Reverse loop length

Discussion in 'DCC & Electronics' started by eric halpin, Mar 18, 2008.

  1. eric halpin

    eric halpin Eric Halpin


    I have noticed, with ever increasing frequency, an issue with a short length of reverse loop (rl) track supervised by an auto polarity changer from Digitrax. It is NOT consistent and therefore a bit confusing to me. Basically, I run trains with metal wheels longer then this short rl. Sometimes, the train momentarily stalls upon entering or exiting the loop! Is it true that rl's must be longer then the train? If so, I don't see why because it is only the engine that triggers the polarity change. Can you folks enlighten me please.

  2. pgandw

    pgandw Active Member

    The metal wheels on the cars also span the gaps at the entrance end of the reversing loop, causing the auto-reverser to sense the short and flip the "polarity". With a train longer than the reversing loop, you can get both sets of gaps being spanned at the same time, which confuses the auto-reverser. If the gap spanning remains, the auto-reverser can get confused enough to release the magic smoke.:cry:

    Part of keeping the magic smoke in is to use train lengths shorter than the reversing section, lengthen the reversing section, or promise yourself never to use anything but plastic wheels on any part of a train that is longer than the reversing loop. My memory isn't good enough to keep the promise, so I use one of the other two solutions.
  3. baldwinjl

    baldwinjl Member

    Right, it is NOT only the engine that trigger the auto reverser. You can do it with a screwdriver! The reverser I have (the DCC specialties one) detects shorts across any of the four gaps, and reversers if any gap has a short, which is caused by a single metal wheel bridging the gap.
  4. 60103

    60103 Pooh Bah

    Depending on the reverser, you can also simulate a telegraph office by dropping a loonie across the tracks in the reverse section.
  5. eric halpin

    eric halpin Eric Halpin

    Okay, I think I get and understand your comments BUT is it not correct that it is the locos metallic wheels that create the short as they touch the mis matched polarity on the rails. My rolling stock while they do have metal wheels are running in non conductive trucks and thus no current can pass from one rail to another unlike the loco??
  6. AlienKing

    AlienKing New Member

    A single wheel could bridge the gap for a very short time if you have small enough gaps. You may want to cut the gaps a bit larger, and use either the plastic insulating railjoiners or a non-coductive material to fill it in. You could also try painting the track around the gaps (1 or 2 mm) with nail polish.
  7. woodone

    woodone Member

    wheels will short

    Well, when your metal wheels come to the gap in the rails, on the same rail, it bridges the gap. The rails are not matched for poliarity. The inside rail or the outside rail, makes no differance. So the wheel bridges the gap and the reverser see a short. Does not care if your have insulated axles or what, the metal wheel is what caused the short.
    At least that is the way I see it.
  8. baldwinjl

    baldwinjl Member

    It really isn't a bad thing, you just have to allow for it.
  9. eric halpin

    eric halpin Eric Halpin

    Okay guys. I think I got it now. I sure wish I had of known about the issue when I was laying the track plan out. None of the books I have read on DCC mentioned the issue and I don't recall any articles in the mags talking about the issue especially now as metal wheels are becoming the standard! Regardless, I physically cannot change the track now. So shall consider increasing the gap method BUT I am concerned if I do that the loco will stall as it tries to pass through the gap or the auto reverse device won't function as well when a loco passes through!!?? Thanks for the comments.

  10. baldwinjl

    baldwinjl Member

    Making the physical gaps wider is not good solution. But even so, you should not have to change the track plan. You might have to move the electrically reversing section, but that should not be too difficult. If you could post plan, I'm sure we can figure it out!
  11. TrainNut

    TrainNut Ditat Deus

    Ditto that.
  12. Harold Cole

    Harold Cole Member

    When i run 3 AC4400 through my reverse loop section the 3 engines are longer than the reverse loop section,It is gapped on both ends with insulated joiners and i've never had a problem.I had a Digitrax reverser and now just use one of the DB200 Boosters as the reverser.I've also run passenger cars through it and no problems either,and the passenger cars have light units in them.Could it be the reversers used,and have you actually tried it out to see if it stalls.It might work ok,but as several have already said,making the gap larger is not a good idea.Try switching the leads to the reverse track section and see what happens.
  13. baldwinjl

    baldwinjl Member

    Hmmm, the only way that you can't be getting a short is if the wheels not quite the length of the section. Here is why, and it makes no difference what reverser you are using. In the two sections that are not the reversing section the "polarities" of the rails on the same wheel are opposite, or you would not have a reversing section. The front and rear wheels of each loco, on each side, are electrically connected. So, if the first and last loco are both straddling the gaps, the reversing section cannot possibly match the "polarity" of the track leading into and out of the reversing section at the same time.

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