When you hop inside a car or aboard a lawn tractor, it's easy to overlook some of the physics problems your vehicle is designed to overcome. When you turn over the ignition and press the gas, everything is hunky dory. The transmission transfers (transmits, really) the power generated within the engine to the axles so the wheels move.
This is all well and good when you're driving in a straight line. But there are few roads that don't curve and you'll only get a fine, single strip of your lawn mowed without eventually going back the other way. What you need is to turn and for that you need to steer.
Steering changes everything in a vehicle in motion. Rather than going in a straight line, the vehicle is now going in a circle (or completing a turn around a right angle). For our purposes, we'll stick to the circular turns that are required to mow a lawn. To mow a lawn back and forth, you will need to turn 180 degrees several times, and to do that you'll actually be completing a series of circles.
Exactly how this works was first patented in 1818 by a man named Rudolph Ackerman. He saw that when the front two wheels on a vehicle moved to turn the vehicle clockwise or counterclockwise, the vehicle turned around the center point of a circle. Although the length of the radii from the center of the circle is different for the inner and outer wheels, all wheels turn around the same center [source: NTBA].
For wide turns, the point is far away from the inside of the turn. For tighter turns, the center of the circle is closer. And for zero-degree turns, the center of the circle is actually found within the vehicle itself. The center of the circle lies within the center of the rear axle of the vehicle.
In 1963, a Kansan named John Reiger became the first to come up with a zero-turn mower. After fiddling in his workshop, Reiger came up with what he named the Hustler, the first line of zero-turn mowers. No longer did lawn tractors require wide lazy turns.
For decades, the zero-turn mower drive system remained largely the same. On the next page, we'll see how zero-turn mowers work and how Synchro-Steer improves upon this invention.