Air 150 Recumbent Trike, Part 2

This article was first published in 2008.

A major problem in the tiny minority field of recumbent trike design is that any assessment of the worth of a new machine seems to be always couched in respectful, group-hug terms. So someone produces a suspension trike design that has clear and obvious design flaws – and then everyone falls over themselves to tell the constructor how wonderful it is! Of course, this approach results in the perpetuation of mediocrity... That’s a major reason why in this article, I present as much hard data as I can assemble and, where that data isn’t obtainable, try to be as honest and objective as I can be in my assessment. That way, those building new (and potentially better) designs can see if in fact they are progressing.

1. Ride Quality

The main objective in developing a suspension trike was to give a much improved ride quality. So, what was the outcome?

Assessment

To data-log ride quality I use a battery-powered accelerometer (a tiny chip on a very small PCB) and place it on the seat. I then sit on it (it's only a few mm thick). I then log to a Fluke Scopemeter. (Incidentally, the accelerometer and board are very cheap and could be logged to an old laptop.)

On the Scopemeter screen, the bigger the peaks and troughs of the graphed line, the greater the vertical accelerations that are being undergone by the rider. That means you can immediately get a feel for the data by thinking that if the ride experienced by the rider was absolutely smooth, the trace would be a flat line. The greater the bumps being felt, the bumpier the trace.

Here I have shown the logged vertical accelerations for three different trikes over the same stretch of road. The road is from my house, up a hill, down a slight descent to a roundabout, then back home again.

The road is absolutely typical of the roads around where I live - bumpyish bitumen. On the ride I am slow up the hills and about 30 km/h down them. It takes about 5 minutes.

The trikes are:

- a standard commercially produced trike, a Greenspeed GTR (20 inch wheels, 60 psi tyre pressures)
- my first suspension trike JET (20 inch wheels, 60 psi tyre pressures)
- Air 150 (20 inch wheels, 60 psi tyre pressures)
- Air 150 with 30 psi tyre pressures (that I normally run)

The comparison is fascinating - and revealing.

The unsuspended GTR is as it feels - lots of bumps, some at about 1/3rd of the way into the ride very harsh. (For comparison’s sake, let's call the max vertical measured acceleration - ie the biggest bump - '10'.)

JET did very well indeed. It was a very heavy trike (so had a high suspended mass) and used polyurethane bushes in every suspension pivot. Steel springs were fitted (see series starting at Building a Human-Powered Vehicle, Part 1). This design both absorbed vibration and also bigger bumps. (On the same scale, the biggest bump is now a measured 6.2.)

The current Air 150 (airbag springs, ball bearing pivot points) does even better than JET on bigger bumps. (Note that while the absolute voltages are different, the scale remains the same so direct comparisons are still possible.) However, you can see that it's not as good as JET on vibration (there are lots more jiggles in the line) and medium size bumps can still get through. However, on the same comparative scale, the biggest bump has now dropped to 4. (The huge bumps at each end of the trace are when I get on/off the trike and also my driveway drop-off, over which I varied in the speed on the different tests.)

The final trace with the Air 150 at the 30 psi tyre pressures I use shows that the 'vibration' type of bumps nearly all gone. The biggest bump achieved in the actual ride is only 2.2 (so over the GTR, the maximum bump acceleration has been reduced by a measured 78 per cent!) and you can see it better approaches JET in small bumps and vibration, while being much better in big bumps.

The reason for the improvement with the lower tyre pressures might be two-fold. Obviously the lower air pressure in the tyres means the tyre vertically deflects more over bumps – it’s a softer spring. But because damping of the front suspension is by track change, a lower air pressure in the tyres also allows them to have more sideways wall deflection, softening the damping on small bumps.

Summary:

I think that in ride comfort the design has been largely successful.

The measured reduction in the affects of large bumps by about 80 per cent over a non-suspension trike is very much as it feels on the road – the Air 150 has a luxurious, cushy but well damped ride that is far less wearing on the rider that the relatively harsh, juddering ride achieved on non-suspension machines over the same terrain. Note also that the above data-logging is on just a bumpy bitumen road at normal speeds – the ride improvement over a non-suspended trike on high speed bumps, and over really bad surfaces at all speeds, is massively improved.

In fact, I would suggest that it has the best ride of any small-wheeled human powered vehicle in the world.

The Drop-Off The following photo sequence is over a drop-off that, at its greatest height, is about 200mm. If you think this is an inappropriate test, consider that just the day before this sequence was photographed, I was riding on an urban cycle path that ended abruptly and without warning in a 45 degree gutter of about the same height. I rode straight off it.

2. Steering

The steering uses modified Greenspeed “non-crossover” steering components.

By far the hardest thing to get right on a long travel suspension design is the steering. This is primarily because it’s very easy for bump steer to occur – this is when toe changes are associated with suspension movement. For example, a vehicle might have toe-out on bump and toe-in on droop.

One result of bump steer is a steering non-linearity when cornering – for example, toe-in on bump will result in more steering occurring on initial turn-in, making the machine darty.

Assessment:

Quite late in the development, I raised the outer steering tie-rod ball-joint height. This was primarily to achieve better clearance between the tie-rod and the frame but at the same time I also dialled-out some bump steer that had previously been occurring. However, this change also apparently altered the Akermann compensation and appeared to increase the turning circle a little (I don’t understand why).

Whether it’s because the dynamic toe change is now better controlled, or because Ackermann appears to be lessened, or some other reason (or combination of reasons!), happily the steering is now much improved. Specifically, it has superior road feel and better self-centre’ing. (For those experienced on recumbent trikes, I think the negative scrub radius steering of the Greenspeed GT3 Series II is still superior to my design, but by only a little.)

The feel of the steering is also quite dependent on the tyre pressures being used – as you’d expect, it gets heavier and less responsive as tyre pressures decrease.

The steering achieves the very difficult compromise of being sufficiently light and responsive at slow speeds on cycle paths and the like, but not too sensitive at high speed downhills. That said, you still need a light touch at high speed – using more a pressure on the steering handlebars rather than a definite push/pull.

Static versus Dynamic Exactly as it is with cars, the suspension angles that are measurable with the vehicle stationary and not subject to dynamic loads may well not be the suspension angles being achieved in hard use. On the bench, the steering and suspension of the Air 150 have been very carefully set up to give zero bump steer. But on the road, some pics appear to show toe-in on bump – but then when you have major changes in camber angles with suspension deflection, assessing toe by eye becomes rather difficult. In short, I don’t know what toe changes occur in action – and there’s no easy way to find out.

Summary:

The modified Greenspeed steering works very well. However, if I was starting the project again, I think I’d look closely at using negative scrub radius for the potentially better road feel and self-correction that could result.

3. Handling

Many believe that, since a suspension trike has to have greater ground clearance than a non-suspension design, the higher centre of gravity will inevitably result in poorer cornering. However, this doesn’t take into account the fact that the front track can also be increased.

Assessment:

Skidpan testing is an excellent way of determining real-world maximum lateral acceleration - that is, how hard the machine can constantly corner. In addition to assessing “over-turnability” (most trikes will pick up the inside wheel before sliding), it also assesses:

- How well cornering can be sustained (it’s usually easy to get a higher peak figure)

- Steering accuracy and response (without good steering, it’s hard to stay tracking accurately around the circle)

- How well power can be applied (apparently, some machines cannot be pedalled at high steering locks...!)

For these reasons, skidpan testing is much better than simply placing the machine on a large board and then tilting it until the trike and rider start to overturn (and then noting that angle of tilt). A tilt test is vastly optimistic in the calculated maximum lateral acceleration. (So be sure to find out how lateral acceleration ["g"] figures were obtained before accepting them!)

Of all the performance assessments that can be made, skidpan testing is the easiest to conduct, is zero cost, can be done nearly anywhere – and yet shows most accurately something normally hardest to quantify.

I live at the end of a quiet street at which is located a smooth, bituminised and flat cul de sac. I 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. (As noted above, 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 results of testing are shown in the table below. Greenspeeds GT3, GTR, GTC and X5 are all non-suspension trikes. JET (Julian Edgar Trike) was my first suspension trike, Air 130 was the first using airbag suspension and Air 150 is the current machine.

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
Air 130	0.41
Air 150	0.36

*ridden by Georgina Edgar – I’m too large to fit safely on the machine

As can be seen from this listing, the Air 150 has gone backwards over my previous two designs. This is because the higher seating position gives a higher centre of gravity and so, with similar track and about the same weight distribution as JET and the Air 130, the trike tips more easily. However, it’s still as good as the Greenspeed GT3 and only a bee’s dick behind the GTR and GTC.

But what if the suspension is lowered? (On a smooth surface like a skidpan, you don’t need bump absorption.) I lowered the airbag pressures (a 10 second job), so dropping the height of the machine. Note that even at this ride height there was still about 1.5 inches (38mm) of bump travel – in most people’s views, more than enough! Maximum measured skidpan figure then rose to 0.39 – the same as I have measured with the Greenspeed X5.

There’s no reason that on smooth surfaces the Air 150 couldn’t be run at this ride height all the time, but I am certainly not going to do that – so the measured number stays at 0.36.

While smooth skidpan testing is very effective, it clearly does not tell the whole story. For example, yaw response (how quickly a machine can change direction) will vary between designs, as of course will bumpy road handling.

I don’t have any way to measure yaw response but subjectively I’d suggest that the lighter, smaller designs (like say a Greenspeed X5) can change direction more rapidly than the Air 150. In comparison the Air 150 feels ponderous when flicking in and out of say a line of witches hats.

On bumpy corners the Air 150 suspension works very well. On bumpy corners taken very quickly, the inside wheel will lighten (sometimes to the point of being off the ground!) and then the outer suspension airbag can be seen working all around the corner. There’s an important point to note here. Because of the semi-leading front suspension design’s high roll centre, even when the trike is on two wheels, the front outer suspension does not unduly compress. That is, suspension jacking helps offset the outer wheel’s suspension compression. In effect, this stiffens the suspension when it’s at maximum roll.

However, corner fast enough on really bad surfaces and the rear suspension can develop a hop. Perhaps better damping could dial this out but since it occurs only in extreme conditions, I figure it’s a good warning that things are about to let go.

Tyre Pressures and Skidpan Testing The skidpan test of the Air 150 was run at the tyre pressures I use – 30 psi. In the recumbent triking world this is seen as a very low tyre pressure but I like it for two reasons – firstly, it improves the ride over high-frequency, low-amplitude bumps (like coarse surface bitumen), and secondly, I think the steering is better. The trade-off is increased rolling resistance. And, after the skidpan test, I have another reason for the low pressures – the trike’s grip level is improved. As noted above, a consistent 0.36g was obtained with 30 psi pressures. At 60 psi pressure, this dropped to 0.32g – again, with consistency. I think that there are two reasons for the improvement in lateral acceleration with lower tyre pressures. I would guess that the round cross-section tyres put more rubber on the road (ie the tyre better ‘keys’ into the surface), and from riding the machine with the two different pressures, I can say that it’s easier to go faster with the lower pressures. This is because when the trike is on the edge of lifting the inside wheel, the tyre deflection makes the steering less nervous when you’re trying to balance the trike at that cornering force. I doubt anyone would have guessed that result – another reason to do careful, real-world testing.

Summary:

I am quite sure that a low-slung dedicated racing recumbent trike could exceed all the lateral acceleration figures listed above, but my aim was to match on smooth surfaces the cornering prowess of general purpose commercial trikes – and that’s mostly been achieved. And, as you’d expect, when cornering on bumpy surfaces, the Air 150 is far better than the non-suspension trikes – the tyres stay in touch with the ground rather than skipping across it. (I'd also suggest that on wet bumpy surfaces my design would be better than all available general purpose trikes.)

Max Angle of Roll The skidpan is also a good place to measure the maximum roll that occurs. The Air 150 uses interlinked front airbags (softening bumps but also reducing roll resistance) with the roll stiffness provided by the large sway bar. However, its affect is diluted by it being connected to the suspension arms well inboard of the wheels. So what is the maximum angle of roll (before a wheel lifts off, anyway!)? As this pic shows, with a line drawn through the axles and another across the top of the rear vision mirrors, the max roll measures about 4 degrees (not including tyre deflection.)

Suspension Travel The sequence of photos used earlier in this article of the trike being ridden over a sharp drop is just one of a number that were taken. The pic shown here, taken from another sequence, very clearly shows the front suspension travel that is being used in this situation. The red arrow points to one of the front airbags. This shows that near to full travel is being used. The purple arrow points to the chain on the slack side – the downwards acceleration that is being experienced can be seen in the chain being left behind (ie appearing to fly upwards).

6. Conclusion

So what are the good and bad points of the design? My summary (with ratings out of 10) is this:

• Seat – roomy, supportive and well-shaped seat; width allows elbows to be tucked against seat sides for stability; wide seat results in higher centre of gravity (because extra sag needs to be taken into account); comfort would be enhanced by a cushion under bum section - 8/10

• Ride quality - outstanding over large bumps, good over small bumps, not floaty, little pedal squat, improves with large loads, some rear suspension extension under brakes, a small front/rear symmetrical bob at high pedal cadence – 9/10

• Handling - predictable, precise, not upset by bumps, adequate although not outstanding maximum lateral acceleration, very predictable when on two wheels, not upset by large loads – 7.5/10

• Steering - precise but not nervous, no bump steer, good at both high and low speeds, can pedal-steer a bit at high speed in top gears when pedalling hard – 8/10

• Brakes - excellent fade resistance, progressive and easily modulated, twin independent discs require even pressure, can lift rear wheel under hard braking – 7/10

• Frame - very little boom bending, small amount boom twist, able to carry large loads, low weight for size of machine but heavy in absolute terms - 8/10

Clearly, from this scorecard I am pretty happy with the end result. For those who think the Air 150 too large (and so overweight), something like perhaps 15 per cent of the mass could be taken out by the use of smaller 16 inch wheels and a proportionally smaller width and length. If the rider was prepared to put up with boom flex typical of some commercial trikes, a little more weight again could be shaved off.

However (I guess as is self-evident since I made it to suit me and no one else!), I love its size and the resulting roominess. I think that ride/handling compromises can always be improved: there’s not a ride I go for on the machine where I don’t think: “Hmm, steering feel was a bit bereft then” or “Impact harshness over that bump too high” – but there’s also barely a ride where I don’t think: “Hell, the trike handled that bump well” or “Gee, the frame is stiff – I am climbing my 40 per cent gradient [no mistake!] driveway and I’m not even in bottom gear!”

But as the negatives above show, I certainly don’t think the machine is perfect...

Specifications All specifications are approximate and where appropriate, with trike loaded with rider and with the carrier fitted Overall length: 205cm Overall width: 94cm Overall height: 78cm Seat height: 41cm Track: 91cm Wheelbase: 98cm Turning circle: 5.5 metres Weight (in base trim): 23kg Wheel diameter: 20 inch Brakes: Front wheel Magura Big hydraulic discs, separate hand levers Gears: 81 Front Suspension: 30 degree semi-leading arms, 25cm high roll centre, anti-dive geometry, interconnected Firestone airbag springs, 32 x 0.9mm anti-roll bar, damping via track change Rear Suspension: longitudinal trailing arm, Firestone airbag spring, custom valved Yamaha R1 motorcycle steering damper, active anti-squat chain pull line Suspension and anti-roll bar pivot points: 30 x 9 x 10mm sealed ball bearings, 10mm high tensile Allen-key through bolts Steering: modified Greenspeed non-crossover, underseat steering Kingpins: modified Greenspeed zero scrub radius, bronze bushes Static loaded camber: negative 5 degrees Maximum dynamic camber: plus 8 degrees to minus 12 degrees (plus 3 to minus 12 normally used) Static loaded castor: 7 degrees Dynamic castor: 6 – 8 degrees Scrub radius: zero Absolute maximum suspension travel front: 170mm (130mm usually used) Maximum suspension travel rear: 130mm Track change with full suspension travel: 100mm Static deflection front: 50mm Static deflection rear: 50mm Calculated front and rear natural frequencies: 2.2Hz Bump steer (over full suspension travel): too low to easily measure!