Dom,
This drawing displays two "artillery wheels": These are the type of dished wooden wheels that were the next step in the evolution of wood wheels after Civil War era carriage wheels. These wheels were developed circa 1870, but I don't know which country should be credited with first developing them. These were the strongest wood wheels ever designed; they had a steel hub (nave), wooden spokes, and usually a one piece wooden rim (no more multiple felloes) that was steamed then bent on a machine, and the first models had steel tyres, later models had hard rubber tires attached to the rims. These were also the first type of wheels used on automobiles; You're probably familiar these wheels from studying the French 75. This is the best drawing I could find, and even though it doesn't show the exact wheels that we want to see, it's still pretty darn good, because this illustration can be used to explain exactly what I was trying to describe in my earlier post. Figure 6, shows a dished wheel on an axle arm that's angled downward, and I'm assuming that the 'balls' are supposed to represent hard rubber tires. This is how I did it, and it seems the easiest way to see it; print the drawing then take a ruler and draw the level surface of the ground the iron 'tyre' (forget about the round ball) is resting on, using the caption "Cambered Wheel" as a guideline (the line drawn for the ground should be parrallel with a line drawn under the caption). Then using the bottom spoke as a reference (its being perpendicular to the ground line), turn the paper upside down, and draw the ground line under what is now the bottom spoke and tyre, making sure that the ground line forms right angles with the line of the downward spoke. Now turn the paper back to right side up, and bisect the hub by drawing a line right through the middle of it. If you now extended these three lines to the left of the wheel, at a given distance the top and bottom lines would intersect the line through the hub (axis). If you extended the lines to the right of the wheel the top and bottom lines would continue to increase the distance between each other and the center line; none of these three lines are parrallel with each other. If you did the same thing on the wheel shown in Fig. 5, (which is intended to rotate on a straight axle arm) all three lines would be parrallel to each other.
I don't know why the caption "Dished Wheel" is under Fig. 5, both figures are drawings of dished wheels; for our purposes it would be better if the caption read, 'Straight Axle Arm'.
