How trains turn

[7j43k]

Randy,

 

The device Dave is talking about has two properties (actually, many, I am sure).

 

One is that the magnetic attraction keeps the wheel on the tracks.  As you have observed.

 

But Dave was referring to the wheel's ability to, as it moves back and forth, to keep itself centered between the rails.  THAT is not due to the magnetization (did I create a new word?), but due to the tapered axle.  If'n the axle were straight, the little wheel would gedaddle (YUP, that one's mine) all over the place.

 

But, yes, magnets are wonderful.

 

 

Ed

[rrinker]

steemtrayn

Remember these? Same principle keeps 'em on track.

 

 That, and the magnets built in to the axles....

Lionel Magna-Traction, anyone?

             --Randy

[7j43k]

BRAKIE

 

 

7j43k

And the INSIDE wheel will lift when the centrifugal force acting on the center of gravity is greater than the weight pushing the inside wheel down. Ed

 

 

So,you're saying wheels on a a 200,000 pound car will lift at restricted curve speeds? I know that will happen at  a higher rate of speed around a curve causing a big mess.

Would the wheels slightly lift at time table resticted speed?

I find such things fascinating that's why the question.

 

 

 

I'm not saying at what speed it will happen, only that there is a speed that it will happen at.  The downward force on the inside wheels will, however, steadily  decrease as the speed in the curve increases.

And the weight of the car doesn't matter.  The height of the center of gravity does.  Consider the extremes:  A 200,000 pound car with the center of gravity 2 feet above the rails, or a car of the same weight with the center of gravity 15 feet above the rails--which tips over first.

Or looked at in another way:  how come fast sporty cars are built low to the ground?

 

 

Ed

[BRAKIE]

7j43k

And the INSIDE wheel will lift when the centrifugal force acting on the center of gravity is greater than the weight pushing the inside wheel down. Ed

So,you're saying wheels on a a 200,000 pound car will lift at restricted curve speeds? I know that will happen at  a higher rate of speed around a curve causing a big mess.

Would the wheels slightly lift at time table resticted speed?

I find such things fascinating that's why the question.

[steemtrayn]

Remember these? Same principle keeps 'em on track.

[cuyama]

basementdweller

Rail cars don't have differentials or even just a simple split axle so instead the wheels are tapered.

+1

[7j43k]

basementdweller

I understood that the reason for the taper is because the axle is rigid. Our cars have a differential which allows the outside drive wheel to turn faster than the inside wheel so our cars turn smoothly. Rail cars don't have differentials or even just a simple split axle so instead the wheels are tapered. The tapered wheel allows the outside wheel to be bigger than the inside wheel therefore having the same effect as wheels turning at different speeds to get through the curve efficiently. Is this what is being stated above but I am just not picking up on it?

 

 

 

 

Yup.

 

 

Ed

[basementdweller]

I understood that the reason for the taper is because the axle is rigid. Our cars have a differential which allows the outside drive wheel to turn faster than the inside wheel so our cars turn smoothly. Rail cars don't have differentials or even just a simple split axle so instead the wheels are tapered. The tapered wheel allows the outside wheel to be bigger than the inside wheel therefore having the same effect as wheels turning at different speeds to get through the curve efficiently. Is this what is being stated above but I am just not picking up on it?

[BRAKIE]

Thanks Ed for clearing that up..It caught was odd that a wheel would lift off the rail by the time you add the truck and car weight.

[7j43k]

Brakie,

Any lift in the illustration is an optical illusion.  I can see it, too, if I "squint".  And, if there were any lift, it would be on the inside wheel, not the outside.  I think maybe they should have added some shadow under the righthand wheel, as they did the left.

The key part is to note the lines that go around the wheel treads.  These indicate point-of-contact for the wheels (hence, no lift) ('cause there's contact) . 

And the reason for the effect is NOT because of speed or centrifugal force.  It will still happen at .01 MPH.  Though the experiment will take awhile.

At a certain speed, centrifugal force WILL force the wheel outward towards the flange.  This will happen when the force overcomes the friction of steel wheel on steel rail.  Which is like pulling a freight car with locked wheels--not easy.

And the INSIDE wheel will lift when the centrifugal force acting on the center of gravity is greater than the weight pushing the inside wheel down.

 

 

Ed

[BRAKIE]

rrinker

No problem following the rails, even around the curve. In slow motion, you could see the side to side shift as illustrated in the graphics above. --Randy

 

Side to side shift yes,but,very little lift that is shown in that illustration the lift comes at high speeds like the illustration..

[rrinker]

 Richard Hammond, whom some may know from the British original Top Gear program, did another series called "Richard Hammond's Engineering Marvels" and one of the shows was on railroads. He set up a brilliant demontration of how railroad wheels work which involved a pair of rails that curved and then a cylinder to try to roll along the rails (failed miserably, as expected) and then a chunk of what look like two cones back to back, with the pointy ends out. No problem following the rails, even around the curve. In slow motion, you could see the side to side shift as illustrated in the graphics above.

                --Randy

 

[BRAKIE]

Also recall unlike modelers that goes full tilt around a curve and through small radius turnouts the prototype uses speed restrictions.These speed restrictions starts long before the restricting curve  or switch and normal track speed will resume after the last car has cleared the restricting track.

The restricted speed and the weight of the car helps keep the flange on the rail instead of lifting.

If we use speed restrictions and if our cars are properly weigh we should not have to worry about our flanges lifting like the example.

[tomikawaTT]

I have used the term, "Flange-screaming curves," to describe our typical model radii.

The effect you see in the graphic is valid for HO curves with radii measured in feet or meters.  Once you're down to inches stated in two digits you're into the range where the prototype roads install flange greasers.

There is a curve on the old IRT Pelham Bay Line just out of the Whitlock Avenue station which I remember as a place where the flanges really let out a howl.  When I checked it out (Google maps) I determined the radius to be just about 36 inches in HO - supposedly big enough for articulateds and full-length passenger cars.

Chuck (Modeling Central Japan in September, 1964)

[ndbprr]

It doesn't always work for real trains either. Often you can hear a flange kiss the rail and sometimes emit an ear piercing long duration high decibel squeal causing dogs to howl and grown men to put their fingers in their ears

[7j43k]

Antoine L.

 

...is it what is applied to models RR wheels we buy? do they have the same physics on the models?

 

 

 

I think we're talking more about geometry here than physics.  But it is the same geometry.  But there's a problem.  Note in the graphic, there's no flange.  And one is not needed (mostly) until the curve gets so sharp that the observed clever effect is overcome.  Then there's lotsa metal squealing (on the prototype).

Our curves are almost always in that "too sharp" range.  So, in our curves, the effect doesn't happen that often.  The flange is used and/or there's wheel slippage.  But on our straight tracks, the wheels should stay nicely centered for the same reason they do on the prototype.  

I suppose someone could get a teeny camera on a piece of model rolling stock to record these effects.  Volunteers????

I think we had a discussion on this very forum, maybe 6 months ago, about what the radius, both prototype and model, would be when the differential effect fails and the flanges get used.  

Nope, it was seven months ago:

 

http://cs.trains.com/mrr/f/13/t/228669.aspx

 

 

Ed 

[steemtrayn]

ACY

 

I've heard people suggest that the flange could be on the outside of the rail and it would work just as well.  This shows why that wouldn't work.

Tom

 

An outside flange becomes useless when a train tilts, whether due to excessive speed, or the "stringlining" effect.

[Train Modeler]

Yes, that's the idea of what RRs are looking for.  So much involved in how well that works.  The taper of course tries to center the truck while going straight.  There is also taper on the railhead. ON a curve there is more distance on the outside vs inside and some centrifugal forces so that gives rise to what you see on the video in your post.    Too much superelevation can keep that from happening and not enough speed with a heavy coupler load can pull the car to the inside and so the truck and wheel.    That's what makes RR engineering so much fun to me.  You try to minimize the forces in play and so it becomes a kind of balancing act.   ONe key reason passenger trains and freight trains need different track.

Richard

[ACY Tom]

Yep.  The wheel treads are tapered for a reason.  In an ideal world, the flange wouldn't be needed because the wheels would follow a natural course and stay on the track.  This also shows why it's better for the track to change gradually from straight to curve and back again through the use of a transition spiral.

THEORETICALLY, the actual contact area between the wheel tread and the rail is a geometric point without any dimensions, which means virtually no friction.  Of course, we know that there is some actual contact area and there is some friction.  But this illustrates the innate efficiency of the steel wheel on the steel rail, as opposed to the large contact area of a rubber tire on an asphalt or concrete roadway.

I've heard people suggest that the flange could be on the outside of the rail and it would work just as well.  This shows why that wouldn't work.

Tom

[Antoine L.]

Hi all I came across this very brilliant image. 

If this is true, is it what is applied to models RR wheels we buy? do they have the same physics on the models?

 

thanks. 

 

Antoine