I am trying to findout how the wheel footprint is figured out? Also the amount of give the rail has when the wheel rolls over a given point.
It’s called a “contact patch”.
Pay a visit to the AAR/TTC Test Center in Pueblo (actually NE of Pueblo on Baculite Mesa on the Colorad high plains desert)…http://www.ttci.aar.com/
The amount of give (deflection) in the rail as a wheel passes over it depends on the weight on that one wheel (or truck) and on the condition of the roadbed. On a well ballasted and drained road bed the rail deflection is usually no more than one inch. The other extreme ocurrs where there is a "soft spot’ resulting from poor sub grade drainage. Soft spots are often seen at the ends of grade crossings and are evident by a coating of mud on the base and web of the rail. This mud has been “pumped” up from under the ties due to the excess rail and tie deflection as trains pass over. Rail deflection in these spots may be 4-6 inches and the cars can rock quite violently as a result.
I know Mudchicken and I have had this discussion before - we have just such a spot right where we sit to watch trains. And sometimes when the cars rock so bad - I make the driver back up just in case.
But - since I am the only one that really seems to get excited by it - how much rocking can they do before we really have a major problem? Is it that what looks like a lot to me may just be SOP?
Mookie
The give in the rail is also partly a function of the rail section – a heavier section will give less, all else equal, than a lighter one will. And a certain amount of give is actually quite beneficial, as it is part of what spreads the axle load through the rail to the ties, into the ballast and eventually into the ground.
Pumping ties are not good – they are a sign (among other things) that the stress is being redistributed through the rail to adjacent ties, which puts a lot of extra stress on the rail. Which can result in rail breaks.
Not to mention that they (and other causes of low spots!) make cars rock, and that can be real scary to look at. How much rocking is tolerable, though, depends on a whole host of factors.
SJ, where you watch the trains, there are two factors that would make them rock: plenty of switches (pounding through frogs will cause low spots eventually) and the “critical speed” on track with jointed rail. Assuming that your track is good for some decent speeds, there is a zone (CNW used to say that it was between 12 and 22; other railroads had different limits, generally encompassing the upper teens) through which the rocking becomes more pronounced. Our crews were encouraged to build up speed quickly to get them out of this “critical” area (either that, or stay below the zone).
I think we’ve had this discussion before, in connection with locomotives bottoming out (was it the thread about wide gauge–4’8 1/2"?). You’d really have to be rocking some for the center of gravity to move to a point over the outside of the rails (and that’s when the car would tip over). Before you got to that point, there would probably be some wheel lift on one side, causing the wheels on the low slide to possibly slide inward and off the rail.
Pumping ties: good point. If you can see that the ballast in a certain area has a covering of mud (wet or dry), that indicates a problem. In rainy weather, the downward movement of the ties when a train passes over them causes what’s underneath to squirt upward. There should be enough ballast underneath so that nothing comes up (and it shouldn’t be moving like that since rock is relatively unyielding). If mud or other dirt is coming up, some stability has been lost, and the ballast needs to be cleaned and tamped to eliminate this.
The guy with the feathers should have plenty to say on this subject.