Six axle trucks

[adrianspeeder]

Why did old GE and Alco six axle trucks have an off center middle axle?
The EMD's (SD-7,SD-9,ect.)had a centered middle axle. What gives?
-adrian

[edblysard]

Hi Adrian,
Part of the reason, but not the only one I am sure, is that the distance between the center of the front axle and the rear axle is too great to allow six axle trucks to navigate a sharp degree or curve in tracks without the center axle binding. This causes the center axle to ride up and over the railhead, and derails the truck.
The reason six axles came into being was to distribut the weight of more heavy, higher horsepower locomotives evenly on the rail. Not all six axle trucks have traction motors on all the axles, notably, EMD E units, theirs were A + A trucks, the lead and rear axle powered, the center unpowered. By moving the center axle back or forward some, the early designers tried to overcome the binding problem, but it didnt work well, because it required the center axle to be unpowered, and placed strain on the two traction motors left, again as locos became more heavy, with higher horsepower. With only four powered axles per loco, the additional horespower was wasted, as it couldnt be applied to the rail.
Today, bith GE and EMD have six axle trucks that flex, sorta, and EMDs have a sort of steering system that keeps all the wheels aligned with the rail at an more optimum angle, to reduce flange and wheel wear.
Ed

[dknelson]

My understanding, probably a poor one, was that the offset truck was done to make room for the traction motors.
Dave Nelson