Haveing made many mounts for drill motors and out of many different materials we found that it all depends on where the motor is supported as to how it will cope with impacts. at the end of the day drill motors are made very cheaply so if you get hit by little spinner the force is so great that it would be better to put a sturdy mount in that will support your motor at the right points. soft mounts will flex a little which will then try and bend the motor/gearbox. As its the motor thats keeping the mount in place rather than the mount solidly supporting the motor. In all cases its best to use as soft a wheel as possible or rubber mount the drill mounts. This way the mount will absorb the shock but wont let the motor take the strain.