mike omalley said:
What makes a locomotive a good puller?
Last summer, I did a lot of experimentation with a Dash-9 and several USA diesels (SD40, GP38, GP30) pulling consists of different weights and configurations up a 6' long six-and-a-half degree slope. Herewith observations based on these trials:
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The number of axles makes no difference. The 6-axle Dash-9 creamed the USA SD40, but so did the USA four-axle diesels.
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More weight can be pulled on a straight without derailing compared to a curve, though a regular bend of appx 15’ diameter wasn’t a limitation in the tests. The four-axle USA locos handled twisty sections marginally better than the Dash-9, but there is a limit to how twisty the track can be before a derailment is inevitable when pulling a heavy load.
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As the weight increases, the strain on the couplers makes them the most likely point of failure, especially on curves. I use H&L couplers exclusively, so can’t comment on whether knuckle couplers behave similarly.
Jon’s point about a “critical balance with weight” is well taken. Our Dash-9 has no extra weights, but the loco is both hefty and well-balanced. I was able to put a lot of weight behind all the locos mentioned above, except the SD40, without inducing wheel spin.
While accepting that thinking by analogy is primitive, let’s disregard Beard and do it anyway. With cars, CCs rule. Ergo, the total rating of the motor(s) in watts is probably a better indicator of pulling power than the number of motors. Again as with a car, the power:weight ratio of a loco is vital. Our Aristo RDC-3s can pull a surprising amount, and their relatively light weight probably has something to do with that.
Tom’s observation about gearing is trenchant. As with a car, a good gearbox can do great things with an ordinary motor, and vice-versa.
A Dash-9 will start pulling from a standing start at lower amps than any of the USA locos we own. Once they get enough juice, though, four-axle USA Geeps are capable of handling almost, but not quite, as much weight as a Dash-9 up that steep test slope.
These experiments provided hours of fun. For me, seeing what these babies can do is one of the most interesting aspects of the hobby.