The "automotive" solution for the suspension
On a tractor, the vibrational comfort greatly improves if the front axle is suspended. The technical evolution of this device, which is designed to remedy the rolling anomalies determined by the imperfect roundness of the tyres. Devices of various kinds are already in mass production
Apart from a few well-known cases that have been on the market for a long time, contrary to what you might think, the application of a suspension to the tractor’s front axle was not originally intended to improve driver comfort, but rather for a well-defined need for safety at high speed.
The introduction (which took place some time ago) of models with a maximum speed of 40 km/h highlighted a serious problem of a vibrational nature, hitherto unknown. In practice, tractors over 130 Hp are fitted with wheels whose size means that at 40 km/h they make 2-2.5 revolutions every second. This is the so-called “rolling frequency” (2-2.5 Hz), and due to the inevitable, albeit small, non-roundness of the outer rubber, it causes rhythmic vertical oscillations, i.e. small jolts. This frequency is very close (if not in some cases equal to) the resonant frequency of the entire tractor. At the resonant frequency, a generic body offers zero resistance to impulses, so it starts to oscillate, or vibrate, more and more markedly, until its (theoretical) destruction. Famous examples of this phenomenon are a few bridges, collapsed disastrously when their resonant oscillation was triggered, caused by the constant speed of winds acting for a long time on their structure.
Without reaching these egregious levels, the vertical oscillation of the tractor’s rear wheels, combined with the alternating horizontal pressure of trailers through the tow hitch (which must necessarily be an elastic connection) caused the onset of a combined vibration on the vertical and horizontal axes, gradually increasing in intensity. This is called “pitching” (basically the typical movement of riding a horse), which interfered significantly with the perfect control of a vehicle, and thus created conditions of significant hazard when driving fast on public roads. The solution was to apply a suspension device to the front axle, to nip in the bud the emergence of the pitching.
The technological evolution
The first versions of these suspensions (and many current ones too) had the option of being locked, therefore making the front axle rigid. This was because when working in the field, especially when a high towing strength was needed, any fluctuation would cause more or less instant changes in the adherent weight (important especially in 4WD tractors), such as to generate functional drawbacks to the work.
With the development of this device, and often with the aid of electronics, these problems have been overcome. Besides avoiding the triggering of the pitch, nowadays the front axle suspension also plays an important role in improving the vibrational comfort of the driver (and any passenger), assisted in this by the seat and cabin suspension, to form a combined “package” to effectively reduce the risk of related medical conditions, especially affecting the spine.
Not only: a technical solution was developed a few years ago, borrowed from the automotive industry, initially applied to high power machines and more recently extended to a specialized tractor model, which enhanced the functionality of this device. We are referring to articulated wishbone independent front suspension, based on a completely different design concept from the “traditional” one, involving some additional operational advantages not only in fast road travel, but also in field work.
Overview from the Market
Since this is a rather expensive device, the front axle suspension is still in many cases offered as an option, compared to the classic rigid axle. Historically, is was requested more often on high-power tractors, those typically employed by contractors, who among other things have more need for fast transfers on public roads and have greater demands in terms of comfort, given that they typically work on the tractor for several hundred hours per year.
Succinctly put, the most common configuration by far includes the hinging of the front axle to the tractor’s body, which is usually supported by one or two hydraulic cylinders, whose operation is supplemented by some nitrogen shock absorbers, with the function of damping any sudden shocks due to the sudden impact of the wheels with potholes, bumps, etc. However, this involves a significantly complicated construction, because we must not forget that the front axle also has to be tilting in the transverse direction, it is part of the steering system, it often also houses the braking elements and, of course, it is driven by the gearbox in 4WD versions.
On medium-high power tractors, New Holland (and Case) are fitted with Terraglide, which in newer versions is electronically controlled to modulate the stiffness according to the stresses.
A similar solution is offered by Deutz-Fahr, which on the renewed 7 series comes with a heavy duty adaptive axle with the “Anti Dive” function, especially useful in high-speed braking with heavy loads. In practice, the system stiffens the suspension in the longitudinal direction, so as to counter the weight transfer that occurs in such a situation, to the benefit of the overall stability of the tractor-equipment set, and ultimately to the road gripping and control of the vehicle.
Compared to the widespread adoption of hydraulics for damping vibrations, Valtra has traditionally avoided this technical choice, exploiting (compressed) air instead of oil pressure. Underlying all this is a precise climatic reason: Valtra being a company of Finnish origin (although in merged into the AGCO multinational group some time ago), the use of oil would have created significant problems for the efficiency of the suspension while operating at very low ambient temperatures (due to an increase in the fluid’s viscosity), problems that air practically does not create, due to its vastly lower density.
The automotive configuration
In terms of vehicles, it often happens that the innovations are borrowed, extending them progressively from more technologically sophisticated areas to the less evolved ones. In the case of front suspensions, compared to the conventional rigid axle, tractors have benefited from the recent introduction of the independent wheel articulated wishbone solution, which offers many advantages. Being a device costing significantly more than the predecessor, its appearance on the tractor manufacturing market initially involved the high-power machines. The hydraulic cylinders enable the manual or automatic trim for the front. It is possible to lock its travel to get only one anti-roll suspension effect. The locking is manual, but can sometimes be handled automatically, based on the forward speed. The solution with independent wheels has a much smaller footprint than the fixed axle one.
This way, the steering angle can be very wide, improving the manoeuvrability of the vehicle, a tangible benefit especially on high-power tractors. John Deere and Fendt, for example, have for some time adopted this architecture on high-end models. Specifically, the Fendt 1000 Vario is fitted with an articulated wishbone suspension whose stroke can be set manually, depending on the load, so that the comfort and safety in driving the vehicle will always remain at the maximum level, up to 60 km/h, the machine’s maximum construction speed (viable where permitted by local law).
The specialized tractors
The emergence of the front axle suspension has led to its gradual spread on increasingly low-power models. In order of time, Fendt was the first manufacturer to fit a device of this type with a rigid axle on specialized models for orchards and vineyards.
Same recently went even deeper, and it is the first in this particular area to offer the Frutteto Active Drive, equipped with articulated wishbone independent suspension. In this case, the aim is to keep the front end in the “central” position in all operating situations, so as to exploit the entire stroke of the damping cylinder, benefiting the machine’s stability, and therefore the driver’s comfort. In the field, and especially with front-mounted equipment, this reduces tyre bounce, typical of the “return” of the masses after the obstacle has been overcome.
This greatly reduces inter-row compaction when repeated passages are involved.
The damping control is electronic and automatic: the optimum level is defined according to the signals of the sensors that monitor the tractor’s speed, the steering angle, the speed of movement, and the position of the cylinders.
Moreover, this configuration offers a beneficial anti-pitching effect, which occurs when overcoming obstacles or during braking, and that causes a rapid displacement of the masses on the front axle, giving a more marked soil compression.
Still, thanks to the independent electronic control, the system counteracts the roll of the suspended masses (important especially when driving on curves at high speed), ensuring lateral stability for the vehicle and ultimately better overall balance, particularly appreciated for work on the row with operators carried laterally, especially those requiring high spatial precision of action, such as pruning, topping, and defoliation.
