Some of the world’s top motocross stars took to the air Friday and Saturday in an acrobatic display that kept people on the edge of their seats from start to finish.   The occasion was the Gravity Slashers tour at Arco Arena. Ten riders from around the United States perform tricks that most riders only dream of doing. The floor of Arco Arena was equiped with four ramps and two landing pads. At one end of the arena was three ramps side-by-side, the middle ramp was wider to accomodate a snowmobile; with an elevated landing pad across from the ramps. A single ramp and landing pad were at the other side of the arena.   The star studded list of riders included: Derek Burlew Brody Wilson