In the beginning...

Building Pospedal Mk V – diary of an obsessive

Sunday 18/11/2012, 5.50pm

Pedalcar defined: Human Powered Vehicle (HPV) powered by 2 people, seated side-by-side. The version I’m working on will have 4 wheels, fully independent suspension and 200 speed gears. Disc brakes on the front. This is the log of its construction and will hopefully be of some assistance if the design works well enough to sell the plans. I’m building it for me, unreservedly selfishly, but who knows, maybe it will be of use when all the oil’s gone (soonish).

A Brief History

This obsession goes back a long way. Sometime in the late 70’s, I made my first attempt. Misguided, perhaps, in view of the fact that I knew nothing about Mechanical Engineering, apart from maintaining and building motorcycles. I built Mk I to tour Tasmania with Sarah. It had two wheels in the back and one wheel up front. The frame was constructed of a sandwich of Aluminium sheet bonded together with fibreglass. I can’t find any photos of it, which is strange considering it took up all my spare time for six months. The word ‘bonded’ is troublesome. I thought sandblasting the aluminium was enough to give it a good bond. It wasn’t. The day before we were due to get on the boat to Tassie, we took it for it’s first test drive. It broke in half before it got to the end of the driveway. To be fair, we had a long and rough driveway. I laughed a lot.

So we had to quickly get our bicycles sorted (adding fuel can panniers) and ride 30 odd Km to the ferry. They kindly delayed the ferry departure for us. Nobody was touring on pushies in those days, and that was fully five years after I’d ridden from Melbourne to Murwillumbah.

Anyway, that’s ancient history. I decided that what I needed for a light and strong frame was M-board (half-inch aluminium honeycomb bonded with araldite between two very thin sheets of high-tensile aluminium), so I had a very expensive 8x4 foot sheet sent out from England. They build planes out of it, most notably the off-ramps for Hercules aircraft. It got lost for a time and they didn’t charge me freight as compensation.

As it turned out, there was no rush. Sarah and I eventually moved from Melbourne to Queensland and the M-board came with us, in it’s corrugated cardboard packaging. Termites love corrugated cardboard, and moved in big time. When I decided that it was time for mark II, about 2006, I opened the packaging and discovered the surface of the panel was a bit corroded from the termite piss and I spent quite some time polishing out the disfigurement.

And so Mk II came into being. In the meantime I had achieved trade certificates in machining and welding at TAFE at night and had also done Engineering (Civil, not Mechanical, unfortunately, but it did give me some understanding of stresses at least) and had worked for a time as a draftsman. So at least I now had some rudimentary knowledge.

Having worked as a draftsman, I was quite aware of the phenomenon that even very carefully drafted plans can sometimes not reveal a very nasty flaw in a design, so I decided to go with the British Aerospace method of ‘suck it and see’. I designed it on the fly without any plans, moving quickly from Mk II, using half-inch plywood sheeting as a central spar and recycled bicycle frames and children’s playground equipment (from the tip) for the subframes. One interesting artifact of Mk II was the use of a flywheel made from a truck disc brake and an ‘infinitely variable’ gearbox of my own design, dating back to the 70’s. Both of these ideas proved hopeless and were quickly jettisoned. That led to Mk III, using conventional bicycle freewheels for propulsion.

Pedalcar Mk II, sans single back wheel, (note through pedal crankshaft and drum brake flywheel). Didn’t take me long to realise that a single back wheel doesn’t work well unless it’s a long way behind the people, as all the mass is outside of the wheel triangle.

At last I was getting somewhere, we could ride it short distances without too much breaking.

Pedalcar Mk III. Driven on one back wheel only. Traction was a problem uphill on dirt roads. The floral seats were a feature. I think this one’s 35 speed.

Sam and Jake give it a burl, Mk IVA, I think. Sam still has hair issues.

Then I finally had the nerve to use some of my precious M-board for the central spar and upgraded to using 6061 aluminium for the subframe, with a bit of chrome-moly for the drivetrain. This was Mk IV. It was quite promising, and gave us some joy for a while. As I gradually tweaked it, it gradually got faster and more reliable. I had no illusions, however, that this (or Mk II or III), was the final product, it was only intended as a testbed, and in this role it was invaluable. I learnt what not to do.

Last version of MkIV, with some hanglider parts and mostly aluminium, shortly before a small fairing was added (gaining 2km/h) and also shortly before its untimely demise. Note no derailleur on back wheels now, both rear wheels driven. The flowers are history. This one is maybe 105 gears. Eventually I settled on 98 gears.

My biggest learn was on Boxing Day 2008, when my son Jake and I were herbing along at about 60Km/h and I steered off the bitumen to give space to a 4WD coming the other way (they don’t like to get off the bitumen). The pedalcar encountered a series of corrugations, which pummelled the worn out second-hand shocks into the excessively lightened front wishbones and caused them to fracture, resulting in us spearing into a concrete culvert (culverts are very infrequent on our road) and coming to an abrupt stop. I was unceremoniously ejected from the driver’s seat on to the road and Jake was hobbling on a sprained ankle for two weeks. Sarah wasn’t impressed and I was understandably chastened. 

One of the ironies of this prang was that I was long aware that the shocks were a problem and had ordered four Fox air shocks from America a couple of days before the stack. So I ended up with four beautiful shocks, but no pedalcar.

The whole idea was put on the backburner for four years, although I spent some time researching design and materials, and putting everything I’d learned down in a set of plans. Lack of funds to do a proper job was a major problem.

Then last month I won the Hugh Sawrey Art Award and was given $10,000. It was now or never. Over the last few weeks I’ve been gathering resources and what follows is a record of the construction of Mk V.

Bill and I. Bill thinks I’m suss.

Building the Beast

So here I am, making a last ditch effort to realise my dream of a two person HPV. Not that revolutionary, really, just imagining a light human-powered coupe. Don’t think anyone’s tried it before (i.e. two pedallers, side-by-side, full suspension practical vehicle), but no doubt someone has (I’ve never seen one). Not that that’s important.

So I started work on the ninth of November and I’ve got a bit done. Here’s a few pics of progress so far.

This is the drivetrain. (Top Row) Trimmed 152mm single speed 28 tooth Sugino cranks feed into (Second Row) Shimano 11/36 tooth 10 speed XT cassettes, which are linked by a cross-shaft to 22/32 tooth Blackspire chainrings, which in turn feed another (Third Row) Shimano 11/36 tooth 10 speed XT cassette, which is linked to 16 tooth sprockets (welded to modified chainrings), which then go to (Fourth Row) 16 tooth flip-flop BMX sprockets on the rear wheels. Whew!

Notes on the gearing

This is essentially the same gearing as established on the Mk IV, though 11/36 tooth cassettes allow a lower and more gradual gearing than the previous 11/34 tooth freewheels. There are four gearshifts. Having two cassettes on the first shaft allows each rider to pick their own cadence. The ‘passenger’ controls his/her own cassette and cadence. But that doesn’t mean they can bludge off.

Highest gear gives 28/11 x 32/11 = 7.4 turns of the (20”) back wheel per pedal rev.

Which is 10.9 metres distance. By comparison, A 27” wheel with 52/11 gearing covers 10.18 metres.

Lowest gear gives 28/36 x 22/36 = 0.475 turns of the back wheel per pedal rev. By comparison, a 27” wheel with 32/36 gearing covers 1.91 metres.

The L/H and R/H drive flip/flop sprockets on the back wheels ensure that only the inside wheel is driven on a curve, thus providing a differential.

The workshop. Yes, it’s messy. But I know where 80% of everything is, at least. Beer helps late in the arvo.

Centre of operations, a small Chinese/German lathe. Crude, but effective. Sorry Neil, I’ll clean the lathe at the end of the day.

Machining a bearing retainer from a salvaged aluminium pulley.

The pulley trimmed before turning. Some nice metal in those pulleys.

Turning the other side

I’ll get back to you when there’s something more to report.

Friday 23/11/2012, 4pm

I finally got something finished today, having spent a few days working on the plans, chiefly refining measurements and detail. It’s the passenger side drive, and here’s a pic –

What we have here is a 10 speed Shimano XT cassette fitted to a cut down Alivio hub. Since it’s the left hand end, I had to use a left-hand thread to fix it to the end of the shaft. For that I used a left-hand-side pedal axle, which via a very tricky manoeuvre (slight grind and beefy weld) has been fixed through the freehub to said Alivio rear wheel hub. Fitted to the end is a 7/8” x 2” hybrid ceramic bearing, which will eventually find its way into a carbon fibre mount. The drive shaft is a piece of 7/8” heavy wall (1.6mm) chrome moly. All bearings except the bottom bracket are hybrid ceramic - low friction and durable. Total costs for the project so far have just passed $6000, not including those Fox shox, which I think cost me $1400.

This is a detail from the plan. I got this far in the drawing and wondered how I might build it, which prompted me to go into the workshop and see what I could find. The pedal axle was just the thing, being high tensile and left-hand thread. It just needed a slight grind to fit it into the freehub. The aluminium piece that the bearing sits on came from a billet that I found at the recycling depot. It’s an amazing high-tensile aluminium that cuts like steel. Hope to finish off the bearing retainers tomorrow.