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Making the trundler

There is no 'proper engineering' involved in making the trundler. It is all what used to be known as 'file and fiddle'. Much of the work is in aluminium, but it is no use quoting exact sizes since countries round the world differ widely in the extrusion sizes and tubes available. Much use is made of odd bits just happening to fit other bits, which will involve you in finding equivalents, making a system that works the same out of, for instance, marine plywood, or taking some other route that has not occurred to me at all.

The most cost-effective plan would be to make sure you can find a drum first. Buying one new drum is not easy, but via the Yellow Pages you should be able to find a supplier of recycled blue HD polythene industrial drums with black moulded lids. You want a 160 or 170 liter drum with flat sides. A shorter cylindrical drum is more common, and there is a longer cylindrical one too, but both of these have 17 inch diameter lids instead of the 14.5 inch diameter of mine, and this makes the bottom wheel cap much heavier and more cumbersome. These drums are used by chemical companies worldwide and unless the US has a 'domestic standard' drum you should have no problems. All the good drums come from Europe, from my experience. The drums have been washed out well, but not well enough. You want one that has contained surfactant, wetting agent or industrial soap. Insist on looking inside, and if it stinks of phenolic resin or similar ask for another, since the drum trader thinks he has found an easy market for a bad drum. The other problem is finding drum traders over the phone who say they have any number of blue plastic drums. You drive over and find one battered one of the wrong size that has been outside for the last ten years, for which the going price (less than $US 10 in Australia) has been doubled. Your good trader will have hundreds of blue drums, and sell you a good one, but when you get home you can expect to spend much of the afternoon washing and scrubbing out your prize until it is really clean. Make sure the black cap fits this particular drum properly, and the clamping ring supplied is not rusted to bits. If you try and put the clamp on, and fail miserably, you will have a bad time with your cart. A good drum, clamp and cap are all required, and you will have your work cut out to get them. If you cannot get a good drum, proceed no farther.

The other critical component is the wheels. I began with golf buggy wheels from Fallshaw, who actually still manufacture in Melbourne. The tyres were Cheng Shin 12.5 inch external diameter x 2.5 inch tubed inflatable, and the wheels were black cast nylon with ball bearings. Both wheels and tyres were excellent, but the cheap bearings could not be remedied by improving the axle mountings. I went to Fallshaw to ask them if they had anything better, and they responded with the same wheels and tyres, but mounted on a proper ground steel stub axle with sealed adjusted industrial bearings and a thread and nut for location. These were I was told for hospital wheelchairs and the like. Current price would be over $US 80 a pair. I gulped and paid up. The wheels have been superb for the last three or four years use, have been swapped from cart to cart, and have carried me some way round Canada and England, as well as heavy Australian use. If you fail in your wheel quest, let me know at breck@permaflate.com and I will see if there is any possiblity of sending out Fallshaw wheel sets. I am not looking for the business.

I have checked the wheel and tyre sets are still available, and Fallshaw now has a site on www.fallshaw.com.au
The tyres are now grey, not white as photographed.

The close-up shows the mount used for the Fallshaw stub axle. From left is the Fallshaw thread and nut, a large spacer washer and the U- shaped extrusion. Visible inside the U-section is not the stub axle, but a pretty close-fitting sleeve made from aluminium tube of the right internal diameter, through which the stub axle passes. On the other side of the U-extrusion is a second spacer washer and an increased diameter stub axle. The close-fitting sleeve is a push-fit inside the extrusion, so as the Fallshaw nut is tightened the close- fitting sleeve braces inside the extrusion, imitating a weld. This light, rigid mount is held to the main board by two internal hexagon cap screws with doubled thick washers to spread the load, mounted as close as possible to the close-fitting sleeve. This is the maximum load point for the design, and has to resist impact loads when the cart falls off fallen trees it has been hoisted over, or jars off rocks. There has been no deformation of the extrusion so far.

A third internal hexagon cap screw locates the end of the extrusion, to spread the load on the plywood rectangle as far as possible. On the other side these three screws terminate in nuts and large thick spreader washers.

The plywood rectangle is
350 mm (13.75 inches) wide
520 mm (20.5 inches) long
half an inch thick

The plywood is quality exterior grade, but not marine owing to the outlandish cost of the marine product. It is proofed with a melted-in layer of candle wax, which can be easily repaired.

The bottom of the cart has the axle mountings covered above, back and front strap mountings for the drawbar, and a circle of screws to hold the black drum cap down to the plywood rectangle.

The long multi-turn straps have a strong stretchy nylon section followed by a Velcro (TM) eye section followed by a Velcro hook section. The straps are fast to apply and produce remarkable force to hold the drawbar in its sockets.

The track, or distance between the tread centres of the two tyres is 25 inches, and is critical. Use less and the cart overbalances easlly, more and it will not go through doorways.

The base is flipped over and viewed vertically. You can see the six large thick double spreader washers that support the axle mountings, and the drum cap mounted in the centre by the ring of small setscrews. The drum cap mounting as photographed would not work, since the centre of the cap is recessed inwards for moulded strength, and all the ring of screws would do is deform the recess as they were tightened. Hidden between the cap and the plywood rectangle is a ring spacer the diameter and thickness of the recess, so as the ring of screws are tightened they draw the cap and the plywood rectangle tight to the spacer. Strength is thus enhanced. My original spacer was made out of plywood of the right thickness, but has been been replaced with two semicircular arcs of bent 20 mm square hollow aluminium extrusion. These were the devil to make and drill, but have cut weight and enhanced strength and water resistance.

The 'extra' screws at the top and the bottom of the cap hold the drawbar brackets to the plywood rectangle, and you can see the oval holes melted in the cap through which the other four drawbar bracket screws are just visible.

This is the back drawbar mounting bracket, with the countersunk heads of the three screws that hold it down. These screws have not loosened over time, so are not being stressed. On the other end of the screws are nuts and big spreader washers. The four short round-head screws through the sides of the U-extrusion are multi-purpose. The two on the right hold a short aluminium strip that anchors the end of the nylon/Velcro multi-turn strap. All four act as guides for the strap to stop it slipping around, and the round heads are carefully filed a little flat on top until the two part drawbar just drops into the space between them with no clearance for rattle. The two holes at the very end of the bracket provide location for twin screws on the end of the drawbar, so the drawbar cannot slip back or forward under extreme pulling such as up steep slopes or over fallen trees.

Right are the drawbar ends and screws.

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The twin drawbar drops exactly into the recesses on the back and front brackets, the multi-turn straps are wound tightly, and the Velcro sticks. I began with much simpler arrangements involving screws, wingnuts etc, but everything bent, got lost and generally caused trouble. This arrangement has worked flawlessly through several emergencies.

The drawbar itself, made of two 2 meter lengths side by side of lightweight 20 mm square hollow aluminium extrusion, does not have much complication until the handle end is reached.

I found a dip-moulded PVC handgrip from a bicycle handlebar, then found an aluminium tube it would fit on, cut and filed two strips of thick aluminium so they would force inside the tube, and drilled a hole and used a cut-off nail to pin the joint. I could then bend the two strips with a hammer and vice, and pin the free ends inside a short piece of the 20 mm square extrusion. All this pinning, rather than screws and nuts, is needed for the handle, since at times you will be heaving on it with both hands, and want no hexagon nuts or screwheads digging into you. I could then fix the square extrusion to a piece of plate trimmed to the right size to fit on the twin drawbar, and the hard work was over.

As well as the multi-turn strap, which is an ancestor of the type used on the drawbar sockets at the other ends, much positive location is needed to resist pulling and twisting forces. The four simple- looking pegs on the handle are 3 mm countersunk setscrews held loosely in the plate by the short 20 mm square section going in on top of them. The threads of the setscrews are covered by short lengths of fine nylon compressed air tube hammered over them. This makes four nice-feeling abrasion resistant pins held loosely in their mounting holes to allow them to align when inserted. The pins locate in the holes visible in the paired drawbars, but these holes are not simple. Each is made by drilling a pair of undersized holes on the 20 mm square tube, then hammering through the hole pair a short length of small diameter aluminium tube, creating four 'holes' for the insertion of the pins that have aluminium liners. This both minimises wear on the pins when hauling the cart around, and provides holes through which nylon cords can quickly be poked when the drawbars are used as tent poles. Other tube-lined holes are put in lower down to provide mounting points for the flysheet, tent and other guys. You will have to find your own 'system of fits' for the small diameter lining tube, the nylon tubing that goes over the setscrews, and the setscrews themselves.

Finally the handle is locked on by winding the multi-turn strap around the mated joint. Wiggling is still a problem, and the four big thick washers held on by self-tapping screws provide sideways location of the handle plate and cut the wiggle down to acceptable limits. As you can see on the previous close-up the washers have over two years use worn into the handle plate.

This is the third handle mount design. It is adequate but not good and some time in the next five years, Mark Four will be built. At the moment I cannot see any other part of the trundler that will require such attention.

Weights

Weight of 170 liter blue poly drum plus cap clamp ring but minus cap is about 5 Kilograms

The entire wheeled base assembly (wheels, cap, brackets, plywood rectangle, mountings and straps) is about 4 Kilograms

The twin drawbar and handle weighs 1600 grams or 1.6 Kilograms

That puts the all-up weight of the empty trundler, ready to roll, at about 11 Kilograms.