Question about radius


The only thing superelevation does is to change the effective CG of the cars and engine. This change allows them to move at a higher rate of speed through the curve.

There are forces at work, centrifical, centripital, inertia and gravity.

Adjusting these via superelevation from the norm helps keep the wheels on the ground.:)
 
bobh said:
The only thing superelevation does is to change the effective CG of the cars and engine. This change allows them to move at a higher rate of speed through the curve.

There are forces at work, centrifical, centripital, inertia and gravity.

Adjusting these via superelevation from the norm helps keep the wheels on the ground.:)
Click to expand...

Not in the model at typical tight radii. The physics are different.
 
Ooh - this one could run & run. A fun debate :)

My 02c;

You're both kinda right ;)

- For 100% sure, the laws of physics are the laws of physics - That's why they're the *laws*! - Doesn't matter (until we get into very esoteric stuff that doesn't concern us ;)) what the scale is - The physics hold true and the same for z scale thru the prototype.

However;

- The result(s) of the laws is different between us and the 1:1 - No doubt I'll be proven wrong - Which is fine btw - But I don't think our stuff is capable of entering a turn so fast that it "flies off" or "lifts it's inside wheels".

There's solid physics behind the 1:1 being allowed to go thru SE turns faster than flat track. OTOH, I believe in our stuff, it's just 'cos it looks cool!

As always, my 02c,
Cheers,
Ian
 
bobh said:
Sorry but physics are physics.. until you get to the nano size then things do change.
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Sorry, not correct. Model railcars are less massive proportionally than the real thing and the curves are much tighter than any prototype curve on which superelevation is used.

At the small radii used in modeling, drawbar forces are at an angle to the track and somewhat directed toward the center of the curve, as opposed to high-speed real-life curves where drawbar forces are generally parallel to the rails.

This, plus the higher relative friction of the wheels on the tight model curves, leads to the force being directed toward the inside of the curve, rather than outside, as would be the case with real-life railcars operated through a curve at high speed.

This is why "stringlining" (derailing across the inside of the curve) is a much greater problem on tight model radii than is any tendency for cars toppling over to the outside of the curve due to centrifugal force.

Yes, Newtonian physics still applies, but the objects to which it applies are completely different in comparing the model and the real thing.
 
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Well I'm not comparing the model world to the real world in the sense that ALL the forces are comparable.

Of course one can model area a lot easier than one models mass. Our rolling stock would be LOTS heavier where it the same relative weight.

This fact doesn't change what forces are in effect. It only effects how and where they work..

I stand behind Newton until he's proven wrong :)
 
I don't intend to participate in the debate side of this discussion, but I think/guess that the fact that 1:1 rolling items are spuspended from their axles might have an effect that should be considered? The models are not sprung. We don't load them in proportion, so they must behave somewhat differently, all things being equal.

Okay, I'm gone....:p
 
bobh said:
Well I'm not comparing the model world to the real world in the sense that ALL the forces are comparable.

Of course one can model area a lot easier than one models mass. Our rolling stock would be LOTS heavier where it the same relative weight.

This fact doesn't change what forces are in effect. It only effects how and where they work..

I stand behind Newton until he's proven wrong :)
Click to expand...

At the real speed our stuff runs at centrifugal forces are negligible. The only significant forces are wheel friction caused by the inner rail being shorter (making the inner wheel want to turn slower) and the drawbars trying to pull the cars off-track towards the center. This friction and how it aggravates the required pull is the real issue, and it is not helped by super-elevating a curve.

With a 20 inch radius curve 1 g is reached at 5 mph. That's a 435 MPH scale speed just to generate 1 g of that fictitious centrifugal force!!! These forces follow a square law, so at 2.5 MPH (217 MPH scale speed) we are at 1/4 g.

At 70 MPH scale speed g-forces are under .026g's. Every time speed is halved, g-force is quartered.

The other consideration is radius, and we can't get 1/2 inch out of the radius with any bank angle. The tracks are not wide enough compared to the radius for this to be meaningful. Clearly anyone proposing the idea that banking the curve increases effective radius any meaningful amount is absolutely wrong. Changing radius is also a linear function, not a square, on g-forces. Halve radius we double g-force, double speed we quadruple g-force.

Super-elevating clearly would make things worse at the speeds our stuff runs at. The dominant force by far should be draw-bar pull caused by friction as the wheels with common axles try to rotate at different speeds, not forces caused by speed.

Banking would tend to push the cars towards the center of the radius, and since the draw-bar is above wheel contact height the pulling force would add to the banking force and tend to roll the cars toward radius center even more!!

Now an opposite bank, where the cars tend to lean outward (outside rail lower in height), might help keep the cars on track better at slow speeds. The draw-bar would then pull the truck DOWN into the rail, not lift them out. Of course backing up would get worse because the draw-bar would be in compression. :)

As we increase draw-bar drag, we want to reverse the normal bank (or lower the draw-bar height above the wheel contact point) when pulling around a curve. As we run the train faster, we need a more typical bank to aid centripetal force.

Anything running around curves at 400 MPH scale speed? Or do you back around the curves? Then by all means, bank them.

Tom
 
So then having wheels that spin independently of the axels ... AND adding weight higher up in the car would give a slightly more realistic result? (not that I am gonna do that .. just a hypothetical question)
 
It may but the degree would be so slight as to hardly be noticable. Adding the weight higher would change the cg for the worse (I think).

The reality is that our trains are very light and our turns very sharp in comparrison to the full scale. But.. the good news is that we don't get a hernia when we lay rail!:p
 
Tygrr said:
So then having wheels that spin independently of the axels ... AND adding weight higher up in the car would give a slightly more realistic result? (not that I am gonna do that .. just a hypothetical question)
Click to expand...

I'm not sure what you are asking, but it seems to me if we could have independent rotational speeds on wheels much of the drag caused by curves would go away. I'm new at this, but looking at the numbers it looks like drawbar tension would be a major player and g-force not.

I think the important point is not everything scales when we scale down.

It would be nice if everything scaled in the real world, but unfortunately that doesn't often happen. Ants can fall a lot futher than elephants without damage. :)

Try scaling a steam locomotive down and you'll see the firebox is too small to work.

Tom
 
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