Class 314 Physics - Are They Correct?

Discussion in 'PC Discussion' started by londonmidland, Jun 15, 2021.

  1. londonmidland

    londonmidland Well-Known Member

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    One thing I've noticed is that with the Class 314 brakes, they seem very weak. Now this may be realistic, but I've noticed at low speeds, say 10-15 mph, in step 2, it still takes really long to bring the train to a halt.

    Surely the slower the train gets, less momentum, therefore the brakes should be more effective? Now I know Simugraph is supposed to simulate things correctly, however it isn't the first time its been wrong, caused by incorrect values to be inputted.

    To me, if the 314 brakes in TSW were inputted into a visual representation as they were, it'd look like this:
    speed example.jpg

    Now this is how I feel like they *should* be:
    speed example 1.png

    Now for obvious reasons, don't take the charts above literally. They are just rough examples of what it feels like.

    What do you think?
     
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  2. acela2163

    acela2163 Active Member

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    I believe disc brakes are less effective at lower speeds. Some trains automatically compensate for this by increasing brake pressure at those speeds, but I don't know if this applies to the PEP units (I think they do have disc brakes though). What I will say is that I've driven the AP 313/314/315 quite a bit on TS20XX, and the physics felt instantly familiar, so I think they are fairly realistic, but they maybe could use a touch more brake force under 10/20mph. If anyone with actual driving experience happens to see this, I'd be interested to know their thoughts.
     
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  3. hyperlord

    hyperlord Well-Known Member

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    I know not enough about the 314 to say "yes you are right" but share your observation. Feels a little bit like the DB BR 422/423 :)
    But hey, let's see if someone knows it for sure, I'm interested now
     
  4. Ben_Broomfield

    Ben_Broomfield Well-Known Member

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    Again, being western region based I have no proper real experience on 314s and their braking.

    However personally it doesn’t seem right, especially when your about to stop and put it into step-1 from step-2 at 7mph and the units braking speed is like a snail.

    Dunno if that’s a effect of disk brakes being simulated as you can visualise the brake pressure chopping up/down; with step-1/2 it seems to only be around 1-1.7 bar being applied on the brake cylinder dial.

    But either way, something doesn’t feel right with the way the brakes handle or their application force/strength.
     
  5. Cameron's Gaming

    Cameron's Gaming Well-Known Member

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    Well, here's a graph from a BR training video:
    upload_2021-6-15_10-46-41.png

    The question is - is the train fitted with disc or tread brakes?
     
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  6. ghawk2005

    ghawk2005 Well-Known Member

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    feels right to me.
     
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  7. Shaun123

    Shaun123 Well-Known Member

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    They seem fine, and that’s from working with another PEP class.

    I will say, technique is generally initial brake in Step 2, and adjust from there. I’ve no issues with stopping/overruns or missing the stopping marked by a great amount.

    To quote the article with the drivers interview, they seem fine too:

     
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  8. Hazawa

    Hazawa Member

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    I can tell you Disc Brakes are by far weaker at lower speeds. Overall, all the "plastic brakes" are kinda weak at low speed, and if you want a high brake force for low speed you need the iron block brakes, which apply on the wheel surface directly. It makes the wheel rough, and gives kind of better traction on the rails. Sadly, Iron Brakes are going to say bubai on newer stocks, because they cause a lot of noise if you run and apply them.
     
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  9. synthetic.angel

    synthetic.angel Well-Known Member

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    I will put my cards on the table and say: that I have no idea what the physics should be on the Class 314, but..... I really like the way that it is modelled on Cathcart, mostly because it is different from most of the other EMUs, and it is a bit of challenge to get the stop right. I really like it... ;-)

    Also, I don't find the braking performance particularly surprising for a train built in 1979, although the "difficulty" in stopping could be expressed with more immersion with a bit of brake squeal, like you get with AP's 412/3 on TS2012... but with whatever the correct sound would be.... ;-)

    You could probably reproduce a fairly decent set of braking curve graphs for the Class 314 with a bit of slowed-down footage from TSW, as you will have yards/metres, speed to 0.1 mph (or you could use km/h for a finer measure), plus the clock...... on different gradients......

    ..... funnily enough, this reminds me of my second ever physics lesson... we were encouraged to bring in a toy to measure acceleration using ticker tape - any toy... preferably a wind-up clockwork car to run along the lab benches trailing the stamped ticker tape, or just a teddy bear to measure accelaration due to gravity (you'd push the bear off the edge of the bench). I brought in about ten metres of model track, a power supply (not needed, cos it was a physics lab...), and my fastest Lima loco (D2785...?)..... and that little thing could really shift on a long straight piece of track... so it put on a nice show.... ;-)
     
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  10. fakenham

    fakenham Active Member

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    I feel that step 2 should give very slightly more deceleration as I find that I'm using step 3 more than normal. I think real drivers say try not to use step 3 unless a mistake is made. It would be good to have a real drivers perspective.
     
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  11. londonmidland

    londonmidland Well-Known Member

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    Thanks for the comments guys. I have no issue with stopping the train in the correct place, you just have to be very gentle with it. As someone else has already mentioned, some brake squeal would be nice, as per reality, so it doesn’t feel like your ‘sliding’ as there’s no noise at all whilst slowing down.
     
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  12. Ben_Broomfield

    Ben_Broomfield Well-Known Member

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    Yeah definitely feels like your wheel sliding with the lack of audible break squeal, can image the units clatter about and squeel a lot!
     
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  13. londonmidland

    londonmidland Well-Known Member

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    They do. This video demonstrates it well.
     
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  14. Shaun123

    Shaun123 Well-Known Member

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    I will add, that during my training, I was onboard a PEP unit that was driven in EP (with the dynamic isolated) and with the dynamic enabled (as is standard) the difference was noticeable, not in terms of braking performance but in terms of feel and sound.

    What is accurate in TSW2 on the 314, is that final bite as you come to stand under 15mph.
     
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  15. lcyrrjp

    lcyrrjp Active Member

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    I’ve never driven this type of unit, but have spent quite a bit of time in the cab. The TSW braking feels pretty accurate to me. Maybe a fraction less powerful than I’d expect, but there is variation between units in real life anyway, so it’s difficult to say definitively that it’s inaccurate.

    I’d also say that the relatively short platforms on the route make the brakes feel less powerful, because if you’re used to hitting the ramp at a certain speed and braking in a certain way, you find the platform end looming sooner than expected, and that can make it feel as if the brakes are under-performing. None the less, 30mph at the ramp with the brake already in step 2 brings you to a stand roughly at the end of a 120m platform, which is about right for this unit type.
     
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  16. cwf.green

    cwf.green Well-Known Member

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    I presume the x-axis is supposed to be time from initiating the braking (there was no axis legend so I have to ask)?

    If so the first plot is most realistic because, as mentioned above, disc brakes have approximately constant deceleration with regard to speed.

    Your second plot visualizes the speed vs time graph of braking with cast iron block brakes quite well, though. :)

    ---------------------------------------------------------------------------------------------------------------------------------

    Assume deceleration is constant, that is

    dv/dt = -a.

    Then integrating once gives us

    v(t) = v_0 - at,

    which is a line with the slope -a.

    in reality, due to train resistance never being zero (forces acting on the train due to wind resistance, wheel bearings and so on) the slope will never be constant, even with disc brakes. But this is more of an issue for DTG or engineers modelling the train dynamics to a high degree of accuracy.

    Just to show that the differential equation I wrote above becomes very nasty as soon as you include train resistance, it looks like this:

    dv/dt = -a -(A + Bv + Cv^2),

    where A + Bv + Cv^2 is according to the Davis equation.

    you get a so called separable differential equation:

    dv/(a + A + Bv + Cv^2) = -dt

    It is analytically solvable, but it looks ugly. The solution will be proportional to the tangent function of time (within some boundary conditions). Much easier to just numerically integrate it if you really need that degree of accuracy.

    When you include non constant deceleration (such as with block/tread brakes) you can basically forget an analytical solution.
     
    Last edited: Jul 3, 2021
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  17. trainsimplayer

    trainsimplayer Well-Known Member

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    From all my time riding on the 314 I can say, the braking was generally in the fashion of the disk brakes.
    The physics on the 314 I think are around spot on.
     
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