In 2012, my old team, 2180, utilized a disk brake on our rear wheel to aid in balancing on the competition bridge with our robot which was basically a 4x4
. I remember that year, once we showed it off in our first few matches, teams would always come to us to make sure that we would balance with them on the center CO-OP bridge. Ahh, those were the days. I'll see if I can find any pictures because it actually was a stupid simple system which included stock bike parts, some fishing line, and a servo. Only a little force was required for our entire robot to be locked in place.