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.
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.
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’.
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
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
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.
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
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
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
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.
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
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
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?
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
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
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.
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
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....
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
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.
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
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
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.
So the proposed spec list reads something like
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
Next week: designing and building the rear
Front Suspension Designs
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.
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.
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
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.
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.
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
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?!