance (weight wanted on the back to stop the trike lifting its rear wheel). Also, completely unlike a bicycle that leans into corners, on a trike the wheels and hubs have to accept very high lateral cornering loads - and the wheel and stub-axle designs used on the Greenspeed have proved to be well up to the forces involved.

Other Greenspeed design positives include zero scrub radius (ie centre-point) steering, modified Ackermann steering geometry and 63 gears.

So the Greenspeed GTR could be used as the design basis of a new HPV, but there were aspects of that machine that I thought could clearly be improved. And making the project something that could really happen was the fact that Greenspeed is happy to sell separately any of components that they either make in-house or buy in. Yep, you can buy their kingpins, their steering arms, the wheels, any parts of the tubular frame – as much or as little in the way of componentry as you want.

But before I could jump on the phone and order any parts, some major decisions had to be made.

Frame

• Material

Nearly all recumbent HPVs use chrome-moly steel tube to form the frame. That’s also the same with bicycles, karts and space-framed low volume cars. The chrome-moly tube is:

• strong for its diameter and wall thickness (and both are usually kept small on HPVs)

• able to be welded by brazing, MIG or TIG techniques

• cheap

• readily available

However, volume for volume, steel is heavier than aluminium, and much heavier than exotic composites like carbon fibre. In short, I thought I could achieve a lighter, stronger frame by using aluminium – without needing to have the huge skill level that’s required to work in carbon fibre.

• Construction

Vehicles can be built using two fundamentally different techniques – space frames and monocoques (or unitary bodies).

In a space frame, the structure is made up of lots of relatively small diameter tubes, positioned to take the stresses that try to bend and twist the vehicle. In a monocoque, thin sheet material is shaped and positioned to provide a stiff structure. The vast majority of current mass production cars use monocoque designs, while one-off cars (eg most kit cars and home-built race cars) use space frames. (Of course, in many cases space frames are stiffened with some thin panelling!)

A monocoque vehicle constructed from aluminium needs to have the design finalised before a tool is picked up – you must know where all the stresses are going and cater for them from the very beginning. In other words, you can’t just add a bracing tube or extra triangulation late in the build. Also, without the presses and dies needed to shape the sheet, a sheet aluminium monocoque can comprise only flat surfaces, so the stiffness and strength that can be gained from compound curves will be absent. (Or you can learn to panel-beat compound curves into aluminium sheet... which would be even more difficult than getting adept with carbon fibre!)

Weighing-up these pros and cons, I decided to make the frame a simple tubular structure that would be TIG welded together. However, there were three further points:

• Most of the aluminium tube would be square in section

• The tube would be considerably lightened by having lots of holes cut in it

• Sheet aluminium gussets (stiffening panels) would be used wherever possible

Further influencing my decision was that I already had lots of aluminium that I’d bought as scrap – primarily square tube 40 x 40 and 50 x 50mm (both with 3mm walls) and also a heap of 3mm sheet.

Aluminium Fatigue? One reason that most manufacturers of HPVs use chrome-moly steel in preference to aluminium tube is the steel’s resistance to fatigue. For example, the Greenspeed design uses two butt-welded cantilevers to form the front wheel supports. The steel tube flexes at these joints (not much, but it does flex) and if these tubes were made of aluminium, they would fatigue and break. However, if the frame design avoids obvious weak points like these, there shouldn’t be a problem using aluminium.

Suspension Design

• Travel

Few road HPVs use suspension and even fewer use suspension on the front and the back. Of those that do use suspension all-round, invariably the front has almost no suspension travel and the rear little. For example, a rear suspension travel of 50mm and a front suspension travel of 25mm seem common. To me, these seem ludicrously small – after all, a 1-inch diameter pebble on the road uses up all the front suspension travel!

The reason that the rear travel is greater than the front (or, on HPVs with suspension at only one end of the machine, it’s the rear that’s suspended) is because the rider feels the behaviour of the back wheel more strongly than the fronts. If a single front wheel passes over a bump, the trike will roll and lift at the same time, decreasing the vertical acceleration. However, the rear wheel passing over the same sized bump will cause an acceleration that’s only vertical – so the rider feels it more.

But the logic of all this seems a bit odd to me. Think instead of keeping the tyres in contact with the ground so that cornering, braking and acceleration can occur (major reasons for suspension on cars) and it’s then immediately obvious that suspension is needed on all three wheels. (On the Greenspeed the front wheels steer and brake and the rear wheel is powered by the pedals.)

Furthermore, for maximum comfort, soft springing is wanted and if the suspension is not to then bottom-out on large bumps, a long travel is needed. In fact, surely what’s required is the very longest suspension travel that can be designed into the machine?

• Rear Design

Once a long travel design on all wheels was accepted as a requirement, the next step was deciding on its design. After looking primarily at motorcycles, I decided on a rear longitudinal swing-arm. This has both pros and cons. On the good side, the basic layout of the Greenspeed GTR showed that there was plenty of room under the seat for a spring and damper – and a swing-arm could position both of these in that location.

Another good point of this approach is that a single pivot axis is used for the rear suspension. So why would this matter? Because the rear wheel is chain driven and the chain has a cyclic varying tension on it as the pedals are pushed, it’s easy to have a design where the rear suspension is compressed or extended with each pedal power stroke. This results in ‘pogo-ing’ which not only is uncomfortable but also wastes energy. Running the tension side of the chain near the rear swing-arm pivot axis prevents this happening and if there’s only one pivot axis, this becomes easier to organise.

The main downsides of a swing-arm are that the wheelbase changes slightly on bump and rebound, resulting in a varying chain length. However, a derailleur easily copes with this variation in chain length in the same way it does with the varying length of chain caused by selecting a different diameter gear.

• Front Design

For the front suspension there was really only one choice – unequal length double wishbones with an anti-roll bar. For the same reasons as on a car (or a quad bike like the one shown here), unequal length double wishbones simply provide too many advantages over alternative designs like swing-arms or struts. These advantages include:

• Wide-based frame attachments resulting in a strong assembly

• Ability to easily control effective front-view swing-arm length and roll centre height

• Dynamic camber control in bump

• With appropriately positioned inner and outer steering joints, control of bump steer

• Zero scrub radius steering easily possible

• Spring and damper able to be positioned in a wide range of locations

• Ant-dive geometry able to be built in

• Dynamic castor control in bump

As the rear wheel has zero roll stiffness, an anti-roll bar is very much needed on a trike. In fact, on the Greenspeed GTR, in very hard cornering it’s possible to lift the inner front wheel off the ground. If the rider and machine weigh 100kg total, about 30kg is normally being borne by each wheel. However, with a front wheel lifted in cornering, the other front wheel has its substantially increased. If the wheel suspension wheel rate is (say) 15kg an inch, the suspension would normally be compressed by 2 inches. But if the load on that wheel is (say) doubled, the compression will increase to 4 inches, resulting in a helluva lot of body roll and the using-up of suspension travel. So an anti-roll bar would certainly be necessary....

Ground Clearance Long suspension travel – especially when a majority of that is in bump – requires greater ground clearance. If the static ground clearance is (say) 120mm and 90mm of bump is provided, under full bump there will be a ground clearance of only 30mm. So what’s a decent amount of full bump ground clearance to provide? Full bump should occur quite rarely and to bottom-out the frame requires both full bump and either a hump midway between the front and rear wheels or a rock that the front wheels straddle and the rear wheel misses. On this basis I figured full bump ground clearance could be made pretty small – say 50mm. Any more than this and the static ride height was going to be way high.

Other

For nearly all the other design details, I looked towards the Greenspeed GTR. That meant 20 inch wheels front and back (the Greenspeed models I have ridden with 16 inch wheels had a clearly inferior ride), derailleur gears front and rear together with a 3-speed internal rear hub, slung hammock-like recumbent seat, similar track and wheelbase dimensions – in fact anything I was unsure of, I copied straight from the HPV I already had.

However, changes were likely in two other key areas – brakes and steering.

The GTR runs cable-operated drum brakes working independently on the two front wheels. And the brakes aren’t great. In normal day-to-day use I am sure they’d be adequate but when plunging downhill at 80 km/h, stopping distances are too long and the need to evenly apply the brakes a bit tricky. I thought perhaps hydraulic disc brakes operated from a single lever would be much better – and since Greenspeed sell these, I put them on my wish-list.

Less easy to potentially solve was the other concern – steering. The GTR uses rods to directly control the movement of the steering arms. The rods are operated through small Heim (rose) joints by the steering levers which pivot around a central, vertical axis. However, the steering – while ultra-sharp at low speed – retains all its sharpness at high speed, resulting in quite a lot of nervousness (of both the steering and the rider!). I wanted slower steering at high speed without losing to much directness in normal manoeuvring.

Conclusion

So the proposed spec list reads something like this:

• Square tube frame made from aluminium with lots of holes cut in it

• Rear swing-arm and front double wishbone suspension with lots of travel

• 20 inch wheels in a ‘tadpole’ configuration (two front steering, one rear driven)

• Recumbent hammock seat

• Hydraulic front disc brakes

• 63+ gears comprising front and rear derailleurs and a 3-speed internal rear hub

• ...and steering mechanism yet to be decided upon

Next week: designing and building the rear swing-arm suspension

Other Front Suspension Designs When trying to do something new, the first step is to have a look around at what others have done. So what does a web search under ‘recumbent trike suspension’ come up with? Well, in short, some pretty horrible front suspension designs. Of course, as I write this, I have no idea if my front suspension design will turn out woeful on the road, but at least on the basis of a comparison made on basic suspension principles, some of these designs look downright bad. At web.uct.ac.za you’ll find an interesting leading link design – but one which apparently has just 20mm of travel! I am hoping to achieve at least 100mm of travel (from full droop to full bump). www.hellbentcycles.com at least has double wishbones but both are very short, the rose joints are not protected from dust or rain, there is no anti-roll mechanism and the suspension travel is listed at only 38mm. Then there’s mrrecumbenttrikes.com which unless my eyes deceive me, uses a swing-arm front suspension without any anti-roll facility! That will give problems in jacking, roll, and - one would assume - bump steer. Then, if you want to see a suspension that is as crude-as, check out www.hotmover.com. Just look at that suspension travel... and the [lack of] damping! As I said, maybe I’ll fall completely on my face when I first roll down the road, but surely even a badly sorted double wishbone system with plenty of travel and a sway bar will be better than these?!

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