Swing Low, Sweet Chariot: I never have found anybody who could really say what that song lyric means.
Chariots appear to have originated somewhere in central Mesopotamia and probably were first used in royal funeral processions. The first ones also seem to have had four wheels and were pulled by teams of onagers, the barely domesticated donkeys of south-west Asia, or by oxen. Early chariots were bulky, with solid wheels rotating on fixed axles, and about the only thing they had in common with later versions was that they were open on top and had side-boards and a high dashboard -- not that they could have done much dashing. Some archeologists think they may have developed from punting boats that occasionally had to be moved -- on wheels -- from one stream to another. Others say they developed from sledges. That's the kind of thing that occupies over-specialized archeologists.
By the time they got to Italy, chariots pretty much matched the standard modern image that we all have. They were swift two-wheeled carriages used in war, ceremonies (specifically, victory ceremonies) and races. All were pulled by horses, but the kind of horses varied. Bulkier war horses pulled most of the military chariots, fancy but strong prancers drew ceremonial wagons. Agile imported "thoroughbreds" (conformation and breeding were regulated) from northern Africa and Spain, pulled the racers. The number of horse also varied as did the way they were hitched. War chariots had stout sides and, yes, they might have blades attached to the wheels. They almost never had more than two horses, because control was absolutely essential. If a charioteer showed up for battle with more than two horses hitched, the other charioteers knew not to get near him in the melee. (Gods, who always seemed to take the field on both sides of all legendary battles, could and did control as many horses as they wanted. That's how you knew you had a god on the battlefield.) Roman ceremonial chariots -- the kinds used in official Victories -- are almost always shown with four horses and lots of decoration, but they moved slowly on well-defined and broad routes and usually had a professional driver aboard. The two outside horses were sometimes ridden by relatives of the victorious general. Racing chariots were extremely light and fragile and could have two, four, or more horses. When audiences became more jaded, exotic animals, sometimes in mixed teams, were hitched up by race promoters.
But for serious racing (and for the record books, which were meticulously maintained) the two-horse biga or the four-horse quadriga were the only types of chariot used. The biga rig had a central pole (a "singletree" in modern wagon parlance) that was fixed at its lower end directly to the stationary axle in early times and later, when rotating axles were introduced, to the light lower framework on which the rider stood. A solid yoke, attached at a pivot on the upper (front) end of the tree, held the necks of the matched pair (iguales). The two center horses in a quadriga rig were also yoked to the tree, and the two outside horses were attached to the two inner horses by strong traces.
All races were run counter-clockwise. Outside horses ran a quarter to a half stride ahead of inner horses on the straights and were trained to pull outward so that the team would stay pretty much in line. The horse closest to the spina, or median wall of the course, had to be the strongest and the best trained. This was especially true in quadriga races where the outside-left had to drop back a half stride and pull the others around the corner. The outside-right horse on a quadriga stepped forward, turned in, and pushed around the turns. It had to have the most stamina since it actually ran farther, and faster around the turns. This outside horse was considered to be the most valuable.
The Circus Maximus, between the Palatine and Aventine hills in Rome, was where Romulus was said to have held races on the day he staged the rape of the Sabine women. It was the standard course on which all other Roman chariot racetracks were modeled, and by the time of Constantine its grandstands were enlarged to hold 250,000 spectators -- one quarter of the population of the city. Races normally were seven laps around the spina, three to four modern miles depending on how close to the inside a charioteer could stay, and more than twice the length of modern race tracks. Up to twelve chariots could run in a single race and often did, making the track, literally, a bloody mess. Twelve races a day were usually run until Caligula doubled it, and thereafter 24 races per day became typical. Domitian once organized a 100-race day (only five laps each race), but the logistics of moving that many horses to the track ensured that it was a one-shot meeting.
On normal race days, novelty events -- strange hitches and animals, strange race formats -- might be added, but these became less popular as fan spirit in support of the four "colors" developed. Hard-core fans rooting and heavily betting for the "reds" and "whites" and, later, also for the "greens" and "blues" did not want their day's pleasure diluted with inconsequential events. (Domitian tried to add two more colors, gold and purple, but they were quickly abandoned -- all the fans were already committed.) Races were always among the "colors" with up to three chariots per color in each race. The four stables eventually trained their own drivers, bred and trained horses and ran most of the on- and off-track betting. Results of Circus Maximus race meetings were followed around the Empire.
By the fourth century AD there were
66 race days per year and, once gladiatorial combat was banned, the races
were certainly the most popular form of Roman (public) entertainment. Horse
racing is, of course still popular, and the follow-on sport, car racing,
even more so. Modern auto-racing tracks are the only sports venues that
have ever had spectator capacity that could compare to the Circus Maximus.
It's hard to keep track, but adherents of the American "stock-car" variety
of auto racing claim that their sport is the best attended in the US.
VRoma (McManus) on chariot racing
and the Circus Maximus:
Photos of Circus Max today (but go out and see it yourself): http://wings.buffalo.edu/AandL/Maecenas/rome/circus_max/thumbnails_contents.html
Currus (chariot) in the Smith
Quadriga images: http://www.perseus.tufts.edu/cgi-bin/sor?type=phrase&lookup=quadriga
Buy an expensive statue of a biga (or just look at the picture): http://www.eleganza.com/detailed/chariot.html
PS: The "-ga" in "biga", "quadriga" and other such words appears to be a contraction of iuga (or juga) meaning bound or yoked (from iugo,iugare) -- "biga would originally have been "bis-juga" = two (horses) yoked. The Indo-European root appears to be yeug-, which also appears to be the ultimate root of the English word "yoke". There is no real proof that the English word yoke was derived through the Latin language.