A CERTAIN RATIO: Achieving mobility in challenging environments is paramount for all military vehicles; however, assuring this in the armoured vehicle domain is far more difficult yet absolutely critical to allow platforms to achieve their missions.

Author:Miller, Stephen W.

Although mobility is vital for armoured vehicles, this must compete with other essential characteristics such as the need to assure the survivability of the vehicle and crew; requirements which can easily find themselves in conflict with maintaining mobility. Yet understandably soldiers reliant on such vehicles demand increased cross-country capabilities, faster acceleration, and higher speed while enhancing survivability. Such needs have pushed the development of automotive systems to find solutions and a range of improvements have become available that begin to address these demands. Yet meeting these requires a combination and balancing of a number of aspects of automotive design. These include the capabilities and performance of a vehicle's suspension which directly influences the ride, the footprint of the tracks or wheels that determines the ground pressure, the vehicle's ground clearance and the engine's power output. The last could be considered both the most important, and the most difficult, to achieve. This is because even within the area of generating and distributing engine power there are traded-offs that must be made. Increasing power in an armoured vehicle is constrained by aspects such as the engine bay volume, the need to maintain the vehicle's operating range, weight constraints and the need to support the power demands of onboard systems like radio communications, navigation systems, sensors and countermeasures, in addition to automotive systems.

It is the need for adequate protection against changing threats, in particular, that place the greatest demands on the automotive side of the vehicle. Protection almost inevitably means armour, and armour adds weight. This creates a paradox that offers uncomfortable compromises: As threats increase the required level of protection also increases. Greater protection typically translates into a requirement for more armour, and additional armour can create a heavier vehicle weight. To maintain, or improve, an armoured vehicle's mobility performance inevitably requires increasing the power provided by the engine, and the efficiency of the matching transmission and drivetrain. However, vehicle weight is also driven by vehicle size: The larger the vehicle, and surface area that must be armoured, the heavier the vehicle will become. Thus; not only must a vehicle's new power pack (its engine, plus transmission and drivetrain) be more powerful but it must, at least, fit the same allocated area, or preferably have less total volume. This is an absolute criterion especially for power packs destined for upgrading existing armoured vehicles, but also highly desirable in new designs.

A commonly accepted measure of the level of mobility provided by an armoured vehicle is measuring the horsepower to weight ratio. Although not accounting for all the various factors that contribute to mobility this ratio is an appropriate rough indication and is useful both as a design objective and as a tool for comparing different vehicles. As a general rule the higher the horsepower-per-ton ratio the better the overall mobility characteristics that the vehicle will exhibit. Despite the fact that top speed is often considered in assessing a vehicle, for a combat vehicle responsive acceleration may actually be a far more important trait. Often overlooked in vehicle specifications, the ability to quickly accelerate and rapidly move to a covered position in response to an attack is invaluable. It directly contributes to the survivability of...

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