Why do locos run at different speeds in forward or reverse?

MP333 Mar 14, 2020

  1. MP333

    MP333 TrainBoard Supporter

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    Title says it. I've noticed many of my locomotives run forward at a different speed than the same throttle setting in reverse. USUALLY reverse seems to be slower, but not always. I can't wrap my brain around any reasons. It's just a motor with some gearing.

    I run Atlas and Kato N-scale exclusively. I would love to know if this is a known phenomenon or just my imagination? To be truthful, I have never actually *quantified* my theory, but I might do that now.
     
  2. RBrodzinsky

    RBrodzinsky November 18, 2022 Staff Member TrainBoard Supporter In Memoriam

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    You are correct, they shouldn't. When they do, it may be due to some binding of the mechanisms in the opposite direction. Make sure the gears in the trucks turn freely in both directions, that there is no obvious of the worm gears, etc. But, even after all that, some motors may just not be the same. That is why most DCC decoders have Reverse and Forward trim settings, to help adjust for this.
     
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  3. viperjim1

    viperjim1 TrainBoard Member

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    Yes, the reason for this is the helix in the worm gear. If you look at the rotation of the gear when it is moving forward it is easier to push than pull. It takes more effort for the gear to remove the slack in the main gears and idler gears than to move in the opposite direction? I don't know, but it sounded good IMHO lol
     
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  4. acptulsa

    acptulsa TrainBoard Member

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    As a general rule, the more they run one direction, the better they run in that direction. How do you explain "broken in"? The friction between parts polishes those parts, reducing friction.

    When a locomotive runs one direction, the surfaces of the gear teeth get polished on one side and not the other. The locomotive only gets halfway broken in. Run it that way, and it behaves like a machine which is broken in; run it the other way and it behaves like a locomotive which never got broken in.
     
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  5. MP333

    MP333 TrainBoard Supporter

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    I guess I should start running things in the wrong direction on purpose, LOL. Thanks for the answers so far. Interesting.

    I'll pay special attention to my new GP38-2s to see about this.
     
  6. gjslsffan

    gjslsffan Staff Member

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    Different motors, same theory, may help explain some of the reasons for the difference in directional motor speeds. Even if the brushes are worn a bit on one side from running in one direction more than the other it changes the timing and therefore speed. Brushes that fit in the brush holders too loosely can explain same. Slight shifting of the magnets can effect this too.

    Just a bit of the info out there.

    "It's due to the timing of the commutator (which on a DC motor affects the timing of the magnetic field generated on the rotor).
    It's usually advanced on one side by a certain angle to cause the motor output to be higher for a certain rotation direction. That's most likely what you're seeing here.
    The motor in reverse would see a retarded timing - which translates to lower output.
    In RC car racing back when brush motors were the common motor racers would adjust the timing on some of the motors to get even higher output than previously possible without changing anything else. (and in some racing classes this was considered cheating)
    "

    Hope this helps, I know I learned something.
     
  7. DCESharkman

    DCESharkman TrainBoard Member

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    It is a property of the motor. Sometimes the motor response is not linear in both directions, meaning spins at the same rpm at the same voltage level. This is related to the hysteresis of the motor core and winding.

    In my locomotives I have seen many have very slight differences and only a few with very pronounced differences. I even have several where the locomotive goes faster in reverse than forward. This all points to the the characteristics of the motor more than anything else.
     
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  8. gjslsffan

    gjslsffan Staff Member

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    Does some of this depend on a relay being charged to reverse the wave?
     
  9. DCESharkman

    DCESharkman TrainBoard Member

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    It is caused by possible materials issues in the cores of the motors. Some sort of non-uniformity, or inconsistent structure at the molecular level that does not produce a symmetric hysteresis response of the magnetic cores, this also be caused to a lesser extent with a possible slight error or inconsistency in the motor winding's as well. The motors are mass produced and so there is always statistical variances between the motors within each production batch.

    The reset of the motor when the direction is reversed is nearly instantaneous, the delay is due the mechanical momentum of the locomotive. It has to compensate for the shift in the weight of the locomotive.
     
    Last edited: Mar 15, 2020
  10. MP333

    MP333 TrainBoard Supporter

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    This is something I never knew. I had no idea what "trim" did, and have never messed with it. I've never found any good descriptions of how to set up deeper things in DCC, aside from YouTube videos lately.
     
  11. DCESharkman

    DCESharkman TrainBoard Member

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    Digitrax was the first decoders I found with Forward and Reverse trim. NCE just had one trim setting. Lenz never had Trim at all. And ESU and Zimo come here later with trim. Using Trim, it becomes so easy to speed match your locomotives with no fighting between the couplers.
     
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