These people make lots of different good types of controllers, for use on anything from go karts to golf carts, and have a large wealth of helpful info on their website. They are located in the UK. The Diagram here is of the Uni controller, the kind I use on my go-kart.
This guy is from the UK and builds lots of different electric go karts and scooters out of wood, among many other cool things. (Using 4QD controllers!) He has his plans up for purchase, and they are very good and detailed. (I bought them.) He also has lots of help for choosing the right controller, motor, etc.
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