The first tracked vehicle was probably invented in the Middle Ages - though the issue of mechanical stress kept the vehicles from being useful until the 18th Century. In 1770, Richard Edgeworth invented the steam-driven caterpillar track vehicle. The concept of these vehicles is that, to all intents and purposes, they take the road with them. Even the Romans where already aware of the fact that roads are vital in order for any community to keep functioning.
Tracked vehicles are mechanical, just as any other wheel-supported vehicle is, the main difference being that they keep the road to themselves. The tracks are closed loops covering all or most of the wheels - which ride on the inside of the track, where the outside uses a relatively large surface to touch the ground. This gives the vehicle the ability to go over a surface without applying too much pressure on that surface. The tracks come in two main varieties: either one single 'rubber band' or a chain of metal links. Tracked vehicles are usually more robust than wheeled vehicles of the same size. Just the space and support required for the tracks makes them heavier. However, due to the spread of their weight they are capable of remarkable things.
How to keep the tracks together
Most vehicles have a guiding system to keep the track in place. This can be a comb on the inner side of the track, or a cover just next to the track on the top side of the wheels. Most use a chain-like construction where multiple toothed wheels keep the chain in place. The toothed wheel system also has advantages in preventing the driving wheel from slipping inside the track.
The smallest tracked vehicles are probably the moles, used to inspect drainage pipes or damaged buildings. More common are small digging machines used to dig narrow trenches for piping and cabling under the pavement. The advantage of these is that they will not easily fall into their own pit as the tracks spread the weight over a large area. Another example of a small-tracked vehicle is the snow scooter. Snow scooters use the 'rubber band' track for traction on the snow. The track on these scooters often has nails or blades for extra contact, with steering via skis.
Mining excavators and rocket transporters are the ultimate in tracked vehicles. These very heavy machines are only capable of crawling at very low speeds. With the spread surface load they can however carry an enormous weight.
Steering tracked vehicles
Although it may look impossible, some tracked vehicles are capable of rotating on the spot. By driving forward on one track and driving in the reverse direction on the other track, it's possible to make a turn on the spot. This is not an easy job, as instead of staying in one place on the outside of the track it has to slide over the surface. A considerable amount of power is taken in order to accomplish this. It can even be impossible if the surface is disturbed by the vehicle sliding over it. It will just dig a hole in the soft ground and eventually stop rotating completely. Another danger is dirt coming between the track and the running wheels; this can damage the vehicle.
Tracked vehicles are also capable of less sharp turns by controlling the speed of each track. The one going more slowly takes the inner curve. This can be done by using a differential, spreading the power over the left and right track, and then using brakes to control the difference. Some vehicles use a computer-controlled system to translate steer movements to running speed making it easier for the driver, though steering this way sometimes gives a little unexpected sliding feeling. Controlling the brakes manually is an extreme sensation - you have to 'become' the vehicle - but once you manage it, it's really sensational.
Normal use of tracked vehicles
Lots of tracked vehicles are used on building sites - they can construct roads to enable wheeled vehicles to take over - and while some heavy machines just need to be there once (like cranes) it is does not always make sense to prepare a road. On some locations it is not possible to make roads at all. Snow stampers prepare ski runs on steep slopes. Tracked vehicles are, besides animals, the only transport to go over dusty desert areas; with enough dust, even a hovercraft will stall. Another extreme is provided by the polar regions. There, tracked vehicles are also popular because they do not slip away, and conquer most obstacles with ease.
Although not especially friendly to look at, armoured vehicles are nevertheless remarkable. Where the more serious vehicles are used to go somewhere or to carry something, armoured vehicles mainly try to hide themselves. Starting before the First World War, the development of tanks took place. After some refinement they evolved into the low-profile, high-speed weapons of today. Some are capable of driving over 100 kilometres an hour with a weight of 50,000 kilograms. Some are amphibious, some submerge and some can be dropped by parachutes. Most explode when hit.
We will not discuss in detail the replacement of a track as it can take days, lots of people and huge spare parts. A very brief summary of how to replace a track is as follows: break the chain, drive off it, replace, drive onto the track and fix the loop.
Another considerable disadvantage is the power consumption. Even at low speeds these vehicles will use more fuel then wheeled vehicles. They are also banned from normal roads.