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The Tractor


"Far better a few tons too much
than one milligram too little."
- Heywood Floyd

After an ambiguous experience with front wheel drive, I chose more conservative rear drive and standard positive trail geometry. Inspiration came from many sources (in short: I copied everything I could :-) ). Thanks to Nink for Steelmachine Mk.3, Jirka for wooden seat and technical details, Cobic for Cycletruck, Radek for lights and company Fote for letting me use their machinery.

Geometry is copied from the original Steelmachine, just some details are different. Steering angle 70°, main boom angle 20° (perpendicular to steering), trail 50 mm, wheelbase 1300 mm, weight distribution front/rear 53/47 % (or cca 55/45 without the lower rack), front wheel 406 (20") with SON Delux dynamo, rear 559 (26") with CS-RK3 hybrid drive, LED lights. Material and parts cost over 20000 CZK (a price of fully equipped good quality standard bike) - not a low-cost prototype anymore and my reserves of usable junk are almost depleted.

The bike feels perfectly tamed since some 3000 km of riding. Of course, compared to a python, this is the easiest bike ever and could be ridden since the first moment, but there were still some reflexes you needed to learn: how and by what amount to turn the handlebars, where to stop your feet at tight turns when they interfere with front wheel, how much lean in turns is possible, how to brace in the seat when going over bumps, how narrow spaces you can go through and so on. Riding behaviour looks very good. Stability at high speeds is good, and sufficient at low speeds (would be even better with "superman" handlebars); even bouncing into the air can be recovered from. It's slightly worse with heavy cargo, the frame was not originally designed for it and even though I used thicker tubing, its torsional stiffness is insufficient and you can feel it oscillate after quick handlebar movements. But it's nothing fatal, the oscillations fade off quickly. Turning radius is tight enough for a U-turn on a normal road. The pedals are a bit too high, I'd put them lower next time (or the seat higher) to increase blood pressure in legs.

The build

"Serial" Steelmachines use 1.5 mm rectangular and square tubes for their frames, I used 2 mm - firstly because that's what all reachable hardware stores stocked, secondly because I can weld 2 mm safely enough, even in less than ideal positions. Main boom is 40×30 mm (positioned horizontally to catch horizontal chain forces, but provide some vertical compliance), rear fork 20×20, cargo racks 15×15×1.5.

I used the movable bottom bracket for welding practice (as you can see by the beads). BB shells are usually mounted on a horizontal flange and bolted by four vertical bolts to another horizontal flange at the top, but that would be too tricky to weld, so I used vertical flanges and horizontal bolts. The good news is it prevents you from installing the BB askew. The bad news is if you (like me) forget to put some spacers under the flanges during welding, you need lots of grinding and scraping before it fits on the boom after paint has been applied.

It's not easy to find a non-kids' 20" fork with a disc brake mount, so I made one by shortening and modifying an old 28" fork. It's made of 2 mm steel, so welding was no problem. The plan was to use the old headtube as well:

But it was mostly brass, which literally evaporates when hit by welding arc. No nearby hardware store stocked a tube of reasonably similar diameter, so I had to go tubeless:

Holes in the booms are larger than necessary, the cups are held by precise holes turned in two reinforcing plates (their weird outer shape was necessary to hold them in a three-jaw chuck). Legs of the "bridge" are machined to a plane parallel to the upper plate, manual filing wouldn't be precise enough. Final position of the bridge was determined by inserting the fork and tightening its bearings. Later I discovered the pure tubeless design doesn't work: the boom is so flexible that the cups travel several tenths of a millimetre up and down and would ruin the bearings quickly. The bearings come from the same bike as the fork and they look perfectly good, despite their age. To be sure, I threw away the original ball cages and replaced them with larger number of free balls.

Rear fork is offset, built around a symmetrically-laced wheel. That improves wheel strength and allows the chain to clear the fat tyre and mudguard when on the biggest cog.

Seat shape was determined by taking a photo of my back and tracing it into CAD. First try didn't work, the lumbar hump was too big and the ends too closed in. Second try was OK, so I used it to make third, final version out of better material. Main beams are made of 15 mm railway-class plywood (unnecessarily thick, but was on hand), side beams of 4 mm furniture-class plywood, rungs of 10 mm beech dowels. If you want to try this seat too, here's a template (print on A4 in 1/1 scale).

The seat is held by two 10 mm rods with threads just at the ends, so the precisely reamed holes in frame and seat fit on the smooth middle section and don't wobble. There are three pairs of corresponding holes, so the seat can be moved forward if needed. The seat held together by friction alone during first test rides, I glued it together after hammering the main beams to correct position to fit around the painted frame (note: soaking your fingers in acetone glue is not a good idea, mine itched the whole next day).

Handlebars came from the bike I slaughtered for the bottom bracket shell. Central part of the risers was cut out, turned upside down and stripped of their brazed stem. The tiller consists of several frame tubes and clamps and is adjustable in all horizontal, vertical and angular directions. The whole assembly tilts upwards on an axle in the stem, making mounting, dismounting and walking easier.

Most Steelmachines have "superman" handlebars: a wide U shape going around knees, which you hold with arms outstretched forward (see photos: 1, 2, 3). Their advantage is better stability because their centre of gravity is slightly in front of the steering axis. The difference is apparent at low speeds where my "tiller" with CoG far behind the axis falls to the opposite side than is needed to maintain balance. I chose it for four reasons: wider steering range, narrower profile, lower weight and mainly for safety during sideways fall. That's the single most frequent type of crash and I already performed it several times. Superman bars can bend, break or stab into your thigh, tiller usually doesn't touch the ground at all.

With handlebars this narrow, there was no other choice than integrated brake/shifter levers ("brifters"). The brake levers had to be shortened a bit to clear my legs. Indexing cog of the left shifter had to have one tooth enlarged because its top gear step was too long for the internal gear hub, leaving the cable slack and bouncing out of its guide slots. Both levers got brake light switches and backlight for their gear indicators (with trigger shifters, this is the only way to tell what gear are you in at night).

It's no problem to get an axle mount for a derailleur. The problem is they are not designed for my dropout angle and big 34-tooth sprockets. But where there's a welder, there are no incompatible parts:

At first the chain sometimes slipped on the second largest sprocket. But it was just an adjustment error, now the derailleur (Shimano Acera) shifts well. Except of slow upshifting on the three smallest cogs, which is a result of the extended mount - the guide pulley is further from the cassette than it's designed for. Barrel adjusters on the shifters came handy, it would have been impossible to tune the shifting without the possibility to screw the barrel and listen to chain response in real time.

Power side of the chain is routed around an idler and protected by floating tubes, return side goes through a fixed tube and hangs on another idler next to the chainring, to completely clear the front wheel. Power idler's groove has flat bottom and is wide enough for the chain to find its optimal position for each selected sprocket. The tubes are polyamide pneumatic tubes 15×1.5 mm, straightened in hot water and with their ends flared by a conical jig over a gas stove.

The second picture shows the bottom tube doing its job. Top rear tube between cassette and power idler looks clean and useless, but maybe the dirt just falls off it quickly. I'm not going to remove it; maybe it would save some friction and noise, but would give me dirty hands every time I strap something to bottom rack. Top front tube is necessary, I'd be all covered in oil without it.

Cranks were originally 175 mm long. On the previous bike, even 170 mm felt too long, so I bought special taps and drilled new holes at 155 mm. They have felt just right since the first moment, but they require two things: to practice faster spinning and to very precisely adjust pedal to seat distance. Faster spinning is necessary because shorter cranks mean less torque with the same force, so power must be made up by speed. Pedal to seat distance is absolutely critical: if it is too close, kneecap tendons hurt; if too far, Achilles tendons hurt. It's a classic RSI, so be careful, you can damage your tendons permanently! Position tolerance is about 1 mm and you can't tell it's wrong before you pedal at least 50 km, and then you must wait several days for the legs to completely regenerate, so you need a lot of patience. But it pays off: I can now ride this bike any distance I want and there is no pain at all, except overworked muscles. This is my first bike ever (upright or laidback) to work this well. (errata added one year later: another critical parameter is angle of cleats in SPD shoes, it also makes your kneecap tendons hurt)

Bottom rack is slung low under the chain and attaches to the bike from below, without requiring to disassemble anything else. Top rack is also detachable, just the two sloping struts - which double as mudguard stays and brake light holder - are welded on permanently.

Mudguards are steel. Rear 26" from a local bike shop, front 20" (actually a rear one too) from a big sale at Azub during 2015 recumbent meeting, where I also got the generator, crankset and several pieces of high quality cable sheath, now used for the derailleur.

First trip was getting home from the garage. Cargo capacity was used to its full to carry the rest of the used material (yes, it works great for long stuff):

Unlike the Python, Steelmachine is stable like a bus, balance is no problem at all. The difficult part was to make it turn: I want to go left, so I must initiate left lean by turning handlebars to the right, which is done by pushing the long tiller to the left, yes, now I'm leaning, so turn the bars left to stay upright, but not that much, darn it, I'm going almost straight again, let's do it all over again...

Second series of test rides occurred together with measuring generator characteristics and the bike also got tested by mud and rain (and it was immediately clear the stock front mudguard won't do). I pedaled up to 35 km/h, steering with one hand, holding a notepad in the other, watching measuring instruments instead of the road ahead. Yep, this will work. Then I took the machine apart, cleaned, painted (the yellow again - still some left), reassembled, adjusted and then spent over half a year tinkering with wires and diodes to make it shine properly.

Notes from run-in period


Looks like the bike has met my expectations. I'm happy with it.

Strong spots

Weak spots