Before a dozen specialized tire bead breakers arrived on the market, I used a large woodworking screw clamp, like you would buy at a hardware store to release the bead on a motorcycle tire. I would place the fixed side of the clamp against a piece of wood that was placed against the wheel rim on one side of the wheel. The screw and flexible foot side of the clamp I would place on the tire rubber sidewall directly against the wheel rim. Then spinning the clamp screw bar would force the tire bead down into the wheel drop rim. I would then be able to use my foot to pop the rest of the bead off around the wheel. Then turn the wheel over and do the same thing to the other tire bead. That never failed me in the past.
Now I use a specialized motorcycle tire bead breaker that uses a long lever attached to a foot to break the bead, just because it is faster and I like gadgets. I might add that I have one of those "bead poppers" and was never able to get it to anything but bounce off of the tire.