Sunday, July 6, 2008

The Modern Chariot (Part 1)

The three part article is provided by Jim Walsh.
Suppose, for the sake of the exercise, that we needed to design a racing chariot for competition, and that competition was as tough as it is in light harness racing throughout the world. What characteristics would we be looking for? In no particular order, I would suggest they are:

* Lightest possible weight consistent with structural integrity,
* Least possible aerodynamic drag,
* Lowest rolling resistance consistent with -
* Reducing the draft animal’s energetic cost of locomotion to a minimum.

Chariots have been around for a long time - 4,000 plus years - and for a fair bit of recorded human history the planet’s top brains, it’s “rocket scientists” if you like, were engaged in optimizing the chariot’s role as the supreme instrument of warfare from 2,000 BC to the time of Alexander the Great at around 300 BC. As with most things, it is a good idea to look at the design of vehicles of similar purpose during the eras when the very best brains were designing them.

Let us start with Egyptian chariots in the era 1500 BC to 1,000 BC. An Egyptian battle chariot weighed (with harness) around 34 kg (75 pounds), but that was designed to carry two men in battle. The Egyptians also had a need for long-distance high-speed communications, and that need was filled by a single person chariot of much lighter weight and therefore higher speed. Like this:

From, “Wheeled Vehicles of the Ancient Near East” Littauer & Crowley.

The platform of the chariot is strung, tennis racquet style, with rawhide thongs, giving a bearing surface, which was light, strong and - very importantly - provided a degree of shock absorption between the vehicle and its driver. This reduced the shock loading on the delicate wheels on rough terrain, contributing significantly to the service life of the 4-spoked wood and leather wheels.

Note that the chariot was connected to the horses by a pair of neck yokes (the inverted “Y” devices in the above illustration). These were placed above the neck and in front of the withers, and were held in place by the downward pressure exerted by the weight of the chariot’s pole resulting from the center of gravity of the chariot being well forward of the axle. Added to that downward pressure was the contribution of the driver, who had no option but to place his feet in front of the axle. This downward pressure increased the load on the horse’s feet and therefore increased the amount of energy those horses had to expend to move at any velocity above zero.

The Hittites of ancient Anatolia made a much heavier three-person chariot which moved the axle to the middle of the floor and revised the hitch, moving the yoke attachment point behind the withers and combining that with a girth strap and a breast strap. This reduced the downward load of the yoke to zero when moving at a constant velocity on level ground.

Around 700 BC, the Greeks combined the lightweight chariot of the Egyptians with the balance of the Hittites and added an improved “dorsal hitch”. The end result of the Greek innovations was a lightweight chariot with a reduced load on the feet of the horses compared to the Egyptian chariot, an increased load on the wheels of the chariot, and a net increase in efficiency. In plain language - more speed for the same effort or less effort for the same speed than either the Egyptian or Hittite chariots.

The balance system of the Hittites and Ancient Greeks is known as “neutral balance” and along with the dorsal hitch was adopted by the later Romans. Every chariot that ever raced at the Circus Maximus in Rome used the dorsal hitch, a fact you would not appreciate if you went by the movie “Ben Hur”, since in that movie NONE of the racing chariots used the dorsal hitch.

The dorsal hitch is optimum for light to medium loads and only passed out of common use after the decline of the Roman Empire and with it the system of smooth roads built and maintained by the Romans. When the first high-speed road was constructed again in England in the 17th century, no coachbuilder (apart from a few in Italy) knew anything about the dorsal hitch, and so vehicles built for speed were by and large lightweight versions of the existing heavy vehicles designed for passage over rough terrain.

No comments: