Average specific ground pressure is an indicator that shows little
In the course of studying the characteristics of this or that tank in various publications, including specialized technical literature, it is almost guaranteed that you will find a line with an indicator of average specific ground pressure, usually expressed in kilograms per square centimeter. It is generally accepted that the value of this parameter allows you to clearly judge the cross-country ability of a combat vehicle on soils with low bearing capacity. But is this really so?
Instead of an introduction
Generally speaking, a tank's cross-country ability is one of its most important parameters. Most important because even the most powerful armament of a combat vehicle, its electronics and armor are of absolutely no significance if it gets stuck in some black soil or marshy ground, before reaching the enemy and, accordingly, without realizing its capabilities in combat conditions.
But what influences this indicator?
In fact, a lot, from the design of the track drive to even the specific width of the tank hull if its bottom is on the ground. However, it is often the average pressure that is perceived as a kind of universal measure by which one can accurately assess the cross-country ability of equipment and, as is fashionable now, distribute it into categories in the style of: "this tank is lighter, so it will go everywhere, and the one that is heavier will get stuck at any convenient opportunity and is not suitable for war."
Well, we won't deny it, average specific pressure is indeed used, including in design calculations when developing combat equipment and its upgrade options. Only this value is in fact terribly conditional - akin to the average temperature in a hospital - and can only give a very distant idea of the cross-country ability of a conventional tank. In most cases, so distant that it does not coincide with practice almost completely.
Discrepancy between calculations and practice
This discrepancy is due to the fact that the average specific ground pressure is calculated using a very simple formula: the tank mass is taken and divided by the support area of the tracks - there are no additional inputs. As a result, at the output we have a value that is valid only for a stationary body whose mass is distributed evenly over the entire area.
Figuratively speaking, it is the same as if instead of a 40-ton machine with a track support area of 20 square meters each, we took two XNUMX-ton blocks with the same area and placed them on the same marshy ground, observing how deeply they would sink into it. After all, the formula does not take anything else into account.
As a result, seemingly paradoxical situations arise, when tanks of different designs with similar average specific pressure behave differently on soft soils, and heavy vehicles overcome off-road conditions better than light ones. An example of the latter, by the way, is the German "Panther", which left a track half as deep as the light American tank M5A1.
Distribution of pressure along the length of the supporting surface of the tracks: 1 - with an open hinge, 2 - with a rubber-metal hinge
Difficulties, or rather, a complete discrepancy with reality, the average specific pressure also creates in the course of theoretical calculations, which is perfectly demonstrated by the tests of the Japanese SPG SS3 (prototype "Type 60"). Based on the specified indicator, engineers predicted an average track depth of 1,25 centimeters on plastic soils with a density of 1,38-4 grams per cubic cm for this product, but in practice they got as much as 13,4 centimeters. That is, in fact, three times more.
The reason is that the track is mobile and consists of tracks, and the tank rests on it with the help of support rollers, which distribute the pressure not evenly, but locally. Therefore, the result of simple calculations of the average specific pressure is a kind of "spherical horse in a vacuum" (something speculative, oversimplified and divorced from reality), which can lead to the wrong place.
Let's put in a word about rollers and tracks
In order to see how much the number and diameter of the road wheels, as well as the design of the track, affect the depth of the track and, accordingly, the cross-country ability of the tank, we can look at two studies conducted back in the Soviet Union.
The first experiment involved a 26-ton T-10 tank with 305 mm diameter road wheels and 254 mm wide tracks with a 100 mm pitch. Its average specific ground pressure was 0,69 kg per square centimeter and did not change during the tests, since the width and length of the tracks remained unchanged. But they played around with the number of rollers, installing from two to ten on board.
The results of these executions were quite expected. They are shown in the attached figure below (z is the depth of the rut).
As we can see, the more rollers per side the chassis of the tested T-26 had, the lesser the track depth it left. And there is no secret here, since in this way the tank's mass was more effectively distributed over the entire supporting surface. So, it is obviously impossible to rely only on the average specific pressure on the ground.
However, the track depth is affected not only by the number of support rollers - their diameter and the width of the track pitch are also of great importance. To prove this, we will cite the second Soviet study, during which a model weighing 8,5 tons was tested with the possibility of changing the size of its chassis. It was driven on sand 0,5-0,7 meters deep at a speed of 4 kilometers per hour.
The model was equipped with rollers with a diameter of 515 and 700 millimeters, as well as double rollers with a diameter of 360 mm. The width of the track with an open hinge is 300 mm, the step width is 123 and 250 mm. The soil resistance coefficient was used as the main indicator - the higher it is, the greater the track depth, and vice versa.
The results, as last time, were predictable:
From the data presented in the table, we can draw unambiguous conclusions that the diameter of the support roller directly affects the reduction of the soil resistance coefficient, and, accordingly, the reduction of the track depth. In other words, the larger the diameter, the larger the support area of the roller and the more uniform the distribution of the tank's mass. The same applies to the width of the track pitch - the larger it is, the higher the cross-country ability.
But these are, let's say, just facts that can be guessed without any special explanations. The question is, how can one get at least an approximate idea of the cross-country ability of a particular tank, given that the average specific ground pressure cannot give anything specific?
There are no universal methods that take into account all aspects, because, as we have already mentioned, cross-country ability depends on many factors. For example, even the presence of a track with a rubber-metal hinge already changes the pressure indicators on the surface on which the tank moves. But there is still one interesting formula.
It was proposed by Soviet researchers in 1979 and is essentially the "heir" to the tried and tested formula of D. Roland, which determines the mathematical expectation of the average maximum of the pressure diagram under the rollers of a moving vehicle. It takes into account not only the mass of the tank, but also the number and diameter of the support rollers, as well as the width and pitch of the track.
The designations used in it are: G is the weight of the machine, n is the number of support rollers per side, b is the track width, D is the diameter of the support roller, t is the track pitch.
The formula shows the peak pressure under the rollers, and with its help, at least, it is possible to compare the cross-country ability of tracked vehicles more productively. Yes, there are more inputs than in calculating the average specific pressure, but the result is much more accurate - it's not a shot in the dark.
Sources:
L.A. Klushin. Complex indicator of tank cross-country ability/ L.A. Klushin // Bulletin of armored equipment. – 1980. – No. 4.
A.P. Belov, V.I. Krasnenkov, Yu.I. Lovtsov. Distribution of normal pressures along the length of track support surfaces / A.P. Belov, V.I. Krasnenkov, Yu.I. Lovtsov // Issues of defense equipment. - 1979. - No. 88.
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