A thought experiment on car weights

Model Railroader Prototype information for the modeler
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OK I know the NMRA has their reccomended weights for various rolling stock, I have heard more than a few times that model ocos are able to pull more cars than their protortype. I’m looking to do a thought experiment on what would give more accurate “trailing tonnage” for our models.

Mind you I’m not looking to turn the reccomended practice on its head, it works well and for most modelers its just fine. But I feel that the RP may be out of date considering the quality of models today vs from the 50’s and 60’s.

Lets be honest, the typical diesel loco in those days had a single powered truck. Many steam models had the drive in the tender. Most were very lightly weighted. All add up to poor pulling power.

Todays diesels are pretty much all wheel drive and tender drive steam is pretty much non existant. Newer locos simply ahve more pulling power than they used to.

Im curious as to how prototype loco weights compare to prottype trailing tonnage and how that can be “scaled down” to modeling. I know theres been many discussions on how weight doesn’t scale very well, and I belive there will never be an acceptable answer to that question. So i’m thinking of more proportional weights.

I think and example of what I’m thinking would give the clearest explanation, so:

lets say a 4 axle diesel weighs 150 tons, and can pull a trailing tonnage of 1500 tons. Thats 10 to 1.

Lets say a HO model of that loco weighs 15 ounces, how close to 150 ounces of trailing tonnage does it pull? Are we still close to 10 to1?

The flip side to this becomes car weights, so a loaded 50 ton hopper weighs 50 tons. If the loco math stays the same (10 tons equals 1 ounce) then a loaded model should weigh 5 ounces.

And of course the flip side to loaded cars is empties. What does an empty 50 ton hopper weigh? What should its model weigh?

While the weights of locos (pr

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When I was working on this same issue awhile back I found a site with very helpful info.

http://www.republiclocomotive.com/locomotive-power-calculations.html

Mel

Modeling the early to mid 1950s SP in HO scale since 1951

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

Bakersfield, California
&

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The purpose of NMRA RP 20.1 Car Weights, is keeping trains on the track, not scaling model weight to match prototype weight. It is well known that a light car[s] inbetween heavier cars can get pulled off the track on curves (stringlining). The solution is to weight all the cars about the same. At the time RP-20.1 was published models contained a good deal of die cast metal and were in general, heavier than modern molded plastic models. It being easier to weight up light cars than to lighten heavy cars, the standard was set a bit heavier than it might otherwise have been. And since model rolling stock lasts a long time (I run some cars that are 50 years old) nobody wants to change the standard.

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Dstarr, like I said, I’m not looking to overrun the RP, just looking atthe situation from a different angle. And if the RP works for you go for it. And yes I know the RP is more to help tracking, hence I pointed out that follwing this thought would make train makeup a more serious consideration for those who choose to go this route instead.

Your point about older cars containg metal bodies and frames (die cast or brass or lead) and newer ones being plastic emphasises my point that RP may be out of date.

Yes the RP works, I’m just considering an alternative.

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I know people that add weight to NMRA and others who have done heavey cars or run as they come. All work fine if the track work is solid (of course one would limit some combos on the as they come bunch. You are right that things have changed but not for everyone, most have tighter turns and steaper grades than real life (mine are 18" radius curves and 4% grade up to one yard with an even steeper grade going back down to the mainline and my layout is 15’x30’ but this section was oringinally loging the rest of the layout being flat with elevation features carved in or built up).

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mel, interesting link. Gives me an idea about looking up tractive effort for a loco and working back to trailing tonnage.

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Hmm OK this got moved.

RRebell, of course models run much tighter curves that the prototype, we just don’t have the space for what would be realistic. But we do tend to follow the prototype in regards to grades.

Yes our tight curves increase the drag on the train and the risk of stringlines. Thats what the RP is for. And I realize that making a wider spread in car weights will amplify the situation. This adds to the challenge that operations types wish to emulate.

Im not saying its a good idea or bad idea, that would be up to the individual modeler. I’m just exploring a possible way to emulate the real world.

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I agree that a layout should be run with some thought to what sort of power a “real” railroad would assign to the train lengths we feature, but the problem is that as a rule our trains tend to be so short that most railroads would hold it in the yard until more cars can be added. As a rule model trains over overpowered, steam or diesel.

Also, having “been there” I cannot totally agree with the OP’s basic premises about model railroad history. There were plenty of diesel models in the 50s and 60s that had all axle drive, thinking back to the Varney F3 that was a “classic” on many layouts, weighed a ton, and pulled like the devil. Ditto for Hobbytown of Boston, whether with their own heavy metal shells, Cary metal shells, or Athearn plastic shells. Even the Athearn rubber band drive F7 was all axle powered, and their gear driven line was all-axle. Penn Line had a low end F unit with one truck powered - but it had a huge lead weight over that truck and pulled rather well. Yes many of the AHM trainset quality diesels had one powered truck (with traction tires but not much weight) but then they were intended to pull AHM trainset quality (and thus uniformly weighted) freight cars on small oval layouts featuring fairly short trains. As for steam locomotives, apart from Mantua’s General, and some of the Rivarossi/AHM smaller engines (and for a brief while, their IHB 0-8-0), tender drive was fairly rare as I recall. I guess Fleischmann had tender drive for some stuff too.

And as far as rolling quality of trucks goes, I am not aware of any current trucks on the market that roll measurably better than the Lindbergh trucks circa 1962. Central Valley trucks rolled well too. Red Ball trucks … eh, not so well.

And then let’s not forget that a prototype locomotive would likely be unable to pull ANY cars at realistic speeds through the equivalent of our model railroad c

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That is basically what I did. I’m one that super weights my rolling stock. All my normal running locomotives are heavy weights, many with well over 5 ounce drawbar. Several of my Cab Forwards & AC-9s have added weight and dual Canon EN22 motors.

I also have a pair of 3½% grades on my mountainous layout. All of my yard turnouts are 18” Atlas with my mainline minimum of 28” radius. My heaviest cars are my Athearn 72’ Daylight passengers at 6 to 8 ounces each. My double E7A/B locomotives weigh in at 2½ pounds each with a total drawbar of just under 10 ounces and easily handle my 3½% grades towing 10 well over weight cars.

Mel

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Actually, this isn’t the case. The laws of scaling mean that model locos aren’t are heavy relative to size as the 1:1.

Yes, there are other factors at work, like MUing diesels, sharp curves, etc. But the fundamental fact is are locos are nowhere near as heavy as they would be if weight/mass scaled the same way that dimensions do.

No, you’re not stuck with the NMRA’s recommendation, but it’s a very good thing to understand it and keep in mind. Narrowgaugers often fudge things by going lighter than NMRA recommendations, but you MUST be consistent still to make this work, as several have already observed.

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on the one hand, it’s not how heavy the car is, but how much force is needed to make it move and on level ground this means overcoming friction

Years ago I though I read it is 8 lbs per ton for a typcial freight car. A 50 ton hopper would require 400 lbs of force to overcome friction.

The excess force, that greater than friction, then accellerates the car up to speed.

I happen to know that a Reading I-10 2-8-0 has a tractive force of 55,000 lbs. Dividing this by 8 lbs is 6800 tons or roughy 100 70 ton cars. But this leave nothing for grades.

on the other hand is that when there is a grade, a fraction of the weight of the car must also be be overcome. If I did the trig correcrly, a 1% grade requires an additional 35 lb per ton. Roughly double that for 2%.

This topic came up last year Prototypical horsepower versus actual pulling power of models

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Let’s look at the real reason…Cars at one tine was made from wood with paper sides or printed on road names these cars did not have any weight other then the trucks and the trucks rolled like the brakes was set…The NMRA worked their math and decided RP20.1 would work.Varney cars was light as well since there was no added weight. Walthers freight and passenger car kits came without weight.Same for the Hobbyline plastic car kits no weights included. We needed RP20.1 back then but,it never was widely received by the modelers of that time…

Today its outdated due to the various modern car lengths-some long cars will be much heavier then a 50 boxcar.Then we have stacks and the front runners and 89’ piggy back cars.

Be that as it may we still have RP20.1 as a guide line-common sense should come into play when weighing our modern cars.

As far as they famous cry of “stringlining” the only time I’ve seen that was when a super long 50 plus car train was ran around sharp 24" curves on a 2% grade. The average 20-30 car trains had no issues traversing this area on the club’s layout… I been told that section of the layout has been redone since I was a member several years ago.

I have read on var

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All depended on brand.Brass diesels had 8 wheel drive and steam used all of its drivers for pulling.

The popular Hobbtown drive used a 8 wheel drive and Athearn did as well on their earilest diesels-these was available along side of their Hi Fi drives.

There was another brand that used a 8 wheel drive-Revelle?

Brass wheels on brass track seem to pull more then nickel silver wheels on nickel silver track.

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Mike, I’m not so much trying to scale the weight, keeping the weight of the locomotive as a percentage of entire train weight is what I’m curious about.

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Dave, good background information. Thanks for the history. And your point of mismatched car weights is noted and expected. Hence what may be a nightmare for those who just want to run trains, may be an enjoyable challenge to others.

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Disclaimer - I have not read all of the replies above.

A few simple points:

Our models have dramaticly different factors of adheasion and coefficients of friction - a direct comparison cannot be made.

I started in this hobby in 1967, and all the diesel locos I had were all wheel drive, Varney, Athearn, etc - but admittedly I did not have TYCO train set stuff…

Having worked in the model train business as far back as 1970, very few steam locos ever had tender drive…

Most of my rolling stock is slightly lighter then NMRA RP, but most also have metal sprung/equalized trucks with metal wheels/axles, keeping weight down low for better tracking.

Generally, model diesels will equal or out pull their prototypes, but the reverse is generally true with model steam, even after adding weight, etc - brass, die cast or otherwise. Again issues of adheasion and friction not the same.

I pull long trains, 40, 50 cars and more no troubles. Two steamers or three and four unit diesels, large curves, etc…

My cars are generally very free rolling - most have Kadee sprung metal trucks refitted with Intermountain wheelsets, a combination which in my testing out rolls any rigid frame plastic truck and tracks better.

I saw a 30% increase in the pulling power of my locos with the implimentation of that as my standard truck for most freight equipment.

And again, most of my cars are weighted to about 80% of the NMRA RP.

Sheldon

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Greg. Similar information in the link Mel provided. But I’m leaving out grades and curve radii as that varies by individual layout. One layout with no grade and 60 inch curves may allow a gp9 to pull 40 cars. That same loco on an 18" and 3% layout may only pull 6. That’s up to the owner to determine.

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O I’m replying to myself, but … Is there a correlation between the MRR drawbar pull and tractive effort of the prototype? Has anyone checked into this?

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i doubt manufacturers are scaling the motor in their models to match tractive force of protoypes which also depends on weight on drivers.

the same DC motor is often used in an 0-4-0 as well as a 2-8-8-2

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An interesting observation. But does it matter? In my experience, lmodel locos tend to slip rather than stall. Have you experienced stalls rather than slip?