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Locomotive Pulling Power Formula
Orjan is hoping someone can assist:
“A friend mentioned a formula I could use to calculate the pulling power of my OO trains, but he couldn’t remember the exact details of how it worked. He said it based on weights, number of cars, gradient percentage and some other factors he couldn’t recall. Does anyone out there know the details and how to use it? I would just like to know. Thank you.”
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Are we model railroading or splitting the atom here ?
It’s just how my brain operates. I like to know how and why things work or don’t. I believe understanding details helps me understand things better.
Isn’t just how many carriages you can pull. Your carriages can derail if you don’t make sure the heaviest carriage first and lightest last
Pulling force is at full throttle in ounces on level ground. This is divided by the the resistance of a 40′ freight car on level straight track. NMRA has a formula for the weight of this car. 1 ounce plus 1/2 ounce times the length in inches. 1 + 1/2 X 5.5″ = 3.75 ounces. MR Oct Atlas SD35 2.9 ounce drawbar pull. Equals 41 standard cars.. 2.9/41 = 0.07 ounce resistance per car. As they say “results will vary” Long passengers cars weigh more so fewer can be pulled. The quality of your rolling stock will greatly affect this. I have a lot of old cars that don’t roll as well as the standard test car. Now for reality. My layout is way to small for 41 car trains. 10 cars look right on my layout. Hills and curves can significantly reduce the number of cars pulled.
1) Test the traction of your locomotive:
Put the loco on a section of straight track on a table.
With small weights on a length of cotton over the table edge, see how much weight is required to move the loco. This is the force the loco can exert. It can be increased by weighting the loco.
(Just for fun, you can apply power to see what the loco will lift, but you will get about the same result.)
2) Use the basic formulas on inclined planes from your school physics textbook to translate this force into the weight of wagons that the loco can pull up a certain slope. In practice, there will also be frictional forces that reduce this, and curves on a gradient can increase this friction.