Backstory (skip if you don't like alternate history junk)
The year is 2239. It has been roughly 210 years since the world was engulfed in nuclear war. Following the war, the United States splintered into hundreds of small statelets. While much knowledge was retained in some form (mostly through books and other printed media), the loss of population and destruction of industrial capability set back society immensely.
Though the Pacific Northwest was less badly hit than other areas, the destruction of Seattle and Portland, coupled with the rupturing of the Cascadia Subduction Zone in 2043, caused society to regress to a mid-19th century technology level. However, in the early 2100s, the Cascade Republic formed, centered near Tacoma. The new nation grew rapidly, expanding to encompass most of Washington and Oregon by 2239. The Cascade Republic now extends from the Klamath River in the south to the Fraser River in the north, and from the Pacific roughly to central Idaho. Over time, the standard of living and industrial development improved (initially through salvaging of surviving equipment, by the late 2100s through new development); the population has grown to about 4.5 million (comparable to 1950 levels), and technology is at about a 1940 level. Automobiles are common, aircraft are less common, but not rare by any means. Computers are nonexistent aside from a few experimental devices; while scientists and engineers are aware of the principles behind microchips and other advanced electronics, the facilities to produce such components simply do not exist. Low rate production of early transistors recently restarted.
The current armored force of the Cascade Republic consists of three armored brigades. They are presently equipped with domestically produced light tanks, dating to the 2190s. Weighing roughly 12 tons and armed with a 40mm gun, they represented the apex of the Cascade Republic's industrial capabilities at the time. And when they were built, they were sufficient for duties such as pacifying survivalist enclaves in remote areas. However, since that time, the geopolitical situation has complicated significantly. There are two main opponents the Cascade Republic's military could expect to face in the near future.
The first is California. The state of California was hit particularly hard by the nuclear exchange. However, in 2160, several small polities in the southern part of the state near the ruins of Los Angeles unified. Adopting an ideology not unfamiliar to North Korea, the new state declared itself the successor to the legacy of California, and set about forcibly annexing the rest of the state. It took them less than 50 years to unite the rest of California, and spread into parts of Arizona and northern Mexico. While California's expansion stopped at the Klamath River for now, this is only due to poor supply lines and the desire to engage easier targets. (California's northward advanced did provide the final impetus for the last statelets in south Oregon to unify with the Cascade Republic voluntarily).
California is heavily industrialized, possessing significant air, naval, and armored capabilities. Their technology level is comparable to the Cascade Republic's, but their superior industrial capabilities and population mean that they can produce larger vehicles in greater quantity than other countries. Intelligence shows they have vehicles weighing up to 50 tons with 3 inches of armor, though most of their tanks are much lighter.
The expected frontlines for an engagement with the Californian military would be the coastal regions in southern Oregon. Advancing up the coastal roads would allow California to capture the most populated and industrialized regions of the Cascade Republic if they advanced far enough north. Fortunately, the terrain near the border is very difficult and favors the defender;
(near the Californian border)
The other opponent is Deseret, a Mormon theocratic state centered in Utah, and encompassing much of Nevada, western Colorado, and southern Idaho. Recently, tension has arisen with the Cascade Republic over two main issues. The first is the poorly defined border in Eastern Oregon / Northern Nevada; the old state boundary is virtually meaningless, and though the area is sparsely populated, it does represent a significant land area, with grazing and water resources. The more recent flashpoint is the Cascade Republic's recent annexation of Arco and the area to the east. Deseret historically regarded Idaho as being within its sphere of influence, and maintained several puppet states in the area (the largest being centered in Idaho Falls). They regard the annexation of a signficant (in terms of land area, not population) portion of Idaho as a major intrusion into their rightful territory. That the Cascade Republic has repaired the rail line leading to the old Naval Reactors Facility, and set up a significant military base there only makes the situation worse.
Deseret's military is light and heavily focused on mobile operations. Though they are less heavily mechanized than the Cascade Republic's forces, operating mostly armored cars and cavalry, they still represent a significant threat to supply and communication lines in the open terrain of eastern Oregon / southern Idaho.
(a butte in the disputed region of Idaho, near Arco)
As the head of a design team in the Cascade Republic military, you have been requested to design a new tank according to one of two specifications (or both if you so desire):
Medium / Heavy Tank Weight: No more than 45 tons Width: No more than 10.8 feet (3.25 meters) Upper glacis / frontal turret armor of at least 3 in (76mm) LoS thickness Side armor at least 1in (25mm) thick (i.e. resistant to HMG fire) Power/weight ratio of at least 10 hp / ton No more than 6 crew members Primary armament capable of utilizing both anti-armor and high explosive rounds Light tank Weight: No more than 25 tons Width: No more than 10.8 feet Upper glacis / frontal turret armor of at least 1 in thickness Side armor of at least 3/8 in (10mm) thickness Power/weight ratio of at least 12 hp / ton No more than 6 crew members Primary armament capable of utilizing both anti-armor and high explosive rounds
Other relevant information:
Any tank should be designed to operate against either of the Cascade Republic's likely opponents (California or Deseret) The primary heavy machine gun is the M2, the primary medium machine gun is the M240. Use of one or both of these as coaxial and/or secondary armament is encouraged. The secret archives of the Cascade Republic are available for your use. Sadly, there are no running prewar armored vehicles, the best are some rusted hulks that have long been stripped of usable equipment. (Lima Tank Plant ate a 500 kt ground burst) Both HEAT and APFSDS rounds are in testing. APCR is the primary anti-armor round of the Cascade Republic. Either diesel or gasoline engines are acceptable, the Cascade Republic is friendly with oil producing regions in Canada (OOC: Engines are at about a late 1940s/early 50s tech level) The adaptability of the tank to other variants (such as SPAA, SPG, recovery vehicle, etc.) is preferred but not the primary metric that will be used to decide on a design. Ease of maintenance in the field is highly important. Any designs produced will be compared against the M4 Sherman and M3 Stuart (for medium/heavy and light tank), as these blueprints are readily available, and these tanks are well within the Cascade Republic's manufacturing capabilities.
Sooooo...after doing a site-wide search and perusing Google, I'm surprised not to have found anything about tank suspension, other than a somewhat doubtful thread on the WoT forums. Would my learned colleagues of SH be able to assist me in understanding and identifying the different types of tank suspension? I think I've got leaf-spring more or less mastered, as well as both VVSS and HVSS (thanks, JGT!) but was somewhat embarrassed not to be able to differentiate between the suspension of a Type 97 Chi-Ha and an FV4201 Chieftain.
UPDATE: I think I understand tank suspension better now. Thanks, everyone!
The mean goons over on SA roped me into writing an effortpost, so I figured it's only fair that you freeloaders get to enjoy it too.
So, suspensions. I'm going to introduce the book as well because it's probably the most Soviet book that ever existed. It is called TANK. What makes this book so Soviet? Well, here's the first paragraph of the introduction: "Under the guidance of the Communist party of the Soviet Union, our people built socialism, achieved a historical victory in the Great Patriotic War, and in launched an enormous campaign for the creation of a Communist society." The next paragraph talks about the 19th Assembly of the CPSU, then a bit about how in the Soviet Union man no longer exploits man (now it's the other way around :haw:), then a little bit about the war again, then spends another three pages stroking the party's dick about production and growth. The word "tank" does not appear in the introduction. The historical prelude section is written by someone who is a little closer to tanks and might be a little less politically reliable, since they actually give Tsarists credit for things. I guess they have to, since foreigners are only mentioned in this section when they are amazed by Russian progress. The next chapter is a Wikipedia-grade summary of various tank designs that gives WWI designs a pretty fair evaluation, then a huge section on Soviet tank development, then a tiny section on foreign tanks in WWII mostly consisting of listing all the mistakes their designers made. The party must have recuperated since the intro since we're in for another three pages of fellatio. Having read so far, you might think that there is very little value in this sort of book, but then the writing style does a complete 180 and the rest of the book is 100% apolitical and mostly looks like this. Which is what we care about, so let's begin. Bonus points to anyone who can identify what the diagram above is about. Sorry in advance if my terminology isn't 100% correct, there aren't exactly a lot of tank dictionaries lying around. The book skips over primitive unsprung suspensions of WWI and starts off with describing the difference between independent suspensions and road-arm suspensions. In the former, every wheel is independently sprung. In the latter, two or more wheels are joined together by a spring. Some suspensions have a mix of these designs. For example, here's a simple road-arm suspension used in some Vickers designs and their derivatives. The two road wheels are connected by a spring and to the hull by a lever. A weight pushing down on top of the pair of wheels is going to compress the spring that's perpendicular to the ground, bringing the wheels closer together. Here's a more complex road-arm suspension, with four wheels per unit instead of one, also AFAIK first used by Vickers and then migrating to an enormous amount of designs from there. This suspension provides springiness through a leaf spring that you can see above the four road wheels. The two pairs of wheels don't have their own springs. The black circles in the image show where the suspension elements can turn, keeping the tank flat while hugging the terrain. Here's another road-arm suspension, similar to the first one. In this case, the spring is made of rubber instead of metal. Otherwise, the design is very similar. Two rubber bungs on the bottom of the axles prevent the wheels from slamming into each other too hard. This design was used by French tanks and nobody else. For some reason, volute spring suspensions are completely absent from this section. This is the best image of a Vertical Volute Spring Suspension (early Shermans) that I could find. It's kind of similar to the first image, except the spring is a volute spring, and it's vertical instead of horizontal. Later Shermans used horizontal volute springs. Of course, as the book points out, these suspension elements are very easy to damage externally and knocking out one part of the suspension will typically take out the rest of the assembly, so independent suspensions are the way to go. The best way to do this are torsion bars. The bar is attached to a lever that holds your road wheel. As pressure is applied to the road wheel, the bar subtly twists, remaining elastic enough to reset once the pressure is off. This image is kind of weird, but the part in the center is the part on the far left, zoomed in, showing you where the lever and the opposite side's torsion bar are attached. As you can see, road wheels in a torsion bar suspension are going to be a little off on one side, unlike what you're used to on cars and such. Now, since torsion bars are metal bars on the floor, they are going to make your tank taller. If you want a tank that's as short as possible at the expense of width, you may want to consider a Christie like suspension. Here, much like in torsion bars, the pressure is transferred inside the tank, but instead of a bar to absorb it, it's a spring in a vertical (or angled) tube. In most tanks with this kind of suspension, the springs are on the inside, but if you want to make the tank roomier on the inside, you can have them on the outside too. If you're really fancy, you can put a spring within the spring like in this diagram. Since this is a Soviet tank book, you gotta have a huge T-34 diagram. Here it is. The T-34 uses Christie springs, which you can see in the diagram. The road wheel configuration is a mix of the externally dampened and internally dampened "Stalingrad type" road wheels. The former have more rubber for absorbing hits from terrain, but the latter use less rubber. When you're in Stalingrad and you have to make tanks with a rubber deficit, that's the kind you want. When road wheels from other factories were available, they would go in the front and then the back to absorb most of the impact from harsh terrain features, and the steel-rimmed wheels went in the middle. The diagram shows how both types of wheels work. Rubber can't really take too much punishment, so the KV, being a heavy tank, went with internally dampened road wheels from the very beginning, with a ring of rubber on the inside around the axle. And finally, idlers. If you don't have big Christie type wheels, you gotta have idlers so your saggy track doesn't fall off. This diagram shows the rubber coating on an idler, and also how the rear idler can adjust to tighten the track. A loose track makes more noise, gets worn more, and is liable to slip off. Keep those tracks tight, and you'll be zooming towards glorious victory in no time flat! Now, the book ends and my own stuff begins. I mentioned rubber, but not what a headache it was to tank designers. In hot weather, the rubber in your tracks and wheels tends to fall apart. If you go fast enough, tires that don't have proper ventilation are going to melt too. There was a lot of pre-war panic in the USSR about the German PzIII being able to do 70 kph on tracks, but once the Soviets started building SU-76Is on the PzIII chassis they found out that the speed had to be limited to a whopping 25 kph to keep the wear to a reasonable level.
Tank design is often conceptualized as a balance between mobility, protection and firepower. This is, at best, a messy and imprecise conceptualization. It is messy because these three traits cannot be completely separated from each other. An APC, for example, that provides basic protection against small arms fire and shell fragments is effectively more mobile than an open-topped vehicle because the APC can traverse areas swept by artillery fires that are closed off entirely to the open-topped vehicle. It is an imprecise conceptualization because broad ideas like "mobility" are very complex in practice. The M1 Abrams burns more fuel than the Leo 2, but the Leo 2 requires diesel fuel, while the omnivorous AGT-1500 will run happily on anything liquid and flammable. Which has better strategic mobility? Soviet rail gauge was slightly wider than Western European standard; 3.32 vs 3.15 meters. But Soviet tanks in the Cold War were generally kept lighter and smaller, and had to be in order to be moved in large numbers on a rail and road network that was not as robust as that further west. So if NATO and the Warsaw Pact had switched tanks in the late 1950s, they would both have downgraded the strategic mobility of their forces, as the Soviet tanks would be slightly too wide for unrestricted movement on rails in the free world, and the NATO tanks would have demanded more logistical support per tank than evil atheist commie formations were designed to provide.
So instead of wading into a deep and subtle subject, I am going to write about something that is extremely simple and easy to describe in mathematical terms; the top speed of a tank moving in a straight line. Because it is so simple and straightforward to understand, it is also nearly meaningless in terms of the combat performance of a tank.
In short, the top speed of a tank is limited by three things; the gear ratio limit, the power limit and the suspension limit. The tank's maximum speed will be whichever of these limits is the lowest on a given terrain. The top speed of a tank is of limited significance, even from a tactical perspective, because the tank's ability to exploit its top speed is constrained by other factors. A high top speed, however, looks great on sales brochures, and there are examples of tanks that were designed with pointlessly high top speeds in order to overawe people who needed impressing.
When this baby hits 88 miles per hour, you're going to see some serious shit.
The Gear Ratio Limit
Every engine has a maximum speed at which it can turn. Often, the engine is artificially governed to a maximum speed slightly less than what it is mechanically capable of in order to reduce wear. Additionally, most piston engines develop their maximum power at slightly less than their maximum speed due to valve timing issues:
A typical power/speed relationship for an Otto Cycle engine. Otto Cycle engines are primitive devices that are only used when the Brayton Cycle Master Race is unavailable.
Most tanks have predominantly or purely mechanical drivetrains, which exchange rotational speed for torque by easily measurable ratios. The maximum rotational speed of the engine, multiplied by the gear ratio of the highest gear in the transmission multiplied by the gear ratio of the final drives multiplied by the circumference of the drive sprocket will equal the gear ratio limit of the tank. The tank is unable to achieve higher speeds than the gear ratio limit because it physically cannot spin its tracks around any faster.
Most spec sheets don't actually give out the transmission ratios in different gears, but such excessively detailed specification sheets are provided in Germany's Tiger Tanks by Hilary Doyle and Thomas Jentz. The gear ratios, final drive ratios, and maximum engine RPM of the Tiger II are all provided, along with a handy table of the vehicle's maximum speed in each gear. In eighth gear, the top speed is given as 41.5 KPH, but that is at an engine speed of 3000 RPM, and in reality the German tank engines were governed to less than that in order to conserve their service life. At a more realistic 2500 RPM, the mighty Tiger II would have managed 34.6 KPH.
In principle there are analogous limits for electrical and hydraulic drive components based on free speeds and stall torques, but they are a little more complicated to actually calculate.
Part of the transmission from an M4 Sherman, picture from Jeeps_Guns_Tanks' great Sherman website
The Power Limit
So a Tiger II could totally go 34.6 KPH in combat, right? Well, perhaps. And by "perhaps," I mean "lolololololol, fuck no." I defy you to find me a test report where anybody manages to get a Tiger II over 33 KPH. While the meticulous engineers of Henschel did accurately transcribe the gear ratios of the transmission and final drive accurately, and did manage to use their tape measures correctly when measuring the drive sprockets, their rosy projections of the top speed did not account for the power limit.
As a tank moves, power from the engine is wasted in various ways and so is unavailable to accelerate the tank. As the tank goes faster and faster, the magnitude of these power-wasting phenomena grows, until there is no surplus power to accelerate the tank any more. The system reaches equilibrium, and the tank maxes out at some top speed where it hits its power limit (unless, of course, the tank hits its gear ratio limit first).
The actual power available to a tank is not the same as the gross power of the motor. Some of the gross horsepower of the motor has to be directed to fans to cool the engine (except, of course, in the case of the Brayton Cycle Master Race, whose engines are almost completely self-cooling). The transmission and final drives are not perfectly efficient either, and waste a significant amount of the power flowing through them as heat. As a result of this, the actual power available at the sprocket is typically between 61% and 74% of the engine's quoted gross power.
Once the power does hit the drive sprocket, it is wasted in overcoming the friction of the tank's tracks, in churning up the ground the tank is on, and in aerodynamic drag. I have helpfully listed these in the order of decreasing importance.
The drag coefficient of a cube (which is a sufficiently accurate physical representation of a Tiger II) is .8. This, multiplied by half the fluid density of air (1.2 kg/m^3) times the velocity (9.4 m/s) squared times a rough frontal area of 3.8 by 3 meters gives a force of 483 newtons of drag. This multiplied by the velocity of the tiger II gives 4.5 kilowatts, or about six horsepower lost to drag. With the governor installed, the HL 230 could put out about 580 horsepower, which would be four hundred something horses at the sprocket, so the aerodynamic drag would be 1.5% of the total available power. Negligible. Tanks are just too slow to lose much power to aerodynamic effects.
Losses to the soil can be important, depending on the surface the tank is operating on. On a nice, hard surface like a paved road there will be minimal losses between the tank's tracks and the surface. Off-road, however, the tank's tracks will start to sink into soil or mud, and more power will be wasted in churning up the soil. If the soil is loose or boggy enough, the tank will simply sink in and be immobilized. Tanks that spread their weight out over a larger area will lose less power, and be able to traverse soft soils at higher speed. This paper from the UK shows the relationship between mean maximum pressure (MMP), and the increase in rolling resistance on various soils and sands in excruciating detail. In general, tanks with more track area, with more and bigger road wheels, and with longer track pitch will have lower MMP, and will sink into soft soils less and therefore lose less top speed.
The largest loss of power usually comes from friction within the tracks themselves. This is sometimes called rolling resistance, but this term is also used to mean other, subtly different things, so it pays to be precise. Compared to wheeled vehicles, tracked vehicles have extremely high rolling resistance, and lose a lot of power just keeping the tracks turning. Rolling resistance is generally expressed as a dimensionless coefficient, CR, which multiplied against vehicle weight gives the force of friction. This chart from R.M. Ogorkiewicz' Technology of Tanks shows experimentally determined rolling resistance coefficients for various tracked vehicles:
The rolling resistance coefficients given here show that a tracked vehicle going on ideal testing ground conditions is about as efficient as a car driving over loose gravel. It also shows that the rolling resistance increases with vehicle speed. A rough approximation of this increase in CR is given by the equation CR=A+BV, where A and B are constants and V is vehicle speed. Ogorkiewicz explains:
It should be noted that the lubricated needle bearing track joints of which he speaks were only ever used by the Germans in WWII because they were insanely complicated. Band tracks have lower rolling resistance than metal link tracks, but they really aren't practical for vehicles much above thirty tonnes. Other ways of reducing rolling resistance include using larger road wheels, omitting return rollers, and reducing track tension. Obviously, there are practical limits to these approaches.
To calculate power losses due to rolling resistance, multiply vehicle weight by CR by vehicle velocity to get power lost. The velocity at which the power lost to rolling resistance equals the power available at the sprocket is the power limit on the speed of the tank.
The Suspension Limit
The suspension limit on speed is starting to get dangerously far away from the world of spherical, frictionless horses where everything is easy to calculate using simple algebra, so I will be brief. In addition to the continents of the world not being completely comprised of paved surfaces that minimize rolling resistance, the continents of the world are also not perfectly flat. This means that in order to travel at high speed off road, tanks require some sort of suspension or else they would shake their crews into jelly. If the crew is being shaken too much to operate effectively, then it doesn't really matter if a tank has a high enough gear ratio limit or power limit to go faster. This is also particularly obnoxious because suspension performance is difficult to quantify, as it involves resonance frequencies, damping coefficients, and a bunch of other complicated shit.
Suffice it to say, then, that a very rough estimate of the ride-smoothing qualities of a tank's suspension can be made from the total travel of its road wheels:
This chart from Technology of Tanks is helpful. A more detailed discussion of the subject of tank suspension can be found here.
The Real World Rudely Intrudes
So, how useful is high top speed in a tank in messy, hard-to-mathematically-express reality? The answer might surprise you!
A Wehrmacht M.A.N. combustotron Ausf G
We'll take some whacks at everyone's favorite whipping boy; the Panther.
A US report on a captured Panther Ausf G gives its top speed on roads as an absolutely blistering 60 KPH on roads. The Soviets could only get their captured Ausf D to do 50 KPH, but compared to a Sherman, which is generally only credited with 40 KPH on roads, that's alarmingly fast.
So, would this mean that the Panther enjoyed a mobility advantage over the Sherman? Would this mean that it was better able to make quick advances and daring flanking maneuvers during a battle?
In field tests the British found the panther to have lower off-road speed than a Churchill VII (the panther had a slightly busted transmission though). In the same American report that credits the Panther Ausf G with a 60 KPH top speed on roads, it was found that off road the panther was almost exactly as fast as an M4A376W, with individual Shermans slightly outpacing the big cat or lagging behind it slightly. Another US report from January 1945 states that over courses with many turns and curves, the Sherman would pull out ahead because the Sherman lost less speed negotiating corners. Clearly, the Panther's advantage in straight line speed did not translate into better mobility in any combat scenario that did not involve drag racing.
So what was going on with the Panther? How could it leave everything but light tanks in the dust on a straight highway, but be outpaced by the ponderous Churchill heavy tank in actual field tests?
Panther Ausf A tanks captured by the Soviets
A British report from 1946 on the Panther's transmission explains what's going on. The Panther's transmission had seven forward gears, but off-road it really couldn't make it out of fifth. In other words, the Panther had an extremely high gear ratio limit that allowed it exceptional speed on roads. However, the Panther's mediocre power to weight ratio (nominally 13 hp/ton for the RPM limited HL 230) meant that once the tank was off road and fighting mud, it only had a mediocre power limit. Indeed, it is a testament to the efficiency of the Panther's running gear that it could keep up with Shermans at all, since the Panther's power to weight ratio was about 20% lower than that particular variant of Sherman.
There were other factors limiting the Panther's speed in practical circumstances. The geared steering system used in the Panther had different steering radii based on what gear the Panther was in. The higher the gear, the wider the turn. In theory this was excellent, but in practice the designers chose too wide a turn radius for each gear, which meant that for any but the gentlest turns the Panther's drive would need to slow down and downshift in order to complete the turn, thus sacrificing any speed advantage his tank enjoyed.
So why would a tank be designed in such a strange fashion? The British thought that the Panther was originally designed to be much lighter, and that the transmission had never been re-designed in order to compensate. Given the weight gain that the Panther experienced early in development, this explanation seems like it may be partially true. However, when interrogated, Ernst Kniepkamp, a senior engineer in Germany's wartime tank development bureaucracy, stated that the additional gears were there simply to give the Panther a high speed on roads, because it looked good to senior generals.
So, this is the danger in evaluating tanks based on extremely simplistic performance metrics that look good on paper. They may be simple to digest and simple to calculate, but in the messy real world, they may mean simply nothing.