Why do i need a powered "B" unit to my F7"A"?

Discussion in 'FAQs' started by YmeBP, Dec 31, 2006.

  1. YmeBP

    YmeBP Member

    What benifit is a powered "b" unit? I've been mulling it over in my head, even did a few experiments and i can't see how it would help pul more cars. Is it just for looks?

    COMBAT Member

    If it is powered it will pull more cars. Its another engine. I dont know how else to explain this. Sorry!
  3. MilesWestern

    MilesWestern Active Member

    A powered "B" unit, is just like a cabless version of your "A" unit. It has the identical drive at your "A" unit, it just looks a but different! :D

    COMBAT Member

    What he said. :oops:
  5. YmeBP

    YmeBP Member

    But wouldn't it have to go exactly the same speed as the "a" unit? If it went faster it would just push the engine in front of it negating any budy gains, and if it were a tad slower it would be a weight on the engine in front.

    And if it went exactly the same speed wouldn't IT (the b unit) be doing all the pulling? Wouldn't the a unit be essientially just driving in front of the b unit?
  6. LongIslandTom

    LongIslandTom Member

    The Athearn locos all have the same gear reduction ratio, so their wheels should spin at approximately the same speed given the same power settings. When both locos are spinning their wheels at approximately the same speed, they should be sharing the pulling load about evenly.

    Hope this helps!
  7. Gary S.

    Gary S. Senior Member

    Making use of my rudimentary knowledge of electric motors, here are my thoughts...

    Let's say the A unit is turning its wheels slightly slower than the B unit. At first, it seems that the B unit would be pushing the A unit. But what actually happens is, the A unit would experience an unloaded condition... and what happens to an electric motor under no load? It speeds up. It would speed up to the point that it would share the load with the B unit. Same thing holds true in reverse. As long as both engines have somewhat similar gear ratios and running characteristics, they should automatically share the load, due to the nature of electric motors. I believe this holds true for the prototype also.

    Considering the prototype, even with today's computer control and monitoring of the drives, it would be impossible to have every truck of a 4 unit mu consist produce the exact same amount of power to the tracks. There are just too many variables to control it so precisely. Fortunately, the very nature of electric motors in tandem causes them to work in sync. Think about it, they used multi-unit consists long before they had sophisticated electronic controls, and they still worked.

    Come to think of it, this holds true for steam, and any other type of engine. A reduction in load on any single unit causes it to speed up. An abundance of load on any single unit causes it to slow down. So they end up working together to share the load.

    Thoughts anyone?
  8. YmeBP

    YmeBP Member

    tooth1 I get it, thanks for the info!! So it's like a mini tug of war going on and in that tug o war they share the load of the cars.

    So this leads to another dillema :), now that i can pull enough trians to litterally take up the entier cross section of my track how do i stuff more linear feet into my layout so i can have more cars to pull!? I have to figure out how to get this layout in the computer to match my alout on the floor and i'll attach it ;).

  9. Triplex

    Triplex Active Member

    No. The pulling force of the A unit is transmitted through the B unit. Speed and tractive effort are entirely different. Speed is governed by how fast the wheels turn, tractive effort by how much torque is turning the wheels.
  10. doctorwayne

    doctorwayne Active Member

    You can easily observe this in action, especially if you're using Kadee couplers. Make up a train, preferrably one heavier than one locomotive can pull. Put one loco on the head-end and another on the rear, as a pusher. It helps if the two locos have similar operating characteristics, but will work in any case, unless the disparity is too great. Now apply enough power to get the train running at a reasonable speed; that is, a speed at which the train can traverse your layout without adjustment. Now, follow the train, watching the slack run in and out on the couplers. On straight level track, any train/loco combination will have some point in the train ahead of which all couplers will be in tension, and behind which all couplers will be in compression. This point will move, either forwards or backwards, as the train encounters curves, inclines, and downgrades, depending on which loco is doing what percentage of the work.

  11. Gary S.

    Gary S. Senior Member

    Very interesting point Wayne. As you allude to, if there was too great a disparity between the locos, then the faster/stronger one may end up pushing/pulling the other loco.

    Now another thought for YmeBP... you may not need a powered B unit. If you will never pull long, long trains where the extra power was needed, it may be money-wise to just buy a non-powered B unit.
  12. doctorwayne

    doctorwayne Active Member

    While that is true, and can also result in some pretty rough running, if the load is heavy enough, both locos will still be contributing something towards moving the train. Unfortunately, a lot of their pulling power is lost because they're "fighting" each other.
    If two locos are similar in speed, regardless of their relative pulling ability, they should work okay with one at the headend and one as a pusher. If there is a large speed difference and you absolutely have to use those two locos, run them together on the front end. It's been often said that the faster loco should lead, but I don't think that it makes much difference.
    If you run a train with a pusher, and the speed difference is too great between the two locos, things can get exciting, particularily at the start or end of a grade, where that "neutral" point in the coupler slack shifts too far, and too fast in either direction.

    It's useful to remember that the sharper your curves, the greater the drag on the train, and likewise, the steeper the grade, the greater the drag. Combining a sharp curve with a steep grade compounds the effect. If your layout is fairly small, this may not come into play, as train length may be limited anyway.

  13. Gary S.

    Gary S. Senior Member

    Wayne, you're making me think, and that is a GOOD thing! There are 2 circumstances to consider:

    1. The train is too heavy for one engine alone and the second engine is needed to be able to pull it.

    2. The train is not too heavy for just one engine, the second is there just for aesthetic reasons.

    In the first instance, a wide disparity in pulling power wouldn't matter that much, as the more powerful unit is already overloaded anyway, and any pull from the less powerful unit would add to the pull of the more powerful unit.

    In the second case, I think a large disparity could actually cause the stronger loco to have to pull or push the weaker loco. Say we have loco A turned up to pretty near maximum force, and it could pull the train at 80 scale mph all by itself. Now, worst case scenario is we attach loco B but leave it turned completely off. It actually is just more load on loco A, and loco A would be better off if loco B wasn't even connected in the consist. Now just barely put power on loco B to where it wants to run at say, a scale 10 mph. The stronger unit would still be pushing/pulling it, and loco A would still be better off if loco B was not connected in the consist. As we turn up the power, there would be a point where each unit would start helping with the load. Where this point is, I have no idea.

    Does that sound reasonable?
  14. doctorwayne

    doctorwayne Active Member


    On my layout, even though it occupies its own room, there's not much straight track and not even much level track. My train length is usually limited by the length of my passing sidings, but even then, some trains require more than one loco. I test all of my locos to ascertain how much they can pull up the worst grade on my layout, then assign them a "tonnage rating". Different divisions of the layout have different "ruling" (steepest) grades, so each loco has a tonnage rating for each division. If loco "A" can handle 5 cars, and loco "B" 12 cars, then the two together should be able to handle 17 cars. Since I've remotored most of my locos, most will run quite well together. However, there are a few that run slower than most, or have a higher starting voltage, so, when including one of these as power, I generally adjust the combined tonnage downward by a car or two.
    Because I also run "live" loads in hopper cars, these are also factored into loco ratings. Generally, one loaded hopper (8 ounces) in a train is counted as one car. Two or more are counted as two cars each. A pair of modified Athearn Mikados can handle a 24 car train of general freight anywhere on the layout, but are rated for only 12 loaded hoppers. Gondolas carrying loose loads of scrap are rated the same as loaded hoppers.
    Because of all the curves and grades, most trains are kept fairly short. A long train can be wrapped around several curves at the same time, with different parts of the train going either uphill or downhill at the same time, not the best scenario for keeping everything on the rails.

  15. Jim Krause

    Jim Krause Active Member

    To answer your original question. You don't "need" a powered B unit if you are running short trains and if you have a flat layout. Assuming of course that you have a good quality A unit.
  16. YmeBP

    YmeBP Member

    O.k. this is likely to be how my layout ends up, i plan to add at least 2 levels of elevation to the back side of my crawl space area for mountain tunnels and then a f1 style tyco race layout to the right w/ some track intercrossing w/ the railroad for later animation.

    Currently the F7a is up to 13 unloaded cars, some w/ crappy, sludgy, friction filled plastic wheels and trucks. I think i'm going to see how many cars it can pull i'll have to pull a couple off some of the other loops i have going ;).

    I plan to have 2 up to 4% grades when i'm finished and i'd like to be able to pull about 20 cars around as my longest consist. I like the way 13 looks :). I guess it's all about the peculiarities (wow sat word) of my layout and how my loco's take to it.

  17. CCT70

    CCT70 Member

    If you don't mind the intrusion, let me throw this curve at the discussion too. A lot of times, while watching local trains work, you'll notice two locomotives on a short local. With a train of about 20 cars, it doesn't appear that the second unit is necessary, when one would pull twice as many cars. However, there are a multitude of reasons why the roundhouse would add a second unit. Usually it is faced the opposite direction and many times only one unit is online even while the other is shutdown and may not even be fired up for the return trip. The main reason behind that being that when you go out, you run unit 1 cab first and on the way back, when you can't just turn the train, you run unit 2 cab first back to the yard. Remember that with multiple unit operation, you don't need to have all units on-line, not even the controlling unit you are operating. A simple explnation of this in action is when Illinois RR Museum runs their UP DDA40X Centennial. They put it on the point of a train with another engine (usually their Ex-SP SD-7, the first SD series engine built). The SD-9 is providing the motive power and neither engine in the DD40 is running, but the engineer is running from the DD40. It makes for a little quieter trip for us hoggers, but I don't like it because I run a locomotive (or a lash-up of locomotives) the old fashioned way, by sound and feel in the seat of my pants to know what it's doing and what it needs, and thus what I need to do to safely and efficiently run the train. The Central California Traction does this with their two GP's. They have the 44 and 1790 back to back and run the 44 up to Lodi and the 1790 back to Stockton, that way they are running nose first both directions. One locomotive is usually fired up and on-line, and the other is not, unless they need the extra power. Same thing.

    Hope that explains another side of things to answer your question.

    Also, be carefull about trying to pull too many cars around a small layout, one of the most common gremlins with that is "stringlining". If you don't know what that particular term means, ask the folks of Dunsmuir California or better yet, look up "Cantera Loop Derailment" on the net. :D
  18. YmeBP

    YmeBP Member

    I always mind the lack of intrusion especially when i get to learn something new if there is an intrusion, but i can think of a couple times when intrusion isn't so nice .... goldtoth1 sorry i have a warped sense of humor and i couldn't let a setup like that pass hahahaha.

    Thanks for the info!! :) I looked up stringling and came up blank, i did however see some images and read about the cantera loop!! Wow! I'm glad i'm not toting anything around my layout, i've had some pretty major derailments but i didn't kill 40 miles worth of fish! Yowza!

    I went to the brandywine river museum today and i saw a consist of 74 cars w/ a plow and 2 fp7's pulling it, it was o gauge and o scale. Picture of the head end attached, more pix at: http://www.skyersfamily.com/gallery/v/Trains/BrandywineRiverMuseum/

    It was setup w/ the front to back 2 engine deal :). All three of these pix are of the same consist.

    Attached Files:

  19. doctorwayne

    doctorwayne Active Member

    Hey Tom, that happened to a train that I was running today. I'll probably take some pictures, but I'm unsure when I'll be able to post them, as my current computer will not retrieve pictures from the camera.

    Information from the scene is sketchy, and photographers have not yet reached the area. The incident occurred on the Grand Valley's high bridge over the Speed River, shown below in a "file" photo.

    According to eye-witnesses, a northbound freight, powered by four locos,similar to the ones pictured below, had safely


    traversed the span, along with 6 or 7 freightcars, when, without warning, the 8th, 9th, and 10th cars in the train were catapulted into the river below. In addition, the 6th and 7th cars in the train rolled down the embankment, narrowly missing a coupe headed north on River Road. The 11th car also derailed, but remained on the bridge. No injuries have been reported.
    Highway traffic through South Cayuga remains blocked by the stalled train

    The crossing at South Cayuga, pictured in less trying times (File Photo)

    and reports indicate that the Railroad is trying to move the train from the south, where the caboose had just cleared the Dunnville station platforms. Unconfirmed reports indicate that the train was 71 cars long, far in excess of usual practices. Railroad officials declined to comment. More news as it becomes available.

  20. YmeBP

    YmeBP Member

    Awesome pictures and the commentary had me rolling!!

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