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I'm trying to design a single wheel trailer for my underpowered VW, much like this one.

Unlike a conventional trailer, it is attached to the car at two points. To prevent the wheel from dragging, it has a swivelling mechanism. Those are hard to come by, so I'm going to build one myself.

I'm not a mechanical engineer by trade, so the questions I have are probably quite basic:

  1. With caster wheel wobble in mind, should the wheel swivel freely, or is some friction desirable?

  2. What kind of bearings should be used for the swivel point? There is definitely an axial load, but is the radial load considerable?

  3. What would be the most effective geometry? How much trail/rake? Is this something I just have to experiment with?

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  • $\begingroup$ I'm a mechanical engineer and I need to get slightly off topic: Surely there is some legel issue with having your own trailer like that? $\endgroup$
    – Dermighty
    Commented Jul 6, 2015 at 9:40
  • $\begingroup$ Good call. Actually, in the Netherlands, if the weight of the trailer is below 750 kg, it doesn't need to be approved or certified, as long as it conforms to the rules. However, I will do my utmost to ensure that the trailer is as safe as possible. $\endgroup$
    – Willem-Aart
    Commented Jul 6, 2015 at 9:48
  • $\begingroup$ I'd be looking at a wheelbarrow wheel, which may have a bearing that you could run a shaft/axle through, depending on how well made the wheel is it would definitely do the job. $\endgroup$
    – Dermighty
    Commented Jul 6, 2015 at 11:06
  • $\begingroup$ Regarding wobble, a fully laden trailer may perform differently to an unladen trailer (queue Monty Python jokes). If you're putting anything heavy on it then you'll need to work out what kind of stress your assembled trailer can take, you do this by selecting each component for the task at hand. For example, if you're putting 4 20kg bags of coal on it, then you don't need a an axle capable of holding 3 tonnes nor do you need an axle that holds 10kg....you get the idea. $\endgroup$
    – Dermighty
    Commented Jul 6, 2015 at 11:10
  • $\begingroup$ Is this an African or European trailer? If you find that you do have problems with wobble, you may want to look at motorcycle steering dampers. They dampen high-frequency wobble while still allowing the handlebars to be turned easily by the rider. $\endgroup$
    – Carlton
    Commented Jul 6, 2015 at 19:37

2 Answers 2

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There appears to be a big fundamental problem with this design in that the trailer is not articulated. While this will indeed make it easier to reverse, in some circumstances it is potentially dangerous as whenever you turn a corner the rear of the trailer will swing out into the adjacent lane or footpath.

I'm pretty sure that this wouldn't actually even be considered a trailer under EU regulations and in any case they are required to be connected to the towing bracket using an approved ball hitch.

There is also the issue that this arrangement effectively couples the mass of the trailer to the rear of the car in the yaw plane which is going to have a very significant effect on its handling in addition to the effective increase in the rear overhang and is likely to be considered a modification to the vehicle itself in the EU and as such would require individual type approval.

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I have a trailer just like this one for my wife who has trouble backing an ordinary trailer. To prevent wheel wobble at speed , I use a motor bike steering damper. The trailer is attached to the conventional tow bar with a spreader attachment to provide 2 fixing points and can pivot up and down.

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    $\begingroup$ That exact same idea went through my head. However, the wheel has to be able to swivel 360 degrees. How am I going to utilise a steering damper for this application? $\endgroup$
    – Willem-Aart
    Commented Jul 7, 2015 at 8:26
  • $\begingroup$ Why does the wheel need to swivel 360 degrees? Can't a motorcycle turn adequately? Also, if the trailer cannot rotate around the hitch, what good would it do you to allow the wheel to turn sideways (90 degrees)? $\endgroup$
    – Chuck
    Commented Sep 6, 2015 at 16:42
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    $\begingroup$ It would need to turn 360 degrees for reversing. $\endgroup$
    – Transistor
    Commented Dec 5, 2015 at 14:49

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