I was thinking of a way to get a bit more life out of MX tyres, I usually replace when the edges are rounded as it's the sharp, square edges which give the grip. After searching the net I came across a cool idea adapting a soldering iron, take a look at the link below. I made one the other day and it works a treat. Will get a few more races out of a tyre now and saving money is always a winner!! View: http://www.youtube.com/watch?v=cbhMCd2HBx8
I read in magazine that you should cut the tops of to square the edges. Cutting the sides weakens the knob and will cause it to bend or flex instead of bite into the dirt.
I'm with ya brother ... I just cut the leading edge off the knobs (the upper part of the knob that is rounded) with care NOT to cut down very close to the bottom of the knob so that the integrity of the tire at its base is not hurt ... Some of my cuts might only be 1/4" off the top of the knob so that as much rubber as possible is left on the tire ...