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Thread: A reminder of how the Numwheels and Adheasion parameters work

  1. #1
    Join Date
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    Default A reminder of how the Numwheels and Adheasion parameters work

    Upon reading some of the discussion regarding the slipping problem it appears that there are some misconceptions about how the sim uses Numwheels to calculate adhesion. Numwheels has nothing at all to do with the number of wheels. Rather, it is just a simple divisor. Here are the exact equations the sim uses:

    Tractive Effort(slipping) = (first Adheasion value)*(third value)*(locomotive weight)/(NumWheels)

    Tractive Effort(normal) = (second Adheasion value)*(third value)*(locomotive weight)/(NumWheels)

    Tractive Effort(sanding) = 0.95*(third Adheasion value)*(locomotive weight)/(NumWheels)


    These same equations are used while braking, but for locomotives only. Note that the first Numwheels value (i.e. the one in the wagon section of engine files) is used for friction braking, while the second Numwheels value (in the engine section) is used for traction and dynamic braking. Contrary to the MSTS documention, NumWheels and Adheasion have no effect whatsoever on the braking of wagons. The only ways to control the braking of wagons are to adjust the MaxBrakeForce and BrakeCylinderPressureForMaxBrakeBrakeForce parameters.

    The sim uses multipliers to account for reduced adhesion under rain and snow conditions(the default values are 0.9 and 0.55, respectively, and these can be adjusted in the default.wag file). These multipliers affect the tractive effort(and braking for locomotives) under slipping and normal conditions only. The tractive effort while sanding remains the same even in bad weather, and interestingly, you can also increase the maximum brake force exerted by the locomotive by sanding while braking. Once again, there is no change whatsoever in the braking behavior of wagons under bad weather conditions. This is dependent only upon the MaxBrakeForce and BrakeCylinderPressureForMaxBrakeBrakeForce parameters, and remains the same in all types of weather.

    Hope this clears things up.


  2. #2
    Join Date
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    ny, usa.
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    Default RE: A reminder of how the Numwheels and Adheasion parameters work

    Soooooo...

    For a 2-8-2 the numwheels in the wag section would be 12
    and in the eng section 8?

    For an A-1-A+A-1-A diesel, 12 and 8?

    For a u33c 12 and 12?

    An EP-2 bipolar would be 20 and 12?

    Lloyd

  3. #3
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    Stalinist,broke United Kingdom (Hey buddy,can you spare �1.3 trillion?))
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    Default RE: A reminder of how the Numwheels and Adheasion parameters work

    Recommended reading is Rudolf Richter's tutorial on eng and wag files at the Steam4Me site.

  4. #4
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    Default RE: A reminder of how the Numwheels and Adheasion parameters work

    Hi Lloyd,
    Note "Numwheels has nothing at all to do with the number of wheels." I am going to start with your U33C. All the wheels are driven. Now if you want sanding adhesion to ~quartz sand/steel coefficient of friction, you have to have Adheasion ( X1 X2 2 0 ) with a NumWheels of 4. Does not matter if there are 28 driven wheels, and no pilots, it is still 4. Now with this adhesion line, X1 is double the slipping adhesion and X2 is double the clean dry rail adhesion. More specifically, for a U33C, Adheasion ( 0.24 0.53 2.0 0 ) with NumWheels of 4 for traction. You may need 8-12 for NumWheels on the braking value (first or wagon value). This is an experimental entry as braking adhesion is about 7-15.5 of weight.
    Now an A1A A1A diesel would be about 12 on first entry, but 6 instead of 4 for the traction NumWheels. This is because only 2/3 of the weight is tractive weight. Therefore, dividing by 6 instead of 4 gives 2/3 of traction. On a 1950 vintage unit this might be Adheasion ( 0.28 0.50 2.0 0 ) with NumWheels of 6. You can also input the adhesion line as Adheasion ( 0.187 0.333 2.0 0 ) with NumWheels of 4. Either way works the same, except in the later, sanding adhesion will be too high, since that is hardwired into the program.
    The key is that NumWheels does not relate to number of wheels as such.
    Bob Boudoin

  5. #5
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    Default RE: A reminder of how the Numwheels and Adheasion parameters work

    Hi Peter,
    Although this is useful reading, keep in mind it was written before a lot of what we have learned about the simulator was known. I think both Joe and I would find a few things in Rudolf's tutorial that we would suggest work differently.
    Bob

  6. #6
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    Default RE: A reminder of how the Numwheels and Adheasion parameters work

    To add to what Bob has stated about the way railcars brake in different weather conditions. When running the real thing I never changed my braking habits using train air based on whether it was raining or not. The train braked the same for this simple reason. By the time the lead engine's last wheel had run over the contact patch of the rail head, the water is pretty much gone or the rail head is gritty enough from the dirt mixed with the water so that the rail similated dry conditions. Also after 20 or 30 hot wheels (from the heat created by the shoes agaist the wheel) pass over the rail, it is dry. So really there is no need for specialized figures for railcars and differing conditions. The only time this would really be a change is on really light cars like empty Centerbeam cars. These cars do have a higher occurance of flatspotting in the wintertime on the Cascade Crossing line as noted to me by RW car mechanics that have to go out into the field to change out wheelsets. Again, an isolated car sliding it's wheels will have little effect on the whole train and how it brakes.
    Bill
    Rule of The Day: 147.7.16.B
    When all previously LISTED Trainhandling braking methods fail to produce desired results......JUMP!!!! Either way it's gonna hurt.

  7. #7
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    Default RE: A reminder of how the Numwheels and Adheasion parameters work

    To add to the differing conditions and wheel adhesion. There are tricks that RW engineer's use to increase wet adhesion to Dry conditions. One is to "ride" the engine brakes with a 10psi application to get the wheels hot enough to "cook" the water off the railhead and keep the wheels dry. Believe it or not I was able to get the C&NW fleet to mimic the results of these RW practices and engines that slip on wet rail, even with sand, will hook up and get a cut of cars moving using RW tricks.

    When working a yard job with switch engines on wet days, I would make my first moves around the yard with a 10psi independant application to get the wheels cooking hot. Once appropriately heated you could hear the water cooking off the railhead as you rolled along. I would then be able to do my work as if the rail were dry. Periodically I would reheat the wheels and continue on. I never had sliding wheel problems when I ran. Now this is not true for all engineers as some of my co-engineers have had to come in on their days off to help drop a set of switcher wheelsets to have them lathed back to a smooth wheelface due to uncontrolled sliding.
    Bill
    Rule of The Day: 147.7.16.B
    When all previously LISTED Trainhandling braking methods fail to produce desired results......JUMP!!!! Either way it's gonna hurt.

  8. #8
    lelandfletcher Guest

    Default RE: A reminder of how the Numwheels and Adheasion parameters work

    Dear Bill,

    I too have used the engine brake to regain adheasion in MSTS when the wheels are slipping while sanding. It works up to about 10-12%, then the braking effect overrides the traction effect and the train slows to a stop.

    I had read that RW engineer's use this trick at times.

    In MSTS it made the difference between making the hill with a heavy train and not making the hill. It got down to 1-2mph before going up again. Without the brake it just slowed to a stall.

    It's experiences like these that confirm I'm close to RW performance in MSTS.

    Also, kicking cars using the front coupler. It worked before Bin, and keeping the same values, it works after Bin (not the latest).

    Yours truly,
    Leland




  9. #9
    Join Date
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    Default RE: A reminder of how the Numwheels and Adheasion parameters work

    Is there a trick to kicking cars in MSTS? I've never had much luck.
    Now there would be something to add, if possible, being able to disable (bleed) the brakes in the F9 window! :D
    but I digress...

  10. #10
    Join Date
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    Default RE: A reminder of how the Numwheels and Adheasion parameters work

    Yep, you can kick cars and do dutch drops. It takes a bit of timing to do, esp kicking. Got to use both hands, one on the keyboard and the other on the mouse.

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