An alternative strategy for larger kilns is to have two pinions acting on the gear wheel, one on each side of the kiln. This was problematic for earlier technology, because of the problems of having two motors competing to supply torque at varying speed, but from the 1960s, advances in motor control allowed dual drives to be installed on larger kilns. Because it is related to eccentric load, kiln rotating power is more or less proportional to speed. The need for higher rates of rotation began to emerge with suspension preheater kilns in the 1960s, and much higher speeds of 4 rpm or more are required for short precalciner kilns. Modern drives, in line with the large, high speed kilns being installed, can be much larger (>500 kW from each motor in a dual drive), with speed varied over a wide range by means of solid state controls. However, the largest British drives appear to have been the pair of 580 kW motors on each of the six kilns at Northfleet