It doesn’t happen often but it does happen. After consisting several locomotives and running them you notice that one (or more) appears to be bucking or surging. This becomes very apparent if the consist is pulling a load of several cars. The cars appear to noticeably speed up and slow. The first thing to do before troubleshooting anything is to make certain that your track is clean and all the wheels are also clean on all the locomotives. Also check that there is no mechanical binding on any of the locomotives. Lastly check that all the locomotives are properly speed matched. If they haven’t been checked in a while, check them!
Assuming they are properly speed matched, the problem
is, almost always, that one of the decoders has the BEMF loop gain (Ki) set too
high for consists. This causes the engine to buck (surge) when it tries to stay
with the other locomotives. You may have heard that BEMF is like cruise control
for an engine when it detects it is moving too slow for requirements it speeds
up and when too fast it slows down. How quickly it responds to these errors is
controlled by the gain. Too high and it appears to buck or surge.
There are some model railroaders that turn off BEMF completely when dealing with consisted locomotives (except the lead). The author doesn’t do this as he consists and un-consists his locomotives constantly and doesn’t want the additional hassle of having to deal with turning BEMF on and off.
The problem now becomes determining which engine(s) is having the problem. In consists of more than 2 locomotives it’s not difficult. Take the locomotive you suspect to be surging out of the consist and see if the consist now moves smoothly. If not, try another until you find the right one. For 2 engine consists it a little more work but we’ll get to that shortly.
In Soundtraxx decoders BEMF loop gain is controlled by CV 210. The value in the CV usually defaults to 16. Set it to half the value in the CV (if 16 set it to 8). Test it by putting it back in the consist and see if it fixes the problem. If it runs smoother but not completely smooth you'll have to play with the value (lower not higher but not zero). Do not set the value lower than you need to. It will cause the locomotive to drag and not react well to speed changes.
For two engine consists since you have no idea which of the 2 locos is causing the problem, take one and note the value in CV 210. Follow the procedure above. If you cannot get it to fix the surging, set it back to the original value and repeat the process on the other loco.
Every time the author has had surging locos with Soundtraxx decoders this has fixed the problem.
Other decoder manufacturers use different CVs for their BEMF variables and also different values so read their documentation.
Read about CV210 in the Soundtraxx reference
manual here on page 62.
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