The Wheel Wobble on the MACH 2 is due to the wheel shaft moving. Here is how to add a set screw to hold the shaft and keep the wheel from wobbling.

Purchase a wood screw at your local hardware
Use a small drill about 1/8" and drill a pilot hole about 1 1/2 " above the wheel shaft on the back (drill at an angle: that will hit the shaft at the halfway point about 1/2" in)
Screw in a #8 screw about 1 3/4 inch long - this will hit the shaft and lock it from moving. This will remove most of the wheel wobble.