A reverse engineered operator’s
manual for computer geeks.
Preface: I’m a mechanical engineer by education, a computer automation engineer by specialization, and a embedded software programmer by practice. I think everything should be electronically controlled, and often (sometimes incorrectly) try to fit everything into a purposeful design.
What is a John Deere SST16?
It is a revolutionary lawn tractor which John Deere began manufacturing in 2001.
It is classified as a Zero Turn Radius mower, although it comes in the “package” or “form-factor” similar to that of a traditional tractor. However, looks are deceiving in this case.
If you are an engineer, tech geek, or
even a gear head, you can probably appreciate some of the articles on
awards that the SST has won. Very interesting reading, IMHO (in my
humble opinion). Just a very well designed
Traditional Rear Drive, Front Steer Tractors
The type of tractors that most people have are rear wheel drive and steer with the front wheels. The technical name for this type of steering control is “Ackermann Steering” (see pic below). This effectively the type of steering used in automobiles. With automatic transmissions, these tractors have a single rear hydrostatic transmission and differential that drives both rear wheels.
What makes a ZTR so special?
Most of the ZTRs on the market have two separate hydrostatic transmissions controlled independently by two levers. The left lever controls the left hydrostatic transmission, and the right lever controls the right hydrostatic transmission. If you’ve played the Atari arcade game called “Battlezone”, the controls are like that.
Normal ZTRs are usually rear engine and rear wheel drive, with front caster wheels (like on a shopping cart’s front wheels.) There are variations that have a single joystick control, although these are less popular and typically harder to operate.
The unique feature of a ZTR is a feature called counter-rotation steering. This feature allows the two rear wheels to rotate in opposite directions and thus quickly spinning the mower around without moving forward or backwards. This is useful for maneuvering in tight spots.
Another feature of a ZTR is the ability to turn with just one rear wheel rotating. This technique is very useful if you mow in the same way a person with a walk-behind mower would, ie. with each consecutive swath in the opposite direction of the last with a bit of overlap. A single wheel rotation allows the mower to like up perfectly for the next pass.
Normal steering with a ZTR is technically referred to as “differential steering”. The two rear wheels turn at different speeds which causes to mower to turn towards the side in which the wheel is spinner slower. So, if you want to turn right, you make the left wheel turn faster than the right wheel!
When you first get in and drive a SST, it looks like a traditional tractor and you instinctively expect it to behave like one. The problem is that it doesn’t behave like a normal (Ackermann) steered tractor. You don’t have any tactile feedback on the steering-wheel of the SST (or any ZTR for that matter). People are so used to diving cars that if you put them behind a vehicle with steering wheel and an accelerator, they expect the vehicle to “feel” like a car. The SST doesn’t feel this way and there may be some discomfort or awkwardness when you first ride it. For instance, turning the steering-wheel when the tractor was stopped, I expecting the front end to “crawl” a little bit from the rotation of the front tires; it doesn't. Turning the steering-wheel does not turn the front tires at all (unless the tractor is moving).
On the other hand, although the SST is truly a ZTR mower, it also doesn’t feel like a traditional ZTR either. If you are use to normal ZTRs, the SST may feel doubly awkward because the first time you drive one, it may also feel like you simply cannot control the SST the way you would like. (Doubly awkward because it also doesn’t feel like a normal “Ackermann” steered tractor.).
Don't lose hope. This is marvelous tractor to drive. The SST is really a hybrid of the two, and its controls are somewhat different and quite unique. Understanding how it operates will give you a much better appreciation of the mower.
The main controls of the SST consist of a steering wheel and two foot pedals on the right side.
The two pedal are a standard accelerator (gas pedal) and a separate reverse pedal (effectively a gas pedal that drives in reverse.) The Steering wheel looks like a normal steering wheel except it is really a spring loaded controller (somewhat similar to the steering wheel controllers of a computer or game console.) When the steering wheel is released, the internal springs in the mechanism bring the wheel back to center. So outwardly, it looks very similar to the controls of a car.
Let’s start with the basics. Pressing the accelerator pedal down moves the tractor forward. Pressing the reverse pedal down moves the tractor backwards. Deere’s SST transmission is unique in that it has DIFFERENTIAL LOCK and thus spin both wheels at the same speed for a truly straight line mow (if so desired). Other ZTRs do not have this feature. Differential lock is usually associated with high end tractors; the next Deere that has this features is the X465 2WS at $8700 retail.
The hardest thing to explain in terms of operation of the SST is probably the control of the steering wheel. It's simple enough that anyone can get it and drive, but different enough that it takes some time to fully understand the differences with a normal tractor. If I have to sum it up in one sentence, it would be “the slower you are moving forward, the tighter the turn”. However, that really would not explain the operation in enough detail.
The way the SST turns is dependent on both the steering wheel, the accelerator pedal and the current forward speed of the tractor. This may be the hardest thing to understand or to “get a feel for” in terms of the operation.
For computer geeks, I think it’s easiest to think of the pedal as having 4 distinct “interaction” zones with the steering:
Rest/Off Zone, ie. foot is off the pedal, no accelerating
Counter Rotate Zone, the accelerator is pressed down just a very small bit
Single Wheel Rotate Zone, accelerator is pressed down lightly
Differential Steer Zone, the speed is linearly variable and the steering is also linearly variable in this zone.
These zones are not distinct or discrete. They are continuous and the transition from one zone to the next is gradual and smooth.
Let me explain each zone and how it interacts with the steering:
1. Rest/Off Zone ( 0-2% accelerator )
ACCELERATOR PEDAL: foot is off the pedal, no accelerating. (approx 0-2% of pedal)
STEERING WHEEL: Steering had absolutely no effect.
FORWARD MOTION: Vehicle does not move, does not even budge in response to the steering wheel.
2. Counter Rotate Zone
ACCELERATOR PEDAL: just touch the pedal so it’s not in the Rest/Off Zone (approx 2-5%)
STEERING WHEEL: tuning the steering wheel full left or full right will command the SST to counter-rotate steer.
FORWARD MOTION: must be stopped or moving very slowly
useful for maneuvering in tight spots.
3. Single Wheel Rotate Zone
ACCELERATOR PEDAL: slightly depressed (approx 5 to 10%)
STEERING WHEEL: turning the steering wheel left or right will rotate the mower by turning the appropriate rear tire and keep the other rear tire (near) motionless.
FORWARD MOTION: stopped or slow speeds (the faster the tractor is going, the more the drivers needs to turn the steering wheel)
this is probably the most frequent mode of turning, especially for mowing in swaths because the rotation puts your mower into the correct potion with a bit of overlap from the previously cut swath.
A single wheel turn has the highest probability of tearing up the lawn. The wheel that is not turning will do the damage. Use a very slight counter-rotate or differential steer for this maneuver if you have problems with turf tearing.
4. Differential Steer Zone
ACCELERATOR PEDAL: moderate to fully pressed. Position is proportional to speed. (approx 10 to 100%)
STEERING WHEEL: proportional to radius of turn
FORWARD MOTION: slow to fast speeds
Tractor behaves the closest to a normal tractor
Used for gradual turns.
It's about the combination of speed, steering, and accelerator.
Again, the beauty of the SST's controls is that anyone can get into an SST and operate it. The interface is intuitive for basic operation. But to fully appreciate everything this tractor can do and to operate it like a pro may take some time. The more you drive it, the more you will like driving it as you get a feel for how it works.
The hood for the engine slopes downward for better visibility.
Although this might not seem significant, it's extremely functional. It allows the operator to see immediately in front of the tractor. Much more so than with a normal tractor. It’s allowed me to see very small toys BEFORE I run over them and shred them to bits. Normally, small object would be lost in the grass, so the visibility issue is quite significant for me.
I’ve never mowed in the dark (I prefer sun burn over mosquitoes) , but perhaps I will one day.
Briggs & Stratton Vanguard Commercial Grade Engine
Long life and
quieter engine. Has a 2 year warranty for commercial use!
( Compared with a Briggs & Stratton Intek V-Twin that only has a 90-day warranty for commercial use. )
Engine will shut off if you get off
the seat while the PTO is engage (blade is turning) or if the mower
does not have the parking brake on.
The mower will not mow in reverse with the PTO engage unless you first pull up the PTO switch from the engaged position.
This puts the PTO switch into its spring-loaded (temporary) position. Hold the switch in this position and you can then mow in reverse.
Mower Deck Control
The cut height is controlled by a knob (which I like), and the whole deck can be lifted up using a foot pedal and then locked in the up (cleaning) position using a slider just below the steering wheel.
I’ve towed a plug aerator
behind the SST and it works wonderfully. This is considered a lawn
tractor so you can’t pull tillers or cultivators with it. You
can get snow blades and chains for the winter!
Deere also has a FRONT MOWER OPTION for the SST which is a 44” mowing deck that extends in front of the tractor.
Differential Lock & Power Steering
The combination is usually only available on very expensive, high end tractors.
John Deere Support
Service and parts very readily available!
Consumer Reports, June 2003:
“Most (ZTRs) steer via two levers like the ones on the Cub Cadet Z-Force 44 and the Ariens Zoom 1640 915035; you push one forward and pull the other back. While that sounds simple, turns with both models were awkward compared with the steering-wheel-equipped John Deere Spin-Steer SST-16.”
Deere does not recommend using this mower on steep hills with a side approach. If you approach a steep hill head-on so you go directly up the hill, then down the hill, you won’t have any problems.
If you try to go along the side of a steep hill such that the SST is slanted from the horizon and one rear tire is higher than the other, this is the worst case situation for the SST.
Normally, the differential locks on the SST and it’s easy to drive in a straight line. When the mower is on a hill, this presents a problem because it feels like the rear differential isn’t locked and there is a tendency to turn, down the hill. After considering the physics of this problem, I believe I understand what is happening.
This is a front engine mower and thus has some weight in the front. Combine that with caster wheels that allows completely free motion for the front and this is where the problems arise. The caster wheels have no ability to prevent the front end from sliding down the hill. This is done completely by the rear tires. If you look at the diagram above, the force of gravity “g” has a “normal” component “n” that is perpendicular to the ground, and a tangential component “h” that has a tendency to pull the front of the tractor down the hill. Normally on a flat lawn, to drive in a straight line I wouldn’t have to touch the steering wheel. However, when navigating a hill from the side, you will likely have to compensate using the steering wheel, and if the slope is fairly steep, then you will need to slow down enough such that the differential steering will compensate for the traction difference of the tires, and also the tendency for the front end to turn down the hill due to weight. Adding more weight to rear counteracts the front (engine) weight and I believe that's how the “Traction Assist Kit” from Deere helps this problem. ie. Weight in front and behind the rear axle (and tires) are closer balanced. The other way the Traction Assist Kit helps is that it adds more weight on the rear tires for traction.
WARNING: This applies to all mower
that I can think of....
DO NOT MOW ON SLOPES WHEN THE GRASS WET. AVOID SUDDEN CHANGES IN DIRECTION WHEN ON A SLOPE.
The bottom line is that this tractor will mow on a grade. Unlike a flat lawn, you will have to steer it on the hill if you approach it from the side, and you may also have to slow down a bit. The Traction Assist Kit helps a lot, especially if the operator is lighter in weight. I can do all of my yard without any problems and the worst slope I have is about 20'. I only have the option of a side approach because the slope is against the side of the house, and it did take me a few tries to get the hang of steering on a slope. Slowing down a bit helps a lot in some cases!
NEW: Regarding weight distribution. The weight of the mower is about 625 lbs. The engine is about 100 lbs. If you consider the transmissions and the driver (150lbs) , the weight distribution might be close to 67% on the rear and 33% on the front. There may be very good reasons for this design. ie. more capable to accept front attachments, less damage to the lawn, etc?
Also, if you add the bagger, that
will add more weight to the rear and remove weight from the front. So
much weight in fact that John Deere recommends adding weight to the
front. My point is that it's really not too far off from the weight
distribution of normal ZTRs as other people would claim.
It just comes in a much nicer package!
CONSUMER REPORTS JUNE 2003 TRACTOR RELIABILITY
John Deere support. Go to the web sites of the manufacturers. Check out what they offer. Check out the online information. With Deere, you can find attachment information, access online users' manuals, etc. I have personally called the Deere 1-800 number on a few occasions and a lady (P.S.) that helped me was extremely knowledgeable and detailed to my surprise. Plus I can find a Deere dealer just about anywhere to get parts. Also Deere's distribution network is such that they can get parts to their dealers in a couple of days (except for the traction assist kit which I've been waiting about 4 weeks for now.)
Easy to get diagrams and parts online through http://jdparts.deere.com
Durable, reliable, commerical grade
equipment. 2 years warranty on the engine under commercial use. (1
year on the rest of the mower under commercial use.) ( 2 year
warranty for residential use.)
Look at the Consumer Reports Repair History. Deere is the most reliabile.
Resale. It's a highly recognized and very reputable brand. You can find parts for Deere's products that date back a long time!
Much better looking!!!! The green and yellow is eye catching.
You wife can drive it, and
she'll want to drive it.
( On second thought, maybe that's a bad thing. )
The SST is considered a lawn tractor and not just a mower because it can push front attachments and pull rear attachments. It does a wonderful job doing so. Not all ZTRs are capable of this.
Differential Lock & Power Steering.
Also, because this mower can be controlled with one foot on the accelerator pedal and one hand on steering wheel, it leaves the other hand free to wave to your neighbors as they watch you ride your mower. Similarly, you could use your free hand to sip a drink! Just sit back and relax while you mow!
Hey, that's gotta be worth somethin'! ;)
Although I don't run heavy-duty construction equipment for a living, I have had the opportunity to operate other machines such as bulldozers, skid steers, exacavators, and backhoe loader.
My opinion is that this is a
wonderful tractor to own and operate. I thoroughly enjoy
driving it and it does a fantastic job cutting grass.
It's one of Deere's hidden gem's! I highly recommend it! It's such a pleasure to drive I wish my lawn would grow faster.
If you have questions or comments,
please email me at:
( please remove the “-123-” and “-456-” below as they were inserted to prevent SPAM bots. )
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