Re: Monday, answer (long)
Tom had the right idea. Product liability indeed. (From various sources:) Unlike a modern bicycle wheel, which depends upon pretensioning slender spokes to such a degree that they never go into compression�even on the side below the axle�a chariot put its spokes into compression. This can result in a stress reversal every cycle.
To prevent spokes from working in their sockets, or tenons, during cyclic loading, it is necessary to apply enough hoop tension at the rim to keep the entire assembly in radial compression. In our millennium, this has usually been accomplished by shrinking an iron rim on a wooden assembly. Chariot builders, who did not have this option, relied on the shrinking of leather. A leather rim on a wheel provided the needed tension, but was subject to wear as it came in contact with the ground. In many cases, a replaceable laced-on "tire" was put over it to protect it. (Did I mention that these were maintenance nightmares) The dependence upon dried leather, along with glued joints, created an assembly that could exist best in a dry climate. It is interesting to speculate upon how long a wheel of this kind would retain its integrity if it were soaked, and its spokes tangled with marsh plants
During the Exodus, around the 13th or 14th century B.C., crossing the Red Sea or Reed Sea compelled the pursuing Egyptians to operate in marshland. The Biblical account states that the wheels fell off the pursuing chariots. The use of dried leather and glues that were probably water-soluble might be an explanation for this.
Some of the linchpins were also made of dried leather. Getting the pins wet might very well cause the wheels to fall off. Lubrication in the form of animal fat or tallow is known to have been used, although the exact composition has not been determined.
You can find a centuries later example wheel at:
http://www.memagazine.org/backissues/membersonly/nov04/features/tutstrib/tutstrib.html
Another interesting historical fact that is recorded: Napoleon tried to move troops in the same geographic area (Suez)in 1797 during his invasion of Egypt and in preparation for a battle with the Turks. He lost more than a few men and he almost drowned when his troops were caught in a tide and wind change--"On the morning of the 28th we crossed the Red Sea dry shod...Near the port the Red Sea is not above 1,500 meters wide, and is always fordable at low water...at high tide the water rises five or six feet at Suez, and when the wind blows fresh it often rises nine or ten feet." Louis Antoine Fauvelet De Bourienne, Memoirs of Napoleon (New York: Charles Scribner's Sons, 1891), V.1 p181 In the evening they entered Suez, but it was highwater; they then ascended to the point of the Red Sea; but the guide lost himself in the marshes, from which he extricated himself with great difficulty, being up to the middle in the water. This guide must have been a descendant of the one who conducted Pharaoh. L.A. Berthier. The French Expedition into Syria (London: Cooper & Wilson, 1799, Reprinted 1990), 22.