### how do you figure out a diesel-electric’s tractive effort?

Sunday, November 4th, 2012*Question by DixieLiner*: how do you figure out a diesel-electric’s tractive effort?

I am building a model railroad with a ruling grade of 3 – 3.5%. Time is early fall 1979 (or late winter 1981). My roster consists of 3 GP9s outfitted with 567D3A prime movers rated 2500hp. 2 Alco PAs and 3 PBs outfitted with 1800hp 251 prime movers, and 3 to 10 SD35s. My trains will consist of 2-12 cars + caboose. I would like to run up to 45 car trains. Most traffic will consist of lumber/forestry products, automotive, and rock from the 3 online quarries.

I found an army site with some equasions

HPx30= starting Tractive Effort : HPx300/speed=TE

and No of Cars = TE/ [3+(20xruling Grade%)]xWeight of cars

Also included was tables for weight of car taking into accout friction and drag like a 70Ton car would be 105 in the equation and a 120T would be 160 Tons.

Does this sound right? or do I need to find a table somewhere with similar grades and use their TE tables or what? Thanks in advance for any help provided.

**Best answer:**

*Answer by HOGHEAD*

Rolling train resistence formula:

HPT x 12 / %G = S, or, horsepower per ton times 12 divided by the per cent of grade equals speed. Drag is figured into the equation and does not need a separate operation to arrive at.

I worked the Siskiyou Branch of SP’s old Oregon Division. Ruling grade was 3.1 per cent. Drawbar tonnage was 2700 tons max. Units were SD-40’s and 45’s and were rated for 900 tons per unit.

But, rule of thumb in real world railroading, about 25% of the locomotives weight is what will be developed for tractive effort. So, a loco that tips the scales at 400,000 lbs will develop at least 100,000 f/lbs torque. In actuality, an average 40 or 45 puts out 105,000 f/lbs. tractive effort.

I never met a GP-9 with 2500 hp. All I ever ran were 1800 hp. So, at 1800 hp each, your three Jeeps would handle around 1350 tons max, on your 3% grade. Your Alco’s would then handle 2250 tons on a 3% grade.

I’m running 70 car trains on my little outfit with needed helper power. I have found that if the power is split according to real world entrainment formuli for helpers, with the helper pulling 2/3s and shoving 1/3 of the total helper tonnage, the helper won’t push any cars ahead off the track.

You sound like a purist, but, weren’t the Alco PAs long scrapped before ’79? They tried using some newer obsolete Alcos (can’t remember the model) as yard engines in Roseville in the mid 70’s, but the rigid trucks, designed for main line rail, just wouldn’t stay on the rails in the yard. If you looked at them cross-eyed they’d fall off the track…..

But, they were nice on the road, rode well, and with the 16 notch throttle the power could be regulated better.

I think I have operators manuals for these critters around here some where. Drop me a line and I’ll try to get some more dope for ya………

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