mikes
I wanted to keep track of miles traveled. I could do it with a log, but then would have to remember every time. There are "hubometers," but they're made for semi-trailers and the tire sizes they use, and quite expensive.

So, I got a bike computer (Sigma BC506) off eBay for <$10 and mounted the sensor to the brake backing plate, and a rare earth magnet on the wheel hub. That lets the sensor be a bit farther away, and sense better at the higher speeds. I used Shoe-Goo to hold stuff on, which is pretty stiff and strong when it hardens. The magnet just clears the lip of the backing plate.
[Odo_sensor] 
The wire just follows the wires for the electric brakes, and I mounted the computer (it's waterproof) to the fresh water fill tube. I may do something different with it later, but it's fine for now. Just need to poke my head under the trailer to check it.
[Odometer] 

The one "gotcha" is that when setup according to directions, it will keep total distance in kilometers, even when set to show MPH. The way around that is to set it up as if the tire size is 1/1.61 (the ratio of km to miles) its actual size. That works out to be ~1375 mm circumference. Then it will totalize miles. If set to display speed in kph, it will actually show mph. (it can give trip average speed, distance, etc., but that's not something I expect to use much)

The battery is supposed to last 2-3 years, and this unit allows you to set the totalized distance, so you can check it then reset it when changing the battery.

It seems to work fine so far, but for my first trip with it I didn't know about it recording km, so the distance was high by 1.6x.
Mike
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mikes
With the axle upgrade, I also updated the odometer. The pictures above are of the new one, which is a slightly better model (I'm hoping it tracks miles instead of kilometers), and the description above has changed. I also put it in a cheap soap holder to keep it out of the weather better. Updated info here.
Mike
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