The Extrawheel is a single wheel trailer, à la the BOB Yak, albeit with one big difference - its monster wheel. Most trailers run 20in hoops- the Extrawheel fits both a 26-inch wheel with an Mtb tyre, or a 700c wheel with a touring tyre.
Running a bigger wheel offers the Extrawheel several advantages. The trailer rolls up and down kerbs and uneven terrain rather than hooking up on to it, so there's less pulsing when you're riding. With bags packed to either side, the trailer is wider but the wheelbase between the rear wheel and the trailer wheel is far closer, so the turning circle is a lot tighter.
The other big advantage is that you're effectively carrying a spare tyre, rim and hub. If these are needed on your bike, your damaged wheel will do for the trailer until you source a replacement. On an expedition tour, I'd run the same hub and spokes for compatibility.
But it's in its performance that the Extrawheel really impressed. Being considerably shorter than a Yak, it acts as less of a lever on the bike, with much less tendency to try to steer and twist you from behind, particularly when the going gets rough. The larger tyre volume also doubles as a dampener once you get the air pressure right. Hit a speed bump, and the trailer does a single jump, then settles itself down.
The finish is a little more home-made in feel than a Yak but it's almost all repairable in the field, including the chromoly main structure of the trailer. In expedition terms, the plastic subframe is a potential weak point and the netting doesn't provide the same kind of protection to kit as the Yak's burly cage. Loading takes more practice than other trailers, as you'll need to split your possessions fairly evenly between both sides, with the heavier items up front.
Care is also needed to make sure bags don't press too hard against the material, causing it to rub against the tyre. A rope cinches the load cleverly into place, though I found a cargo net bungee strapped over the top helps stabilise things off-road - and provides additional space to stash kit you need to get to quickly, like a waterproof slicker.
Single wheel trailers are inevitably more fiddly to hitch than two wheel trailers, as you need to balance them at the same time. The Extrawheel uses a novel sprung fork system, which does away with any pins, allowing it to be lighter, simpler and more reliable. Initially a little dubious, I didn't managed to dislodge the trailer once during several months of testing, despite my best efforts over fast, technical and rocky riding.
The Extrawheel is rated to 30kg, but like all single wheels that are rotationally coupled, handling starts to get distinctly dubious with that kind of weight. I'd be happy up to 20kg, especially if I was heading off-road. At around 5kg for the trailer, it's really light too, especially bearing in mind you're effectively carrying spares.
Lastly, £125 for the trailer alone, or £150 with two 60 litre waterproof canoebags, makes the Extrawheel a great value.