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Building a Human Powered Vehicle, Part 7

Tuning the suspension

by Julian Edgar

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At a glance...

  • Fitting a sway bar
  • New tie rods
  • New lower wishbones
  • New upper wishbone bushes
  • Damping
  • Data-logging the suspension
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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 design of the suspension; 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; 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...

Road Testing

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So now I was on the road. After hundreds of hours of work I could assess steering feel and weight, ride, handling, suspension bob with pedalling – and all the rest that I’d tried to optimise in the design and build process.

So what is it subjectively like?

Let’s start with the ride quality. Realistically, the ride quality provided by the new trike depends on two things – (1) the surface on which the machine is being ridden (amplitude, sharpness, wavelength of bumps), and (2) how fast you’re going.

The latter is very important – even a brick can be ridden over with little harshness if you do it at a snail’s pace. (That’s why non-suspended bikes can be ridden up and down kerbs – you just do it very slowly!). So if an HPV is to be never ridden faster than about 10 km/h, I don’t think an elaborate suspension system is worthwhile.

So while at very slow speeds you can see the suspension working (on typically bumpy bitumen roads around where I live, wheel movement in these conditions is about 20mm) and while the sharpness is certainly taken off bumps, the ride isn’t really transformed over a non-suspension HPV that uses the same 20 inch wheel diameters.

But even that’s not quite true. The entrance to my driveway is a 75mm (3 inch) high, 45 degree chamfer where the bitumen road finishes and my driveway starts. (I leave the bump there because it’s perfect for testing the impact harshness of press cars at low speeds!). And in the same way, it’s also a very tricky bump for an HPV – especially because you often negotiate it while braking hard after riding down a steep slope. When braking, the non-suspended Greenspeed GTR is very harsh indeed across the bump, whereas the suspended trike is far better.

So in some low speed conditions, the suspended trike is far better that the non-suspended trike, but as a general statement, at low speed there’s usually not a lot to pick between them.

The differences really start to show as you go faster than 10 km/h, and rise exponentially from that point. At 15 – 20 km/h (a very easily achievable cruising speed on the flat) the ride of the suspended trike is clearly far better than a non-suspended machine. A road that with the Greenspeed GTR gives constant impact harshness and three or four back-jarring bumps is despatched by the suspension HPV with zero of both. You can feel the road isn’t smooth but you have to look down at the road to see the bumps coming - you simply cannot feel their magnitude. (Also see the section later in this story on data-logging vertical accelerations...)

At these speeds, typical urban riding (cycle paths, footpaths, ramps from roads up to footpaths) uses about 60mm of the front suspension travel (bump and rebound) while the rear uses about 33mm. Note: most of this suspension movement occurs when negotiating ramps from roads to footpaths.

At 40 - 50 km/h (eg downhill) the difference between a non-suspended HPV and my suspension design is like chalk and cheese. Bumps which literally launch the non-suspended GTR into the air (not nice – a huge impact and very hard to keep steering control) are on the suspension trike simply a rapidly passing bump-thump. You know you’ve gone over a sharp bump but the tyres stay on the road and the rider isn’t disconcerted – or sore afterwards!

So the sweet spot for the suspension trike is when you get moving. In fact, at 15 – 60 km/h the ride is simply transformed over a non-suspension HPV - you just glide along.

One of the reasons that I have described in detail such a variation in speeds is that my local roads are all very steep. How steep? Well, some are hard to walk up – you need to be pretty fit to walk up one particular road without stopping several times for rests. Therefore, on any ride that I do around here, I might be in 1st gear for 5 or 10 minutes while climbing a hill at 4-5 km/h, and then, seconds later, be in 72nd gear, pedalling as fast as I can at 65 km/h! On the way up the hill, I curse the extra mass imposed by the suspension; on the way down the hill, I revel in the ride and control that it gives me.

The suspension trike is exemplary in off-road work. My test road doesn’t have gutters and in some places the soil adjacent to the bitumen has been channelled by the rain to a depth of 10-15cm, forming steep-edged depressions about 40cm wide. I can ride straight through these channels while the suspension simply articulates its way over them. The HPV moves vertically but there’s absolutely no impact harshness or sudden bounces. In these conditions the front and rear suspension travel used can be as high as 90mm.

Note: for all the ride tests described, tyre pressures were set at 90 psi, far higher than I can run in a non-suspended trike without experiencing back pain.

It’s a lot of fun heading down a bumpy hill on the suspension HPV, noting with delight how well it copes with the bumps I know so well from testing dozens of different cars over the same roads.

So the ride is pretty good – what about the handling and steering?

At this stage the HPV exhibited quite a lot of body roll when turning hard, although even with this amount of roll, the outside tyre still gripped extremely well. The cornering limit was reached with understeer, to be expected with a three-wheeled machine with most of the mass positioned between the two front wheels. But unlike a trike without suspension, the machine did not pick up an inside wheel when cornering very hard – the outside suspension compressed and the inner suspension drooped so that both wheels stayed on the ground. But the body roll was disconcerting.

Dynamic (Im)Balance

On my Greenspeed GTR I find that if I am pedalling very quickly in a tall gear (which means travelling at 50 – 60 km/h), the front of the trike develops a sideways oscillation. This translates to a steering input, causing the machine to swerve from side to side down the road. If pedalling fast enough and travelling fast enough, this swerving can get bad enough that I have to stop pedalling – then the machine settles down again.

Many people suggest that this movement is the result of the rider inadvertently steering as their arms move in sympathy with their legs, but I found that even if holding my arms as still as possible (or even taking my hands off the steering levers!), the same thing occurred. I then decided that it was a result of insufficient self-centre’ing castor – one of the reasons I decided to give my suspension trike greater castor. Another way I attempted to get rid of this trait was by using a slower steering ratio.

In the main these two approaches have been successful, but some weaving still occurs when pedalling fast at high speed. Why?

I now think it is the dynamic imbalance of the rotating assembly comprising the legs, feet and pedals. In Fundamentals of Automotive Engine Balance (Thomson, W. ISBN 0 85298 409X) the author actually uses cycle pedals as an example of a system that has a fundamental rotating imbalance.

When the pedals are in one position, the centrifugal force acting in opposite directions and offset from the centreline tries to rotate the whole assembly anti-clockwise. When the pedals are 180 degrees different in position, they try to rotate the assembly clockwise. Therefore, in each full rotation of the pedals, there is a twisting force one way then the other way.

In a normal bicycle, where the pedals are located about midway between the front and rear wheels, this force is easily resisted. But in a recumbent trike (and, I assume a recumbent bike), as the pedals are ahead of the steered wheels, these forces actually steer the machine. Of course, it’s not just the pedals that are rotating: you also need to add the mass of the feet and perhaps half the mass of the legs – quite a lot of weight whizzing around!

This affect could be overcome by adding balance weights – but of course the mass of the whole HPV would also then rise.

Tie-Rods

Toe is set as close to zero as the adjustment allows. This decision was made not on the basis of handling (significantly, toe changes seem to make little difference) but on the basis of tyre scrub and drag. Setting a large amount of either toe-in or toe-out considerably increased drag, to the extent that when the HPV was pushed along by hand, the extra resistance could be easily felt.

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However, a problem developed with the tie rods. As shown in Building a Human-Powered Vehicle, Part 5, the steering system uses what are effectively four tie rods; two of the traditional sort connect the steering mechanism to the steering arms on the uprights, and another two transfer the motion of the two steering levers to the rest of the system. All four rods were made from aluminium tubing. To allow adjustment of length, and to allow the ball-joints to be easily connected, threaded spuds were turned-up from aluminium and then TIG’d into the ends of the tubes. Jam nuts were also fitted on these threads.

All worked fine for a while. But then one day I noticed that the toe had changed. That was especially significant because only the day before I’d very carefully adjusted the toe to be as close to zero as I could make it. Now it was out by quite a few millimetres. I inspected the tie-rods to find that the ones connecting to the wheel uprights were bending, not as might be expected in the tubular section but at the threaded spuds.

I replaced all four aluminium tie rods with threaded steel bar.

While previously mentioned in this series, it is worth repeating: if you run a lot of castor and/or steering axis inclination, the loads in a steering system can be very high indeed.

Sway Bar

With one of the major deficiencies being body roll, I decided to make and fit a sway bar.

This was difficult – the front suspension is very tight for clearances, primarily because of the combination of compact dimensions and 100mm of suspension travel. The long suspension travel and the limited length of the lever arms that could be incorporated in the sway bar shape means that long sway bar links were necessary if the angularity of these links wasn’t to become excessive as the suspension moved through its full travel.

Long sway bar links meant either positioning the sway bar high and attaching the links to the lower wishbone, or positioning the sway bar low and picking the links up off the upper wishbone. I did the latter.

Click for larger image

The sway bar was made from 10mm spring steel (a guess in thickness) which was cold-bent using a vice and brute force. Heating the bar would have made bending it much easier, but the bar would then have needed to be re-tempered. Because I didn’t want to weld to the bar, and using a die to place a thread on spring steel is very difficult, the sway bar had to be constructed so that the links could be attached to the bar without the need for threads or eyes.

This was achieved by using nylon bushes (machined on the lathe – hell, doesn’t nylon machine beautifully!) press-fitted to aluminium tubes that in turn were welded to the sway bar links. The sway bar links were made from square tube. The nylon bushes simply slipped over the ends of the sway bar with the bushes lubricated with plastic bush grease. Spring steel clips were then press-fitted over the ends of the sway bar. This gave strong and lubricated lower pivot points for the sway bar/link connections. (However, the disadvantage of this approach is that sway bar stiffness cannot easily be adjusted by moving the sway bar links.) The upper suspension connections were made with ball-joints.

The sway bar pivots in nylon bushes. To get these one-piece bushes onto the centre portion of the sway bar, a cut was made in each bush so that it could slide over the end of the bar and then be expanded sufficiently to get around the bends in the sway bar.

The other advantage of this mounting approach was that if the sway bar needed to be changed in thickness, all four bushes could be easily replaced to suit.

The body supports for the sway bar were formed by a transverse aluminium tube and flat plate mounts. The stainless steel clamps holding the sway bar pivot bushes in place were sourced from a boating supplies shop.

Testing the Sway Bar

The thicker a sway bar, the more that body roll will be resisted. However, the firmer will be one-wheel bumps and the less independent will be the suspension. In addition, the thicker the sway bar, the more the inside wheel will be picked up when cornering.

To be honest, I simply didn’t know what was desirable in terms of sway bar roll stiffness. I wanted to reduce body roll without needing to change springs or dampers, but I didn’t want to be often flying an inside wheel because the inside suspension no longer drooped sufficiently. I also figured that the sway bar was an area where a dozen iterations could easily be trialled, all with slightly different results.

But, whether by luck or the fact that any of a variety of bars would have been far better than having none, the 10mm swaybar gives an outcome with which I am happy. Body roll has been halved and the HPV takes a cornering ‘set’ far earlier. Flip-flop weight transfer in S-bends is vastly reduced... something I’d have thought was more dictated by damping rates than the swaybar. However, there is still enough inner suspension droop that the inside wheel stays on the ground.

But one-wheel bumps are firmer – in fact, big one-wheel bumps are quite a lot firmer! If a sway bar is to be fitted, it’s important to realise that spring rates should be considered in the context of the ‘bar stiffness. Had I known that a sway bar was going to be required, I would probably have used slightly softer front springs.

New Lower Wishbones

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As described in Building a Human-Powered Vehicle, Part 3, the lower front wishbones (which carry the whole forward weight of the machine: the upper wishbones don’t carry any weight) were made from square tube with lots of holes drilled in them for lightness. However, after the failure of the main frame where cracks had propagated from holes (see Building a Human-Powered Vehicle, Part 6), I decided I wasn’t happy with this design. I replaced these wishbones with two new ones, made from 33mm x 3mm round aluminium tube. In addition to being very strong indeed, these also look better!

New Upper Wishbone Bushes

Because the whole front suspension can be observed while the machine is being ridden, I was able to see that under hard braking something interesting was happening. What was occurring was that the upper wishbones were deflecting quite a lot on their bushes, with the outer upper ball-joints consequently moving forwards. This decreased castor under brakes and also dulled the initial brake bite.

To overcome this problem, the upper polyurethane bushes were replaced with ones machined from nylon.

(Note: if replacing polyurethane with harder material, be careful to size the new bush to suit the required dimensions, rather than just copying the poly bush. Poly bushes tend to ‘flow’ when being press-fitted and so are often a little oversize in outside diameter and undersize in inside diameter.)

With the new bushes in place, the upper wishbones are rock solid under brakes. The on-road difference isn’t huge but the brakes have more initial bite and the steering is a little sharper around centre (the bushes must also have been deflecting sideways a bit).

Dampers

In Part 4 of this series (see Building a Human-Powered Vehicle, Part 4) I described the suspension dampers that were being used. These are based on steering dampers from Suzuki GSXR 1100 motorcycles. Because of their original function, they have equal damping force in each direction. Testing of the rolling chassis showed that having equal bump/rebound damping resulted in a much firmer ride than was achieved without the dampers in place. After all, every bump wasn’t being resisted just by the spring but also by the damper! And the sharper the bump, the more the damper resisted the compression – dampers being the sort of device that rapidly increase in firmness with higher shaft speeds.

After initially considering internal or external valving changes, I decided to simply fill the dampers with very thin fluid - 2.5W 5 oil, designed for use in the front forks of motorcycles.

However, following extensive road testing, I decided that the bump damping was too great, especially on the front. To overcome this, I decided to pull the steering dampers apart and change the internal valving.

Then followed days of frustration!

Fitting an internal one-way valve that allows bump damping to be much weaker than rebound damping sounds simple but I found it unbelievably complex. The best results I could get were obtained by using a thin nylon washer backing a custom made piston with slots cut in it. On bump the nylon washer deflected, allowing fluid the pass through the grooves in the piston. On rebound, the fluid had to squeeze past the edges of the piston as the nylon washer closed off the slots.

Sounds good, eh? Yeah, but I found that anything positioned inside the damper within about 10mm of the nylon washer also affected dynamic flow – even something as simple as a circlip or small diameter washer on the shaft. The fluid inside the damper was obviously rushing around at such high speed that the presence of other obstacles totally changed the damping behaviour. And there was a further problem. After a few strokes, the fluid would get aerated, which changed the damping rates still further. After spending dozens of hours painstakingly building tiny assemblies that simply didn’t work, I decided to have a radical change of heart, and try the trike with no external dampers at all.

And the fascinating thing was that the front suspension was far better without external dampers! A ‘step’ input (like jumping on the trike) died away to nothing within 2.5 bounces.

But where was the damping coming from? The conventional answer is that friction damping was being provided by the upper and lower wishbone bushes, and also the ball-joints and the sway bar bushes. But how could that be when I’d taken such great care to make all these joints free-moving? The answer is that only a little of the damping was coming from this source.

The major form of front damping is in fact being provided by lateral movement of the tyres. Because the roll centre is not located at ground level, and because dynamic camber increases in bounce, the front track changes by about 20mm as the suspension moves from full droop to full bump. The only way that this can occur is if the tyres are dragged laterally across the ground. Given that each front tyre typically supports a mass of 55kg, the strong damping affect of this movement on the suspension is easy to imagine. The greater the bump, the more of this friction damping that occurs (and of course the greater the tyre wear!).

But the rear suspension is a completely different story. There is no lateral tyre movement through the full suspension travel, and with only two bushes, also less bush friction. The result is that running the rear without an external damper resulted in lots of oscillations. In fact, pedal torque inputs in first gear (that can cause squat) could be timed to be close to the resonant frequency of the rear suspension, so resulting in greater and greater bouncing. (See below for an explanation of suspension resonant frequencies.) To damp the rear suspension I fitted it with my most successful experimental asymmetric hydraulic damper.

Natural Frequencies

When I decided to forgo external front dampers I was initially a bit worried: would the trike develop a high speed suspension oscillation that could throw me off? They key to predicting the behaviour of the suspension system is to look at the natural frequency of the suspension. That is, the number of times per second it would naturally bounce if pushed down and released.

The natural frequency in cycles per minute (divide by 60 to get cycles per second, or Hertz) can be found by: 188 divided by the square root of the static deflection, measured in inches.

On my HPV, without dampers fitted, the front drops by 2.75 inches when the weight of the person and trike settles the springs. This indicates a natural frequency of 1.9Hz. So a problem would occur if you met a bump 1.9 times per second, that is, bumps 0.52 seconds apart. In that case, the natural frequency of the HPV suspension would cause the suspension movement to grow and grow.

At speed (say 50 km/h or 13.9 metres/second), the bumps would need to be regularly spaced at 7.2 metre intervals – pretty unlikely. However, at slow speed (say 6 km/h – 1.7 metres/second) the bumps would need to be spaced at only 88cm – quite likely if traversing a concrete footpath with regular expansion gaps. But then again, it’s only at slow speed so it’s very unlikely to be dangerous.

The further the frequency of bump input from the natural frequency of the suspension, the less excited the suspension will naturally get. A very high natural frequency is achievable only with almost no deflection – ie a bloody hard suspension! So that’s no good. A very low natural frequency requires a lot of deflection, in turn needing a lot of suspension travel and relatively soft springing. In fact 1Hz is deemed as optimal, but that requires a static suspension deflection of 10 inches!

And what of those HPVs running very small suspension travel? A suspension which compresses ¼ inch with load has a natural frequency of about 6.5Hz, or will get very excited by a bump occurring every 0.15 seconds. At 50 km/h this translates to a bump every 2 metres – a quite likely scenario. (Well, a much more likely scenario than a recurring bump every 6.2 metres!) At slow speed (6 km/h), it’s a bump every 25cm – again a quite likely scenario.

So in addition to a short suspension travel having limited compliance, it will have a higher natural frequency that is more likely to be excited by the frequency of bumps found in the real world. That’s why most cars have a suspension natural frequency in the 1 – 1.5Hz range, rising in performance cars to 2 – 2.5Hz.

So what differences occur in ride with relatively small changes in natural frequency? Lots! We’ve all experienced how cars ride better with extra people in them. If the car has a static suspension deflection of 4 inches, the natural frequency of the suspension is 1.6Hz. If the static deflection increases by another 1.5 inches when 4 people get in the car (giving a total static deflection of 5.5 inches), the natural frequency of the suspension has decreased to 1.3Hz. That’s why the ride has improved! (Of course, if there is insufficient suspension travel and the extra static deflection causes the suspension to hit the bump stops, that’s another story...)

It’s interesting to note that increasing natural suspension frequency from 1 to 2 Hz causes vertical accelerations experienced by the sprung mass to increase by several hundred percent.

References:

Dixon, John C., The Shock Absorber Handbook, ISBN 0 7680 0050 5

Gillespie, Thomas D., Fundamentals of Vehicle Dynamics, ISBN 1 56091 199 9

www.rqriley.com

Data-Logging the Suspension

  • Suspension Travel

When I started describing the suspension of the HPV – and the on-road results I was getting – many people in the recumbent trikes on-line community suggested that the amount of suspension travel I had was way in excess of what was required. “What sort of roads are you riding on,” they said, “paving stones?” I’d already measured damper travel by means of cable ties on the shaft but now, without any dampers fitted to the front suspension, travel was harder to measure.

So, primarily to show for doubters that at real speeds on real roads a lot of suspension travel is used, I decided to data-log the front suspension.

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I mounted a rotary potentiometer near the suspension and attached to its knob a sprung lever. This contacted the upper wishbone of the right-hand front suspension. As the wishbone moved, the lever also moved, rotating the pot. To log the output of the pot I used a Fluke 189 Scopemeter, measuring pot resistance. By manually moving the lever and checking the Scopemeter reading, I could work out what resistance corresponded to what suspension deflection. The relationship wasn’t perfectly linear but it was pretty good.

With the Scopemeter mounted on the trike, I pedalled to the top of my local test hill. This is a steep road with a bumpy bitumen surface. However, it’s just a local suburban road – it’s not a little-used dirt back track. On this road I achieve a maximum speed of about 65 km/h. At these sorts of speeds the non-suspension Greenspeed GTR launches into the air over the short, sharp Big Bump and is harsh and uncomfortable over many others.

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I stopped at the top of the hill, stepped off the trike and activated the logging. I then sat back on the trike, which settled to its normal ride height, fiddled a little with the pedal toe-clips, then rode onto the road and accelerated down the hill. I then started hitting the bumps and depressions (the Big Bump is clearly visible on the data log record), with the trike (and me!) remaining unfussed by the road surface beneath. At the end of the street, about 90 seconds later, I negotiated part of the roundabout which is placed in the cul de sac – the extension of the right-hand suspension through body roll when negotiating this radius can be clearly see. I then stopped, got off, and stopped the data-log record.

When analysing this record there are a few points to note. Firstly, on this road at this speed, the spring rate and travel are almost perfect – the suspension does not use full bump or full droop once. This means the tyres are far more likely to always be in good contact with the road.

Secondly, lots of travel is needed! The data log record shows that, if you don’t want to subject the rider to high vertical accelerations, the suggestion that only one or two inches (25 – 50mm) of suspension travel (ie the total of bump and rebound) are needed is completely wrong. In fact, to keep the natural frequency low (2Hz or below) requires soft springing – and so you must have lots of travel if you’re not to bottom out under bump, or fall into holes.

Thirdly, major suspension deflections were more often in droop than bump (although the Scopemeter-calculated average for this stretch of road was very close to normal ride height), which implies that on a vehicle of this sort, the static deflection should put the vehicle at about the mid-way point in suspension travel.

Fourthly, the friction damping provided primarily by the lateral movement of the front tyres looks pretty good – big suspension movements are not immediately followed by further large oscillations.

Finally, notice how at both the beginning and end of the data log record shows less suspension movement? This primarily because here the HPV was moving slowly – and if you’re going slowly, lots of suspension travel isn’t used.

To some extent the data-log record contained very few surprises for me – it’s largely what I could see and feel the suspension doing. However, I have the feeling it might surprise lots of other people!

  • Vertical Acceleration

So the front suspension data-logging was interesting, but what about vertical acceleration? Vertical accelerations are the key indicator of ride quality. To measure these, I bought an electronic accelerometer.

I logged the output in two ways – (1) on a fast reading multimeter that allowed the recall of average, min and max voltages - corresponding to maximum upwards acceleration, maximum downwards acceleration and average vertical acceleration; (2) a Fluke Scopemeter, which could continuously log vertical acceleration over the course of a ride.

Let’s look at the peak readings first.

The test track was the section of grass/bitumen/concrete shown in these two videos –

www.youtube.com

www.youtube.com

As it looks, this is a very bumpy, difficult section. The accelerometer was mounted on the seat of the suspension trike (in fact I sat on it – it’s only 25 x 25 x 2mm) and the test was ridden. This gave a maximum vertical acceleration of plus/minus 0.49g.

I then mounted the accelerometer on my non-suspension Greenspeed GTR trike and rode the same course. (Tyre pressures the same, same wheel diameters, same seat accelerometer location.) On the GTR the maximum vertical acceleration on the seat was a staggering plus/minus 1.3g! An upwards acceleration of more than 1g will have you literally accelerating out of the seat, while a downwards acceleration of 1g doubles your effective body mass. So as can be expected, a machine that gives plus/minus 1.3g of vertical accelerations is impossibly hard in ride.

So, compared with the Greenspeed GTR, the suspension trike reduced the magnitude of the peak vertical accelerations by 62 per cent.

However, the GTR is not intended for riding over such surfaces. What were the differences on normal roads?

This time I continuously logged using the Fluke Scopemeter. The test course was a ride from my house to a local roundabout – a very slow steep climb up my street (say 5 km/h) followed by a slightly downhill run to the roundabout, and then return. Max speed was about 30 km/h and the road surface is slightly bumpy. To me it represents a pretty typical ride. (It doesn’t include any massive bumps taken at high speed, for example.)

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I rode the course (it takes about 5.5 minutes) on the GTR, logged the result, then rode the same course on my suspension trike. The results are shown here. The top graph is the Greenspeed GTR and the lower graph is my suspension trike.

As can be seen, the difference in bumpiness is astounding. Not only are peak accelerations far lower, but for much of the ride, there simply aren’t any bumps. Remember, this is just a normal road, at normal speeds, measured at the seat and so very accurately replicating what the rider feels.

Another comparison can be made. My suspension trike gives a similar maximum vertical acceleration to the non-suspension GTR – but that’s when when my trike is ridden across the ultra bumpy off-road course and the GTR is ridden on the normal road!

For those really into this, I also measured peak vertical acceleration on a front wheel, ie the input acceleration. On the off-road course the max was plus/minus 1.7g. So my suspension trike reduced this by 71 per cent, and the Greenspeed GTR (assuming the same acceleration inputs), through the flexing of its frame and seat, reduced this by 24 per cent.

Other Design Aspects

  • Boom – the forward-mounted boom that supports the pedals has proved to be stiff in both torsion and bending. No changes were needed in this area. However, I found that the initial height at which I set the pedals was much too high. In fact, it seems to me to be as important to get their height right as it is to get the leg distance to them correct. (Lots of discussion is made of the latter – and all commercial HPVs are adjustable in this regard – but the former almost never seems to get mentioned.) I therefore made an adjustment mechanism that allows the height to be varied over a wide range.

  • Seat shape – before the front seat extensions were welded in place, the forward part of the seat was angled upwards (initially it was too flat). Some have suggested that the straight backrest (ie lacking a forward curve to allegedly match the shape of your back) is a retrograde step but I find the seat very comfortable.

  • The rear damper tends to rattle in its mount and needed neoprene strips inserted to allow some lateral movement but stop the rattles.

  • The braking system, which comprises Magura Big hydraulic front discs, was initially operated by a single lever. However, I decided to change to two individual levers. This was done for two reasons – (1) It suddenly occurred to me that with only a single lever, a hydraulic fluid leakage would result in total brake failure - not good when riding down a hill towards a dead-end at 60 km/h... and (2) Braking when cornering is much more efficient if the brakes can be applied unevenly – eg the rider can prevent the inner (unloaded) wheel from locking. But braking at high speed needs to be done with finesse – yank on one brake harder than the other and (of course) the trike will swivel around the slower turning wheel.

  • Spring preload – at one stage I inadvertently ran a different spring preload on one side at the front. (This was caused by a damper being wrongly adjusted for length). Despite the additional preload being only 7-8mm, this had a dramatic affect on that side’s suspension compression and ride quality. It also made cornering asymmetric, to the degree that when turning one way, the HPV would dramatically lift an outside wheel, but when turning the other way both front wheels would stay on the road.

So that's it for this series. But you might be interested to know that I almost immediately began a new, much better design - see Another Human Powered Vehicle!

The Carrier

Designing the rear carrier was difficult for a number of reasons.

Firstly, I wanted it removable so that the extra mass can be easily lost when load carrying isn’t needed. Also, making it removable means that when transporting the HPV, it can be easily disassembled to fit in a 1 cubic metre box. Incidentally, to fit into that volume, the rear suspension swing arm and wheel are removed (four bolts) and the front boom carrying the pedals and chain wheels unbolted (two bolts).

Secondly, the carrier needed to have a large capacity – I’d decided that if the HPV was too heavy to be a sports machine, it was definitely going to be a good tourer! And while I may never do it, the idea of self-supported long distance touring is very attractive – and for that, the more carrying capacity, the better.

Thirdly, the carrier needed to mount as much as possible of the load mass low and symmetrically about centreline of the machine

Click for larger image

The last one needs more explanation. Picture the rear carrier on a motorcycle and you can immediately see that it must be high enough above the rear wheel to clear the vertical movements of the wheel on its suspension. Apply that to an HPV and you can see that the load will be positioned very high relative to the centre of mass. Furthermore, it will be a long way rearwards of the wide-spaced front wheels, potentially making the three-wheeler unstable in cornering. The solution is to carry gear in low-mounted panniers, one positioned each side of the rear wheel. However, I wanted more carrying capacity than even large panniers could provide.

To solve the problems, the load-carrying areas of the carrier were positioned at two heights. One storage area (arrow) was placed immediately behind the seat and above the rear suspension swing-arm. This put it in front of the rear wheel (so within the wheel-base) and only 46cm above the ground. This area can be used for heavy gear. Behind that - and high enough to clear the 100mm of rear wheel travel - is positioned a large carrier, suitable for lighter objects. Furthermore, four panniers can be suspended low each side.

This gives a total carrying capacity of a massive 240 litres – two 40 litre panniers, two 38 litre panniers, 35 litres in the ‘heavy’ area, and 50 litres in the ‘light’ area. (Note: you can see how from a touring point of view, a trike totally wipes a bicycle!)

The carrier was made removable by the following process. Two machined spigots are located low on the frame. The angled carrier supports slide over these spigots. The upper supports are bolted into place. With appropriate tools, this allows the carrier to be removed or replaced in a minute or two.

The carrier was fabricated from 33mm x 3mm round aluminium tube and 3mm aluminium sheet. It has a mass of 5kg.

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