How Synchro-Steer Works


Image Gallery: Sports Cars Legendary race car driver Barney Oldfield preparing for a 1905 spin around the block. See pictures of sports cars.
Hulton Archive/Getty Images

According to Doug Milliken, author of the racing bible, "Race Car Vehicle Dynamics," legendary race car driver Barney Oldfield coined the term plowing. Oldfield was racing a front-heavy car on a dirt track when he took a corner at a high speed. Rather than fully direct the car into the turn, the front wheels continued their forward momentum, creating trenches in the track the way a plow would. Oldfield's estimation of the problem was summed up in the term he's thought to have invented [source: CMA Consulting].

Today, plowing is still in use. It describes the effect of understeering, essentially when the forward momentum carries front wheels straight ahead, rather than in the direction of the corner they're turned into. Since the steering wheel has turned the front wheels, they are no longer aligned straight ahead. Instead, they create flat edges that dig into the ground rather than roll smoothly along on top of it, as they're designed to.

Advertisement

On a dirt track, plowing can create divots. On a hot asphalt track it can shred tires. And when a rider mower plows, it can cause some serious turf damage.

When a tire loses contact with the ground -- whether a track or a lawn -- the other tires can lose control and lead to sliding or even rolling. Depending on the grade of the incline that the machine is on, this can be bad news for anyone aboard.

This is precisely why the Synchro-Steer system was created. Designed by Cub Cadet engineers in 2009, Synchro-Steer is intended to lend the most control possible to rider mowers. The system allows for a zero-turn radius, meaning the entire mower pivots on a single point along the axle of the rear wheels. The Synchro-Steer system isn't the only zero-turn system, but it has the added benefit of four-wheel control. This cuts down on plowing, leading to less turf damage, and provides greater stability and traction regardless of weather conditions or the grade of an incline that requires mowing.

To truly appreciate what Synchro-Steer is all about, let's take a quick look at the physics of steering and what the system is intended to overcome.

 

Basic Steering Physics

none

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.

Advertisement

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.

The Synchro-Steer System

Not all lawns (or fields) are the same. Bumpy terrain, wet turf and inclines all pose potential trouble for your tractor.
Not all lawns (or fields) are the same. Bumpy terrain, wet turf and inclines all pose potential trouble for your tractor.
©iStockphoto/SteveStone

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.

Advertisement

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.

Related HowStuffWorks Articles

Sources

  • Bednar, Jim. Cub Cadet product manager. Personal interview. March 9, 2010.
  • CMA Consulting. "Frequently asked questions." September 23, 1995. http://www.ibiblio.org/rdu/usenet-arc/sub00045/msg00000.html
  • Cub Cadet. "Comparison; Synchro-SteerD." Accessed March 10, 2010.
  • Cub Cadet. "Cub Cadet reinvents the (steering) wheel with new Z Force S, offering alternative to lap bar controls for zero turn riders." Accessed March 9, 2010.
  • Evanson, Jimmy. Cub Cadet chief engineer. Personal interview. March 9, 2010.
  • Grunt Laborer. "Who really invented the zero turn radius mower (ZTR)?" April 24, 2008. http://gruntlaborer.wordpress.com/2008/04/24/hello-world/
  • Hydro-Gear. "Lawn and garden solutions." Accessed March 9, 2010.http://www.hydro-gear.com/LawnGard/IZT.aspx
  • National T-Bucket Alliance. "Ackerman steering." Accessed March 9, 2010. http://www.nationaltbucketalliance.com/tech_info/chassis/ackerman/Ackerman.asp