Scale Weight

[cjcrescent]

I have to agree with Larry about the RP20. People seem to forget, its NOT a standard. Standards in the NMRA begin with a "S". Recommended Practices begin with a "RP".

Many people seem to think that this is law and must be followed. It doesn't. I know several modelers that add no weight at all to their cars as they want the engineers on the trains to have to think about the problems that can be associated with a couple of real light cars, (empties), coupled between heavy,(loaded) cars or ones located at the head end of a heavy train.

I believe that John Allen also practiced this. (Andy S would know). I think I read somewhere that when he built a new car he didn't add any extra weight to it at all to bring it up to RP20.

[Pruitt]

QUOTE: Originally posted by edkowal



Mark:

I wanted to do a minimum of number crunching, and not too many examples, because most folks eyes just glaze over when you begin to throw numbers and equations around. Also, I was just too lazy to do the head-scratching necessary to come up with more than one example [:D].

Things like kinetic energy, power generated by motors, dynamics in general, or other quantities that depend on area, volume or other complex relationships don't scale in a linear fashion. That's why I said that you'd have to change the laws of physics to get our models to behave like the prototype does in the full size world.

While these posts may have gotten too theoretical for most, the original posting was actually a fairly theoretical question, so it would be hard to give a good answer without it.

-Ed

Ed -

It's been many years since I've done these sorts of calculations, and it took me well over an hour to compose the posting and calculate some of the data. Even so, when I was finished I almost cancelled the post, for the same reason you took pains to simplify your explanation. Then I thought "What the heck!" and posted it anyway. There might be some technically-minded folks out there who's eyes won't glaze over!

Cheers!

[tatans]

What a great bunch of informative responses to this scale weight forum, I must agree with all replies, these questions and responses are exactly what makes this site work, informative, intelligent along with a few laughs, and let us not take ourselves too seriously,-----keep trainin'

[edkowal]

QUOTE: Originally posted by Brunton

You're on the right track ([:D]) Ed, but it's more complex than that. The Kinetic Energy of the model is substantially less than that of the corresponding prototype...
...So, in a collision between a train and a brick wall, the kinetic energy of an exact-scale HO model is reduced from the prototype's kinetic energy by 87.1 (scale ratio) to the fifth power, or by roughly 5 billion times!

Meanwhile,stress in materials is calculated as force / cross-sectional area...
...Since stress does not scale, exact-scale HO parts made of the same material as the prototype are over seven thousand times stronger than the prototype. (It's somewhat more than that even, since the smaller a cross-section gets, the closer the material gets to its theoretical strength limit).

That probably got kinda deep for some folks, but the upshot is that even exactly-scaled parts are much stronger, proportionally, in the model than in real life.

I hope I didn't make a mistake in all that! [:o)]

Mark:

I wanted to do a minimum of number crunching, and not too many examples, because most folks eyes just glaze over when you begin to throw numbers and equations around. Also, I was just too lazy to do the head-scratching necessary to come up with more than one example [:D].

Things like kinetic energy, power generated by motors, dynamics in general, or other quantities that depend on area, volume or other complex relationships don't scale in a linear fashion. That's why I said that you'd have to change the laws of physics to get our models to behave like the prototype does in the full size world.

While these posts may have gotten too theoretical for most, the original posting was actually a fairly theoretical question, so it would be hard to give a good answer without it.

-Ed

[Marty Cozad]

I was chating with an Amtrak engineer friend who said the other day that they had to put a Mac 70 on the lead becuase they hit something and damaged the lead loco of the Amtrak unit, any way he said the Mac had the specs of just under 400,000 pds for the loco
Now the Aristo Dash-9 weights in around 23 pds, thats still a little under scale 400,000 pds; but the tractive effort is outstanding for its scale.
This scale weight could be helpful when we realize that many of our models won't pull anything near what the 1:1s do. Per loco

[BRAKIE]

QUOTE: Originally posted by tatans

"Scale" weight, very interesting question, just why is there a recommended weight for a car? with all the trouble people seem to have pulling a long string of cars(engine slippage) is this weight the actual "scale" weight of the prototype? would it not be practical to make the car as light as possible so the engine can pull more cars? adding weight defeats the efficiency of the locomotive and the slightest incline will stall an engine, the key would be to have a highly efficient motor, a LOT of weight in the loco and pull a long string of very light cars, would this solve the problem? forget "scale" for efficiency, [u]real locomotive are not the lightest equipment around, any comments??

First RP20.1 was needed when cars was made of wood and lack weight.Then the plastic cars was introduce and lack weight..Then Athearn introduce plastic car kits that included weight.
With todays cars that has free rolling trucks and weight added RP20.1 is has outdated as brass wheels and brass track..

Now,these RP20.1 advocates are the same ones that cries the loudest or yell junk when their new locomotive won't pull 3 over weighted cars up a unrealistic 5,5% grade..The reason this is because they have their minds made up by those dang so called and usually self imposed "expert" modelers that in all reality thinks a king pin is found in bowling.[:0]
As you can plainly see there are those that that still pu***he outdated RP20.1 or as many mistakenly call it a NMRA standard when in all actuality its not.
Sad but,laughable by the thousands that know the extra weight really isn't needed.

[Pruitt]

QUOTE: Originally posted by edkowal

.....So your model is 10,000 times lighter than the prototype, proportionally. This is the reason that it doesn't behave in the same way as the prototype. When civil and mechanical engineers study models in wind tunnels or water tanks, they have to mathematically correct for such differences, or else their results will be useless.

This is true because you can't scale the materials that you use to construct your model. That is, you are using full size metal and wood, with real world densities, to construct your models. The only way that you could get an HO scale car that would behave the way the real world car does would be to magically change the laws of physics for your model railroad room, in such a way that it would cancel out the changes in the volume to linear size ratio. A difficult problem to calculate, and of course impossible to achieve, since the laws of physics can't be changed.

-Ed

You're on the right track ([:D]) Ed, but it's more complex than that. The Kinetic Energy of the model is substantially less than that of the corresponding prototype - much less than just scaling down the mass and the speed appropriately. Kinetic Energy = 1/2 mass*(velocity)squared. Scaled mass of an HO car is (1/87.1)cubed of the prototype's. Velocity is (1/87.1)squared of the prototype's. So, in a collision between a train and a brick wall, the kinetic energy of an exact-scale HO model is reduced from the prototype's kinetic energy by 87.1 (scale ratio) to the fifth power, or by roughly 5 billion times!

Meanwhile,stress in materials is calculated as force / cross-sectional area, or F/A. Since force = mass X acceleration, the ratio of prototype force to HO force in, for example, pulling a train (assuming scale acceleration) is (1/87.1) to the fourth power. Cross-sectional areas of exact-scaled parts would be at a ratio of (1/87.1)squared. So the ratio of stress in the drawbar of the prototype vs. the stress in the drawbar of the HO model, we get Stress=F/A, or ((1/87.1)4th power)/((1/87.1)squared), which equals (1/87.1)squared, or about 1/7587.

Since stress does not scale, exact-scale HO parts made of the same material as the prototype are over seven thousand times stronger than the prototype. (It's somewhat more than that even, since the smaller a cross-section gets, the closer the material gets to its theoretical strength limit).

That probably got kinda deep for some folks, but the upshot is that even exactly-scaled parts are much stronger, proportionally, in the model than in real life.

I hope I didn't make a mistake in all that! [:o)]

[jrbernier]

Overduff,

You are mistaken. This is something folks get confused about all the time. Look at the stenciling on a prototype car. If you have a 100 ton capacity car, the 'CAPY' will show something like '200,000' - This is the 'payload'. The 'tare' weight or 'LT WT' stencil on the car gives you the weight of the 'unloaded' car.
For example, a 100 ton grain hopper would have the following stenciling:

CAPY 200000
LD LMT 206100
LT WT 59900

The 'capacity' is 100 tons of grain(200000 lbs)
The 'load limit' is a 'do not exceed' weight(206100 lbs)
The 'light weight' is the 'tare' weight or 'empty' weight of the car(59900 lbs)

Your example of a 'ore' car is that of a 70 ton capacity Great Lakes ore car.

CAPY 140000
LD LMT 156600
LT WT 43400

Again, that '14000' lbs is the payload 'capacity'. One must add the CAPY & the LT WT to arrive a the total weight of a loaded car. CAPY is the 'nominal' weight of the payload. Since the density of the payload can vary, and the payload can be 'heaped' as in open hoppers, the LD LMT can come into play.
I hope this helps explain the marking system used on railroad equipment. To add confusion, the stencilling has been changed in the past few years.

Jim Bernier

[IRONROOSTER]

While a number of people have made light cars work very well, others have found it to be more trouble than it's worth. Some of the problems include pullng cars off to the inside of curves - our curves are much sharper than prototype. This problem is aggravated if you get a heavy car near the end of the train with lighter ones in between. Pushing operations are more prone to derailment. Also your trackwork needs to be very good.

If long trains are your desire you can use light cars with very broad curves and very good trackwork. But you can also use multiple engines like the protoype. Or you can cheat and use powered boxcars.

Prot87 is concerned with using accurately scaled wheels and track, while I haven't seen any discussions on weight I would think that light weight would be even more of a problem since the flanges are smaller and the tolerances are tighter.
Enjoy
Paul

[Anonymous]

The example of the 100 ton capacity hopper is slightly miss stated, since capacity refers to the weight that the frame and trucks can carry. Thus if the car weighs 30 tons the net capacity of the car is 70 tons. Ore jennies are short with relatively small capacity volume wise, while coal hoppers are larger in length and volume because coal is much less dense than iron ore.
The NMRA recommended practice for car weight is based on the needed weight for the trucks to track well through turnouts, diamonds, and curves. In theory the tapper of the wheel tread should keep the wheel on the track, without flanges. This is why Proto 87 wheels have smaller flanges than RP-25 wheels,as the "87s" have a prototype tapper to the wheel tread. I have not read the proto 87 specs to see if the recommended weight of the car could be reduced with improved tracking performance of the wheels, though I doubt that the real world of occasional pushing a string of cars into a siding would result in the derailing of part of the string as the under weight cars get skewed under compression.
Darn Physics keeps cropping up fouling up our fun.
Will

[tatans]

"Scale" weight, very interesting question, just why is there a recommended weight for a car? with all the trouble people seem to have pulling a long string of cars(engine slippage) is this weight the actual "scale" weight of the prototype? would it not be practical to make the car as light as possible so the engine can pull more cars? adding weight defeats the efficiency of the locomotive and the slightest incline will stall an engine, the key would be to have a highly efficient motor, a LOT of weight in the loco and pull a long string of very light cars, would this solve the problem? forget "scale" for efficiency, real locomotive are not the lightest equipment around, any comments??

[edkowal]

QUOTE: Originally posted by jcbeard

The reason that I asked this question is, It just seems like the size to weight ratio of a model train is different than that of a prototypical train. If my model train hits something and comes to a dead stop quickly, chances my train will probably not come off of the tracks, or very little. But if a prototypical train came to a dead stop from hitting something there would be a large pile of derailed locomotives and railroad cars.

J C Beard

You're exactly right JC. The size to weight ratio does change, and that's why your model does not behave in a similar fashion to the real thing.

Your 87 ton prototype car, let's say it's 87 feet long, just to make the math easier. Now if you are going to make a ratio of size to weight, using length, the prototype car will have a size to weight ratio of 1:1.

Your model will weigh 4.23 ounces, as mentioned previously. That's only 0.000132 tons. (Remember, that for the ratios to be meaningful, you've got to use the same units in each ratio.) And your model will be 1 actual foot long. So, for the model, the ratio will be 1 : 0.000132. So your model is 10,000 times lighter than the prototype, proportionally. This is the reason that it doesn't behave in the same way as the prototype. When civil and mechanical engineers study models in wind tunnels or water tanks, they have to mathematically correct for such differences, or else their results will be useless.

This is true because you can't scale the materials that you use to construct your model. That is, you are using full size metal and wood, with real world densities, to construct your models. The only way that you could get an HO scale car that would behave the way the real world car does would be to magically change the laws of physics for your model railroad room, in such a way that it would cancel out the changes in the volume to linear size ratio. A difficult problem to calculate, and of course impossible to achieve, since the laws of physics can't be changed.

-Ed

[jcbeard]

The reason that I asked this question is, It just seems like the size to weight ratio of a model train is different than that of a prototypical train. If my model train hits something and comes to a dead stop quickly, chances my train will probably not come off of the tracks, or very little. But if a prototypical train came to a dead stop from hitting something there would be a large pile of derailed locomotives and railroad cars.

J C Beard

[Anonymous]

1/29 * 1/29 *1/29 = 1/24389
In decimal = 0.000041
This is the scale volume (or weight).

100 ton * 0.000041 =0.0041 tons, weight of model in tons

0.0041 tons * 2000 lbs/ton = 8.2 lbs, weight of model in pounds

[Marty Cozad]

Can you show me how you figured the 1:29th of 100 ton?
Please
I am so bad at math it hurts.
Thanks for the kind words and the help.
I record this info for future work on my RR.
I have to keep track of how much my cars weigh because i have broken couplers with long trains and bad stops.
Larger the scale the more the dinamics
My Arsito 100 ton hopper with metal wheels and my false load wieghs 4.5 pds,
thus a 30 car train is alot.
My USAT passenger cars are 10.5 pds out of the box.
Gunderson double stacked is 6.5 pds, etc.
I have to use double ball bearing wheels on the gunderson to cut down on wheel drag when we run (6) five packs.[B)]
cars are 32" long each. i think??

[jrbernier]

Dave is correct. A typical '100 ton capacity' covered hopper weights about 30 tons empty. Thus, the total weight of a loaded car is about 130 tons. Older 40' steel boxcars weighed about 23 tons 'empty', and have about a 50 ton payload capacity.

Jim Bernier

[Anonymous]

When HO scale is said to be 1:87, that is for a linear dimension. Weight is loosely related to volume which is a cubic dimension. I agree with Dave's method.

BTW, going in the opposite direction, this is why the old movies like where a giant spider comes along could not be. If the giant spider was, say, 100 times as wide as a real spider, its volume would be 1,000,000 times as big (and so would its weight). If its legs were scaled in the same proportion as a real spider, the giant's legs would not be strong enough to hold it up.

I always had assumed that a hopper, for example, that was said to be a "100 ton" hopper, it meant that it could carry 100 tons of material. But I could be wrong (and it wouldn't be the first time).

[DavidJ611]

QUOTE: Originally posted by Marty Cozad

David
Heres a question.
A 100 ton hopper, is that gross weight or load weight?
And what would a 1:29 scale model be ?
Thanks
I really don't know

By the same theory as outlined above, a protoype hopper weighing 100 tons in prototype would weigh ~8 lb 3 oz if magically shrunk to 1:29 scale.

I'm not sure, but I think in common RR nomenclature a "100 ton" hopper is capable of carrying a nominal 100 ton load (net weight), which would mean gross weight would be greater (by the weight of the empty car itself). For a "quick and dirty" sanity check, look at the reporting marks for a "100 ton" hopper; that should answer that question.

BTW -- Marty, you have one AWESOME "layout"; I can't stop drooling over your pix on the other threads! [:p] (Your rotary plow really takes the cake! [bow])

-Dave

[jcbeard]

Thanks for the info.
J C Beard

[Marty Cozad]

David
Heres a question.
A 100 ton hopper, is that gross weight or load weight?
And what would a 1:29 scale model be ?
Thanks
I really don't know

[icmr]

[#ditto][#ditto][#ditto][#ditto]



ICMR

Happy Railroading.[swg][swg]

[DavidJ611]

QUOTE: Originally posted by jcbeard

I was just wondering about the scale weight of a piece of railroad equipment. I know that there are the weights recommended by the NMRA for a certain length of boxcar, or tank car etc. But is that the true scale weight? Or would, for instance, in HO scale a prototypical 87 ton railroad car, weigh 1 ton since HO scale is 1:87 scale

J C Beard

The "scale weight" of an 87 ton railroad car would be about 4-1/4 ounces in HO (1:87).

Scale volume is 1/87 the width * 1/87 the length * 1/87 the height--or (1/87)^3--the prototype volume. Therefore calculated "scale weight" (if a model were built exactly to scale of the same materials as the prototype) would be (1/87)^3 of protoype.

For your 87 ton car, the theoretical "HO scale weight" would be:
87 tons * 2000 lb/ton * 16 oz/lb * (1/87)^3 = 4.23 oz

Of course the NMRA has a standard for recommended rolling stock weight which is not based on the theoretical calculation outlined above. I believe the NMRA receommended weight is based on a fixed "base weight" + a certain additional amount of weight per unit length of the car.

Hope this helps.
-Dave

(edited first response to clarify wording)

[jcbeard]

I was just wondering about the scale weight of a piece of railroad equipment. I know that there are the weights recommended by the NMRA for a certain length of boxcar, or tank car etc. But is that the true scale weight? Or would, for instance, in HO scale a prototypical 87 ton railroad car, weigh 1 ton since HO scale is 1:87 scale

J C Beard