How Synchro-Steer Works

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.

Zero-turn mowers use two rear transmissions to distribute energy, one powering each of the rear wheels. This is beneficial for creating zero-radius turns; with one rear wheel moving faster than the other, the moving wheel can initiate the turn and serve as the pivot point for the zero-radius circle. The transmissions of both rear wheels are operated independently, usually with a system called lap bars.

The big trouble with zero-turn mowers is found in the front wheels. Standard zero-turn mowers use casters in the front, wheels that pivot 360 degrees. The casters aren't attached to any part of the drive train; they move based on the motion initiated by the rear wheels. This allows for quick and easy movement around a zero-radius turn -- in theory.

As we've seen, lawns feature inclines and bumps and when the turf is wet, traction can become an issue. When the rear wheels begin a turn, they have to overcome resistance found in the front casters. This can lead to plowing in the front wheels, and when the front casters hold fast, the rear wheels can lose traction, leading to skidding and slipping. All of this adds up to turf damage.

The Synchro-Steer system was designed to overcome resistance found in the front wheels by never allowing it to generate in the first place. The solution is to initiate the turn at the front wheels, but control the speed at the rear wheels.

Like traditional zero-turn mowers, the Synchro-Steer system uses dual rear transmissions that distribute power to the rear wheels independently. When the steering wheel found aboard Synchro-Steer system mowers is turned, the steering box turns the front axle. But a pair of direct linkage connections that runs from the steering box to each of the rear transmissions adjusts the speed in the rear wheels as necessary.

For example, when a left (counterclockwise) turn is initiated, the linkage to the left rear transmission slows the speed of the left rear wheel. This moves the power to the right rear wheel, which carries out the zero-radius turn. Since the turn was initiated among the front wheels, there's no resistance to overcome, and hence no turf damage from plowing in the front or skidding in the rear.

It seems the Synchro-Steer engineers managed to combine Ackerman's steering with John Reiger's mower, while cutting Barney Oldfield out of the picture once and for all.

For lots more information, ride on over to the next page.