Some people have asked me about these parts so here goes:
All these parts were second hand from ebay from go-kart shops. Karts are small petrol engined one seat racing machines with a wheel at each corner. They range from toys to very serious machines and form a lower cost entry point into motor racing for many people (Lewis Hamilton in Formula 1 started out in karts for example).
These parts may look expensive but take a look at the price of big combat robot mechanicals from robot stores and then you realise that, if bought second hand from ebay, they are not too bad.
The back axle on a kart is a 4ft long tube driven by a central sprocket. There are long keyways cut into sections of this axle –at each end for the wheels and further in for the drive sprocket. The sprockets come in different sizes from about 6 inches diameter to about 8 inches. On ebay you often see a set of 3 for example so you can experiment. These sprockets all have the same inner hole pattern and bolt to a central alloy casting called a sprocket carrier. The sprocket carrier slides over the axle and clamps to it. It also has a keyway cut in it that a rectangular lump of metal fits into (the key) locking it to axle so it cannot slip.
The back axle is mounted on 2 bearings which slide over each end and lock to the axle with a grub screw in the side of the bearing. The bearing is usually then held sandwiched between two plates (bearing carriers) shaped to enclose it which then bolt to your chassis each side.
The wheels come with a central casting which also has a keyway in it. It clamps to axle and keyway stops it slipping.
So you need:
a) A kart rear axle tube. Some are solid but you don’t want all that weight so get a hollow one if you can. Also, a slightly bent one will be useless for racing (so you can get it for free) but OK for you as you are only going to use a short length of it. Note the diameter of the axle as there are several sizes.
b) Get a pair of bearings and bearing carriers (they may come together) in same diameter as your axle – check and double check.
c) Get a sprocket carrier, with middle hole same diameter as your axle.
d) Get a wheel. I found someone selling a single wheel on ebay. I went widest possible rear kart wheel that is made as I wanted it to be stable in terms of side to side tilt. Others have used one size lower than this (which is smaller diameter so more room for batteries etc in the board). Again the wheel centre has to be the same as your axle diameter.
e) Get some sprockets.
f) Motor sprocket must be same tooth pattern, as must the chain.
g) Assemble whole lot as in the kart but saw out a section of the axle just wide enough for your monowheel board. Hint: Make it a bit too long to begin with then trim it down later when everything assembled and working right.
A Kart. Smaller sprocket bolted to a sprocket carrier, which in turn clamps to axle tube.
Sprocket carrier. Inner ring of each sprocket is a standard size even though outer diameters vary.
End of an axle showing the keyway. 2 bearings top left. The triangular things clamp around the bearings and are bolted to
sides of chassis through bolt hole at each corner of the triangle.