Yes, I have only the front brake. I was planning a rear, but have not felt I needed it.
The Big Wheel has a 2 piston caliper, on a 300mm rotor. It is a heavy machine, and the little caliper will lock the 19" motorcycle tire if you use more than two fingers on the lever:
When I put the sidehack together, I was concerned that I was going down to a 180mm brake disc, so I planned for a rear brake. I found a 180mm rotor I thought was a bicycle rotor:
But it was about 1mm thicker, and as you can see I had to re-drill the 6 bolt pattern. If you look at the pad marks on it, I think some of the bicycle rotors might work. As long as the braking surface is mostly round. Bike parts tend to be thin - the spandex crowd worries about grams.
I looked around on e-bay, and found the same rotor I used. Apparently it was used on some sort of scooter:
Scooter brake rotor
while not a direct bolt on, it is beefier than the Avid G2 Rotor
, and about the same price.
I know there are some 290mm bike rotors out there, and that would work too. I had to remove a little forging flash from the caliper to clear the spokes - a larger diameter rotor would move the caliper away from the spokes.