Another Human Powered Vehicle! Part 9 - Building the Front Suspension

This article was first published in AutoSpeed.

Earlier in this series we’ve covered the ideas behind the new semi-leading arm suspension design.

So what advantages did that approach have?

It’s a mouthful but we’ve described the way in which the design dynamically adds camber and castor in bump and (to the outside wheel) in roll; how it has the potential to be lighter than many other systems because of having only single arms, and only two pivots for each of those arms; how it leaves plenty of clear space forwards for pedalling legs; and how it allows the use of a low motion ratio spring.

That’s all well and good – but how to make it?

Semi Leading Arms

Let’s take a look at the stresses and strains each front suspension arm will be subjected to.

1.  Vertical forces trying to bend the arm primarily upwards. These forces are developed by the weight of the trike and in vertical bumps. However, because the spring is out near the wheel, this component is quite small.

2.  Horizontal forces trying to bend it backwards. These forces are developed as the wheel runs into a bump, but, much more importantly and in the other direction, when the brakes are applied. Braking develops the greatest forces these arms need to withstand – and braking forces can also be applied very suddenly. Braking also causes the leading arms to twist – torsion is applied.

3.  Buckling and bending forces. These forces are developed when the vehicle corners and a sideways load is developed.

Purists can pick holes in these points – most times, the arms are subjected to multiple variations of these forces. But the only reason I went for relatively thick 1.2mm wall thickness (a decision that caused me physical pain with the thought of the extra weight!) is to withstand braking forces. There’s nothing like actually being able to see the suspension in action to make you realise how high braking forces are...

So the arms, each about 380mm long, were made from 35mm OD, 1.2mm wall chrome moly steel tube.

Pivots

On my first trike I used for suspension pivots the pictured hardened, chrome-plated steel rods (ex car shock absorber shafts) that rotated in greased polyurethane bushes (off-the-shelf suspension bushes from an obscure car). In terms of strength and durability, this approach is a wonderful way to go. The assemblies absorb road vibration and are very strong and durable.

But (1) they’re heavy. And (2) they allow deflection.

In fact, even when using the widely spaced double wishbones on my first design, I found that in hard braking, the upper bushes noticeably deflected – and so I replaced them with nylon. With this new HPV design, using a front suspension design with just a single semi-leading arm (and so closely spaced mounting points at the chassis end of the arm), it was very important that the pivots have effectively no deflection at all.

And they needed to be light....light....light.

I looked long and hard at all sorts of pivot systems, including:

1.  steel shafts running in greased bronze bushes (the system used in the Greenspeed trikes for the kingpins and steering arm pivot)

2.  plastic bearings made from acetal rod

3.  plastic bearings injected as hot plastic (there are car ball-joint repairers who have the equipment to do this)

4.  rod-end bearings (ie Heim or rose joints)

5  ball bearings

But when I looked at the available load data, the cost and the weight, plain old sealed ball bearings were way out in front.

I have an old NSK roller bearing book and it lists, for example, the stats on a sealed roller bearing with a 30mm OD and a 10mm ID as 520 kg force dynamic and 229 kg force static - and yet each bearing weighs only 32 grams. That seems to me to beat a plastic bearing with completely unknown specs and durability... (Incidentally, note that mountain bike suspension systems have available very high performance [cage-less] roller bearings, but they’re rather expensive.)

The new design uses a suspension arm in the shape of a T (arrowed). The length of the long arm of the T is 380mm and the cross-piece is 80mm long. The ball bearings mounted in each end of the top of the T. The 30mm by 10mm bearings are a loose fit in the 35mm OD tube, but if a 32mm x 0.9mm tube is slipped inside the 35mm tube, the bearings are a nice fit inside the paired tubes. By cutting off short sections of the 32mm tube, placing them inside the 35mm tube and then brazing them into place, a very lightweight bearing mount could be made. Bearing adhesive was used in the final assembly to absolutely secure the bearings.

Passing through the bearings are high tensile steel, counter-sunk head bolts, one entering from each end of the assembly. To reduce weight, the two bolts screw into high tensile nuts brazed to each end of a 19mm x 0.9mm wall chrome moly steel tube positioned between the bearings. Weight is reduced because a single through-bolt is not used, and this internal spacer also holds the bearings correctly spaced apart.

The swing arms pivot on brackets folded from 1.6mm chrome moly steel sheet. Rather than the bearing through-bolts passing through just the 1.6mm wall thickness of the brackets, steel sleeves are used to spread the load. Furthermore, the steel sleeves are flared so that the countersunk through-bolts self-align when they are tightened. The flared sleeves were made by using an oxy torch to heat the sleeves red-hot and then forcing them down over a sacrificial inverted countersunk bolt. The resulting sleeves are very light, fit the countersunk bolts perfectly, and were vastly easier made in this way than by a lathe.

A very important advantage of the semi-leading arm design is the reduction in the number of pivot points. As described above, the chassis suspension pivots of my first trike were made from hardened steel rods working in polyurethane bushes. Taking the front double wishbone suspension, there were four pivots per side, one for each base of the upper and lower A-arms. That makes in total 8 chassis suspension bushes. Then, on the wheel upright, there were two more pivots – small (but relatively heavy) ball-joints. That makes another four pivots – so we’re up to 12. In comparison, the semi-leading arms design has only four pivots in total. Two are the chassis pivots (formed by the two pairs of ball bearings) and two are the steering swivel bronze bush kingpins. Whatever way those pivots are formed, a reduction from 12 to 4 is a very major weight saving.

Roll Centre? As any good suspension textbook will show you, swing arm suspensions (ie where the suspension pivots are parallel with the longitudinal axis of the car) have a roll centre that’s very high. On the other hand, pure trailing arms have a roll centre that’s at ground level. So what of semi-leading arms? This diagram, taken from 30m3performance.com, shows how the roll centre position is calculated for semi-trailing (or semi-leading) arms. Basically, depending on the angle of the arms (and as you’d expect from the above) it’s somewhere between ground level and very high! So of what significance is the position of the roll centre? Well, the roll centre height (lateral position isn’t an issue - it should be on the centreline of the vehicle) will influence the amount of roll that occurs in cornering. The roll centre is almost always lower than the centre of gravity (COG), and the closer it is to the height of the COG, the less roll that will occur. So having a high roll centre sounds a pretty good thing, no? The trouble is, the higher the roll centre, the more that the lifting force being developed at the tyre tries to corner the vehicle. This diagram shows the situation - there’s an upward component in the force vector. Now if the vehicle rises on its springs during cornering, the rising pivots cause positive camber on both wheels, resulting in a loss of grip. This development of positive camber is most likely if the vehicle passes over a mid-corner hump. The result is sudden sliding: the trait for which early Volkswagen Beetles and the Chev Corvair were known with their swing-axle rear suspension designs. But of how much importance is this to my HPV? The short answer is that until I ride the thing hard, I honestly don’t know. That’s not to say that I haven’t considered it in great detail - but it’s just all too hard to try to work out on paper. For example, say that there’s 60kg vertical load on the front suspension. I corner hard enough that I lift the inner wheel: now there’s 60 kg vertical load on just the outer wheel. If I am cornering at 0.5g, that translates to 30kg lateral force. With a 30 degree angle between the tyre contact patch and the roll centre, that 30kg lateral force translates to (...thanks for the maths, Dad...) a 15kg vertical force. So the front suspension gets lifted as if the weight has dropped by 25 percent. But – so what? There’s still the same downwards force on the tyre – the trike hasn’t got any lighter. And even with a 15kg reduction in front ‘weight’, the camber will still be negative, or at worst, zero. In short, yes the virtual swing arm is very short, but that’s what I want to give me the major camber variation on bump. As a result of this major camber variation and short virtual swing-arm, the roll centre is very high – but again, does it matter? You won’t find the answer in suspension design textbooks, and nor will you find it in on-line resources devoted to suspension and handling of normal race cars. However, I did find a gold mine of pertinent information on a discussion group devoted to those who build their own off-road racers – people not afraid to push the boundaries in terms of dynamic castor, huge camber variations and yes – short virtual swing-arms. One clue came from the point: "We usually don’t need much anti-roll bar, so our roll centres are pretty high" – the more you think about it, the more interesting that line is.... But really, for this HPV, it’s very much a wait and see exercise.

Kingpins

Off-the-shelf Greenspeed kingpin assemblies are used. These set the scrub radius at zero (that is, for 20 inch wheels, the steering axis intersects the tread at the road half way across the tyre’s width) and they’re cheap and well made. Using them was a case of using a known entity – in terms of strength, contribution to total mass and scrub radius, I knew what I was getting. The kingpins use bronze bush greased bearings.

Spring Supports

In a way the term ‘spring support’ is misleading: it may be supporting the top of the spring but in fact its most important function is to take the weight of the HPV acting through that wheel.

But it’s an interesting type of weight – a very interesting weight indeed.

Let’s assume that the spring support takes only the forces acting through the spring – in other words, the bump-stop is mounted somewhere else. This means that the weight is cushioned and absorbed by the spring: the very high peaks that would be experienced without the springs are gone. Hit a big bump and instead of the impact being immediately fed into the frame, it’s ameliorated by the compression of the spring. In short, the instantaneous loads that need to be carried by the spring support are much lower than are carried by the frame of a machine that doesn’t use springs.

Furthermore, and this is not obvious until you think about it, the maximum weight acting through the spring support is only present when there’s weight on the machine!

Huh? What does that mean?

Well, like a bicycle, the majority of weight of a loaded recumbent trike comes from the presence of the rider. The trike itself might weigh only 20kg unloaded – but 100kg when loaded. To put this another way, 80 per cent of the weight of the loaded trike is made up of the person riding it. So, assuming one-third of the weight acts through each wheel, an unloaded trike will have a maximum force acting upwards on a spring support of about (20/3=) 7kg. That’s very little. However, with the trike loaded, that jumps to about 33kg, with dynamic increases to 66kg on 1g bumps.

Now, remember that all the rider’s weight is pushing down on the seat. This in turn makes the loaded seat able to cope with pretty high vertical force inputs – in fact, you need to push up with a combined force of more than 80kg before the seat even tries to lift from its mounts. So where is this discussion going? In short, the front spring vertical loads can be fed into the seat. You wouldn’t want to apply full loads to the frame without someone being in the seat, but then why would that ever occur?

In fact, I initially decided to use simple extensions of the front seat supports as the upper spring mounts. This meant the front spring supports were made from small tube – just 19mm diameter x 0.9mm wall. The lower spring supports, which mounted the base of the spring on the suspension arm, were also made from the same size tube.

However, testing of the frame showed that the upper spring supports had major deflection – as much as 15mm! This occurred for two reasons – (1) the 19mm tube was not strong enough, and (2) my design placed part of the upper spring support in torsion – and in torsion, the 19mm tube was especially weak. So despite the increase in weight, there was nothing for it but to remake these supports in much larger 32mm x 0.9mm tube (arrowed). The new design places this tube only in bending (not torsion) and still incorporates the seat supports.

The 19mm tubular lower spring supports were retained.

(More on spring supports in the coming Part 11 of this series – which is on the design and construction of the frame.)

Track Change One thing that immediately struck me when the suspension was first built was the amount of track change that occurs as the arms move through their arcs. From full bump to full rebound, the track alters by something like 130mm. (Since that suspension movement should almost never occurs, in more normal use the lateral movement is more likely to be about 20mm). In other words, the tyres are forced to scrub sideways across the road as the suspension moves up and down. The disadvantages of this include increased tyre wear and greater straight-line drag. But there is also a potential major advantage: the suspension movement is damped by this scrubbing. In other words, the sideways movement of the tyres creates resistance to suspension movement. On my first HPV, where double wishbone front suspension was used, the track changed through full suspension movement by about 20mm. Together with friction in the suspension pivots, the damping caused by this track change was sufficient that external dampers were not needed. To put it another way: If the use of external dampers can be avoided, but tyre wear is greater, it’s a good trade-off in terms of reducing HPV weight. But of course, the degree of trade-off depends on the amount of tyre wear! (Another thought: if the tyres’ sideways movement is providing through damping, the lower the friction of the surface, the less damping that will occur. This implies that on wet roads, the suspension will be less damped – something that’s normally desirable!

Bump Stops

Full deflection bump stops are normally considered to be intrinsic to good suspension design: they’re the rising rate springs that take over to absorb and cushion the last movements of the suspension. And especially when dealing with the miniature Firestone airbags, which don’t have internal bump rubbers, the presence of bump rubbers would seem to be critical.

However, after some thought I decided not to use front bump rubbers. Why? Well, with 5 inches of suspension travel (yes – 130mm!) at the front and an appropriate spring rate, the bump rubbers should be impacted hard only extremely rarely – if ever. So with full bump rubbers you’re carrying around with you the extra weight that may never even be used. Because the airbags are clearly visible to the rider, it will also be fairly obvious when full bump is being reached, and, even when that does occur, the frame and suspension is built strongly enough to take it. I may later fit some very light expanded foam bumpers, but at this stage I decided to leave them out of the front suspension.

However, full droop stops are absolutely required. This is because whenever there’s no rider on the machine, these stops are brought into action – get off the trike and the airbags immediately expand to their full constrained length. The force the airbags apply in this situation is quite large, so the full droop stops are subjected to high and frequent forces. After looking at lots of options (lots – including stainless steel cable loops, chrome-moly-tube-and-rubber-stops located at different positions, simple ropes – and more!), to keep weight down to a minimum and provide high strength, very heavy duty plastic ties were used.

Anti-Roll Bar

As described earlier in this series, an anti-roll bar is a prerequisite if linked front airbag springs are to be used. However, the anti-roll bar caused me quite a lot of concern – it was very hard indeed to make it light.

Not knowing how stiff an anti-roll bar would be needed (that depends on maximum cornering lateral acceleration, roll centre height, dynamic spring stiffness and tolerable roll angle, sway bar geometry and linking) I went on the experience of my previous suspension HPV design. In short, I thought that a bloody stiff anti-roll bar would be needed! (Note that on a three wheeler, the rear wheel contributes zero to roll stiffness....) I used 19mm x 0.9mm wall chrome moly tube and linked to the leading arms about half way along their length.

The chassis mounts are fabricated steel D-shackles, mounted on the same brackets that support both the front and rear suspension pivots, while the bushes are single piece high density polypropylene, a self-lubricating plastic.

I initially intended to use rubber bushes each side of mounting plates to cope with the angular change between the sway bar links and the suspension arms. However, after spending nearly a full day making different prototypes (none of which were light, strong and didn’t put side loadings on the links during full suspension movement), I decided to use rose joints (ie rod-ends). These are 6mm designs (ie the threaded ends and ‘eye’ holes are 6mm in diameter).

Note that the mounts do not place the through-bolts in double shear, which is normal best engineering practice. This was done deliberately: double shear mounts fail catastrophically, while single shear through-bolts will first bend if the loads are too great. Without knowing the magnitude of the loads, I wanted an early indication if the links weren’t up to the task. (I think a sway bar failure at high downhill cornering speed would likely throw you off the machine....)

Conclusion

So the front suspension is built but how will it perform? With its short virtual swing arms length and high roll centre, will it have problems in ‘jacking’? Will the huge variation in camber cause steering problems? Will the change in castor (something really radical!) cause steering wander or massive changes in steering weight? Will the changing track wear out the tyres in just a few kilometres?

Watch this space!

Welding I had intended to tack-weld the trike together using an arc welder, and then take the frame to a welder to have it TIG welded. However, tacking the very thin wall tube with an arc welder proved impossible, even when using low hydrogen rods designed for high tensile steel. Instead, I changed approach dramatically and invested in an oxy acetylene set-up. This allowed me to braze the frame together using high strength flux-coated nickel-bronze rods. The huge advantage was that welding could be done on-site and different design approaches could be tried and tested (eg changing the upper front spring supports – see above). This speeded-up the construction process immensely. So how strong is the brazing? That’s a hard question to answer – but I don’t think as strong as good TIG welding. I brazed together some sample tubes and then took to them with a hammer. I couldn’t break the join. However, during construction I broke several short tacks that I’d made. These tacks failed much more easily than similar TIG’d tacks. To retain maximum strength, I decided not to grind back any of the braze welds. I also brazed fillets (ie built up the weld to quite great thickness) where I thought additional strength was required. The flux-coated rods use a flux that’s very hard to remove – a normal hand-powered wire brush just runs over the top of it. The brazing material itself is also very hard. As a result, during construction I decided to give each join just a cursory wire-brushing. The finished frame will be sandblasted before being powder coated; I’d expect the blasting to remove the flux properly. (Black paint has been used here just to stop rusting during the testing process.) A final note: the flux-coated nickel-bronze rods are very expensive –as high as AUD$7 each – and can be hard to obtain.