what is the expected pulling power of a loco?

Model Railroader General Discussion (Model Railroader)
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on another forum, someone asked what locos pull more than expected? what’s expected?

i estimated that the pulling power of a loco is 20-25% of its weight and that the effect of gravity going up a 1% grade is approx that percentage of a car’s weigh – the grade requires 0.04 oz to pull a 6" car weighing 4 oz.

so 50 4oz cars adds 2 oz of resistance to be pulled up 1% grade which is 20% of a 10 oz loco and the # of cars the OP on the other forum said his loco could pull

i’ve gone thru my trucks, measured the grade they roll freely on and after tuning the worst truck would roll freely on a 2% grade. some rolled uphill :). But Armstrong’s chart indicates the full cars roll more freely than empty cars and I would expect better performance with a weight car body.

is 1% of the weight an accurate estimate of the resistance of model cars on level ground?

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I’m just finishing up four custom E7s, Cary bodies on Athearn SD40-2 frames with dual Mabuchi SF266 motors. All four weigh in at 35 ounces and the drawbar at wheel slip is 10 ounces each on all four locomotives.

About 28%.

All four draw 630ma each at 12 volts at wheel slip.

Mel

My Model Railroad
http://melvineperry.blogspot.com/

3

I have always had small layouts in HO scale, longest trains are 12 cars, no grades anywhere, dead flat.

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I expect all locomotives to pull a 12 car train, and so far, all have been able to do it.

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A double headed pair of powered Stewart F3s can simply walk away with a 12 car train with ease. I am satisfied.

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I think each person’s expectations will be different.

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-Kevin

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Not familar with all the math & physics involved…but I do know that most, if not all, of my HO Athearn & Atlas 6 axle locomotives will easily pull up to 30 free rolling , truck tuned, properly weighted cars by themselves on level grade. Meaning most single loco trains are about 18-24+ ft long depending on car type…auto racks, 50’ box etc

These are straight out of box locos with no modifications or added weight

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loco weight?

6

Hello All,

Then there is this thread…

Prototyical horsepower versus actual pulling power of modlels

Hope this helps.

7

Here is some info to start with, I will post some more in a bit:

Model railroad track elevation and drawbar pull.

Caveats, If you believe you are mathematically challenged… consider stop reading now.
Following uses information based on testing by others and my own testing. Your results may vary depending on the controlling factors of your test environment.

1 inch of elevation to flat run of 100 inches of track = 1.0% (0.01) grade.

Rolling Resistance
Rolling resistance for real railroads is about 0.2%, for model railroads it is more like 2.0% i.e. a model railroad car will roll down a 2% grade on its own. This factor must be taken into consideration when calculating tonnage ratings.

Curve Resistance
Another factor is curve resistance, which is .04% per degree of curve for the real railroads and about the same for model RRs. The big difference is model RR curves are much sharper and this resistance is a very big factor. It is equivalent to around a 30%/R where R = curve radius in inches. An 18-inch radius curve will produce about 1.7% additional resistance. Most mountain model RRs are all curves, so compensating for them usually isn’t worth the time, but if you have to cheat on the ruling grade, do it on tangent track.

Locomotive Traction
An all-metal wheel model locomotive will have about a 20% adhesion factor just like the real ones. This means 20% of the locomotive weight can be turned into pulling force (locomotive weight * 20% = estimated Drawbar pull).

So how many 4-ounce cars will a 12-ounce loco pull up a 3% grade with 18" radius curves?

The equation 12 * 0.20 = ((4*N)+12) * (.03 + .02+ .017)
where 0.03 is the grade, 0.02 is the rolling resistance, and 0.017 is the curve res

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My own pulling power tests and observations from a few years ago:

Rolling Tests/Pulling Power Tests

I have recently done some testing of various wheel set and truck side frame combinations to improve the rolling qualities of my equipment. Up till now I had not concerned myself with this much and had installed Kadee sprung metal trucks on most of my freight cars for the following reasons:

Sprung/equalized design

Non magnetic axles

Improved appearance - brake shoes & rigging, metal color/texture

These trucks rolled OK based on older standards and tests but compared to most new offerings are they are very stiff.

In an effort to increase train length for specific steam locos I began my research.
I will not bore everyone with all the details - the following are the results and important observations:

Several tests on my layout and a friends layout lead me away from semi scale wheels like REBOXX. I use Atlas code 83 turnouts and while no derailments where experienced, I simple did not like the way looked or felt as they went through the frogs of these turnouts.

As advertised the REBOXX wheels did improve the rolling qualities of almost anything I put them in.

One principal goal was to see if the Kadee trucks could be made to roll as well as any of the following:

New Athearn RTR with metal wheels

Proto 2000

Any thing with REBOXX wheel sets installed

After trying reamers, lubrication and various replacement wheel sets in the Kadee trucks, most with little or no improvement, a magic bullet was found.

Intermountain Railway Produc

9

My degree is a EE so I have no idea if the way I measure the drawbar on my locomotives is accurate.

I bought a digital hanging scale that measures from 0 to 5 pounds in tenths of an ounce. I bought a calibrated weight set to make sure the meter is accurate and it is.

I replaced the hook to connect better to my trains from .032” Phosphor Bronze wire.

This is the way I connect the meter to a locomotive with Kadee couplers. The track is a 18” piece of Atlas code 83 track with wires soldered to the rails that plug into my bench power supply.

I crank up the voltage until wheel slip then use that reading on the digital meter for drawbar and the voltage and current on the po

10

James Wright the youtube guy measures loco pull with probably a micromark scale that looks like that. A “spring scale” could do the same.

The question was the “expected pull” That would depend on the loco weight, the quality of the motor, the number of driven wheels. Weight and wheels are obvious, though some locos have traction tires, so that implies they need extra friction. Motor quality must be determined empirically.

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I have read that the maximum pulling ability of most locomotives, model or full-size, is equal to about 25% of their weight, and some modified Athearn U-Boats verified that figure, almost exactly, in tests done at a nearby model railroad club.

Each of the three locos tested weighed 33oz., and each tested at a drawbar pull of 8.3oz., plus-or-minus a bit.

I don’t have much level track or straight track on the lower part of my layout, but I did do tests with most of my locomotives on the two most severe grades (2.8%). One is fairly short, about 15’ with two 34" radius curves in opposite directions.
This helped me to determine (very roughly, as I would consider the rolling qualities of my freight cars to be rather disparate) “tonnage ratings” for most locos.
I did test the U-Boats on that short grade, and was disappointed to see that one unit could not pull a 44 car hopper train completely up the grade, slipping its wheels about 6’ from the crest of the grade. Much of the trailing train was still on straight and level track.

I backed the train down the hill, and added another unit to the one used in the first run. The two units easily took the train up the hill, with no wheelslip whatsoever. I would have liked to have added more cars, but I had no more hoppers, and no “coal” to add to them either. I use Black Beauty blasting medium for Anthracite coal, so the trailing train’s weight was 22lbs.

When I backdated my layout, those diesels, along with most of my other ones, were sold.

I did another test with four remotored Athearn switchers (the SW7s, originally sold as SW1500s), and they were able to pull a train of 71 cars, with various rolling qualities, up the 2.8% grade to the second level (or at that time, where the second level was to be built).
I repeated that test several times, with the locos in various places within the train, and substituting modified Athearn Mikados and/or Bachma

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Most modelers own way more locomotives than they need for the number of cars they have. So you can always throw another engine on a train.

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All HO model locomotives that I have ever seen have enough motor power to spin the wheels. In that case, maximum pulling power is given by the coefficient of friction for metal on metal. Which is something in the order of 0.20 to 0.25. So the maximun pulling power of a locomotive, be it full size or a model, will be 0.20 to 0.25 times the locomotive’s weight.

To increase pulling power, you need to add all the weight that you can cram into the locomotive. Or you can increase the coefficient of friction with rubber traction tires or Bullfrog Snot. Coefficient of friction for rubber on metal is 0.75 to 0.80. Or you can do both, add weight and use traction tires.

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many have confirmed the pulling power of a loco is 20-30% its weight

has anyone done a pull test on a long string of cars?

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At the Bucyrus HO club the front layout had ridiculous steep grades yet four Athearn BB GP38-2s pulled a 36 car train around that layout which means while half the train was on the downside of the grade the other half was still on the upgrade side…

My cars weighed 3 oz and uses IM wheelsets. I usually ran 18 car trains with three Athearn BB GP38-2s simpily because the train look better on those ridiculious grades.

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17

The only test I have ever done for pulling was on a mock-up of the 5% grade with a 22"R curve at the top as proposed for my next layout to get the “local turn” into staging.

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A Walthers Proto FM Switcher will pull four cars and a caboose up that grade and around the curve with no problem.

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Good enough for me.

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-Kevin

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18

I have a small layout, the main line, about a 50’ run at most, with a 1 1/2" grade, my longest trains are 20 cars.

It’s an "L"shaped dog bone, with a 24"r. loop on each end.

Any of the 6 axles of have, Kato, and Genesis, all handle it well. Most start to creep at speed step 1. Spectrums, not quite.

4 axles, not quite, more like 12 or 15 cars.

I have plenty of power I can add on, if needed.

It is kinda fun to see how much a loco can pull, considering the curves I have on the layout, with the L shape.

Mike.

19

Long trains are a primary interest for me.

In the data I posted above:

Broadway N&W 2-6-6-4, 75 cars up 1.8% grades

And lots of other 30 plus car examples, many with my uniform set of piggyback cars.

Most of of my testing is with steam, since diesels all pull much better. All my four unit diesel lashup (typically F units or GP units) easily pull 100 cars up 2% grades.

Point of both my long posts? There is much more to this than engine weight or a single value for rolling resistance.

As an operator of long trains, typically 35 cars and longer, with many being 50 cars or more, knowing what it will take in motive power is important.

Sheldon

20

just interested in a reasonable measurement of train (>30) resistance on level straight track, an average.

i understand that curves and grades add to this.