Ackermann Steering: The Inner Wheel Turns Further

A cart turns by swivelling its whole front axle on a pin — simple, but the axle sweeps under the body and the cart grows unstable. In 1816 the Munich carriage-builder Georg Lankensperger pivoted each front wheel on its own stub instead, joined them with a tie rod, and angled the steering arms so the inner wheel turns further than the outer. Rudolph Ackermann patented it in England on his behalf in 1818, and the geometry has carried Ackermann's name ever since. Every wheel should roll round one common centre on a turn; this linkage gets very close to that without any clever mechanism at all. This rung works out the wheel angles and checks a trapezoid linkage numerically, then builds one and measures it with a protractor.