Scale vs. mass vs. weight for

[bicknell]

Plus 'properly' weighted N scale cars are much heavier than their prototype counterpart.

I'm not entirely sure "much" heavier is always accurate....

http://www.nmra.org/standards/sandrp/rp-20_1.html

40' box * 12 inches a foot / 160 = 3"

0.5 oz + 0.15 * 3 = .95 oz

.95 * 160 * 160 * 160 / 16 oz per lb = 243,200 lbs

Using a PRR as a standard from here (http://prr.railfan.net/freight/classpage.html?class=X23), 100,000 lbs capacity, 46,000 lbs car weight, so a real car would have been 146,000 lbs fully loaded.

So yes, fairly well over.

Modern though: http://www.uprr.com/customers/ncsc/bcsize.shtml

50' box car comes to 1.06 oz, which is 272,000 in N scale....

Real life?  90T, or 180,000 lbs, and probably at least 60,000 car weight or 240,000 lbs.  Not that far off, about 10%.

Point taken that N scale cars even without load are more like fully loaded real cars, when scaled 160^3.

[wcfn100]

"Real life?"

In 'real life', a loaded 40' boxcar (in the 60's) was rarely over 60 tons with most between 40 and 50 tons, at least on the couple of thousand cars Ive' looked at.  A loaded 89' autorack weighed in at 75 tons.  What would that be by NMRA specs?

So I would call twice the weight significant.  And as you know, an empty 50' gondola is a huge differential.


Jason

[asciibaron]

Plus 'properly' weighted N scale cars are much heavier than their prototype counterpart.

I'm not entirely sure "much" heavier is always accurate....

http://www.nmra.org/standards/sandrp/rp-20_1.html

40' box * 12 inches a foot / 160 = 3"

0.5 oz + 0.15 * 3 = .95 oz

.95 * 160 * 160 * 160 / 16 oz per lb = 243,200 lbs

Using a PRR as a standard from here (http://prr.railfan.net/freight/classpage.html?class=X23), 100,000 lbs capacity, 46,000 lbs car weight, so a real car would have been 146,000 lbs fully loaded.

So yes, fairly well over.

Modern though: http://www.uprr.com/customers/ncsc/bcsize.shtml

50' box car comes to 1.06 oz, which is 272,000 in N scale....

Real life?  90T, or 180,000 lbs, and probably at least 60,000 car weight or 240,000 lbs.  Not that far off, about 10%.

Point taken that N scale cars even without load are more like fully loaded real cars, when scaled 160^3.

the calculations are incorrect - mass does not scale

-Steve

[wcfn100]

the calculations are incorrect - mass does not scale

Why not if you had a 2 pound box in N scale, the size of that box would be 160(boxes)x160x160 in real life.

Why wouldn't the weight be 2x160x160x160?


edit: Who's talking about mass anyway, we're talking about how much something weighs.

Jason

[DKS]

the calculations are incorrect - mass does not scale

Why not if you had a 2 pound box in N scale, the size of that box would be 160(boxes)x160x160 in real life.

Why wouldn't the weight be 2x160x160x160?


edit: Who's talking about mass anyway, we're talking about how much something weighs.

Jason

Not to mention that the friction inherent in models is very high compared to the real deal. 1:1 freight cars will roll on a fraction of a percent grade. Models can't match that. In fact, I'd say that friction is a bigger performance issue than weight.

[bicknell]

Most 40' boxes don't weigh close to the NMRA specification, they are lighter.  How much depends on manufacturer and body style, and yes, there are exceptions.  Gondolas and hoppers, for instance, are way under.  I think my 33' twin bay hoppers are probably less than half the recommendation, but were 55T capacity cars.  The flip side of course is that most N scale locomotives weigh 3-5 times what a real one would, by scale.

When it comes to tractive effort friction is more key, and in more than just bearings.  First, good roller bearings in real life are very low friction, if you've ever seen a strong man competition you know one man can pull a car on level track no problem.   Indeed, on PERFECTLY level track a 44T could probably "move" 50 cars, very slowly, and in a way that burns up the traction motors.  But the other thing the prototype has going is adhesion with the locomotive.  The way the wheels and rails deform under a locomotives weight greatly increases adhesion, add in sand and there are properties there that don't work in the model.  I believe model motors are higher torque and "Horse Power" than real life, when scaled as well, and at least when up to speed.

All the calculations aside though, I based my guess more on a gut feel compared to other N scale locomotives of what should be expected.

[asciibaron]

edit: Who's talking about mass anyway, we're talking about how much something weighs.

public school?

-Steve

[asciibaron]

Why not if you had a 2 pound box in N scale, the size of that box would be 160(boxes)x160x160 in real life.

Why wouldn't the weight be 2x160x160x160?

if you had a 1 ton piece of lead that was 4 feet square and 4 inches thick, would a piece a 1/4 the size weigh a 1/4 of a ton if made from styrene?  the answer is no because mass does not scale.  the material used has a specific mass and simply scaling the dimensions does not change the mass of the material used.

you are doing volume calculations.

-Steve

[bicknell]

if you had a 1 ton piece of lead that was 4 feet square and 4 inches thick, would a piece a 1/4 the size weigh a 1/4 of a ton if made from styrene?  the answer is no because mass does not scale.  the material used has a specific mass and simply scaling the dimensions does not change the mass of the material used.

you are doing volume calculations.

Err.  Um.

Let's try this again.

Steel weighs about 500lbs per cubic foot.  You can google for that, and yes, it's 400-600 depending on what alloy we're talking about, but I'll go with 500 as a round number.

So a 10'x10'x10' cube of steel would weigh 500,000 lbs.

A 1:160 model made out of the same material would be 0.75"x0.75"x0.75", or 0.412875 cu^in.  A cubic foot is 1728 cubic inches, 500 / 1728 is .28935 lbs per cubic inch, so the model block would weigh 0.122 lbs.

Note that 500,000 / (160 * 160 * 160) is also 0.122 lbs.

Mass scales with volume, indeed, that's the way it is measured.  500lbs per cubic foot.  Weight scales with volume as long as gravity is constant (weight is mass * gravity).

So if I took a real car and shrunk it to 160th the size using the exact same materials it would weigh 1/(160^3) as much.  A 50,000 lbs car would weigh about 2 ounces.

Now, someone is going to pop up and say, "That can't be right, plastic weighs less than steel, and yet our cars weigh about the same amount".  Yup.  Often we don't realize how oversize our model parts are.  For instance a wheel is a solid bit of steel (prototype) or plastic (model).  Those plastic grabs really are 4x the size of scaled down grabs.  The floor really is a foot thick solid plasic, and would have been 2" thick steel on the prototype.

[asciibaron]

made out of the same material

that was my point - that you can not scale the mass of steel using feathers.  sorry i was less technical, i'm actually working today.

-Steve

[wcfn100]

edit: Who's talking about mass anyway, we're talking about how much something weighs.

public school?

-Steve

Yup, that's where I learned that mass and weight are not the same thing.


Of course, I'm still waiting for you tell me what the weight on an N scale car should be if I'm so far off.  I shouldn't matter what they're made of for this arguement, an N scale scale represents a full size car regardless of materials.  If it were made of feathers, I'm pretty sure it'd be lighter than the prototype, if it were made of lead, heavier.


I've always been curious about this and thought I had it figured out.  I would like to see what the actual math is.


Jason

[wcfn100]

I've been looking for just the right description and I found it.


"The weight of an object increases as the cube of the scale factor."

If anyone has a problem with that, you can go argue it here:

http://www.worsleyschool.net/science/files/scalefactor/factors2.html


It also says this:

"The mass of an object depends on its volume. For example, if you increase an object's volume by eight times, its mass will also be eight times greater."

Sound like scaling....  Go get 'em.


Jason

[asciibaron]

I've been looking for just the right description and I found it.


"The weight of an object increases as the cube of the scale factor."

If anyone has a problem with that, you can go argue it here:

http://www.worsleyschool.net/science/files/scalefactor/factors2.html


It also says this:

"The mass of an object depends on its volume. For example, if you increase an object's volume by eight times, its mass will also be eight times greater."

Sound like scaling....  Go get 'em.


Jason

have you factored in the empty space of the car?  simple 3 dimensional measurements do not account for the empty space inside the car  sure you can scale a solid objects mass, but a boxcar is not a solid object, there is plenty of air in the car, even when loaded.  a box car's walls would be super thin if scaled correctly

-steve

[wcfn100]

I was going to account for the air, but an N scale car is much closer to the ground than a prototype car and the concentration of gasses would be different which could effect the actual weight. ;D

Of course that's a joke because the space inside could be anything.  Air, lumber, washing machines.  They would all scale up as well.  Air does have weight like everything else.

I think you're over thinking this.

A simple question then for you steve, how much would an N scale box car weigh if you scaled it up 160 times?


Jason

[DKS]

Maybe I'm stating the obvious, but why is anyone even thinking in terms of scaling down a real object using the same density of matter? Presumably when one thinks about shrinking a full-sized boxcar, one must also think about shrinking the matter that comprises it; if the model boxcar had precisely-scaled dimensions but the same (non-scaled) matter density as the real one, then the microscopically thin steel sides of the model boxcar would disintegrate if even touched. Shrinking the matter density along with the dimensions, a 50,000 lb. boxcar in 1:1 would weigh 50,000 lb. in N scale (and of course it would promptly sink through the layout, and the earth's surface, to merge with the earth's mantle). If you're going to scale anything down, you must then scale everything down, including matter, gravity and time; not doing so forces us to make up all kinds of kludgy explanations for things like tractive effort, friction and so forth of our models--which, in fact, is what we must do, since we can't scale matter.

Boy, talk about straying off topic...