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Building a Human-Powered Vehicle, Part 2

The rear suspension

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Suspension bushes
  • Pivot location
  • Spring location
  • Swing-arm design
  • Damper
  • Adjustable spring seat
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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...

Last week in Building a Human-Powered Vehicle, Part 1 we covered the basics of the proposed design:

  • Square tube frame made from aluminium with lots of holes cut in it

  • Rear swing-arm and front double wishbone suspension, each with lots of travel

  • 20 inch wheels in a ‘tadpole’ configuration (two front steering, one rear driven)

  • Recumbent hammock seat

  • Hydraulic front disc brakes

  • 63+ gears comprising front and rear derailleurs and a 3-speed internal rear hub

  • A steering mechanism yet to be decided upon

Click for larger image

...and with the basic layout of the wheelbase, track, seat shape and so on based around the pictured Greenspeed GTR.

But now it’s time to forget the theory and start the practice, beginning with the rear suspension.

The Design

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As mentioned last week, the rear suspension design is a longitudinal swing-arm. Two interconnected parallel arms support the rear axle, being pivoted near their opposite ends. As the wheel moves upward, the part of the arm forward of the pivot moves downwards, in this case compressing a coil spring and at the same time, extending the damper.

Oddly enough, the first important decision that had to be taken about the rear suspension was the nature of the pivot.

Pivots

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In automotive applications, suspension pivots almost always comprise rubber bushes. An inner crush tube is bonded to a rubber cylinder that in turn is bonded within another metal tube. The rubber twists torsionally when the inner and outers move relative to one another. In racing machines, pictured Heim (or rose) joints are often used, taking out the flex associated with rubber. Car manufacturers use rubber bushes for these reasons: they’re cheap, absorb vibration, and allow suspension rotations that are not perfectly axial. (A Holden engineer once told me their trailing-arm-with-an-extra-link rear suspension would bind solid if rubber bushes weren’t used.)

On the other hand, most makers of HPVs use Heim joints, while high performance mountain bikes use ball bearings or small diameter graphite tubes.

My preference was for conventional car-type bushes, but using polyurethane instead of rubber. (The use of the plastic necessitates that the bush rotates on the crush tube.) The benefits are extremely good durability (the loads are far less than on a car but the bushes are similarly sized), some vibration absorption, and custom sizing easily catered for.

I wanted to use as the outer bush housing aluminium tube with an ID of 25.4mm (ie 1 inch), an outer diameter of 32.4mm, and a through-bolt diameter of 8mm. Polyurethane bush manufacturer Super Pro were able to offer off the shelf bushes to suit this – the SPF0107K which are normally used as replacements in the front spring shackles of a ‘85 Jeep (or the rear shackles in a ‘57-‘65 Gordon Keeble or Tempest!).

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These bushes are 25.4mm outside diameter and have a total length - including a single end flange - of 30mm. The end flange is chamfered and including the chamfer, is 7.7mm thick. The diameter of the flange is 34mm. The bushes are designed to be inserted from each end of the tube that holds them. The flange gives lateral location and the chamfer reduces the amount of polyurethane which is in contact with the bracket, reducing stiction and potential squeaks.

In the rear suspension application the bushes in use have a small (~7mm) gap between them within the sleeve. But this is of no consequence as there’s still plenty of polyurethane to take the forces.

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The crush tube used through the middle of the bush (on which the bush rotates) was made from 12.5mm (just under 1/2 inch) shock absorber shafting. Using a lathe, the shaft was shortened and then drilled and tapped to take an 8mm bolt at each end. The huge advantage of using this material for the crush tube is that the shock absorber shaft is hard chrome plated (for wear resistance) and is strong with a very smooth surface finish. The downside of the whole assembly is weight – but IMHO it is a weight penalty well spent.

Pivot Location

If you think of a swing-arm as being like a see-saw, the wheel is at one end, the pivot somewhere along the length, and the spring at the opposite end to the wheel. The closer to the far end that the pivot is placed,

  • the greater the leverage on the spring – and so the stiffer the spring has to be for a given wheel rate

  • the smaller the compression of the spring for a given wheel movement

  • the less the ends of the spring will become angled during compression and extension

  • the less the spring intrudes on the space available within the wheelbase

That’s quite a list to consider and in the end I made the decision to place the pivot point about one-third way along the swing-arm. (The detailed geometry of the front and ear suspension is covered later in this series.) However, unlike every other HPV with a rear swing arm that I’ve seen, I decided to use two pivot points widely spaced but located along the same axis. Widely spaced? How?

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Looking straight down on the rear wheel, the swing-arm comprises two parallel arms, one that goes to each end of the wheel’s axle. In the case of a recumbent trike, the rear axle is quite long as it needs to accommodate the wide hub and gear cluster. Widely-spaced arms give greater strength and if the forward pivot points are equally widely-spaced, the rear wheel will be well supported in side-load, such as generated when cornering. (Contrast this with if the widely-spaced arms join at one forward point, with a single bush used to pivot it.) It’s an interesting fact that when people make HPVs using the suspension rear forks from a mountain bike, the lateral loadings of the trike usually end up breaking the rear suspension arms...

And there’s another reason for widely-spaced pivots. As mentioned last week, if the tension side of the chain pulls along a line which is greatly above or below the pivot axis, with each pedal stroke the swing-arm will be either extended (if chain pulls below the axis) or compressed (if chain pulls above the pivot axis). Either effect will cause suspension ‘pogo-ing’.

Most mountain bikes and suspended recumbents place the chain axis a little above or below the suspension pivot, because the physical presence of the pivot prevents the designers doing anything else. However, if two in-line widely-spaced pivots are used, the chain can pass through this axis, running between the two pivots. (Or, as was later actually done, an adjustable height guide pulley can be used in development, with the chain location able to be moved over a wide range without it fouling anything.)

Spring Location

Looking at the shape of the Greenspeed GTR (on which the basic dimensions of the new machine were being based), it could be seen that there’s plenty of room under the inclined hammock-like seat for the spring and damper. This allowed the spring to mounted vertically, bearing at its lower end on the main frame longitudinal (or in fact on the adjustable lower spring platform, but we’ll get to that later), and the upper end bearing against an extension of the swing-arm.

But to achieve this, the swing-arm had to be an unusual shape – so maybe I’d better cover the swing-arm design now!

Oops – the First Swing-Arm

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The first swing-arm looked like this. In operation, the arm was going to be subjected primarily to the upwards push of the wheel (red arrow), the downwards pull of the suspension bush (blue arrow), and the upwards push of the spring (green arrow). I was pretty happy with this design, which during bump placed all but the gusset (tension) and the spring-arm (bending) in compression.

However, at 4am I suddenly awoke with a start. The design was wrong! If roughly one-third of the total weight of HPV and rider is borne by the back wheel, the vertical load would be only about 30kg – say 60 or even 90kg, the latter if the vertical acceleration over a bump reached 2G. But the pull by the chain could be substantially more than that.

A recumbent allows the rider to push against the seat, not against just their weight. As a result, the pull on the chain could be expected to be way more than the upwards force of a bump. And my grand design failed miserably when the forward pull of the chain was considered. In fact, in that case, the gusset would be subjected to compression – exactly what it was never meant to do...

I started the rear suspension again...

Better – the Second Swing-Arm

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Starting on the basis that the greatest force on the rear swing-arm would be the pull of the chain required that a member be positioned to take this in compression. This was made from 40 x 40mm x 3mm square aluminium tube, located largely horizontally. This tube connected at an acute angle to a larger 50 x 50 x 3mm tube positioned at its forward end.

Next, the upwards forces of bumps had to be taken. A 3mm sheet gusset was positioned beneath the horizontal member to take this bending force and turn it into a tension – ie the upwards force was trying to stretch the gusset. That was fine in bump, but in rebound (where the damper resists the downwards movement of the wheel) there was little strength in the assembly. To absorb this force, a second gusset was positioned above the horizontal. This gave a rigid assembly with the tube extremely strong in compression (caused by chain tension), and very strong in upwards bending (caused by bumps) and downwards bending (caused by the damper extension in rebound).

To operate the spring, an extra piece of rectangular tube was placed parallel to the horizontal part of the arm, extending forwards from the top of the larger upright-angled tube. (This small extension piece was the only bit I could re-use from the first design!)

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The round aluminium tube to take the polyurethane bushes was positioned in a hole drilled right through the 50 x 50mm tube. But this wasn’t the only hole drilled – numerous holes were strategically made to lighten the whole assembly. (Incidentally, when the first arm proved a flop, I decided to test part of it to destruction. As can be seen at Making Things, Part 2 , bending it proved very difficult – even when supporting the back-end weight of a car!)

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So far I’ve only described one half of the swing-arm. But of course there are two mirror-image halves – and they needed to be joined together. Two tubes are used to make this connection. The first is in 40 x 40mm tube and joins the two ‘spring operating’ extensions. In fact, it is this tube that bears down on the spring. The second connector piece is in larger 50 x 50mm tube and is positioned as close as possible to the wheel axle. This piece also carries one of the rear wheel mudguard mounts.

Welding

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The rear swing-arm was the first part of the HPV that I built. As a result, it was also the first part that I needed to have welded.

Welding of aluminium can be by MIG or TIG, with MIG the most commonly used welding technique. However, while it is faster, it is also less suited to small components and gives a less pleasing visual result that is potentially weaker. TIG needs a very steady hand, a good welding machine and takes much longer. Good TIG tradespeople are also much harder to find than good MIG welders.

However, after chasing around on the phone (in this sort of search, being passed by word of mouth from one welder to the next is best) I managed to locate a brilliant TIG welder who was close, cheap and flexible in working hours.

Welding the rear swing-arm took about four hours of careful work but the results were stunning – superb welding and a very rigid, relatively light weight (2.2kg) assembly.

Damper and Spring

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The details of the springs and dampers will be covered later in this series but – in brief – what do they consist of?

In fact, the very first components bought in the build of the HPV were the dampers. After looking at mountain bike spring/damper combinations, I decided to steer well clear of any of them. Why? Because of their ridiculous cost. Many are just air springs (which should cost next to nothing) while others are oil/gas with external concentric coil springs and adjustable damping.

But as far as I could see, all the rear bike dampers were way overpriced - AUD$500 seemed common. Also, there didn’t seem to be the professionalism of support that I thought I’d need – at minimum, I expected to have to rebuild the damper to achieve the front and rear damping behaviour I desired. And when I mentioned damper rebuilding, bicycle shop staff backed away in terror... Perhaps I just went to the wrong shops, but these dampers do seem incredibly overpriced for what they are.

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Instead I bought three second-hand motorcycle steering dampers. This design uses a large diameter steel shaft which passes right through the body of the damper (so reducing their required size by avoiding the need for a twin tube design), aluminium bodies, and are rebuildable and so can have their damping behaviour altered. They’re also cheap – I paid AUD$75 each for them second-hand. The downside is that their overall mass is high (500g each) and the damping behaviour is symmetrical (ie when used unmodified in a suspension application, bump and rebound damping will be the same).

Spring Seat

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As already mentioned, the spring is a conventional coil spring mounted vertically. It needs to be adjustable for preload, primarily so that the same ride height can be achieved with different weights on the rear carrier. I used my lathe to turn-up a cup that formed the lower spring seat. This sits on the shoulder of a long externally threaded upright that rotates within an internally threaded tube welded vertically within the main longitudinal frame tube. A large knurled aluminium knob allows easy adjustment. While I am happy with the end result, a quick check with the scales showed the assembly adds about 200g over the weight of the bare lower spring cup. I was starting to find that every feature added mass...

Pivot Adjustment

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So that the rear wheel could be aligned, adjustment was built into the rear pivot points. This took the form of castor adjusters used in karts which incorporate an eccentric able to be adjusted by rotating the collar.

Next week: the nightmare of designing and building the front suspension

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