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Another Human Powered Vehicle Part 11 - The Frame

Making a frame stiff but ultra light

by Julian Edgar

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At a glance...

  • Design
  • Tubing sizes
  • Deflection testing
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This article was first published in AutoSpeed.

When designing a Human Powered Vehicle, it’s one fight over weight after another. We’ve already covered in an earlier part of this series how weight of the machine has increased, seemingly without the addition of any parts! So when it came to designing the frame, keeping the weight low was an absolute priority.

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As with the suspension arms already covered, the frame was built from thin-wall chrome-moly steel tube, brazed together with nickel-bronze rods. If you haven’t worked with chrome moly tube before (and I hadn’t), the tubing is incredibly strong for its weight. And, despite having wall thicknesses of only 0.9mm or 1.2mm, it’s very easy to braze together.

Frame Design

In all vehicles, the frame (or chassis or monocoque, depending on the type of construction), locates and supports the engine and suspension. (Of course, it also locates and supports the occupants, but they’re not the items creating the biggest forces.) To achieve these traditional functions, the frame has to accept the:

1.  input of loads from the suspension - loads which include propulsion, braking and cornering forces

2.  forces generated by the engine – torsional drive forces, and static and dynamic weight forces

3.  vertical weight of the vehicle (normally achieved by supporting the top of the springs)

Because of the wide spacing of the suspension chassis attachment points, spring mounts and engine mounts, the frame usually needs to be quite extensive.

But in the HPV being designed here, the situation, although apparently similar, is actually very different.

Firstly, the heaviest load that has to be borne is the occupant – the rider typically weighs four times as much as the unladen vehicle!

Secondly, with the semi-leading front suspension and the rear trailing arm suspension described earlier in this series, the braking/cornering/propulsion loads are all being fed into one small area of the frame. Why? Well, the front semi-leading arms have their chassis pivot points located near the centre of the machine, while the long rear trailing arm suspension also has its pivot points located near the centre of the machine. (Furthermore, another high load input – the anti-roll bar mounts – are located in the same area!)

So to cope with front and rear suspension force inputs, the HPV frame needs to be little larger than a shoebox – all the suspension cornering/braking/propulsive forces are being fed into one area. This approach was deliberate – there was a major weight saving achieved by needing only one small area of strong frame structure to support the suspension input loads.

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But what about the big load – that of the rider’s weight? Before we can discuss that, we need to talk about the seat. The seat design being used is a Greenspeed ‘Ergo’ seat. It comprises two parallel 19mm (¾ inch) x 0.9mm tubes bent into the curved shape of the seat. The two parallels are supported by tubular cross braces while the synthetic fabric material of the seat is stretched between the parallels, making a kind of hammock. As the name suggests, in a recumbent HPV the rider is semi-recumbent, in this case having a reclined angle of about 35 degrees to the horizontal. (This pic shows a bunch of Ergo seat frames.)

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So the seat takes the rider’s weight (and also reacts against the pedalling pushes!) but what supports the seat? The 19mm (¾ inch) x 0.9mm tubing of which the seat is made isn’t strong enough to cope on its own with the rider’s weight – it needs further support. In the Greenspeed designs, this support is provided by a large diameter main backbone that extends from the pedals to just ahead of the rear wheel. One support for the seat is near its leading edge; the others comprise struts that go down to the rear axle (arrowed).

But in a suspension trike, the seat can’t connect to the rear wheel – after all, the wheel is moving up and down by up to 150mm!

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OK, so let’s take stock. We have the front and rear suspension cornering loads being fed into a small area of the frame under the leading edge of the seat. That area of the frame needs to be strong, and so, if required, can also be used to support the front of the seat. Then we need some further support for the rear of the seat, so a relatively strong framework needs to extend rearwards of the suspension pivots. For least use of material, this rearwards frame extension needs to follow the angle of the seat, so to "kick-up".

And since with a low motion ratio rear suspension, the spring needs to be close to the leading edge of the rear wheel, the rearwards frame kick-up can also support the top of the spring. Furthermore, because the maximum weight on the machine is the rider in the seat, this weight force gets transmitted straight from the seat to the rear seat support to the spring to the suspension arm to the tyre to the ground.

Click for larger image

OK, now let’s move forwards. The front springs have to be positioned out near the wheels. But it’s a terrible extravagance of material and weight if we put in frame tubes with the sole function of supporting the top of the springs. So what else can these spring supports do? Well, if they are angled correctly, they can also support the front of the seat. That is, the section of frame that supports the springs can also support the seat. This has another benefit – like the rear suspension, the weight of the rider gets transmitted straight from the seat to the front spring supports to the springs to the suspension arms to the tyres to the ground.

So now we have the seat supported, the front and rear suspension pivots well supported, and the front and rear springs well supported. Now what about the ‘engine’? On a HPV this comprises the rider and the pedals. And from our existing frame design it’s easy enough to extend a tube forwards and upwards to the pedal shaft to perform this pedal support function.

Frame Refinements

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The above description summarises the approach taken. However, a few refinements were added.

Firstly, another seat cross-brace was installed. This helps stiffen not only the seat but also strengthens the rear frame kick-up (the seat then acts more as a structural member).

Secondly, by putting a small extension piece on the frame kick-up, a carrier can easily be added. Because the carrier then feeds its loads directly into the main backbone, it’s very well supported – much more so than in most trike designs.

Finally, to prevent the otherwise unsupported parallels of the seat being pulled together by the tension in the seat material, a small diameter cross-brace was placed on the upper back of the seat.

Structural Seat

In all recumbent trikes where a steel tubular seat frame is used, the seat can be said to be a structural component. However, the strength added to the frame by the seat is aided if two things occur: the seat is welded into place (or bolted so firmly that absolutely no movement at the bolts can occur) and if when viewed from the front, the seat plane is vertically well offset from the backbone. That last idea is hard to picture so let’s look at two simple diagrams.

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Hear is the view from the front. The green line is the seat material stretched between the two seat tubes which are blue. The red tube is the main longitudinal backbone tube. The black lines are the tubular struts that support the seat frame. As can be seen, the green and black lines form a triangulated structure that is quite stiff in bending.

Click for larger image

However, as the seat tubes and the main backbone become closer in plane, the bending strength of the assembly decreases, even though much the same amount of material is used.

The approach shown in the first diagram above was adopted. The downside is, as the seat base is positioned further above the ground, the centre of gravity is higher.

Tubing Sizes

The actual weight of the frame depends not only on its design but also on the tube diameters and wall thicknesses used.

What can be called the main backbone (the tube that extends from the pedals to under the seat) was made from 44.5mm (1.75 inch) x 0.9mm wall tube. (This had the added advantage that an off the shelf Greenspeed pedals/adjustable boom assembly slides straight in the end.)

The angled mounts for the front suspension pivots, rear suspension pivots and anti-roll bar mounts were made from 35mm (1 3/8 inch) x 1.2mm wall tube.

The rear kick-up was made from 32mm (1¼ inch) x 0.9mm wall tube and the seat and its supports from 19mm (¾ inch) x 0.9mm tube. The front spring mounts, which also support the front of the seat, were made from 32mm (1¼ inch) x 0.9mm wall tube.

Deflections

While it’s an anathema to car designers, these ultra-lightweight human powered vehicles will always have frame flex. But it’s where the flex occurs that’s important! If front suspension arm flex occurs laterally when steering, or torsionally when braking, the results will be poor – the steering and braking will be sloppy. If torsional flex of the rear suspension arm occurs in cornering, the chain may come off the sprocket. So flex in these areas would obviously be bad.

But if the seat frame, spring supports or even suspension arms flex only up and down, there will be little impact on dynamics. (The effective spring rate will simply be softened a little.) However, you don’t want the seat flexing backwards when the rider pushes forwards on the pedals, and you don’t want the pedal support moving around.

To reduce to a minimum suspension arm deflection, the arms themselves were made as strongly as I could achieve while still keeping weight low. The area of the frame that these loads are fed into is very strong: again, there is very little flex.

However, the spring supports flex upwards when loaded. To check on this vertical frame flex, the easiest approach is to firstly remove the suspension arms and springs. The spring seats are then supported on blocks of wood and the rider sits in the seat. Any movement closer to the ground of the main backbone is evidence of frame deflection. When tested in this way, the frame has a front deflection of 2mm and a rear deflection of 1mm, both for a load of 89kg. (Spot my mistake? Next week you’ll see the results of it.....)

Weight

The bare frame – comprising the seat metalwork, front spring supports, boom, rear kick-up – weighs 4.0kg.

Conclusion

Light-weight HPV design requires that:

1.  structural members are only as strong as required – and therefore, material strength (eg tube diameter and wall thickness) is matched to the application

2.  each section of the frame performs as many functions as possible

3.  areas where high loads are experienced are grouped so that the required amount of strong (and therefore relatively heavy) material is minimised

4.  direct, short load paths are used that reduce to a minimum the amount of material required to cope with the forces

Despite its apparent simplicity (or perhaps because of it?) I found designing the frame extremely difficult. In a construction where more = less (ie more tubes = less performance because of higher weight), what looks easy simply isn’t. But then again, that’s the challenge of building an HPV...

Next: the first test ride. Will the radical front suspension prove to be a complete flop? Will the frame flex like candy when subjected to real on-road forces?

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