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Question: Rolling/air Resistance On Level Grade.

Discussion in 'General Discussion' started by Kuu Lightwing, Apr 1, 2018.

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  1. Yes, they should slow down faster.

    6 vote(s)
    46.2%
  2. No, that's about right.

    7 vote(s)
    53.8%
  1. Kuu Lightwing

    Kuu Lightwing Active Member

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    Aside from stuck brakes on some of NEC wagons, do trains roll too well, or that's about right? They seem to lose speed on level grade, but sometimes it feels like they maybe should slow down faster, but I'm not sure about that. HST can roll for like 4 miles, and still requires some active braking to conform to speed limits, for example.

    P.S. If you come here to complain about "Boo, DTG is ass and can't get anything right", please don't. I want to see actual discussion, preferably informed discussion, not people repeating same complaints over and over again.
     
  2. animalkosmik

    animalkosmik Well-Known Member

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    All right, maybe not 100%, but very realistic!)

    Train during traffic getting huge kinetic energy (live power of train).

    If you take a train of the weight, for example 2000 tons, then at a speed of 50 km/h it will have a kinetic energy of about 20000000 kg·m, and at a speed of 100 km/h it will already have about 80000000 kg·m.

    The movement of the train is always impeded by the resistance power of the oncoming air flow, the friction force of the wheels with the rails. These force of resistance are overcome by the train at the expense of traction, and when the traction is stopped due to accumulated kinetic energy, the supply of which gradually decreases. When the force reserve of the train is exhausted, it will completely stop. The distance that the train will pass to a complete stop, continuing to move by inertia, depends on the speed at the moment of stopping the traction and the path profile.

    The higher the initial speed of the train, the longer the distance it will go by inertia. If the train-driver stopped delivering traction at a speed of 60 km/h, then the distance traveled by the train along the horizontal path to a complete stop is 5 km, with an initial speed of 70 km/h the distance will increase to 6.8 km.

    Therefore, when the kinetic energy is absorbed only by the force of resistance, the train will go a very long way.
     
    Last edited: Apr 2, 2018
  3. Kuu Lightwing

    Kuu Lightwing Active Member

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    I understand that trains are heavy and hard to stop (BTW, air brakes on all US trains feel a bit too good), and while with more railcars you add more mass, you also add more friction and rolling resistance. There's also air resistance, which depends less on the number of cars and more on the speed of the train.

    Another thing I noticed: I rolled around Cumberland Yard a bit, and found out that AC4400CW goes down 0.2% grade at 10 mph pretty much steadily, very-very slowly losing speed. But if I add some railcars to it, it starts accelerating slightly down the same grade. So, apparently cars roll better than the locomotive, which is something I can see, but I'm not entirely sure if it's accurate.

    In any case, I'm afraid without more concrete data from an actual train or something, it's pretty hard to evaluate whether TSW does it close or not much. It's one of those things that requires fine-tuning.

    On a side note, I don't quite understand your measurement units. It should be Joules, not kg·m, right? It's more like kg·m^2/s^2
     
  4. Jef-F

    Jef-F Active Member

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    4 miles? Oh sweet summer child :D From 125mph it would coast for 20+ miles like no big deal.

    Yep, and they're also inconsistent between freight wagons. Some of them on minimal application brake as hard as others on full service.
     
    Last edited: Apr 1, 2018
  5. Kuu Lightwing

    Kuu Lightwing Active Member

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    The question is how much would it slow down on it's own. Apparently they designed it with 4,500 hp total to be able to get it up to that speed, so I'd assume without engine power, it should lose speed pretty fast. There's a reason why all high speed trains are electrics, after all.
     
  6. Jef-F

    Jef-F Active Member

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    Well, I'm answering exactly that. From 125mph it slows down to 85mph in 15 miles on its own.
     
  7. Kuu Lightwing

    Kuu Lightwing Active Member

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    The real one? And if so, how do you know that?
     
  8. animalkosmik

    animalkosmik Well-Known Member

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    Kilogram-meter 1kg·m=9,8J
     
  9. Kuu Lightwing

    Kuu Lightwing Active Member

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    Well, then you multiplied it by free-fall acceleration, (which is measured in m/s^2). Otherwise it's not a unit of energy :P
     
  10. animalkosmik

    animalkosmik Well-Known Member

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    kilogram-meter a unit of energy or work, being the amount needed to raise one kilogram one meter ;)

    Energy Unit Converter
    ENERGY UNIT.jpg
     
  11. Kuu Lightwing

    Kuu Lightwing Active Member

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    Welp. Never seen it used before, and also can't quite find when is it normally used.
     
  12. pschlik

    pschlik Well-Known Member

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    Thats not a unit of work...work is force x distance, but that’s mass x distance. Mass x distance is equivalent to force x time x time, which, to be honest, is a very useless measurement to make. Whoever made that has clearly confused the concept of weight (a force) and mass, because force x distance is work.

    However, to make it a proper unit of work all you gotta stick in is an acceleration. And the fact that it is designed to be for lifting something up explains why the conversion to J is suspiciously like the acceleration due to gravity...gravity is actually in the equation, but is hidden. 1 kg x 1 m x g is indeed eqaul to 9.8 J, but that will have units in terms of N x m...which is actually what work is. kg x m is not work like you think it is.


    But why the heck would you ever use this like ever? This is a horribly designed unit of mesaurement that is fooling you into thinking that a joule is equivalent to anything measured in kg x m, which is totally untrue. Also, this only works when you are lifting things up vertically; if you are moving a train forward on a flat surface, multiplying kg x m will not be equal to some factor of 9.8 J at ALL. In any other situation, this is a completely useless relationship as it fundamentally depends on the acceleration due to gravity. (So it also doesn’t work on other planets.)


    And people like to say the imperial system is dumb while people are going around telling you kg x m is a unit of engery. What BS, I’m an American and I understand this better!
     
  13. Kuu Lightwing

    Kuu Lightwing Active Member

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    pschlik, I can assure you that this isn't a part of International System of Units. It just abbreviates kilogram-force (which is not a part of SI either) as kilogram for some reason. Yet several dictionaries list it as a measurement unit. Dunno, maybe it' a British thing. :)
     
  14. StratPlayer62

    StratPlayer62 Well-Known Member

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    I've often wondered this and even asked DTG but never received a response. There is rolling resistance, wind resistance, and also resistance when going around curves, I have no idea if any of this is accounted for in the game or not.

    I seem to remember reading once that the rolling resistance of a train is the same as going up a .2% grade but I don't remember where I read it at.
     
  15. animalkosmik

    animalkosmik Well-Known Member

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    Foot-pound (energy) -is a unit of work or energy in the Engineering and Gravitational Systems in United States customary and imperial units of measure. It is the energy transferred upon applying a force of one pound-force (lbf) through a linear displacement of one foot. The corresponding SI unit is the Joule.

    1ft·lb=1.35J
     
    Last edited: Apr 1, 2018
  16. pschlik

    pschlik Well-Known Member

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    Ok...your point is? A foot pound is force x distance, which IS energy, so that does make sense. Don’t know much of a use for that though, usually foot pounds come up in torque.
     
  17. Kuu Lightwing

    Kuu Lightwing Active Member

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    Hey, guys, can we stay a touch closer to the topic? Somebody decided that since ft·lbf is a thing m·kgf should also be a thing, and called it a "kilogram-meter" making everyone confused. That's it.
     
  18. Jef-F

    Jef-F Active Member

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    No, the one from TSW.
    You need quite a lot of power just to gain kinetic energy accelerating all that mass, and do in in a reasonable amount of time, rolling and aero resistance notwithstanding. HST needs about 6 miles to reach its top speed, it definitely isn't possible for it to lose it all coasting for the similar distance. So in my amateur opinion HST behaves pretty mush reasonable in TSW.

    Hey, UK members, you could use GPS speedometer next time you ride on HST and measure deceleration rate ;) (that's assuming you can clearly hear those pesky MTUs from inside coaches, I have no idea)
     
  19. Kuu Lightwing

    Kuu Lightwing Active Member

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    Oh. Well, that doesn't tell much.

    That is true. However, we know that top speed of HST is 148mph, which I'd assume would be the speed at which the resistance balances out the engine output.

    If I haven't messed up my calculations at that speed two engines with total power output of 4500hp should produce about 50kN of tractive effort (since power = force times velocity). If we assume that it has been balanced out by resistance forces, then we can estimate them to be about the same. Which would mean that our 7-coach HST with two power cars, with total weight of 370 tons would be decelerated at 0.13 m/s^2 or 0.3 mph per second. Which doesn't seem to be that far off actually... as with such deceleration the train could roll pretty much forever.

    Plus, these calculations assume that train moves at 148 mph, while at 125 mph the resistance forces would be smaller, and falling off as train slows down. So, yea, the HST's behaviour is probably about right... unless I messed up my calculations somewhere.
     
  20. DominusEdwardius

    DominusEdwardius Member

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    Lets just do a rough estimation for a 2+8 HST, at 125mph the rolling resistance (+ air resistance requires) requires approximately 2850hp to overcome (you'll have to trust me on that one as I don't have permission to reproduce the graph).

    so converting to metric that is 2125kW and 55.88m/s. So the force at that speed is Power/Velocity, or 2125/55.88 = 38.03kN which is about right for a hst, a pendolino 9 car set is approximately 30kN from memory. Now this is a very bad assumption since it totally isn't true but lets assume that the force is constant right down to 0mph (it isn't it exponentially decreases but that is much to complex to calculate quickly!). So in this utter worst case scenario the distance to stop is calculated as follows:

    stopping distance = (Initial Velocity^2)/(2*deceleration rate)

    we know the initial velocity (55.88m/s) and the deceleration rate is force/mass, so 38.03/413.98 which is quite a high 0.091m/s. so...

    stopping distance = (55.88^2)/2*0.091 = 17157m

    17.157km is the absolute worst case distance from 125mph, it will in fact be vastly greater than that since as I say, the rolling resistance decreases exponentially with speed. Because I don't want to do any complicated maths tonight I will do it stage wise and break it down into small segments with an average acceleration in each (don't ask me why I chose these intervals, call it re purposing another spreadsheet :P )

    125mph - 38.03kN
    110mph - 30.32kN
    105mph - 27.80kN
    100mph - 25.02kN
    95mph - 22.82kN
    90mph - 21.34kN
    75mph - 15.56kN
    60mph - 11.12kN
    45mph - 9.27kN
    25mph - 6.67kN
    15mph - 5.56kN

    So breaking that down into segments the distance covered in each section
    125-110 - 4266m
    110-105 - 1529m
    105-100 - 1605m
    100-95 - 1685m
    95-90 - 1773m
    90-75 - 5551m
    75-60 - 6277m
    60-45 - 6391
    45-25 - 7266m
    25-15 - 2705m
    15-0 - 1525m

    so a total distance of approximately 40573m or 40km!
     
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  21. Kuu Lightwing

    Kuu Lightwing Active Member

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    Oh, that's very helpful, thank you. So, like I suspected (well, and honestly, who would be surprised by that conclusion?) trains are very good at rolling. :D

    Although, according to Jet-F, the HST is slowing down from 125 to 85 in 15 miles, which is much longer than it appears by your numbers. There's a lot of 0.1% grade in GWE, so that might affect things quite significantly, though.
     
  22. DominusEdwardius

    DominusEdwardius Member

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    Mine was purely based on real world data, albeit subject to ambient effects such as head winds, tail winds etc. I suppose one way to check is to run the opposite way so you are aided by the 1 in whatever gradient and if it is still short then the rolling resistance is likely a bit too high. I suppose it depends how DTG have implemented the rolling resistance, whether it is a constant value, or a function or speed etc and how that function varies with speed. That bit is a mystery at the moment and probably will remain so until we can get some tools to play with.
     
  23. Kuu Lightwing

    Kuu Lightwing Active Member

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    I'm pretty sure it is a function of speed, since trains at high speed slow down much faster than they do at low speeds. They probably use an exponent too.

    Also, it appears that it's not too high, but a touch too low, since in your figures train should slow down from 125 to 90 mph in about 10 kilometers, while in the other example - it's about 15 miles or 24 kilometers (but down to 85 mph), which is a significant difference.
     
  24. Jef-F

    Jef-F Active Member

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    DominusEdwardius thanks, that was quite interesting to hear. Will try to measure it going other way through the same stretch soon
     
  25. animalkosmik

    animalkosmik Well-Known Member

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    Yeah, and that's why you decided that they should slow down faster :D
     

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