Momentum Oddity

[MarkInLA]

Hi, I was under the impression that a momentum value can be added while loco is rolling or not..Of course I have done both..But I don't understand why, when idling-loco receives a momentum value/change, it lurches/jerks forward a few scale feet...I could swear my first HO DCC loco, Bachmann Ten Wheeler never moved a scale inch when adding momentum at idle..Now, having added Bachmann Mogul, both lurch forward when adding/changing momentum ( I don't mean at same time)..I think Mogul is not a Spectrum..If so, has she possibly affected Spectrum Ten Wheeler ? I know this sounds crazy...But ,again, I could swear Ten Wheeler didn't jerk at idle...I realize, too I can add momentum while under way with no lurch and or leave loco this way..But the jerking action at idle is so unrealistic; a loco jumping forward on the rails...I've even coupled on to cars I was not meaning to couple to due to lurch...Well, fellas in NCE/DCC forum say the lurch is normal as it is a programming action , seperate from F1-F0 function keys...What's the bottom line ?? ...In a related topic, is Mogul capable of having a CV/ binary-code changed in order to add brake squeal, which is so nice on Ten Wheeler ? ...Thanks much,   Mark In L.A.

[Jim Banner]

The decoder inside your locomotives is designed to send a signal back to your DCC command station when it has successfully received and installed a program change.  This signal is in the form of a momentary increase in the current drawn by the locomotive.  The only way that a decoder can momentarily increase its current draw is by sending that current somewhere, and that somewhere, as you have noticed, is to the motor.  That momentary burst of current to the motor is what makes the locomotive jump a bit.  All decoders do this, not just Bachmann ones.

How big the jump is depends on the quality of the locomotive and the motor inside it as well as the state of lubrication of the locomotive and how well broken in it is.  The higher the quality, the better the lubrication, and being properly broken in all make the locomotive jump farther for a given pulse of power.  If your locomotive had an inefficient motor, loose gears, and/or desperately needed lubrication, the pulse of current probably could not move it at all.

Your observation that one of your locomotives did not originally jump and now does sounds quite accurate.  When new, the motor and running gear can be a bit stiff and the pulse of current may not move them or may move them only slightly.  But once the locomotive has run a few hours, the motor and running gear run freely and now the pulse can move the motor, running gear, and the locomotive along with them.  It may be annoying but it is a sign that you have a quality locomotive in fine running shape.

Surprisingly, this jump happened occasionally in the real world in the days of steam.  This very dangerous occurrence was normally limited to old, worn out locomotives which were a bit out of time.  Because of the timing problem, the locomotive could high centre, that is, you could apply steam pressure to the cylinders but the locomotive would either not start at all or would suddenly start which a jump and wheel spin.  If the locomotive also had a leaky main valve, it could jump unexpectedly, throwing people around in the cab or hitting them if they were too close to the locomotive.

Jim

 

[MarkInLA]

Hi Jim, I just now got to read your explanation of my jumping locos via momentum change at idle..It is a very informative response ...Thank you for easing my mind about it all. That sounds spot on about Ten wheeler not jumping at first due to new drive train parts and such and how the jump shows it's a quality loco..Surprised too to learn it occured on the Realroads  ( as I've coined it ) too !!  I have learned to set momentum and just leave it alone longer or change whilst rolling..While you are reading this I do want to ask: If Mogul has no brake squeal do I assume correctly that I can not add it with present/factory decoder ?   Thanks again,   Mark