DCC Consisting And Speed Matching

Fellow blogger John R has challenged several of us to come up with 7-unit loco consists. He knew I'd been thinking about putting together a post on this, so he encouraged me to make that post as part of the project. I've been converting to DCC for about five years, and one of the big reasons I chose to go that route was the ability to speed match locos and run them in consists. So here's my current take based on the experience I've had.

On the other hand, I don't want to get into the technical weeds -- in fact, my whole approach is not to do this, and I think a lot of writers and YouTubers make things much too complicated, including Model Railroader and MR Video Plus -- over the past year, I've ended my subs with both in part due to what I think is poor coverage of DCC. I will be writing this from the perspective of an NCE PowerCab user.

There are some gotchas to start with. The first is that Digitrax handles consisting differently from other manufacturers. The newest form of consisting is "Advanced", which I use, but I'm not going to go into detail over it. Just follow the PowerCab prompts if you use one. While NCE will set up a consist in Advanced using Digitrax decoders, you must change a CV on any Digitrax decoder to avoid "unpredictable results". If you are not using a Digitrax system, you must set CV 57=102 on all Digitrax decoders if you want to use Advanced consisting with them. Not a biggie, just remember to do it.

The second big gotcha is that Bachmann factory decoders do not support all NMRA DCC CVs, including those that support speed matching. (The Soundtraxx decoders that come with the Sound Value Bachmann range do support all speed matching CVs, although some sound-related CVs aren't supported.) However, many of the Bachmann factory decoders are also noisy, and some have a compulsory speed and acceleration curve, so it's best to replace them if you're upgrading your DCC game no matter what. The NCE BACH-DSL decoder is a drop-in replacement for Bachmann factory decoders and is reasonably priced.

The next issue is how you run your layout. I like switching operations, and my layout has mountain grades, so my average train speed is pretty low -- in general, I'm not modeling the BNSF Transcon with 70 mph intermodals or 90 mph Amtrak. More typical freight train speeds in the 20-40 mph range come off as quite low on a model layout. As a result, I match speeds only at the lower end of the model loco's speed range -- and many model locos, especially in N scale but still in other scales as well, have unrealistically fast speeds in the top half of throttle voltage settings anyhow.

As a result, while the DCC standard allows elaborate speed curves, I find these unnecessary and too much work. I got frustrated with the MR Video Plus discussion of speed matching, which involved using a scale speedometer to mach loco speeds across a whole speed curve. Some guys may well like to do this, it's a hobby after all, but it's not my thing.

Instead, I have a very simple approach, which works for me. In general, so far, I run two- or three-unit consists, and this works. I use a small number of CVs. CV 6, the mid-range speed, is the most important for me. The range is 0 to 255. 0 is the default, while values between 1 and 255 regulate the mid-range speed (which for my purposes may as well be the maximum for normal operation.) What I do is take two locos that I want to speed match. I set up a consist for the two locos on the NCE PowerCab and run them uncoupled on the same track with no modification to the CVs. I simply observe which loco is faster, and all I do is adjust CV 6 on the faster loco to slow the faster one down to the speed of the slow one. Typically a value between 30 and 60 works. If, after resetting the CV on the fast loco, the two locos run visibly at the same speed in the slow range, I'm satisfied.

The next question is the starting voltage. The two locos need to start at the same speed step to avoid one pushing the other at the very lowest speeds. What I normally do is when I install a decoder on a new loco, or get one with DCC already installed, I adjust the starting voltage CV, CV 2, so that the loco will start at speed step 1. A really good loco, like a Kato, will start at speed step 1 with no adjustment. Others will need values between 10 and 40. If I'm consisting locos, and after adjusting CV 2, both or all start at the same voltage and they have the same mid-range speed, I'm pretty close to satisfied.

The one additional factor is CV 3 and CV 4, acceleration and deceleration momentum. Here the gold standard is Kato, whose flywheels have good dwell time, along with very smooth running motors. The problem with consisting is that a loco with better flywheel dwell time will be pushed by a loco with less momentum when accelerating and will push a loco with less momentum when decelerating. So, for instance, if you're consisting an Athearn RTR with a Kato, CV 3 and CV 4 should be set to 1 on the Athearn, with the Kato left to 0.

But there are no solid rules, each consist will be unique, and locos can vary widely even from the same manufacturer. It will also depend on how you run your layout, and your preferences can well change over time on your own layout.

It's a good idea to keep a file with notes on what values you've set on every loco. JMRI Decoder Pro can be used for this, but I don't completely trust it, and I would rather have my own comments and my own format for this kind of thing. If you keep the file on a computer, be sure it's backed up!