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Building a Human-Powered Vehicle, Part 6

The frame and chain drive... and catastrophic failure

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Frame design
  • Chain path
  • Preventing power-induced suspension movement
  • Frame breakage!
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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, many 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...

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The original intention was to make the frame solely of 50 x 50 x 3mm aluminium tubing. And that’s just how it started out: a longitudinal backbone of square tube. At the rear, a cross-member in the same material provided pivot points for the rear longitudinal swing-arm. At the front, a more complex double L-shaped lateral member provided the upper spring and upper wishbone mounts. (The lower wishbone mounts are on brackets welded to the main longitudinal tube.)

That was all well and good but then I had the seat frame bent up. I’d intended that this was going to be made of large diameter, thin wall aluminium round tube – and in fact sourced some 40 x 1.6mm wall thickness tube very cheaply. But could I find anyone who could mandrel bend it? Nope! I tried a crush bender – even though the bender didn’t have exactly the right mandrels – but the result was terrible.

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So the main criteria in selecting the seat frame tube became: what aluminium tube could be mandrel bent? The answer was 32 by 3mm wall, a much heavier and stronger tube than I’d envisaged. I got the seat frame tubes (two of them) mandrel bent at a cost of AUD$15 for each of the six bends – ouch! Just as well the bends were superb...

About this stage I started to become worried about the increasing weight. It’s hard to accurately weigh everything when it’s still in bits and pieces (parts aren’t accurately sized, lightening holes often yet to be drilled, etc) but it was starting to look like the seat frame and its supports were going to weigh more than the main square tube frame! And if that was the case, the seat frame had better do a lot more than its original non-structural role had suggested.

Structural Seat

The seat has two main forces acting on: it supports the weight of the rider and very importantly, it provides the back support when the rider is pushing forward on the pedals. This latter force is trying to push the seat rearwards; the weight of the rider is of course trying to push the seat downwards.

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The weight was supported by two short members that angled down to the main longitudinal square tube. It would seem logical that the push of the pedals could be catered for by two tubes angled well forwards and upwards to the seat frame from the rear cross-member. But there was another factor: I wanted to be able to install a heavy duty – but removable – rear carrier. This would – of course – be part of the sprung weight and so had to be supported by the back of the seat and the rear cross-member. If more vertical struts were used to support the rear of the seat, these members could also be used as part of the carrier support frame. However, when the carrier wasn’t there, pushing on the pedals would tend to move the top of these tubes backward, putting bending loads on the lower welds. The solution to this problem was to slightly angle the seat supports forward and to place small braces at each of the corners.

The seat frame therefore triangulated the rear half of the frame. The full length of the frame could be triangulated if the forward edge of the seat rails were braced to the front suspension pick-up points – but was the extra 400 grams of tube (at this stage, a total mass increase of about 8 per cent) worth it? I decided to have the seat welded into place without these forward tubes and then do some deflection tests – ie jump on it and see if it moved. (Little did I realise that this decision would come back to haunt me!)

Watching Weight

By this stage I was drilling holes in everything but the round tube (very, very hard to put symmetrical holes along a length of round tube.) The rear swing-arm was mostly air, the front wishbones had dozens of holes in them and the main square tube frame and cross-members were, in part, like Swiss cheese. While holes in the right places reduce mass without affecting strength by anywhere near the same proportion as the loss in mass (see Making Things, Part 2 ), I now had a LOT of holes. But I was determined that the weight was to be kept down.

In fact, I was actually appalled at how heavy some things were that I’d never bothered even considering. The long chain – a full kilogram! The front cogs, bearing, cranks and pedals – 1.6kg! The hydraulic disc brakes – 1kg! The gears – derailleur, cables and selectors – 0.8kg. This was all top quality, brand-name gear and so it all sounds light (and it probably is), but these incidentals added up to 4.4kg.

Then add the rear wheel (including its internal gearbox) at 3kg and the front wheels at 1.5kg each, and suddenly there’s a mass of 10.4kg – without any frame, suspension arms, ball-joints, bushes, dampers, springs, steering or seat! The dampers I was using weighed 500g each and the steel springs totalled no less than 3.5kg – so there’s 15.4kg.... still with no frame, suspension arms, ball-joints, bushes, steering or seat!

Clearly, I wasn’t going to get anywhere near my goal of under 20kg, so instead I resigned myself to a heavy HPV – but one that was engineered very well in terms of durability and strength.

Talking about durability and strength, have I told you about the clothes pegs man? No? OK, well listen to this.

When I started construction of the machine, I joined a few web discussion groups and mailing lists, hoping to bounce ideas off other people who had built pedal-powered recumbent trikes. Unfortunately, very few had had anything to do with suspension and so most of my questions were met with blanks. However, one guy had built some suspension machines and – according to his web site – pedalled them far and wide in the US. He was the only person who seemed to understand concepts like scrub radius and roll centre, and so I looked with interest at his replies to my posts.

Until he mentioned the clothes pegs.

I’d brought up the topic of suspension dampers, telling people that I intended to use modified motorcycle steering dampers as my shock absorbers. These are hydraulic, very well made and compact. The downside is – as mentioned above – their mass, which is about 500g each. Lots of people on the discussion group suggested that using hydraulic dampers was a poor decision, not only because of the mass but also because of odd ideas like “lack of reliability” (huh? How ‘bout a billion cars, trucks, trains, planes equipped with hydraulic dampers...?) and being overly complex for the application.

Then the guy who’d built more than a few HPVs chimed in. He suggested what I needed to do was to get rid of the hydraulic dampers and use wooden clothes pegs. Yep, wooden clothes pegs. He’d used clothes pegs (clamping them around an aluminium rod that moved with the suspension) and he could tell me that wooden clothes pegs were just the right thing for damping the suspension of Human Powered Vehicles...

When I recovered from my mirth I pointed out that this approach seemed to have a few minor problems, ones like getting rid of heat, having separate adjustment of bump and rebound, and durability.... But nope, this guy had none of that – for example, heat problems could be solved by tipping some water on ‘em....

So you can see that when I decided I was prepared to wear some extra mass, it was with the realisation that some of the machines I’d previously been making weight comparisons with weren’t very well designed....

(I should add that I found three very helpful people on-line. One was a guy in New Zealand who has made himself a superb fully suspended trike with front double wishbones, and who sent me some good pics. Another was an Australian AutoSpeed reader who noticed my name on the discussion groups and contacted me directly. He’s built several recumbent trikes and consistently made excellent points during the construction of my machine. And the third was a Swede who has done lots of long distance touring on bikes and trikes and sent me a brilliant list of what he’d taken with him and the volume it’d consumed.)

Chain Drive

Recumbent pedal trikes use a very long chain, about 2.5 times that of a conventional bicycle. The pedals and front cogs are mounted on a boom extending forwards and upwards. The tension side of the chain is guided and supported by toothed rollers that run on sealed ball bearings. The non-tension side of the chain is largely left to follow its own path from the derailleur back to the front cogs, although – like some lengths of the tension side – it’s partly guided by plastic piping. The pipe guides also prevent the rider’s legs getting tangled-up with the chain.

My HPV uses two rollers – the chain passes under one positioned under the leading edge of the seat and over one positioned near the rear of the frame.

Unlike a conventional pedal bike equipped with suspension, the pedalling force of a recumbent trike is largely fore-aft, rather than up and down. On conventional bikes the suspension has to be designed (especially in damping behaviour) so it doesn’t compress with each downwards thrust of the pedals, otherwise an unpleasant bobbing motion occurs. For this reason many bike dampers have a ‘lock’ position that is manually set.

However, the other design aspect of conventional bikes than can cause pedal-induced suspension movement still applies. In fact, because of the immense torque a recumbent ride can develop (it’s not limited by the rider’s weight: he/she can push back against the seat), the design aspect applies even more strongly. So what’s the problem then? Basically, if the chain alignment is not correct, the pull of the chain will cause the rear suspension to extend or compress with each power stroke.

The conventional wisdom appears to be that to avoid this, the chain’s tension path should be through the rear suspension’s pivot point. In other words, the chain pulls along the same path as a line joining the axle to the suspension pivot point. This is then claimed to place no resulting vertical force (up or down) on the wheel.

But this is wrong.

Click for larger image

In fact, the chain should be parallel to a line that connects the axle to the trailing arm’s pivot. The pull of the chain is then parallel to the resistance (ie normally compression) of the suspension arm, so no upwards or downwards forces are applied to the wheel. However, when rear derailleur gears are used, maintaining this relationship is easier said than done because a chain idler location that gives a parallel chain in one gear will result in a non-parallel chain in another gear! So in this area, a compromise will be needed.

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However, when the wheel moves up and down another, entirely different, vertical force is generated that tries to extend or compress the suspension. To see why this is the case, think of how the backwards push of the tyre on the road results in forward push on the body of the machine. If the suspension arm (either actual or virtual) is parallel with the road, this push results in just a forward push on the vehicle, with no vertical forces involved.

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But if the suspension arm is not parallel to the road (perhaps because the wheel is experiencing bump or droop), there will be a vertical component to the force being transmitted by the suspension arm to the vehicle. If the arm is parallel at normal ride height, during suspension bump it will head downwards to the suspension pivot, resulting in a downwards force on the pivot. This will cause the suspension to compress – effectively the suspension bump will be increased. When the suspension arm is angled the other way in droop, there will be an upwards force transmitted to the suspension pivot, effectively causing the droop to be increased. Significantly, the actual effect of these changes will not only depend on the ‘at rest’ inclination of the suspension but also on the amount of bump and rebound travel that is available (since this will help determine the angle that the suspension arms adopt in each direction).

And there are (at least!) three other aspects to keep in mind.

  • The developed tractive effort is highest in low gears. Therefore, the compression/extension behaviour is better optimised for low gears than high gears.

  • The spring will resist efforts to compress it to a much greater extent than efforts to extend it. Partly offsetting this is that the damper is (usually) stiffer in rebound (ie extension) than bump (ie compression).

  • The rear of the vehicle will try to squat under sudden torque inputs so if the suspension is extended a little under power, this may be used to offset the squat.

Doing It

The above points are the result of external engineering input and much experimentation – I couldn’t find any HPV resources that dealt with any of these points in detail.

I started off with the ‘chain through the pivot point’ philosophy but even simple testing with just a bare frame and the rear wheel showed that this created a very large suspension extension force. Unfortunately, I’d been so confident that there wouldn’t be problems in this area I’d had the idler pulley mounts welded into position – so the rear mount needed to be ground off and radically revised.

To find the best position for the idler, I rested the back wheel on the ground, replaced the spring with a very soft, short spring and placed the chain over the lowest rear gear. I placed packing under the spring to give normal ride height and then applied pedal torque by hand, being very careful not to input any vertical forces into the pedals as I did so. The rear chain idler pulley was supported on blocks at different heights; the best height was found when the suspension neither compressed nor extended before the tyre slipped on the concrete.

(Note, as described above, it is vital that the tyre is transmitting torque to the pavement during this testing. It is not the same just jamming something into the wheel to prevent it turning!)

Having then found the best idler height for this ride height and bottom gear, I changed the spring packing to simulate movement of the suspension arm from full bump to full rebound, testing each few centimetres of travel. As a result of this testing, the idler pulley height again needed changing to retain the best compromise.

Then, when this was done, I moved the chain to the smallest cog and did it all again.

Having to move the idler from its previously welded position was a pain but every cloud has a silver lining: I was able to reduce the lateral angularity the chain was forced to adopt when moved from the highest to the lowest gear. Previously, because of clearance issues with an inner bolt holding one of the suspension pivots in place, I’d needed to have the idler mounted pretty well in line with the largest rear cog (ie lowest gear). That was fine for that gear but when the tallest gear was selected, the chain had to bend pretty hard.

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But with the idler pulley mounted further rearwards and much higher, the clearance to the inner bolt was fine. This meant the pulley could be positioned so that it lined up with a middle rear gear. However, I went one better - there was the space to insert a mechanism that allowed the pulley to float laterally, so that it was always in a straight line between the front idler pulley and the selected rear cog.

This mechanism comprises a 12.5mm chrome-plated steel rod (another one salvaged from a car shock absorber) on which the guide pulley is mounted. The rod slides in wide-spaced high density polypropylene bushes, allowing the lateral movement to take place.

Chain Forces

The forces acting on the chain are huge. But huh, isn’t this just a pedal-powered machine? Let’s first take a look at the torque being generated.

The pedal cranks on my machine are 170mm long. That is, the force applied by a pedal is 170 mm from the shaft centreline. On a normal bicycle the maximum force that can be applied to the pedal is dictated by the rider’s weight, so an 80kg rider can apply a max of 80kg. However, a recumbent pedaller can push harder than their body weight because they’re pushing back against a seat. So the force that can be developed is high, perhaps by an 80kg person as much for a short period as 120kg.

A force of 120kg applied 0.17 metres from the axle is a torque of 20.4 kg/m or almost 200Nm!

(kW is Nm x rpm divided by 9550, so if the pedals are being turned once per second, the power being developed is 200 x 60 divided by 9550, or 1.25kW, which is possible for a human to develop for a very short period of time. A continuous pedalling force of 15kg is easily able to be achieved by humans, which at a cadence [ie pedalling] rate of 1.5 per second calculates out to 235W.... so the figures make sense.)

So don’t think that the forces involved are trivial...

Frame Breakage!

At this stage I was able to ride the machine. Clearly, it was unfinished but it was always my intention to do lots of on-road development so changes could be made as necessary. At this stage I was having problems with a slipping chain, something that was eventually traced to the use of a secondhand rear gear cluster. (I hadn’t realised how critical it is that the chain and cogs ‘wear in’ together.) However, when chasing the slipping chain, I’d been really throwing the HPV around, thinking that the suspension movement might have been causing gear cables to pull and so cause inadvertent ‘half‘ gear changes.

During this process I’d been heavily stressing the frame in a way I’d never considered: torsion. Torsional (twisting) stiffness is regarded as very important in cars, where any frame twist will cause the suspension geometry of a wheel to be altered. But in a three-wheeled vehicle, any twist in the frame is of little consequence because the three wheels will still find an equal footing. But what I’d forgotten is that with the frame design I’d built, all the torsional forces were being concentrated in one short section of frame.

Click for larger image

This plan view shows what the problem was. Shown are the main longitudinal, front wishbones and wheels, and rear trailing arm and wheel. (The scale is arbitrary!) The seat frame is shown in blue. Remember that the rear wheel has no lateral roll stiffness, so any roll forces (cornering or even simply leaning over in the seat) are all resisted by the front wheels and suspension. The rear two thirds of the frame were much stiffened by the seat assembly, so concentrating all torsional forces on....

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...the bit shown in the pic by the arrow!

Making matters worse was this section of frame had had several 32mm holes drilled through it. These holes were positioned for the steering system – later abandoned – that used two laterally sliding rods mounted in poly bushes. The holes were originally strengthened by having 32mm aluminium tubes welded through them but when the steering system was changed, I ground-out the welds and removed the tubes, leaving the holes behind.

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And here was the result. A spectacularly failed part, I think you’ll agree. The breakage didn’t dump me on the road (it just made for some weird lean angles) but clearly things had to be changed. The seat frame extensions (mentioned above) were no longer optional; they were critical!

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So I replaced the failed part with some new tube that was undrilled....

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...and extended the rails to the front crossmember.

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This shows how the seat extensions provide much better torsional stiffness – and bending stiffness too.

Conclusion

Even something as simple as the path of the chain turned out to be pretty damned complex in real life. The frame – and its failure? This was an engineering lesson in itself! The end result is a frame very stiff in bending and moderately stiff in torsion, although with the latter I wouldn’t want to quote it in degrees per Newton... I (still) don’t think it would be very high.

In the next part of this series we’ll look at the on-road tuning of the suspension

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