main wheel
noun
1The first, or driving, wheel in a series of gears, especially in a clock or watch.
2 Aeronautics each of the wheels which are located near the centre of an aeroplane and bear most of its weight when it is on the ground, as distinguished from a nose or tail wheel.
Origin
Late 17th century; earliest use found in Joseph Moxon (1627–1691), printer and globe maker.