Multifunctional army all-terrain vehicle M561

The success of modern combat is achieved not only by superiority over the enemy in forces and means, but also by the ability to maneuver troops and use the terrain during combat. The maneuverable nature of modern combat is a consequence of technical progress and equipping the army with special equipment, which makes it possible to quickly transfer the actions of the main forces from one direction to another.
M. G. Lazebnikov, 1958
The idea that it is possible to create an effective vehicle design for operation in complete off-road conditions based on road-type vehicles is wrong. An all-terrain vehicle must be built from scratch, abandoning many generally accepted designs. And American designers understood this well.
In 1960, in the United States, as part of the Detroit Arsenal project, a decision was made to develop a new model of a military cargo-passenger vehicle with high cross-country ability, which was supposed to expand the range of vehicles in the American army. Engineers from the American company Ling-Temco-Vought were involved in the design of the vehicle. It is interesting to note that the company never manufactured cars; it was engaged in the development of aerospace technology. Despite this circumstance, its talented engineers designed and built a magnificent three-axle all-terrain vehicle Gama-Goat with all-wheel drive, designed to transport people and cargo in severe off-road conditions.
Based on this, a prototype was designed to meet the requirements of the military customer (the US Army), and received the index XM561. With a length of 5620 mm, a width of 2135 mm and a height of 2700 mm (1585 mm with the awning removed), the vehicle weighed 2650 kg and could "take on board" 10 soldiers with full gear or a load of up to 1350 kg in total. In order to minimize its own weight, the all-terrain vehicle was made with the maximum use of aluminum alloys and with a load-bearing body. At the same time, the engineers managed to build such strength into the load-bearing system that the XM561 could retain functionality even after parachute landing with a payload of 1140 kg.
Ground clearance of 380 mm and the use of large low-pressure tires with a profile width of 300 mm and an external diameter (height) of about 1010 mm provided the vehicle with excellent cross-country ability. Moreover, the designers were able to achieve such a ratio of the vehicle weight to the area of the supporting surface of the wheels, at which the specific pressure on the ground was (according to my calculations) only 0,6 kg / sq. cm. This is even less than the values of the specific pressure of many tracked vehicles and ten (!!!) times less than that of some wheeled vehicles. For comparison: the specific pressure on the ground of the tires of the GAZ-63 trucks (front / rear) was 5,14 / 6,28; ZIL-151 - 6,24 / 5,22 kg / sq. cm. But the main highlight of the design was that the all-terrain vehicle consisted of two sections.

The first was a 4x4 tractor with an engine mounted behind the cabin. The second was a trailer with a power drive connected to the axle (the so-called "active trailer"). When driving on a good road with a small load, it was possible to disconnect the front and rear axles from the power drive, while the car moved due to the traction force of the wheels of the middle axle.
The trailer was connected to the tractor via a hinge of an original design, allowing a relative rotation angle of up to 30 degrees in each direction around the longitudinal axis (torsion) and up to 40 degrees around the transverse axis (bending). In addition, the middle axle of the vehicle (the rear axle of the tractor) could rotate relative to the longitudinal axis of the chassis at an angle of 15 degrees in each direction from the middle position. This technical solution brought several advantages at once:
a) all 6 wheels of the all-terrain vehicle, even in the most difficult conditions, constantly followed the terrain changes, rolled over uneven surfaces, maintained contact with the ground regardless of the terrain profile and, as a result, could develop the maximum possible traction under the conditions of adhesion to the supporting surface;
b) a good smooth ride was ensured, which increased the maximum possible speed off-road and reduced driver and crew fatigue;
c) the torsional stress acting on the body when passing over ridge obstacles was significantly reduced;
d) the threshold cross-country ability was increased; the all-terrain vehicle could overcome a threshold obstacle (barrier) up to 0,66 m high.
But the tricky hinge did not allow changing the position of the tractor relative to the trailer in the horizontal plane, since this would have caused the breakage of the intermediate cardan shaft, which supplied torque from the tractor to the trailer wheels. On the one hand, this was even good, since it eliminated the "folding" of the trailer on a slippery road, when braking and reversing. But it worsened the maneuverability of the car, since turning the car in the way usual for articulated vehicles became impossible.
Engineers addressed this issue by arranging the steering to act on both the front and rear axles, resulting in a minimum turning radius of 8,9 m. The rear wheel steering angle was 50% of the front steering angle.
At the same time, this solution improved cross-country ability, since the presence of steerable wheels on the outer axles ensures stable movement of the car on steep climbs, whereas in conventional cars, due to the redistribution of weight to the unsteerable wheels of the rear axle, controllability may be lost. And to improve the lateral stability of the car when turning corners on the highway at high speed, the rear steerable wheels were locked in a straight-ahead position, and only the front ones turned.
If necessary, the rear section could be quickly detached, resulting in a small two-seater 4x4 vehicle with a length of 3135 mm and a wheelbase of 2010 mm, which had excellent cross-country performance parameters and could be used as a reconnaissance, patrol, communications vehicle or a tractor for various purposes.

The XM561 powertrain was equipped with a transfer case with a 1,8 demultiplier, and the designers abandoned the mandatory for all-terrain vehicles forced locks of the interwheel differentials, using limited-slip differentials (self-locking) in all axles, having rightly decided that with the chosen type of design, wheel suspension is unlikely, and if so, then there is no particular sense in full differential locking. The axles were made split, power was supplied to the wheels by half-axles with constant-speed joints.
The XM561 suspension was independent with double wishbones. The elastic element of the front and rear axle wheels were springs, the wheels of the middle axle were suspended on a single-leaf transverse spring.

Note the monumentality of the lower arms of the 1st and 3rd axles.
Independent suspensions, compared to dependent ones, have the following advantages in terms of their impact on the vehicle's cross-country ability:
the possibility of obtaining a softer suspension with greater wheel travel without raising the vehicle's center of gravity;
lower unsprung masses and, therefore, lower probability of high-frequency resonance;
better adaptability of wheels to uneven roads;
the ability to eliminate self-oscillations and involuntary rotation of the steered wheels;
the possibility of obtaining a smooth bottom without protruding parts.
The disadvantages of independent suspension include its greater complexity and the presence of numerous rubber-metal connections that require frequent replacement.
The shafts that transmit rotation to the wheels are located on the outside of cars with independent suspension, not on the inside, like the axle shafts in solid axles, which increases the risk of damage and reduces their service life. It should also be borne in mind that the independent suspension must have a higher ground clearance than the dependent one, since it is significantly reduced by body vibrations and increased payload.
Therefore, in most mass-produced military vehicles with increased cross-country ability, intended for long-term operation off-road, as well as in all trucks with normal cross-country ability, solid axles are used. But in specialized all-terrain vehicles, such as the XM561, the presence of independent suspension is quite justified.
The interior space of the trailer was also carefully thought out. The free width of the body between the wheel arches was 1320 mm, which allowed for the placement of two standard containers, and its length of 2350 mm was chosen based on the requirements for army ambulances.
To enable self-extraction, the vehicle was equipped with a stationary winch.
The technical characteristics of the car did not include the column "maximum fordable depth", and this is not without reason. The fact is that the XM561 did not ford, it ... swam!

The height of the above-water part was at least 23 cm, and the wheels of the car were the propeller, which is why the speed afloat was low - 3,2 km/h. But if necessary, it would be possible to significantly increase the speed of movement on the water by installing a pair of screw propellers in the back in the form of ordinary easily removable boat motors.
The designers were absolutely right in thinking that the XM561 is a land vehicle, not an amphibious vehicle, it was not tasked with landing from a ship or swimming across wide water areas, therefore, it does not need high speed on water. And if so, then there is no need to complicate the design of the vehicle with additional stationary propellers (and their drive), which will remain unused for most of the operating time and will only add weight (and therefore reduce the load-carrying capacity), complicate the design and increase the cost.
The XM561 was equipped with a three-cylinder, two-stroke diesel engine with a power of 103 hp and a torque of 297 Nm at 1500 rpm. It was also possible to install a multi-fuel Lycoming AVM 310 engine with a power of 103 hp.
On hard roads the car could travel at speeds of up to 93 km/h.
In 1963, this magnificent all-terrain vehicle, along with other experimental and production vehicles, underwent lengthy military testing. It was the only one of the eight vehicles to successfully complete the entire testing program.
As a result, the experimental XM561 was accepted into service by the US Army as meeting all the requirements for vehicles of this class.
Exploitation
But when the M561, which had shown itself so remarkably in testing, went into production and entered service with the troops, American soldiers encountered the same problems when operating the vehicles as the soldiers of the former German army back in 1941, who had had enough of suffering with standard all-terrain passenger vehicles during the fighting on the Eastern Front.
The fact is that in combat conditions, reliability and maintainability come to the fore, which is ensured by the simplicity of the design. The technically overcomplicated M561 often broke down, and its repair in field conditions by the crew was most often impossible. And this negated all the advantages in cross-country ability that were achieved by using load-bearing hulls, independent suspensions and an articulated design. Because the efficiency of a vehicle operating in off-road conditions is assessed by a set of operational properties, therefore, an insignificant increase in one operational property (cross-country ability) with a decrease in several others (reliability and ease of repair) is often unjustified.
The car's designers blamed numerous failures on third-party component manufacturers, who (in their opinion) were unable to ensure the proper quality of the supplied units.
It was not possible to establish who exactly was to blame, it is only known that the vehicles remained in service with the US Army until 1983, until they were replaced by the newly-introduced universal military vehicle "M998", better known by the abbreviation HMMWV.
Conclusions
It is well known that a significant increase in the reliability of complex structures requires the use of highly resistant materials, which entails a several-fold increase in the cost of the car. This explains the fact that, despite technical progress, most modern high- and high-cross-country vehicles of mass production still use many design solutions that came out of the early twentieth century.
However, already in the mid-60s of the last century, designers, having analyzed all the experience of the world automobile industry accumulated during this time, came to the conclusion that, while cars were created according to traditional schemes, their cross-country ability did not go beyond certain limits. A significant increase in the cross-country ability of wheeled vehicles can only be achieved by abandoning the usual design solutions.
It should also be taken into account that the optimal design solution cannot be universal, i.e. the most advantageous solution applied to some specific operating conditions may turn out to be erroneous in other conditions.
Acceptance of boundary conditions of operation also sometimes leads to uneconomical, and therefore incorrect decisions. The most economically feasible was considered to be the creation of specialized vehicles intended for a certain, relatively narrow range of operating conditions.
And sometimes, in the case of mass transportation, it is more economically advantageous not to develop and produce a new special vehicle, but to build a road and use ordinary road vehicles to transport goods.
These obvious conclusions have not lost their relevance to this day.
References
Ageikin Ya.S. Vehicle cross-country ability. Moscow, Mechanical Engineering, 1981.
Grinchenko I.V., Rozov R.A., Lazarev V.V., Volsky S.G. High-cross-country wheeled vehicles. M., Mashinostroenie, 1967.
Lazebnikov M.G. On the cross-country ability of vehicles on unpaved and snowy virgin soil. M., Military Publishing House of the USSR Ministry of Defense, 1958.
Laptev S.A. On the vehicle's cross-country ability. Motor, 1938, No. 8-9.
Selivanov I.I. Cars and transport tracked vehicles of high cross-country ability. M, publishing house "NAUKA", 1967.
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