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what is the expected pulling power of a loco?

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Posted by mvlandsw on Monday, January 13, 2020 2:30 AM

ATLANTIC CENTRAL
 
mvlandsw

ATLANTIC CENTRAL wrote the following post 2 hours ago:

"Prototype railroads did lots of testing and calculations, but at the end of the day, they just needed to know how much tonnage a given loco could move (and stop/hold still) over the ruling grades of a given section of the line."

Almost all of the "holding still" is done by the air brakes on prototype cars. I did hear of a modeler who ran long trains of cars loaded with real iron ore up steep grades using rear end helper locomotives. After the helpers were cut off if the train went down grade too fast it could not be stopped. Even with the power shut of the weight of the train would slide the locomotives down the grade.

Mark Vinski

 

 

 

Not exactly, read this:

http://hm.evilgeniustech.com/alkrug.vcn.com/rrfacts/hp_te.htm

Sheldon

 

From the linked article " Therefore these locomotives, with their brakes set, will hold the train on the hill "

This assumes that the locomotive brakes can keep the wheels from turning. My experience is that they cannot be depended upon to do that. Then as they roll along the rails they find rail conditions where the adhesion is lower and they begin to slide.

Mark Vinski

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Posted by doctorwayne on Saturday, January 11, 2020 2:40 PM

Thanks for posting that link, Sheldon.  I've always enjoyed the way Al explains things so that even I can understand it.

Wayne

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Posted by ATLANTIC CENTRAL on Saturday, January 11, 2020 9:47 AM

mvlandsw

ATLANTIC CENTRAL wrote the following post 2 hours ago:

"Prototype railroads did lots of testing and calculations, but at the end of the day, they just needed to know how much tonnage a given loco could move (and stop/hold still) over the ruling grades of a given section of the line."

Almost all of the "holding still" is done by the air brakes on prototype cars. I did hear of a modeler who ran long trains of cars loaded with real iron ore up steep grades using rear end helper locomotives. After the helpers were cut off if the train went down grade too fast it could not be stopped. Even with the power shut of the weight of the train would slide the locomotives down the grade.

Mark Vinski

 

Not exactly, read this:

http://hm.evilgeniustech.com/alkrug.vcn.com/rrfacts/hp_te.htm

Sheldon

    

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Posted by gregc on Saturday, January 11, 2020 5:42 AM

SeeYou190
I have a terribly hard time believing any mathmatical formula that ends with basically a 12 ounce locomotive should pull X many cars.

while nothing is exact ("~"), expectations should be within limits and variables that matter understood.

balanced weight determines pulling power of model locos just as it does prototypes.   size (length) and number of drivers do not.   Weight can be added.

the resistance (oz) for a # of cars also depends on weight and wheel friction.  Wheel friction varies but can be improved.   Observing that a loco cannot pull an "expected number of cars" suggests upgrading to metal wheels or making the effort to check the rollability of the trucks (simple incline) and tune them.

 

SeeYou190
I guess that is why Model Railroader actually measures the pull of every locomotve they review, rather than just weigh it.

not every modeler has an appropriate gauge to measure pull strength, but it's easy to weigh a loco and estimate the pulling power (i.e. min 20%)

 

SeeYou190
I'll bet that if I take two locomotives that are mathmatically indentical, load them up with cars, and send them up a 3% S-curved grade made with Kato Unitrack 28" radius sections that they WILL NOT both max out at the same number of 4 ounce cars all with indentical Kadee trucks.

sure.   but how much of a difference, 1 or 2 cars, not 10.

greg - Philadelphia & Reading / Reading

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Posted by SeeYou190 on Friday, January 10, 2020 4:22 PM

I have a terribly hard time believing any mathmatical formula that ends with basically a 12 ounce locomotive should pull X many cars.

.

There are too many other variables.

.

Quality of the gear train, center or gravity, are the trucks sprung and equalized, are the trucks balanced, what material are the wheels made of, gear ratio, bearing slop, worm end-play, and so on, and so on, and so on.

.

I guess that is why Model Railroader actually measures the pull of every locomotve they review, rather than just weigh it.

.

I'll bet that if I take two locomotives that are mathmatically indentical, load them up with cars, and send them up a 3% S-curved grade made with Kato Unitrack 28" radius sections that they WILL NOT both max out at the same number of 4 ounce cars all with indentical Kadee trucks.

.

-Kevin

.

.

-Kevin

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Posted by gregc on Friday, January 10, 2020 3:51 PM

rather hastely measured a pull of 2.4 oz  for 30 cars ~4 oz each, total 120 oz.   comparable to going up a 2% grade.   still expected it to be a bit less.

if max TE is ~20% of weight, a 12oz loco should be able to pull this train.

greg - Philadelphia & Reading / Reading

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Posted by gregc on Wednesday, January 8, 2020 4:03 PM

thanks for the data

20% of 4.7 oz loco wt is a Tractive effort of 0.94 oz

8 2.5 oz cars is 20 oz.

that the equivalent of a 4.7% grade (0.94/20). 

I was expecting < 2%

 

greg - Philadelphia & Reading / Reading

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Posted by Wolf359 on Wednesday, January 8, 2020 2:52 PM

gregc

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

 

I don't know if it would be considered a long string or not, but I did a pull test on a Bachmann 0-6-0-T Porter side-tank engine not that long ago after reading a review of that model. The locomotive weighs 4.7 oz., and was able to pull 7 40' quadhoppers weighing 2.5 oz. apiece on fairly level track at about 30 scale mph without any wheel-slip. At 8 cars, it started to slip. I should also add that the quadhoppers still have stock plastic trucks and wheels. For such a small engine, I'd say it has pretty good pulling power.

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Posted by mvlandsw on Wednesday, January 8, 2020 2:39 PM

RR_Mel

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 power supply for wheel the wheel slip measurements.
 
I’m open for suggestions if the above isn’t correct.
 
 
Mel
 
 
 
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I'm beginning to realize that aging is not for wimps.
 
 
 
 

My Kadee gage does not slide along the rails very freely. It seems like it would affect the scale reading. It might be better to hook the wire directly to the locomotive coupler.

Mark Vinski

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Posted by mvlandsw on Wednesday, January 8, 2020 2:28 PM

ATLANTIC CENTRAL wrote the following post 2 hours ago:

"Prototype railroads did lots of testing and calculations, but at the end of the day, they just needed to know how much tonnage a given loco could move (and stop/hold still) over the ruling grades of a given section of the line."

Almost all of the "holding still" is done by the air brakes on prototype cars. I did hear of a modeler who ran long trains of cars loaded with real iron ore up steep grades using rear end helper locomotives. After the helpers were cut off if the train went down grade too fast it could not be stopped. Even with the power shut of the weight of the train would slide the locomotives down the grade.

Mark Vinski

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Posted by ATLANTIC CENTRAL on Wednesday, January 8, 2020 11:50 AM

Prototype railroads did lots of testing and calculations, but at the end of the day, they just needed to know how much tonnage a given loco could move (and stop/hold still) over the ruling grades of a given section of the line.

That is my goal. Once the new layout is operating I will do tests with the piggyback cars, which are for the most part my heaviest rolling stock, and set tonnage ratings for selecting motive power for trains.

The layout plan stores about 30 trains in staging, most being able to be 35 to 50 cars long. 

Most will require two steam locos or three to four diesels.

Sheldon.

    

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Posted by gregc on Wednesday, January 8, 2020 10:10 AM

ATLANTIC CENTRAL
rolling resistance can very greatly

i agree

ATLANTIC CENTRAL
This combination rolls as freely or more freely than any rigid plastic truck/wheel combo I have tested. But it works better on heavier cars. Meaning that as the cars get heavier, the rolling resistance does not increase in proportion to the weight.

in the link from above

 

SeeYou190
What about gear ratio? Does that effect pulling power of a diesel?

i'm guessing the gearing may increase TE, but weight limits max TE

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Posted by SeeYou190 on Wednesday, January 8, 2020 9:50 AM

What about gear ratio? Does that effect pulling power of a diesel?

.

There were "Super Gearing" kits available for Athearn Diesels that made the run very slow and was supposed to make them better pullers.

.

How does this factor in?

.

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Posted by ATLANTIC CENTRAL on Wednesday, January 8, 2020 8:47 AM

gregc

 

 
ATLANTIC CENTRAL
There is much more to this than engine weight or a single value for rolling resistance.

 

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.

 

I understand. My work with free rolling trucks has shown me that rolling resistance can very greatly, and is not automatically linear in relation to car weight.

I prefer sprung metal trucks, I use mostly Kadee trucks refitted with Intemountain code .110 wheels. 

This combination rolls as freely or more freely than any rigid plastic truck/wheel combo I have tested. But it works better on heavier cars. Meaning that as the cars get heavier, the rolling resistance does not increase in proportion to the weight.

But pulling long trains, I find the lower center of gravity with metal trucks, and the equalizing, to provide much better tracking.

Sheldon

    

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Posted by gregc on Wednesday, January 8, 2020 7:54 AM

ATLANTIC CENTRAL
There is much more to this than engine weight or a single value for rolling resistance.

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.

greg - Philadelphia & Reading / Reading

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Posted by ATLANTIC CENTRAL on Wednesday, January 8, 2020 7:45 AM

gregc

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

 

 
gregc
worst truck would roll freely on a 2% grade.   some rolled uphill :).

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

 

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

 

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

    

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Posted by mbinsewi on Wednesday, January 8, 2020 6:32 AM

  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.

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Posted by SeeYou190 on Wednesday, January 8, 2020 6:10 AM

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.

.

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

.

Good enough for me.

.

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Posted by BRAKIE on Wednesday, January 8, 2020 5:10 AM

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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Posted by gregc on Wednesday, January 8, 2020 4:14 AM

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

gregc
worst truck would roll freely on a 2% grade.   some rolled uphill :).

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

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

greg - Philadelphia & Reading / Reading

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Posted by dstarr on Wednesday, January 8, 2020 12:42 AM

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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Posted by Bayfield Transfer Railway on Wednesday, January 8, 2020 12:33 AM

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.

Disclaimer:  This post may contain humor, sarcasm, and/or flatulence.

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Posted by doctorwayne on Wednesday, January 8, 2020 12:15 AM

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 Bachmann Consolidations for some or all of the diesels.  Every combination was able to reach the top of the grade, but there was nowhere to go beyond that.

I did learn that two of those switchers or two of the Bachmann and Athearn steamers (and various duos of each) can handle 12 loaded hoppers and a caboose up both of the 2.8% grades.  The upper grade is about 45' long, and features two horseshoe-type curves in opposite directions, plus several other curves just before the top of the grade.  Weight of the trailing train in this instance is only 100oz.

Using two such steamers on such a short train is not likely all that realistic, but I have an unmodelled coal-fired power plant on the upper level, and that coal traffic is part of the operations.  I could, I suppose, use unmodelled coal in the hoppers, too, but I like the idea of the challenge.

Most regular freights will be between 12 and 20 cars long, adjusted somewhat if there are too many heavy cars - I have a lot of gondolas with "live" loads, and many of them are in excess of 10oz.

This "doodlebug", with only the front truck powered, can easily handle 19 cars on level, albeit mostly curved, track...

In normal service, a coach or two and maybe a freight car should look believeable, though.

I don't know if this  old video will still work.  This is DC operation, and while the video, if it works, has sound, it's that of electrically-powered models, along with some wheelslip and other noise.

I checked the video in "Preview" and it works for me.  If you click on the link, please let me know it it works.  If it doesn't, I'll remove it, as I don't wish to waste anybody's time.

Wayne

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Posted by dbduck on Tuesday, January 7, 2020 11:43 PM

gregc

 they vary  but most between 15-18 ounces

 
dbduck
locomotives will easily pull up to 30 free rolling

 

loco weight?

 

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Posted by BigDaddy on Tuesday, January 7, 2020 11:22 PM

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. 

Henry

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Posted by RR_Mel on Tuesday, January 7, 2020 11:04 PM

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 power supply for wheel the wheel slip measurements.
 
I’m open for suggestions if the above isn’t correct.
 
 
Mel
 
 
 
My Model Railroad   
 
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I'm beginning to realize that aging is not for wimps.
 
 
 
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Posted by ATLANTIC CENTRAL on Tuesday, January 7, 2020 9:49 PM
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 Products .110 metal wheel sets installed in the Kadee trucks instantly produced a truck that is better tracking and freer rolling than anything else tested. This combination is on average 15%-20% more free rolling than even the best of the others. And is a 300% improvement over the stock Kadee wheel sets.
 
Early pulling power tests show 50% to 100% increases in the number of cars certain locos will pull.
 
Most of this testing has been done with a large fleet of identical Athearn 50' piggyback flat cars, each weighing 4.3 oz. Results with lighter cars such as 34' hoppers are expected to produce similar results.
 
Early tests on passenger equipment appear just as promising.
 
Anyone interested in more details can e-mail me directly or postquestions or comments here.
 
3/35/06 – update – reaming Athearn plastic side frames and using Athearn RTR metal wheels provides performance that nearly equals the intermountain wheel sets in the Kadee trucks.
 
With minor modification the Kadee brake shoe assembly can be added to Athearn trucks.
 
36” Intermountain wheels work well in the MDC challenger passenger trucks and in the Athearn 6 wheel Pullman trucks. The rolling test results with the ConCor and Athearn cars where similar to the 50’ piggy back flats in both before w/Kadee wheel sets and after with intermountain wheel sets.
 
Ultimately, to reduce side play, all my MDC and Athearn plastic passenger trucks received the newer Athearn metal wheelsets resulting in even lower rolling resistance.
 
STEAM LOCO PULLING TESTS
 
Level track – 50’ Athearn piggyback cars, Kadee trucks, Intermountain wheels, 4.3 oz ea.
 
Number of cars pulled with a no slip start.
 
BLI                 2-6-6-4            75 cars (Bob’s layout – not controlled test – 1.8% grades)
Bachmann       2-8-0                14 (23) cars*                           REVISED 18 CARS  80oz
Proto 2000      2-8-8-2            40 cars                                     REVISED 32 CARS
PCM                4-8-4 T-1         40 cars (no traction tires)
PCM                4-8-4 T-1         limit not yet found with traction tires
Bachmann       2-6-6-2            22 (36) cars*                           REVISED 29 CARS
Bachmann       4-8-2 H            38 cars                                     REVISED 30 CARS
Athearn           2-8-2                35 cars (weighted version 2.5oz above stock)
Athearn           2-8-2                26 cars (stock)
Proto 2000      0-8-0                40+ cars (new traction tire version)
Bachmann       4-6-0 (63”)      17 cars                                     REVISED 13 CARS
Mantua            4-4-2                17 cars (weighted 2oz)
Mantua            0-6-0T             15 cars
Mantua            4-4-0 Gen        36 cars
Mantua            4-6-2                                                                REVISED 22 CARS                         
BLI                 2-8-2 lt            18 cars (Bobs layout) 1.8% grades
 
 
TEST DATA IS SUSPECT - LAYOUT SECTION MAY HAVE HAD AN UNNOTICED DOWN GRADE! RETESTED RESULTS NOTED BY * OLD RESULTS IN (X)
 
ALSO, * TESTS DONE IN WARM WEATHER - IS THIS A FACTOR?
 
MYSTERY SOLVED, SLIGHT GRADE ON TEST TRACK - 0.15% +/-.
 
NEWER TESTS CONDUCTED IN OPPOSITE DIRECTION. 40% DIFFERENCE SEEMS CONSISTENT; ADJUST ALL FINDINGS BY 20%.
 
AMAZING - CARS ARE SO FREE ROLLING THIS SMALL GRADE HAS A 20% EFFECT ON CAPACITY.
 
CARS ARE SO FREE ROLLING THAT CAPACITY CAN BE MEASURED IN TRAIN WEIGHT - SUM OF CAR WEIGHTS IS ROUGHLY EQUAL REGARDLESS OF CAR TYPE.
 
One Bachmann 2-8-0 up .5% grade =
18 X 4.3oz (piggyback flats) = 77.4oz   OR   34 X 2.3oz (empty hoppers) = 78.2 oz.
 
MORE DATA BASED ON WEIGHT - FURTHER TESTING WITH HOPPERS (2.4oz) AND 2-8-0 = 2-8-0 ON .5% GRADE PULLED 40 HOPPERS OR 100oz. ON 2% GRADE CAPACITY WAS REDUCED TO 12 HOPPERS OR 30oz. THIS IS EQUAL TO 7 PIGGYBACK CARS WHICH IS THE CAPACITY OF THE 2-8-0 IN THOSE TESTS.
 
CONCLUSION - CARS ARE SO FREE ROLLING THAT WEIGHT AND TRACTION ARE THE DETERMINING FACTOR IN CAPACITY, ON LEVEL OR ON THE GRADE.
 
ALSO - CAPACITY ON GRADES SHOULD BE EASILY MEASURED BY JOHN ARMSTRONG’S CHART.
 
34 X 2.3 OZ (EMPTY HOPPERS) - WHICH SHOULD ALSO EQUAL 25 AVERAGE 40’ BOX CARS
 
HIGH WEIGHT OF PIGGYBACKS MAY BE AFFECTING THE TRUCK PERFORMANCE - TEST LUBRICATION ON THESE HEAVY CARS TO SEE ANY IMPROVEMENT CAN BE NOTED. THIS SHOULD STILL BE TESTED BUT IS UNLIKELY.
 
Actually, lubrication did make a big difference.
All Intermountain wheel sets now installed in Kadee sprung metal trucks are lubed with light oil which seems to soak into the cast sideframe like an oilite bearing and causes no dirt or operational problems.
 
Sheldon
 
 
 

    

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Posted by ATLANTIC CENTRAL on Tuesday, January 7, 2020 9:38 PM

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 resistance (see above)

 Answer: N = 6 cars (5.955)


Use actual drawbar pull figure in place of L*E
L*E is a way to estimate drawbar pull

More accurate equation
L*E = (C*N*(G+E+R))+(L*(G+R))

L is  Locomotive weight in ozs.
E is  Rolling resistance factor of 0.02
C is Car weight in ozs..
N is  Number of cars
G is  % track grade
R is  curve resistance determined by 0.3/r  where r is curve radius in inches

12*0.20=(4*N*(0.03+0.02+0.017)) +(12*(0.03+0.017))

N=  7 cars (6.851)

(this information provided by another modeler known to some as "Hunt")

Sheldon

    

  • Member since
    September 2014
  • From: 10,430’ (3,179 m)
  • 2,310 posts
Posted by jjdamnit on Tuesday, January 7, 2020 4:54 PM

Hello All,

Then there is this thread...

Prototyical horsepower versus actual pulling power of modlels

Hope this helps.

"Uhh...I didn’t know it was 'impossible' I just made it work...sorry"

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