On motorcycles a grabby disc is a sure sign of a kink or warp in the rotor or a thickness variation that in some way tries to wedge the pads apart moreso than the other portions. The amount of variation that can result in this sort of pulsating braking action is so slight that it cannot be seen with the naked eye in any but the most gross of conditions. It requires a machinist's dial gauge capable of reading in thousandths of an inch to measure runout and thickness for signs of whatever damage is causing the issue.