If you do not know what BEMF is, go here: Back-EMF (BEMF)
BEMF is a nice feature if used properly. However defualt CV value of early People compared BEMF decoder on how well the performed when creeping along a 1 Tie/Minute type speeds with pulling torque. Eventually people started noticing that sometime when you put a consist together with multiple engines having BEMF turned on would sometime fight each other
Modern BEMF decoders give the user some options on how to set the effectiveness of the BEMF intensity when involved in a consist. Unlike running a locomotive by itself, running a consist of BEMF engines together potentially. opens the door to some performance issues that vary with decoder manufactures. Specifically if your consist has engines with BEMF using decoder from different manufactures, engines may appear to be "Fighting" each other.
This section talks about the Identification and adjustments required to get the locomotive to cooperate in a consist. This is a bit different than tuning a decoder for low speed performance.
How do I know if I have a problem?
Simply put each engine with BEMF wants to control the consist speed. If the engine are not speed matched well while operating in consist mode, one or more engines may:
1) slide it's wheels down the rail while the rest of the consist is going faster. Stated another way, it wheels are turning to slowly relative to the establish consist speed.
2) spin it's wheel down the rail while the rest of the consist is going slower. Stated another way, it wheels are turning to fast relative to the established consist speed.
Why?
a) The default settings for BEMF decoder are often tuned at the factory for awesome performance at the slowest speeds as opposed for the needs of consisting.
b) The decoder does not support BEMF in Advance Consisting mode in some way or by defualt.
Solutions
The following should be done in this order:
1) If you plan to use Advanced Consisting, check to see if the decoder supports special BEMF setting in Advance Consist Mode. Use this mode if possible. See CV's for BEMF below for more information.
2) Speed Match the engines. This means working with Start (CV2), Mid(CV6) and Max (CV8) or use the Speed Tables. Time it takes for a engine to go around a defined distance on the layout...a test track. Set one engine as the standard and adjust all others relative to that engine in terms of speed matching. Test the engine speeds a low, mid range and full scale speeds. (Not full blast on the throttle). The other way is to use a Train Speedometer and determine what scale speed a given speed step will correspond to. Pick a standard and adjust all locomotive to that standard.
3) Adjust the BEMF intensity DOWN such that is fades quickly as speed steps increase. In other words, by the type the train gets up to normal train speed, the BEMF is almost off.
CV's for BEMF
Defintions:
Any = CV's that are active in any mode. The mode of consisting does not apply.
Basic = CV's that are active when the decoder is NOT part of an Advance Consist. Covers "Universal", "Standard", "Double Header" consisting. To learn more go here: Consisting Term Confusion.
Advanced = CV's that are active when the decoder is part of an Advanced Consist.
Nibble = 4 bits is called a Nibble. This is a computer term. It is used to describe the breaking up the normal full 8 bits of a CV value into a lower 4 bits and a upper 4 bits that have independent functions.