Note that when using CV’s 2, 5 and 6 you can only speed match at one predetermined throttle setting. You will not be able to match speeds through the whole range of the throttle.
Locomotives should be speed matched when consisted together to avoid possible damage to motors and couplings and to prevent derailments and wheel grinding caused by mismatched speeds. The following describes how to match the speed of the locomotives you regularly use in a consist and/or a helper that may only be used on part of a route, such as up a steep grade. Keep in mind that you don't have to match every locomotive in your stable - just those that will be used in consists or as helpers.
Not that it doesn't exist, but we've yet to find any article on speed matching when only using CV’s 2,5 and 6 that even mentions, never mind explains, why you can only match locomotive speeds at only one throttle setting. That setting can be 20% of full power, 42%, 57%, 70% or whatever but only the one setting and no others. Be aware of this when attempting to speed match using the procedure that follows.
The reasons for this and how to speed match through the whole throttle range using CV’s 67 through 94 are described in ’Speed Matching ‐ Advanced’.
First thing you must do is make yourself comfortable with the process for changing decoder Configurable Variables (CV’s) with your DCC system. Don't be afraid to experiment, you can always reset any locomotive decoder to its factory default if you think you've got it wrong. It’s suggested you start experimenting with CV’s 3 and 4, which vary the acceleration and deceleration of locomotives. The default is zero (instant start and instant stop) but entering values between, say, 3 and 15 for CV’s 3 and 4 will teach you how to change CV’s and how such changes affect performance.
You don't have to use Ops mode programming (a.k.a. Programming on the Main), but you'll find it much more convenient to do so when speed matching. It is possible to reprogram the wrong locomotive when in Ops mode, but you'll be OK so long as you make sure the locomotive you want to change is the one actively displayed on the throttle.
To give yourself a comfort level when first changing CV’s in Ops Mode, take every locomotive but one off your layout and practice changing its speed characteristics "on the fly" with Ops mode programming.
If you use 'Page' mode rather than 'Ops', you'll have to keep transferring the locomotives between the main and programming tracks.
Don't forget to press 'write' when changing the value of a CV, and 'Exit' when you've matched speeds and finished programming.
DECODER SPEED STEPS
It's important to ensure that the decoder speed steps of every locomotive you're going to use in consists should be the same, i.e. 14, 28 or 128. The reason for this is because for 14 speed steps there is an approximately 7% difference between each one (in other words, the speed will jump by 7% each time the throttle is turned to the next step), for 28 the difference between steps is less than 3.6% and for 128 there is only a 0.78% change between each step. If your consist locomotive decoders are using different speed steps then it's highly unlikely they'll run at the same speed at the same throttle settings.
If some of the CV’s beside the address have been changed from the default then it's probably prudent to record them beforehand so you can re-enter them if things go wrong. The most commonly changed CV’s are:
- CV1 - Address
- CV2 - Kick Start
- CV3 - Acceleration
- CV4 - Deceleration
- CV5 - Maximum Speed
- CV6 - Mid Speed
You should also record CV’s 29 (Configuration Register), 56 and 57 (Dither frequency and voltage), 65 (Kick Start Value), and 66 and 95, which are Forward and Reverse Trim respectively.
Keep track of the changes you make to the CV’s on every decoder. Try the sample record we use. DCC Decoder Record
Most decoders can be reset to their defaults by programming in either CV8 to the value of 8 or CV 30 to the value of either 2 or 30, although these values are not cast in stone. Some require the use of a reed switch. See the instructions that came with your decoder for the proper CV and its value. If you don't know the type of decoder in your locomotive you can put it on your programming track and read the value of CV#8. The number that displays is unique to the manufacturer and a complete list can be found athttp://www.nmra.org/standards/DCC/mfgnumbers.html
THE THREE-STEP PROCESS
'Speed Matching 101', or the Three Step procedure, uses the following CV’s
- CV2 = Volts Start (V-Start)
- CV5 = Volts Max (V-Max)
- CV6 = Volts Mid (V-Mid)
Using the three-step method of speed matching will not make every locomotive run exactly the same, but in most cases it will suffice. If you can get the two locomotives' speeds within a percentage point or two of one another at the same throttle setting you can consider it a success. Keep in mind the locomotives will likely work differently under a load, so you may want to consider speed matching all your locomotives with the same load of, say, 10 cars.
For ultra-fine speed matching through all throttle settings use Decoder Pro on the JMRI program (downloadable and it's free).
- Decide which locomotives you're going to use on a regular basis in consists or as helpers and warm them up for a few minutes.
- Ensure all locomotives being adjusted are programmed with the same speed step mode, i.e., 14, 28, 128. If so equipped, turn off Back EMF and sound.
- Determine which locomotive is going to be the "Master", which is the standard to which other locomotives will be adjusted. Note: this is not the same as the leading locomotive in a consist. It can be the one that runs the fastest at full throttle, or the slowest or even one you like because of its paint job. What is important is that the locomotive is smooth running from a standing start to top speed.
- Adjust the Start Voltage (CV2) of the Master locomotive so it crawls at a minimum throttle setting. Record the throttle setting, which will be either a speed step number (1 to 255) or percentage of full power depending on your DCC system. You can adjust CV# 2 or, with some decoders, 56 and 57, to send pulses of extra power to temporarily boost a sticky motor.
- Turn the throttle up to maximum speed (step 255 or 99%) and then adjust the value of CV5 of the Master locomotive until the locomotive runs at the maximum speed that you like. A method of calculating scale speed appears below.
- Now run the Master locomotive at the consist speed that you like. Record the throttle setting.
- Warm up the second locomotive and adjust the value of CV2 so that it and the Master locomotive both start at the same throttle setting and, when first starting, run at more-or-less the same speed, ideally a crawl. If this second locomotive will only start at a higher throttle setting than the Master, adjust the Master's start-up setting to match this second locomotive. If the second locomotive starts at a lower throttle setting, adjust its CV2 upwards to match that of the Master. You want both locomotives to start moving at about the same speed at the same throttle setting.
- From this point forward you may find a stopwatch (nowadays frequently found on cell phones) useful to roughly gauge when the two locomotives are running at the same speed along the same section of track.
- Staying with the second locomotive, adjust CV5 so that it runs at a maximum speed that you like. It does not have to be the same as the Master locomotive. A switcher, used as a helper up a grade or in a consist, is unlikely to have the same maximum scale speed as the Master.
- Now run the second locomotive at the same throttle setting as that used by the Master locomotive in its consist speed. It will most likely run either faster or slower than the Master at this setting.
- Do NOT adjust the throttle setting. Instead, adjust the value of CV6 of the second locomotive until its speed more-or-less matches that of the master.
- N.B. If the value of CV6 (V-mid) is higher than that of CV5 (V-max) then CV-6 will be the maximum voltage/speed. CV6 should always be lower than CV5, even if only by one speed step.
- Set up the two locomotives as a consist on your throttle (the terminology is likely 'MU'). Do not couple them together. For now keep them about 12 inches apart.
- Run both locomotives at the consist throttle setting. Both locos will move more-or-less in unison although one will likely be a bit faster than the other.
- Cancel the consist (MU) on the throttle and, preferably in Ops mode so the changes can be seen instantly, adjust the second locomotive's CV 6 (V-mid) until its consist speed is similar to the first loco.
- When the consist speeds are as close to identical as possible, you can couple the locomotives together and operate them from a standing start to their consist speed. They should run more-or-less in unison. Do not, unless the maximum speeds of the two locomotives are the same, increase speed past the consist speed - one of the locomotives will not be able to keep up, possibly resulting in either a derailment and/or damage.
- You can also, at this time, adjust CV’s 3 & 4 (acceleration and deceleration) so the locomotives match one another as they speed up and slow down.
- Adjust any other locomotives that will be used in consists to match the Master, but keep in mind the start speeds of all locomotives used in a consist should be the same, so adjust the value of CV2 in the locomotives as necessary (Step 7).
- Turn on Back EMF and sound.
i. If the value of CV6 (V-mid) is higher than that of CV5 (V-max) then CV-6 will be the maximum voltage/speed. CV6 should always be lower than CV5, even if only by one speed step.
ii. The decoders must be using the same speed steps - 14, 28 or 128
iii. For ultra-fine speed matching through all throttle settings use Decoder Pro.
Estimating Scale Speed
- Measure a length of track – preferably straight – in inches. If it’s sinuous, lay a length of string between the rails then measure the string. Use the following formula:
- O Scale Miles = measured track (in inches) x 0.0007575
- HO Scale Miles = measured track (in inches) x 0.0013731
- N-Scale Scale Miles = measured track (in inches) x 0.0025252
- Run your locomotive over the measured distance and time as accurately as possible. Use a stopwatch for more accuracy.
- Scale MPH = (60 ÷ t) x d x 60 where t = time in seconds and = distance in scale miles.
Example: Measured HO track length is 200 inches (0.27462 scale miles), and it takes a locomotive 40 seconds to cover the distance. 60 divided by 40 = 1.5 x 0.27462 x 60 = 24.716 scale miles per hour.
|Scale Speed over FIVE (5) feet of Track
Times are rounded to the nearest 1/10th second>
|Scale Speed||N. American N-Scale
To calculate the EXACT scale speed over five feet timed in seconds, use the following:O Scale: Divide 163.636 by the time, in seconds, to travel five feet
OO Scale (1:76): Divide 258.794 by the time, in seconds, to travel five feet
HO Scale (1:87.1): Divide 296.591 by the time, in seconds, to travel five feet
N Scale (UK 1:148): Divide 504.545 by the time, in seconds, to travel five feet
N Scale (N. America 1:160): Divide 545.454 by the time, in seconds, to travel five feet
|Time to Travel FIVE (5) Feet at Scale Speed|
|Secs.||N Scale MPH||HO Scale MPH||O Scale MPH||Secs.||N Scale MPH||HO Scale MPH||O Scale MPH|
The results in both tables can be extrapolated to determine intermediate times/speeds