Another Human Powered Vehicle! Part 12 - First Testing

This article was first published in AutoSpeed.

Testing started off well, but then came the problems...

The first step was to take the HPV onto the road, some of the brazed joins still just tacked together, an old seat quickly looped into place and the brakes only temporarily bolted on.

To say I was looking forward to this is the understatement of the year: what would the HPV actually be like on the road? Especially with the radical front suspension - that causes such dramatic changes in camber, castor and track – but also in the use of front linked air-bag springs and a rear air-bag working with a separate pressure canister?

Initial Test Results

1    Steering – Slow - 9/10; Fast - 2/10

At low speeds the steering feel was good, and didn’t dramatically change in weight during cornering (given the varying castor, a real concern). It was never heavy enough to be a worry – but neither was it eerily light.

But at high speed it was twitchy. To some extent this is typical of Greenspeed trikes (and the steering being used is a direct lift from a Greenspeed) but in this case the trait seemed even more pronounced. At over 30 km/h I thought it would be easy to lose the trike – small steering inputs resulted in very rapid changes of direction!

The turning circle (about 5 metres) was fine and the rose-joints of the steering tie-rods provided sufficient angularity that binding didn’t occur in all normal combinations of steering and suspension deflection.

The front suspension arms also appeared sufficiently stiff that quick steering inputs didn’t cause noticeable deflection. Partly responsible for the good slow speed steering feel (and high speed twitchiness?) was the lack of any bushes in the front suspension and steering – small steering inputs weren’t lost in bush deflection.

2    Ride Quality – 9/10

The ride quality was exceptional, feeling every bit as if the spring frequency was as low as previously measured.

Especially noticeable when compared with my previous suspension HPV was that the lack of ride ‘jiggliness’. For example, riding off the edge of the bitumen onto coarse gravel and dirt provided almost no change in ride quality.

Big bumps were also absorbed very well, especially those experienced when riding over steep drops. However, rising ‘step’ input harshness (eg running over a 40mm high block of wood positioned on the road) was higher than expected, perhaps the result of the anti-dive built into the front suspension.

3    Ride Height – when revised, 8/10

This initial testing showed that the front ride height was a little short. This was a problem because (1) when the airbag pressures were lower than normal, the rider’s heels could get very close to the road, and even touch the road over big bumps, and (2) chain ground clearance was too small.

The upper air-bag mounts had only been tacked into place so they were easily able to be moved downwards by about 25mm, solving this problem.

4    Damping – 9/10 Front, 7/10 Rear

Front suspension damping occurs primarily through the track change that takes place during suspension deflection. Because the tyres are forced to move sideways across the road (the camber changes a lot, remember), the tyres are effectively damping the suspension movement. This affect is very noticeable when first getting on the trike: until it rolls forward, the suspension stays in full droop position. That’s very much like cars with swing-axle suspensions – when lowered by a jack to the ground, their suspension initially stays at full droop.

The front suspension felt very well damped, however, the rear suspension damping was not as successful. The rear air-bag was designed to be damped through being connected via an adjustable flow valve to a separate canister. (See Another Human Powered Vehicle! Part 4 - Testing Interconnected Airbags for more on this.) This set-up proved less effective on the road than on the test bench.

5    Body Roll – 8/10

Body roll, even with the 19mm x 0.9mm anti-roll bar and high roll centre, was greater than expected. Interestingly, the body roll was largely the same irrespective of whether the front airbags were pneumatically connected or separated. This result differs from what was observed on the air-bag test bench; however, the on-road HPV runs a much stiffer anti-roll bar than the test bench set-up, so the anti-roll bar is the dominant determiner of body roll.

Should the body roll prove to be excessive at high speed, a simple swap to a 19mm x 1.2mm tube (ie thicker wall) sway bar can easily be made.

6    Handling Grip – 9/10

Handling grip was good: with the increasing bump camber, and the increasing bump castor (the latter in turn giving even more camber!), the outside wheel could be seen and felt to be working hard. The wide track (the trike has a higher than normal track:wheelbase ratio) gave good stability. (See Lateral G’s breakout box below.)

7    Power Suspension Compression – 9/10

The propensity of the rear suspension to compress (or extend) with pedal strokes could also be assessed during this testing.

Different rear idler pulley heights were used until pedal suspension extension cancelled-out compression due to weight transfer. The amount of rear squat/extension that occurs will also depend on whether the rear carrier is fitted (and how much load there is on the carrier) so the rear idler pulley height was made adjustable over a 50mm range. However, the centre of the adjustment range was determined during this testing.

8    Brake Attitude Change – 6/10

A dynamic deficiency was found under brakes.

When braking hard, two things occurred in sequence. The first was that the rear suspension immediately extended to full droop position. Then, when braking harder, the rear wheel would start to lift. The lifting of the rear wheel was no big deal: it’s to be expected with the chosen weight distribution. But the two-stage non-linearity in vehicle attitude was disconcerting. That’s especially the case because the front suspension’s anti-dive works well: all the vertical movement felt like it was at the back!

Improved rear spring rebound damping would help stop the fast spring extension under brakes.

The Party Trick On the Greenspeed trikes that I have ridden, and my first suspension trike, getting up on two wheels (ie lifting one front wheel) was, um, a pretty big warning. In fact, in skidpan testing of my first suspension trike, I nearly rolled it – I had to quickly put down a hand on the ground to stop the trike tipping too far. So when I cornered hard on the new trike, and a wheel lifted, I figured a pretty quick turn out of the corner was required to bring the errant wheel back down. But then, when I practiced a bit further, I found that the new trike has the most amazing ability to be ridden on two wheels, one of the front wheels about 30cm off the ground. Turn-in hard to get a wheel lifted, lean a bit the wrong way, and then ride along, balanced on two wheels. The ease of balance is so great that I can ride in circles, ride in a straight line, pedal, look around – the lot! – all on two wheels. Given that once you’re up on two wheels on a suspension trike, the suspension roll centre ceases to be relevant (the roll axis is at ground level and joins the front and rear wheel), the exceptional ease of balance is nothing to do with the roll centre. Instead perhaps it’s the fact that when it’s up on two wheels, the front wheel that remains on the ground has relatively little camber and lots of castor, allowing precise and light steering. However, that doesn’t explain the fact that even on smooth surfaces, the suspension can be seen to be working quite a lot when two-wheeling – perhaps it softens the roll response. Whatever the reasons, the trike is amazingly easy to pedal on two wheels - if nothing else, a pretty impressive party trick!

Conclusion

So the most major dynamic deficiency found in this testing was the steering twitchiness at speed. I tried various combinations of toe, but while toe-out improved matters, the resulting drag was easily noticeable when pedalling. It looked very much as if the amount of castor would need to be increased – something for which there is no adjustment...

And then, after two days of thrashing the machine (by this time fully welded) across bumps and jumps, up on two wheels, braking hard and cornering hard, I found a structural failure in the frame. Where the two front spring support arms join the main longitudinal, the main backbone tube was being crushed and distorted as the spring supports bent upwards. No wonder ride height seemed to be dropping and the front of the seat seemed to be getting narrower...

So despite most of the testing showing excellent results, in two key areas – steering and frame integrity – more work needed to be done. And not minor work either....

Lateral G’s Maximum lateral acceleration – or how hard a machine can constantly corner – is easily measured on a skidpan. I live at the end of a quiet street at which is located a smooth, bituminised and flat cul de sac. I decided to use this area to lay out a temporary skidpan testing circle. A 6 metre diameter circle was marked out by the simple expedient of temporarily driving a nail into place in the middle and then stretching out a 3 metre rope, looped at one end around the nail. At the other end I placed a piece of chalk and then stretching the rope tight, marked out the circle. (And then removed the nail!) With the diameter of the circle known (or actually half its diameter – the radius) the equation to work out the centripetal (lateral) acceleration is this: 39.48 x radius Centripetal acceleration = ---------------------- time squared ...where radius is in metres, time is in seconds and the answer is in metres per second per second. So with the circle 6 metres and a time of (say) 5.8 seconds, the lateral acceleration is 3.52 metres/second/second. Divide this by 9.81 to get the results in g’s – 0.36 g. The tyre pressures were set at 60 psi and then testing of the new trike was undertaken. (Incidentally, 60 psi is much higher than I normally run but it’s in the middle of the ballpark most people seem to use on recumbent trikes.) The result of this testing showed that the new suspension trike develops a maximum continuous lateral acceleration of 0.41g. So, is this good or bad? Previously, I’d tested the other trikes to which I have access, and these results are shown in the table below. Greenspeeds GT3, GTR, GTC and X5 are all non-suspension trikes. JET (Julian Edgar Trike) is my first suspension trike. Trike Lateral Acceleration (g’s) Greenspeed GT3 0.36 Greenspeed GTR (old model) 0.37 Greenspeed GTC* 0.37 Greenspeed X5 0.39 JET 0.41 *ridden by Georgina Edgar – I’m too large to fit safely on the machine As can be seen, despite its higher centre of gravity and soft suspension, the new suspension trike matches my previous suspension trike and betters all the Greenspeeds. However, note that the track of the new trike is also the widest of the lot...