Different loco makers placed their eccentric cranks in various locations due to the size and shape of the locomotive they were building. For instance. I have seen an 0-6-0 with their crank at the second driver spot and others at the third. Why?

Also, I have seen a loco with 8 drivers have the crank at the second and or the third driver. Why? What difference does it make where it is? Wouldn't make more sense to keep it near the back for more pulling power as opposed to near the front of the driver set?

And lastly why is it that the rods are connected to the drivers so close to the center? Again, wouldn't it make more sense to keep it closer to the edge of the driver to maximize effort? I can understand not wanting to smack the ground on a downward stroke but is that the only reason?
Were they built that way for a reason or could a steam loco function properly and efficiently if the rods were placed further from center?

Distance from center = half stroke of cylinder. How much steam do you want it to eat?

dave

Deciding which driver the main rod connects to was, like many engineering jobs, a balancing act. In the case of a 6-drivered or 8-drivered locomotive, a longer main rod connected to third driver results in less rod angle at the cross head. This produces lower vertical forces on the cross head which is good. (The cross-head can be designed lighter, less wear, etc.) OTOH, this usually results in a long main rod which will be heavier and more difficult to balance. Early U.S. 4-8-2's had the rod connected to the third driver which resulted in either a LONG main rod or an awkward long piston rod with the cross head mounted much further back than normal. Later U.S. 4-8-2's had the main rod connected to the second driver.

In the case of crank pin placement and stroke, it's like Dave said "how much steam do you want to eat?" A long stroke gives a high piston speed and shifts the locomotive's horsepower curve lower. OTOH that long stroke makes it more difficult to balance the reciprocating parts. A short stroke is easier to balance but results in maximum horsepower only being reached at higher speeds. Once again it's a balancing act for the designer and they were still playing around with the best combinations of bore and stroke when steam design ended. A further complication is that with roller bearing rods, the rod eyes get MUCH bigger, so the stroke is limited by how close you can allow the rods to get to the ground to clear obstructions like switch stands.

Kind of 'damned if you do and damned if you don't' deal huh?
A Catch 22 if you will...right?