Air 150 Recumbent Trike, Part 1

Its design and build

by Julian Edgar

Click on pics to view larger images

This article was first published in 2007.

In Another Human Powered Vehicle! Part 16 - The Conclusion I wrote that I was building another trike, one that incorporated a host of subtle changes over the design that had been covered in that series. And now, some 6 months later, here it is.

Rather than describe the day by day ups and downs of this build (and there were lots of problems as well as successes!), and also attempt to relate this trike to the one previously covered, I’ve decided to present this new design as a standalone pair of stories.

If you want detail on why, for example, airbag springs were used rather than (say) polyurethane elastomers, or why an odd-looking semi-leading arm front suspension design was used, refer to the previous series where the design decisions for each part of the trike were covered in detail.

What you can see here is, I believe, the best riding human powered vehicle in the world.

In its suspension design it incorporates apparent simplicity with a ride vastly better than a conventional non-suspended trike and very much better than any other suspension trike whose design I have seen. In addition, it has outstanding frame stiffness that results in much more power from the legs getting to the ground. It also performs well in skidpan testing, matching many commercial trikes. In bumpy cornering, I think it would be superior to nearly all trikes.

At a bare 23kg it is certainly no lightweight in the recumbent trike world, but it is also a very large machine, with a width of 94cm and a total length of 205cm. With the addition of the rear carrier and pannier mounts (adding 1kg) it can carry up to 40kg of gear with little trade-off in handling and an actual improvement in ride quality.

It is equally at home racing down a hill at 80 km/h as it is toddling along on a cycle path.

And of course it also has some deficiencies. We’ll look at the range of good and bad points later, but first, let’s look at the technical make-up of this machine.

1. Frame


Light and stiff design using commonly available and easily brazed 4130 chrome moly thin-wall steel tube.


The frame is constructed primarily from 0.9mm wall thickness chrome moly tube. All joins were made with flux-coated nickel bronze welding rods and an oxy-acetylene welding set. The tube was sourced from

The central backbone, that extends from the pedal axis (“bottom bracket”) to near the top of the seat, is made from 44.5 x 0.9mm tube. The seat frame is curved to fit the shape of the human back and is made from 19 x 0.9mm tube. The seat rails were bought off the shelf from Greenspeed – they are for that company’s ‘Ergo’ seats. Supporting the seat frame are V-shaped tubes connecting to the central backbone. These are also formed from 19 x 0.9mm tube. The seat is 43cm wide – that’s very wide for a recumbent trike.

It was found that in the frame form so far described, excessive frame flex existed. This flex occurs when the pedals are being pushed, with the reaction forces being taken by the seat. The flattened ‘V’ that the main backbone forms in side profile was being further flattened by these forces – that is, the bottom bracket was moving away from the seat when very high pedal forces were applied. (This phenomenon is normally called ‘boom flex’ but in reality it is not just the boom that is flexing but the whole seat/frame/boom assembly.)

To reduce this flex, a 19 x 0.9mm tension brace was placed between points halfway along the boom and where the rear backbone kicks up behind the seat. The brace was made adjustable for length and also removable (the latter to allow the steering to be inserted). The installation of this brace first involves pre-tensioning the frame so that the elongated ‘V’ arms are pulled together. The brace is then inserted and bolted into place and the frame is then allowed to spring back, applying a tension to the brace. The result is a frame that is incredibly stiff under pedal forces.

The upper mount for the rear spring is located on the main backbone. Initially this tube was simply butted against the lower wall of the main backbone but testing at full load (ie 90kg rider + 40 kg luggage) when riding over a 250mm high sheer drop resulted in the wall of the backbone buckling.

This mount was subsequently redesigned so that it connects to both walls of the tube and in addition, has a strengthening collar over its upper half.

The front spring mounts use separate arms coming forward and up from the main backbone. These arms are brazed together within the main backbone and in addition, use a cross-brace between them. Initially these arms were formed from 32 x0.9mm tube.

However, when S&S couplings (that would have allowed the trike to be disassembled to a smaller size for transport) were silver-soldered into these arms (the stainless couplings cannot be nickel-bronze brazed), one of the arms failed, again under high load testing. It was subsequently decided that the S&S couplings unacceptably reduced the structural integrity of the trike (not the beautifully made couplings themselves but the silver soldering and heating of the chrome moly tubes) and so the broken sections of tube were replaced with heavier wall and larger diameter 35 x 1.2mm tube.

The rear suspension arm bearings are supported on brackets that hang from a crosspiece constructed in 44.5 x 0.9mm tube. These brackets were formed from a mixture of 1.6mm chrome moly plate and 32 x 0.9mm tube. The bearings are a press fit in the tube.

The front suspension bearing mounts were formed from folded 1.6mm chrome moly steel plate. The high tensile 10mm, countersunk head, internal Allen-key bolts nestle in cones formed in steel tube and then brazed to the folded mounts. The results in very accurate and sustainable bearing location - and so suspension geometry is rigidly maintained.

These two curved brackets support a bag carrying a 7.2V nickel metal hydride battery for the lighting system. Incidentally, the battery is one stick from a current model Prius high voltage battery pack. The rear-facing curved brackets are the front mounts for the panniers (the rear mounts are on the removable carrier).

A small bracket was made to mount the Schrader valve fill point for the front airbags.

2. Boom Extension and Bottom Bracket


Adjustable leg length, off-the-shelf assembly.


The bottom bracket boom extension uses a Greenspeed boom extension assembly. Two changes were made: the front derailleur mount was inclined further backwards to suit the different chain-line and the headlight mount was offset and reduced in size to suit the custom headlight. The extension slides within the boom of the main frame, allowing adjustment for different leg (and crank) lengths.

3. Rear Suspension Arm


Low motion ratio, laterally stiff rear suspension with long travel and anti-squat.


The rear suspension arm comprises a long length of 44.5 x 0.9mm tube. At its leading edge a ‘T’ shape is formed by a cross-piece of the same type of tube.

At each end of the cross-piece a high tensile nut has been brazed to the inside of a 1.6mm thick chrome moly disc that covers the open end of the tube. The 10mm high tensile internal Allen-key bolts that pass through the sealed ball bearings screw into these nuts.

During cornering, high lateral bending forces are developed at the top of the ‘T’. To better resist these, triangular gussets were folded from 1.6mm chrome moly sheet and brazed into place.

To support the wheel, two lengths of 32 x 0.9mm tube (one arrowed) extend back to the rear wheel lugs (“drop-outs”). These tubes were heated and then pressed so that their trailing end thickness matches that of the drop-outs. The drop-outs were sourced from Greenspeed. Because the wheel is asymmetric (it has the rear gear cluster on one side) the rear suspension member is also asymmetric, so placing the wheel on the longitudinal axis of the trike. To feed loads into the whole tube and not just the walls, the rear tubes extend right through the rear support tube. Also note the lower rear spring mount and the mount for the mudguard (fender). A Greenspeed-sourced generator mount is also brazed into place.

The rear damper bracket is located so that the travel of the damper (65mm) matches that of the wheel (130mm). A design deficiency is that the damper is a little offset from the longitudinal centre axis. The damper is a revalved Yamaha R1 steering damper.

The chain pull line and idler mount position have been designed to cause rear suspension extension under high torque inputs. With the rearwards weight transfer that occurs with acceleration, the two effects cancel out, resulting in no squat.

4. Front Suspension Arms


Light weight, low motion ratio, high roll centre suspension with dynamic camber and castor change. Sufficient suspension arm stiffness to avoid torsional wind-up under braking. Anti-dive.


The front suspension arms are of a semi-leading design. Their angle to the longitudinal axis is 30 degrees. Each arm is made from 32 x 0.9mm tube. At each wheel end is a cylinder carrying two ¾ x 5/8 x 1 inch bronze bushes in which the kingpins rotate. These are fitted with grease nipples. At the other end is a cross-piece made from 32mm diameter tube. Triangular, folded gussets stiffen this join. Two 30 x 10 x 9mm roller bearings are press-fitted in the ends of the cross-tube. In between the two bearings is a spacer, formed from two high tensile nuts brazed to the ends of a tube. The 10mm high tensile internal Allen-key bolts that pass through the sealed ball bearings screw into these nuts.

A sway bar pick-up point is provided part way along the suspension arm. Maximum load testing resulted in a pair of short (2-3mm), hairline cracks developing in the lower wall of the arm at this point. A 120mm long ¾ reinforcing collar of the same tube was then brazed over the top.

The lower spring mount is formed from 19 x 0.9mm tube, bent in a hand bender. The tube passes right through the suspension arm, feeding loads into both walls. Note the design deficiency of the force of the spring being offset from the centre-line of the arm so, causing some torque to be applied to the arm. Despite their diminutive dimensions, the spring supports have never shown any signs of stress.

Fitting the Bearings

The observant will have noticed that 30mm OD bearings are stated as being a push-fit into 32 x 0.9mm tube. However, the ID of this tube calculates at 30.2mm – so isn’t the bearing a loose fit? The answer is ‘no’ because in each case, the tube has previously been brazed to another. The distortion (and shrinking?) effect of the brazing makes the bearings a good fit. Some anaerobic adhesive could also have been used to hold the bearing in place but I haven’t bothered.

5. Springs


Low natural frequency, adjustable for differing loads, light weight and slightly rising ratio.


All three springs comprise Firestone 4001 Airstroke/Airmount units. These each have a mass of 350 grams, a maximum stroke of 3.6 inches (91mm) and a maximum internal air pressure of 100 psi. They use a rolling bag design - not a piston or a simple pressurised bag.

The rear spring is mounted with a (very low for a HPV) motion ratio and typically uses an unladen air pressure of about 20 psi. When running at maximum load this is statically raised to 60 psi or so and dynamically rises to near 100 psi.

The front springs also have a lower motion ratio; they have a typical static air pressure of 15 psi. They are interconnected with no restriction placed in the interconnecting line.

6. Anti-Roll Bar


Very stiff, light weight design that does not restrict suspension travel.


The anti-roll bar is made from 32 x 0.9mm tube. It is supported each end by 30 x 10 x 9mm sealed ball bearings located in mounts brazed to the main frame.

The anti-roll bar links are formed from 12.7 x 0.9mm tube. One end has a nut brazed within it and the other end a threaded bolt protruding at not quite right-angles. Rose-joints are used at each end of the links and these are 6mm in size. The links connect to the suspension arms well inboard from the wheels. This is good in that it reduces the required length of the anti-roll bar but bad in that it requires a stiffer anti-roll bar to gain the required roll stiffness and also increases bending loads on the suspension arms.

7. Steering


Largely off-the-shelf design that integrates well with the suspension to provide zero bump-steer.


The steering system is fundamentally the Greenspeed “non-crossover” system. The steering handlebars are standard adjustable Greenspeed, however the central pivoting section has been modified by further offsetting the handlebar line from the pivot point. The top end of the hollow shaft has been filled, a grease nipple has been fitted to this assembly and a lubricating hole drilled at right-angles through the bearing shaft.

Standard Greenspeed aluminium steering tie-rods connect to the Greenspeed kingpin assemblies. The kingpin assemblies have had the outer balljoint locations raised by about 30mm. In addition, the kingpin bolt thread has been shortened. When used with 20 inch wheels, these kingpins give zero scrub radius.

8. Seat


High comfort off-the-shelf design.


The seat uses a Greenspeed synthetic material equipped with lacing eyes. Lacing is by means of elastic ‘bungee cord’. Despite the wider spacing of the seat frame rails, the standard Greenspeed seat still fits.

9. Carrier


Removable, strong and lightweight design that incorporates rearmost pannier mounts.


The carrier is made from 12.7 x 0.9mm and 32 x 0.9mm tubes. It incorporates the rear mounts for side pannier bags, placing the pannier bags within the wheelbase of the trike. The bends were formed with a hand bender. The carrier slides on a double-wall spigot extending rearwards from the main backbone tube. This spigot passes right through the main frame tube, so feeding loads into both walls. The carrier is rated at 40kg.


In use, the airbags are inflated to give a suspension compression of about 50mm, irrespective of the load and the type of terrain to be tackled. For touring use, the tyre pressures are set at about 30 psi (because of the suspension travel, this low pressure doesn’t cause tube pinching problems) to better absorb vibration from coarse bitumen surfaces. For sport cornering, the tyre pressures are raised to 60 psi. This increases vibration but gives better turn-in performance.

Next: Assessing the trike – and full specifications.

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