Four wheel drive systems and their effects on your turf
There are numerous 4WD/AWD technologies and systems. Most have evolved over many years. This article explains the differences and attributes of some of those that are commonly used in the outdoor power equipment sector.
Many turf care, tractor and utility products used in the professional grounds maintenance industry have Four Wheel Drive (4WD), or All Wheel Drive (AWD) systems to improve traction, but how often have you stopped to consider the relative merits of each of them. Specifically, how might that performance interact with your particular turf conditions and topography, especially when turf damage caused by tyre scrubbing needs to be kept at a minimum.
The simplest, and often very efficient, type of 4WD uses two mechanical axles, one at the front and one at the rear of a machine. The main gearbox or hydraulic transmission are often combined with the rear differential and axles which produces a 2WD machine. Adding a propellor output shaft (Prop Shaft) to the two wheel drive transmission is the simplest way to create 4WD. The prop shaft is taken from the gearbox to the front axle which also has a differential and drive shafts to the front wheels. There are many variations on this theme with various sophisticated and smart additions, all of which have advantages and disadvantages, but fundamentally all four wheels are driving the machine along, improving traction.
Today, the front wheels of a 4WD tractor are usually smaller in diameter than the rear wheels. This ensures a tighter steering lock can be achieved, it improves visibility to the front of the tractor and enables front mounted attachments, such as loaders, to be more easily installed. However, this presents a minor technical problem for the 4WD engineer. Smaller wheels have a smaller rolling diameter and thus, for a given forward speed of the machine, must rotate faster to cover the same amount of ground in a given time as the larger diameter rear wheels.
The problem is easily resolved using final drive gear ratios between the axles. In fact, when 4WD is engaged it can be desirable for the front wheels to rotate very slightly faster (circa 2%) than the rear wheels to ensure that they help “pull” the rear wheels along, which is usually hardly noticeable on turf or loose surfaces. However, it causes tyre wear and sometimes torque twisting of the front drive shafts on hard, less slippery surfaces, which is why many 4WD systems are best disengaged when on roads at high speed. Some more sophisticated 4WDs have “smart” auto engage/disengage mechanisms that reduce turf damage and tyre wear on hard surfaces.
It’s useful to know what type of system you have and whether you should disengage it on hard surfaces. Some equal sized wheel machines have all four wheels rotating at the same speed which, in the turf sector, reduces turf damage to minimum. Whichever technology you have, in most cases the front wheel speed is inextricably linked to rear wheel speed, by gear ratios or hydraulic flow, to ensure the engineered design operates as intended and offers the optimum all-wheel drive performance.
Most of the machines mentioned so far have a fixed chassis. The front (or rear) wheels must steer to enable changes in direction. The inner steered front wheels will follow a different path to the inner rear fixed wheels as the vehicle pivots around the centre of the fixed axle. This is managed by making the inner steered wheel turn sharper than the outer, using what is known as Ackerman steering geometry and is a fundamental steering design principle. It ensures that both front wheels (or rear in a rear steer machine) turn at slightly different angles when cornering. Both wheels then trace arcs with a common centre point minimising tyre scrub.
Usually, more weight is transferred to the outer front tyre, increasing ground pressure when turning (for front steered machines), sometimes causing turf damage in delicate conditions. This fixed chassis arrangement can be reversed to provide rear steer traction units for ride-on mowers in the turfcare sector, but, of course, is used in telehandlers and forklifts to mention but two. The fixed chassis tractor has numerous advantages, not least of which are generic hitch systems to accept a wide range of third party attachments, high spec cabins, aircon and smart controls, all adding to ease of use. There are also All-Wheel-Steer, AWS, machines using a fixed chassis with both axles steering in unison. These are less common in the turf care sector primarily due to cost constraints.
The Principles of Ackerman Steering.
The alternative to a fixed chassis is an articulated one, where a front and rear chassis pivot in the middle of the machine to allow steering. In this case, all wheels can more easily be the same size and follow each other when the machine is steered if they are all the same distance from the pivot point. This causes less slip at the point of ground contact which, in turn, will cause less turf damage. Ackerman angles are not required as the inner wheels, and for that matter the outer wheels, follow each other’s paths when turning. A few manufacturers have chosen this less common but very versatile architecture in the turf care sector and it is used extensively in viniculture where low ground pressure and slope stability are required in mountainous regions. Some robotic machines are adopting this design too as it is quite simple to manufacture and usually offers a low centre of gravity to improve stability.
Another feature which needs consideration is axle oscillation. This determines how much undulation the machine can accommodate before a wheel leaves the ground. In fixed chassis 4WD systems, the front axle usually has a central longitudinal pivot under the chassis. As this type of tractor usually has large diameter rear wheels, the centre of gravity is inherently higher reducing the angle at which the tractor is likely to tip or at least lose traction on the wheel that is trying to lift. Differential locks reduce wheel slip but also damage turf if any steering corrections are required when they are engaged.
An articulated machine usually has a ball joint or similar that allows the front and rear chassis members to oscillate.
The oscillation point can be in line or below the axle line thus inherently reducing the centre of gravity making such machines very stable on slopes. Articulated machines often have an almost 50/50 weight distribution between the axles. This distributes the weight of the tractor evenly between all the wheels, lowering ground pressure which can, in turn, reduce turf damage in wet conditions.
In addition to mechanical 4WDs, there are hydraulic and, more recently, electric systems. Many of the numerous commercial rider-on, job specific mowers have hydraulic 4WD systems; sometimes configured with two axles with hydraulic motors driving them whist others have individual motors on all or some of the wheels. These motors are driven by a hydraulic pump with variable flow control often referred to as hydro transmission. Suitable motor capacities need to be calculated to ensure smaller wheels are rotating at the appropriate speed compared to larger wheels, and valves or sensors are required to complete the circuit, often adding 4 wheel drive when slippage is detected from the front wheels. Wheel motors have the advantage of light weight and small size so can usually be positioned to become part of the wheel hub.
They are not mechanically attached to the transmission (pump), allowing more ambitious or creative wheel architecture to suit the positioning of mowers, engine and operator.
They usually have the disadvantage of being less efficient than mechanical or electric final drives, producing heat and requiring a large oil reservoir and cooling packages to maintain performance. They do have the advantage of allowing for more function focussed chassis design, for instance greens and fairway mowers. Finally, electric drives have the advantage of being highly efficient, fewer or no gear reductions, no pumps, no oil, but, for the time being in larger machines, higher cost. The battery weight is challenging too if enough autonomy is to be provided, but the advantages of the function focussed chassis design are retained. In addition, oil reservoirs can be reduced or eliminated and leak potential is reduced or eliminated.
Finally, we should mention three wheel drive machines. These are usually specialist mowers, such as greens, tees or fairway mowers, and are hydraulically or electrically driven. They do not require complicated steering geometry as only the single rear wheel steers. Depending on the length and width of the machine the rear wheel often follows the arc of the outer front wheel in a turn, thus minimising turf damage as there is little tyre scrub, ideal for fine turf but potentially less stable on a diagonal side slope.
In summary, numerous AWD systems have been developed, all of which have attributes which serve the turf care sector. It can be worth evaluating which attributes of the various drive systems best suit your priorities and might offer the best performance in your circumstances.