Another Human Powered Vehicle! Part 16 - The Conclusion

This article was first published in AutoSpeed.

Writing 16 parts in a series seems rather excessive – especially when it’s about the design and construction of a pedal tricycle! But as those you who have closely followed the build will know, it’s anything but kids’ stuff. In fact, I think that building an ultra lightweight, fully suspended recumbent trike that both rides and handles well is an amazingly difficult engineering challenge.

So what have I ended up with?

In summary, the design uses 4130 chrome-moly steel tube for its frame, seat support and suspension arms. Most of the tube is 0.9mm in wall thickness but the front suspension arms use 1.2mm wall. The tubing was brazed together with nickel bronze rods.

The front suspension comprises a semi-leading arm design that has inbuilt castor and camber change. The roll centre is high. The steering uses Greenspeed steering and kingpins. The rear suspension is a trailing arm design. The front and rear springs are Firestone airbags. A rear hydraulic damper is used (a modified Yamaha R1 steering damper). Front damping is through the mechanism of track change. An anti-roll bar is fitted.

The most important aspects of its design is the long wheel travel (130mm) and the very low frequency suspension - with the airbag pressures typically used, well under 2Hz. The next most important aspect is the use of the semi-leading arm front suspension with a high roll centre and short virtual swing-arms.

Riding

I have now ridden quite a lot of kilometres on the trike.

I have trailered the trike down to the smooth paths and roads of the Gold Coast; I have taken the trike to Brisbane and ridden on the wonderful cycle paths to the north-east of the city. I’ve towed a Burley trailer carrying my 3 year old son (and could easily feel the change in unsprung weight - the trailer connects to the rear wheel axle); and I have ridden the sort of routes and roads that most people with recumbent trikes ride on. And on these surfaces the trike is wonderful: quick steering, a smooth ride and absolute comfort. Cushy luxury in fact.

But I also ride on my local roads, roads that are extremely hilly, very bumpy and often narrow. One hill is an especially difficult test. The first trike I bought, a well known non-suspension design, would when ridden down this hill launch into air over one of the bumps. It would fly through the air and come crashing down with a bone-shaking thud. Along the same stretch of road there were 5-10 other bumps that were harsh; harsh enough to cause headaches after a ride. This is at 40-50 km/h down the hill – toddle along at 10 km/h and the bumps are relatively minor.

With the new suspension trike I can ride down the hill, at night, as fast as I like. The bump that used to launch the previous trike into the air is now literally only barely felt. In itself, that’s quite incredible. I have to strain to even remember how bad the road used to be: in ride comfort it’s like stepping out of a leaf sprung, unloaded utility and straight into a Lexus.

I have also headed off the back of the mountain on which I live, plunging down the winding, steeply descending, narrow bitumen road. Free-rolling down this road I do between 30 and 50 km/h, the speed depending on the grades. The road is edged on one side by a vertical cliff and on the other side by a discontinuous guard rail. Where there is no guard rail, the slope falls downwards into a creek, tens of metres below. The bitumen surface is awful: lumps, bumps, surface roughness, patches, occasionally a few potholes. Corners are sufficiently tight that some are marked with advisories of 30 km/h.

And I did it at night, all the vision being provided by my 5W white LED headlight. I was throwing the trike into these corners, not knowing the precise nature of the surface, not knowing if the corner tightened (I’m familiar with the road but there are perhaps 100 corners), but knowing full well that if I made a mistake it would be nasty.

The wind was whistling through my helmet, the suspension hammered over the terrible surface, and all my senses were absolutely attuned to staying on the road. One left-hander tightened-up to the extent that I could either go up on two wheels trying to make it - or momentarily cross the white line. I chose the latter and then became more committed – turning-in as early as possible and then holding the line. Sometimes that meant deliberately putting the inside wheel onto the (largely unseen) dirt; other times it meant steering with absolute fingertip precision, sensing the shape of the corner way past the sharp cut-off of the headlight’s beam.

When I finally pulled over, my breathe was rasping, my heart was pounding and my fingers tingling.

I’d taken on the road and won – and won in a vehicle of my own creation. It was an awesome, real world ride. I’d dearly love to put the handful of full suspension HPVs developed world-wide to the same test...

The Lessons

I think that the fundamental design of my trike has proved itself. It has what I am sure is the best ride quality of any human powered vehicle of its size in the world, and in handling it can match or exceed those four commercially produced - and internationally very well regarded - recumbent trikes to which I have access. (In part it can do that because of its very wide front track – it would be too wide for some people and some riding environments.) The semi-leading arm front suspension - with its high roll centre and dramatic dynamic changes in camber and castor - has been completely successful; the rear suspension with its damper and new design of arm is now also working very well.

The two suspension trikes that I have built have convinced me of some very important points that are germane to any vehicles that are:

• fully suspended

• ultra lightweight

• narrow tyred

• trikes of tadpole configuration (ie two front wheels, one rear)

1. The combination of a suspension with long travel (eg 100mm +) and low natural frequency (ie at least 40mm of static deflection) is required for a good ride quality. There is no way around this: those machines that use short travel and stiff suspension have a ride that is, in comparative terms, awful.

2. The suspension should be designed to maintain as low a motion ratio as feasible. That is, spring and wheel travel should be as similar to one another as can be achieved.

3. The combination of (1) and (2) above preclude the use of elastomers and rubber as the springing media. Steel coil springs and rolling-edge airbags are both very successful springing choices.

4. If roll angles aren’t to become excessive, and (1) and (2) above are followed, an anti-roll bar will be required.

5. Dynamic track changes are effective at damping suspension; therefore with a suspension that achieves this, external dampers are not required. The downside is that tyre wear will be greater than in those systems that do not alter track.

6. Toe-out on bump can be used to settle twitchy steering. The downside is increased rolling drag on bumpy surfaces, so a steering system should always be initially set up for zero bump steer and then assessed by road testing.

7. Narrow, round-shoulder tyres develop increasing cornering grip with negative camber. Therefore, suspension systems that dynamically develop negative camber of the outside wheel work very effectively. This is advantageous in that more complex and heavier suspension systems (eg double wishbones) need not be employed.

8. Rear suspension requires damping. Friction dampers give poorer ride quality than hydraulic. Damping should be very carefully set up to give relatively soft bump damping (especially in high speed bump) and much firmer rebound damping.

9. The rearmost chain idler should be made adjustable for height. Reducing pedal squat of the rear suspension to a minimum cannot be done by design rule of thumb – it must be done by experimentation.

Solid suspension pivots (eg ball bearings and bushed pins) will transmit vibration to the frame. Rubber or polyurethane bushed pivots will not. However, the latter approach needs multiple pivot points to gain sufficient rigidity in dynamic wheel location.

Those are the technical aspects to focus on but there are some more general points that can be made from my experience:

• Trying to build anything like this without having access to your own welding or brazing gear is madness: my first trike, which was made from aluminium and TIG’d piece by piece by a local welder, took perhaps three times as long to build as it would have, had welding gear been available on the spot.

• I am now a complete convert to chrome-moly steel tube. I find it immensely strong for its weight and size, resistant to fatigue and having the ability to be heavily (over)loaded but still flex back to its original shape. It is simply nothing like mild steel tube to work with. Brazing it together with nickel bronze rods is easy and the joins have proved to be very strong. When making nickel bronze welds, ensure that the flux is completely removed after the part has cooled. The flux is resistant to removal, but soaking the parts in very hot water and then using a wire brush will get it off.

• When using tube it needs to always be kept in mind that the strength is in both walls. That is, loads need to be fed into both walls, not just one. For example, where one tube butts against another and is welded to it (think of a ‘T’), the forces are being fed into only one wall – in this case, the lower wall of the cross-piece of the T. Forces will therefore tend to buckle this wall. Where one tube is smaller in diameter, it is a good approach to pass the smaller tube right through the larger tube, brazing each side. This connects the smaller tube to both walls of the larger tube. If this is impossible, or the tube sizes are the same, triangular gussets can be used to spread the buckling load.

• Using many small tubes instead of single, large diameter and thin wall tube is an approach frequently taken in lightweight constructions. However, large diameter tube is vastly stiffer than smaller diameter tube and you’ll often find that a structure using many small tubes is heavier than one using a single large diameter tube. That’s not to say that a truss made from small diameter tubes is poor engineering, but when in a very lightweight structure you can expect every member to be subjected to bending, torsion and extension, a single tube is usually the lightest solution. (In part this outcome is because the smallest wall thickness commonly available is 0.9mm. If you could easily buy small diameter tube with a wall thickness of – say – 0.5mm, the outcome would probably be different.)

• During the building of both my Human Powered Vehicles there have been many who have said to me that car suspension design and technology is irrelevant. I consider that this idea is completely wrong: cars are the most highly developed form of road transport and an enormous amount can be learned by looking at how their suspension systems work. That said, the earliest automotive textbooks are the best to look at because they analyse absolute fundamentals – things like suspension frequencies, camber and castor.

• While I have often belittled on-line discussion groups, I think that in an extreme minority pursuit like recumbent trike building, a lot can be gained from them. But you need a very strong self-belief and to carefully weigh up the veracity of all the advice you are given.

• If building a recumbent trike, I strongly recommend buying some off the shelf parts. Greenspeed will sell you any of the parts used in the manufacturer of their trikes, including parts they custom produce. (Disclaimer: my wife is a Greenspeed dealer – www.speedpedal.com.au.) Using parts like the machined and welded kingpin assemblies and the steering handlebars and pivot – parts that are well proven, strong and relatively light – makes things much easier while still allowing plenty of creative freedom. The curved ‘Ergo’ seat rails and seat fabric are more off the shelf parts that can be adapted a variety of custom designs.

Where to From Here...

So now that I am pretty happy with the design....I am building another trike. (Nope, we won’t be covering it in a multiple part series in AutoSpeed!) The new trike will be closely based on the one that has been developed in this series.

I want to make the new version more accurately, with a frame that is exactly symmetrical. Jigs will be used for parts of the build.

I’ll also change the anti-roll bar design and make some changes to the rear suspension arm that will decrease weight. The front suspension arms will drop in wall thickness from 1.2 to 0.9mm - the current ones have never shown any visible deflection, even under hard braking.

The seat will be quite a lot wider and the motion ratio of the front suspension increased a little (ie the air springs will move inboard about 20mm). The combination of these two factors will move the springs closer to the edges of the seat, providing better stiffness.

The frame will additionally be stiffened to better resist flex under high pedal forces.

I’ll also include some couplings that will allow the frame to be disassembled for transport. I hope that the slight decrease in front and rear suspension arm weight will offset the addition of the disassembly couplings. (But then again, the stiffer frame will weigh more.)

So the changes will be relatively subtle but I think will make for a better overall machine and one with which I will feel happier. In my estimation, the current trike is perhaps at 8/10 – and I’d like to make the next a personal 9.

Why Do It? Think of a vehicle you can design and build yourself. It’s like a race car formula where you’re strictly limited only in engine power. But, unlike any racing formula, you can drive this vehicle on the street. And, in the design of the vehicle, you can do pretty well anything you like. Furthermore, it will cost you nothing to run and it’s light and compact enough that you can build it in a small space. You’ll be rewarded by innovation and skill – and heavily punished in performance if you go even a little over-weight. In terms of suspension and steering – castor, camber, scrub radius, roll stiffness, toe, bump-steer, anti-squat, anti-dive, steering ratio, Ackermann, spring rates, wheel travel and lots of others – the more informed and knowledgeable you are about these aspects, the better will be the end result. I’m lucky enough to live in an area of sufficiently steep hills that real speed can also be part of the riding experience, but even without that, I’d be convinced simply by the fact that with no design approval, registration or rules, you can steer your own vehicle on the road. And if you can’t get past the fact that your legs propel it, start thinking of electric assist. The local rules might say 200W maximum, but the reality of electric motors and batteries is that a continuous 200W can be a peak of nearer 1kW...