I’m curious about the difference in adhesion or the tractive effort created between a GP and an SD. The point of confusion for me is… if you have a GP and an SD of identical weight and horsepower, which can pull more. Intuitively I’d guess the SD because is has more wheels making contact w/ the rail, however the weight is divided amongst more wheels, so is there actaually a gain? … or does the GP have a benefit by having more weight concentrated on each of its fewer wheels?
I’m guessing the answer is “it depends.”
Clearly you see more SD’s for heavy lugging, but then again, they probably don’t follow the premise above of, " a GP and an SD of identical weight…" I’m sure the SD’s are much heavier (i.e. the CSX super-heavy coal luggers they’ve developed w/ GE and EMD). The SD’s create greater traction, but is it more a result of greater weight or as a result of the added axles.
Normally, most SD weigh about 50% more than GP’s, so you are talking about unit reduction in most cases. Weight on drivers is only a real factor in calculated Stating TE. A more critical value is the TE generated at the ‘minimum continous speed’. Most GP’s will have a speed of 12-13 mph, and SD’s will have a value of 7-8 mph - This means they can lug at lower speed without burning up the electrical system. With AC motors, this has not been a big factor anymore, and the computerized wheel slip systems haved improved engine performance.
Comparing a GP40-2 with an equal-weight SD40-2 (if it were possible for the SD to be that light) the SD40-2 should pull somewhat more. A 62:15 geared GP40-2 has a continuous TE around 55000 lb, which will often (usually?) be the limiting factor for it; the SD40-2 is rated 50% higher by that criterion, so if it only weighs 135 or 140 tons (like the GP) it will be limited only by adhesion. Comparing a GP50/60 with an equal-weight SD50/60, the SD advantage won’t be as great.
I know a couple of GP’s that will pull. How bout the only 2 GP26’s ever built. SW 2601 and SW 2602-567 power, DASH 2 elec. GE752A traction motors, and those rough riding ALCO trucks, and you have a sweet combo. On 4-5-08, I had these 2 plus a GP30-CVR 3004 which was DIC, for a total of 44 loads weighing 5,720 tons up a 2.5-2.7% grade. Pulled it at 8 mph holding between 1450 and 1500 amps. The picture below is this pull up the hill east of Ulysses, KS. Our track speed is 10 mph. And on another occasion, I had just the 2602-41 loads and 11 empties,for a total of 4,660 tons and I pulled a 1.5% grade @ 5 mph.
Just as another tidbit, we at the railroad have nicknamed these 2-The Clydesdales, the traction motors are geared @ 74:18. But they do alot of fuel, I have spent 125 gallons on the 2601 in just 25 miles, that may not sound like alot but when you are 10 mph it doesn’t make a lot of sense. It kind of blew me away when I saw that it used that much fuel. But they are worth it.