2. A Systematic Comparison
2.1 Mobility
2.1.1 Strategic mobility
Today's tracked armored vehicles weigh between 3.5 and 65 metric tonnes (t). The respective figures for wheeled armor are 3.5 and somewhat over 35 t. An important qualification is that most wheeled types fall into the category of "up to 20 t". Only about 10 percent are heavier. In the case of tracked vehicles, about half of current types are in the lower weight category, whereas the other half consists of heavier machines (mostly between 35 t and 65 t).
In addition to their relative lightness, wheeled vehicles tend to consume significantly less fuel (and other lubricants) than tracked armored vehicles of equal weight. (I will elaborate on this phenomenon below). Their relative "lightness" and reduced logistical needs together give the wheeled family an edge over the tracked in strategic mobility -- meaning the transport of forces over continental and intercontinental distances.
2.1.2 Operational mobility
Operational mobility refers to the ability to swiftly allocate and relocate forces within a theater of crisis or war. The challenge it poses is more on a regional than a continental scale. One factor relevant to operational mobility is the "rolling resistance" of a vehicle traveling on ordinary roads. On roads the rolling resistance of tracked vehicles equals four percent of their weight, on average, while that of their wheeled counterparts (fitted with cross-country tires) equals only 2 percent of their weight. Consequently, wheeled vehicles need less fuel and can cover longer distances by road before they need to be refueled.
This advantage of wheeled vehicles disappears, however, when they move off roads. Then their fuel consumption may be at least as high as that of tracked vehicles (of equal weight). Still, if patrolling and area control missions are emphasized, road travel predominates and, thus, the advantage of fuel economy accrues to the wheeled class. Even in the context of typical warfighting scenarios, off-the-road activities constitute less than 50 percent of overall travel. This is because, within a sizeable theater, many movements have to be devoted to marching the troops to the combat areas in a timely fashion, rather than to maneuvering in the thick of battle.
There are two reasons that forces equipped with wheeled armor are more likely to deploy operationally in a timely fashion:
- First, there are fewer and shorter refueling stops. (The average road range of wheeled vehicles exceeds that of their tracked counterparts by 50 - 100 percent.)
- Second, the average marching speed of wheeled vehicles is, on roads, also 50 - 100 percent higher than that of tracked vehicles.
The fact that wheeled armor can cover longer distances faster than tracked vehicles is complemented by yet another advantage: There is much less fatigue for their occupants because the wheeled platforms do not suffer the vibrations generated by tracks.
In actual practice most armies recognize the overall advantages of wheeled vehicles with respect to operational mobility. Typically, they use wheeled carriers -- "tank trailers" -- for the theater- wide allocation of tracked armor. This measure, which temporarily puts tracked vehicles on wheels, makes sense only as a stopgap; its disadvantages are quite obvious: It is expensive and makes marching columns clumsier and more vulnerable.
2.1.3 Tactical mobility
Tactical mobility is the kind needed when a force is in immediate contact with its adversary. Direct confrontation with an enemy imposes at least two mobility requirements:
- Good off-road mobility is an important precondition of being able to evade enemy action and exploit unexpected avenues of approach.
- Agility -- a combination of high speed, good acceleration, and the ability to "zig-zag"--is also key to being able to respond flexibly to rapidly changing opportunities and challenges.
Relevant to off-road mobility, wheeled vehicles tend to have a ground pressure considerably higher than that of their tracked counterparts. The Mean Maximum Pressure (MMP), which is the average peak pressure under the tires of wheeled vehicles or under the road wheels of tracked vehicles, varies between 200 to 270 kN/m2 for the latter and 300 to 450 kN/m2 for the former. This implies inferior performance for wheeled vehicles on soft ground. There is at least one notable exception, however. The French Panhard VBL M-11 (a 4 x 4 vehicle weighing 3.55 t) has an MMP of only 220 kN/m2. In this case, a very light wheeled armored vehicle achieves an MMP in the range of tracked vehicles.
Generally speaking, the ground pressure of wheeled vehicles rises significantly with the platform's weight. In the case of tracked vehicles this correlation is not as evident. In light of this, the renowned British tank expert Ogorkiewicz has argued to abandon concepts of wheeled combat vehicles weighing significantly over 22 - 23 t. Even a multi-wheeled configuration (8 x 8 -- that is, eight powered wheels) with variable tire pressure can not solve the problem -- resulting only in a very complex, hence expensive, design.
This is a principal matter: It is difficult, if not hopeless, to conceive of technological solutions that could radically solve the problem of wheeled armor's relatively high ground pressure. (And we certainly should not contemplate resurrecting the failed "solution" attempted during the 1920s and 1930s, which was to equip wheeled vehicles with auxiliary tracks.)
Although wheeled armored vehicles cannot escape their principal dilemma, there have been some interesting and worthwhile examples of such platforms in the 20 - 30 t weight range. One is the South African mechanized howitzer, RHINO, with a weight as high as 36 t. Several other vehicles of interest, mostly in the experimental or blueprint stage, may achieve around 30 t -- for example, the new British/Dutch/German infantry carrier. But the willingness of advanced militaries to invest in such vehicles does not mean that Ogorkiewicz' concerns are being over-turned. These programs do not indicate a belief that wheeled armored vehicles could generally be heavier than he argues and still exhibit good cross-country performance. Instead, in most cases, the fielding of heavier wheeled vehicles reflects special, limited circumstances or goals.
-- In two cases -- the South African RHINO and the Czech/Slovak DIANA/SUZANA -- the vehicles in question are mechanized artillery. For these, tactical mobility is not a high priority. They are wheeled because the resulting operational mobility facilitates the flexible allocation of fire -- a key concern for artillery.
-- In the cases of France, Germany, and some other nations, military planners and designers appear to have deliberately down-rated soft-terrain capabilities. This probably has to do with increased emphasis on peace support and peace enforcement missions, which also put a premium on operational mobility for vehicles of relatively high payload.
Turning to the question of agility: Wheeled armored vehicles tend to excel in speed -- on the road, of course, but also in open terrain, if it is fairly negotiable. When it comes to zig-zagging and acceleration, the advantage also seems to go to wheeled armor. It is true that most tracked vehicles can pivot in place, while wheeled vehicles cannot (except for those with brake-steering). Otherwise, however, wheeled vehicles are more easily steered and their running gear is more responsive. Compared to a tracked counterpart of equivalent weight and engine output, we can expect a wheeled platform to have not only higher speed, but also better acceleration. Interestingly, these advantages are especially pronounced with respect to relatively light armored vehicles. It is plausible that high agility is associated with "smallness" and "lightness."
Tactical mobility has another important precondition: protection. As one legendary expert, General Israel Tal, has argued: Without proper protection even the most agile and cross-country capable vehicle could not move forward in harm's way. We will deal with protection and survivability in the following section. Suffice to say for now that there is a dialectic interplay between tactical mobility (in the narrow sense) and protection that is relevant to the challenge of moving under fire.
