Vr is defined as the speed at which the rotation of the aircraft should be initiated to takeoff attitude.
Rotation speed (Vr) cannot be less than V1. If it is greater than V1 and it is found that, at Vr, rotation cannot be achieved, a subsequent rejected take off may not be possible within the remaining runway length and is likely to result in a Runway Excursion.
Vr is a function of aircraft weight and flap setting but may also vary with pressure altitude and temperature.
In the engine failure case, Vr must allow for acceleration to V2 at screen height - 35 feet above the level of the runway surface for aircraft certificated as meeting Performance 'A'.