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Cargo trailer Mk. 2

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After tasting the benefits of a trailer, I can't live without one now. Mk. 2 is my first prototype. The width is just right to be road-legal (900 mm) and all space between the wheels is used for a cargo box. An advantage of the axle-mounted drawbar is stiffness, disadvantage is it can't be used as a hand cart. The hitch is inspired by Peter Eland's design. It's a three-axis joint with M8 bolts for axles; slightly cumbersome to connect, but I trust it much more than commercially available cables and springs. The other equipment are reflectors, improvised aluminium mudguards (added after a very wet experience) and built-in lights. Some rainproof tarp or lid would also be nice to have, but it will probably never get realized because I plan something better.

The longest trip so far measured over 80 km, the worst problem was the extra aero drag and weight. The heaviest load weighed about 100 kg and it was too much, practical limit is cca 75 kg.

Design

geometry plan

The trailer hitches to the left end of rear wheel axle of the tractor bike. At first I set the drawbar length so that the bike was exactly between the trailer's wheels. But that was a mistake because a trailer doesn't care where the bike is - only the hitch position relative to trailer's centre of gravity is important. This decides whether the pulling force causes the trailer to bounce sideways or not. The stability improved when I extended the drawbar a bit to get the trailer exactly behind the hitch (see picture on the right). It looks weird from the outside (the trailer is offset to the left), but it works.

The frame is made of 20×20×2 mm steel rectangular tubing. It's unnecessarily strong, ×1.5 would be strong enough and lighter, but these were not available at our local ironmonger's and I was too lazy to find another one. The box is made of leftover roof polycarbonate, wooden bars and screws. Plywood would be more practical - holes could be drilled in it to allow hooking up bungee cords or the like. The floor is padded with foam to reduce rattling.

Drawbar is also made of 20×20×2 square tube, attached to the frame by three M6 bolts. It is stiff enough up to cca 30 kg of cargo. Over 40, you have to watch for longitudinal oscillations and shift to avoid getting into resonance. With 100 kg the oscillations become a really big problem. Next time I would make the drawbar 30×30.

The hitch joint is made of two identical steel cubes, each with two perpendicular threaded holes in it. Three M8 bolts are loosely screwed in them. The threads work as plain bearings and don't need any lockrings to hold them together. Friction is negligible if greased properly. I'd recommend to use only first two taps for cutting the threads, third one gives them too much play which causes rattling. The vertical bolt in the drawbar end has no head; its bottom end is held tight between two nuts and one of the joint cubes is threaded loosely at the top.

The wheels are 24" and run on cartridge bearings. This is probably good for rolling and durability, but not so good for occasional rattling noises caused by loose inner ring of one bearing I can do nothing about. Next time I would use 20" wheels, they are stronger, lighter and easier to store.

Lights

At first I used a battery-powered blinky stuffed under the cargo, flashing through the translucent box. But it was not very reliable (the blinky didn't hold in place) and the bike wasted a lot of light illuminating front wall of the trailer. An obvious solution was to divert rear light power to the trailer through a suitable connector and prepare a matching set of LEDs there:

All diodes are built into the hollow transparent walls so they are out of harm's way. Yellow ones are on the sides, red ones at the rear (three on the left and two on the right). Both types have the same power consumption, so they can be connected in series; this solved the odd number problem.

Conclusion

Strong spots

Weak spots

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