8005 at Trestle


Speed Matching - Advanced
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We'll start off by showing how changing CV's 2, 5 and 6 affects locomotive speed. Then we'll give some examples of how they can be set up for different types of locomotives, such as a GP38 and an Alco S2 switcher or a helper and how they can be speed matched.

Be aware that using CV's 2, 5 and 6 you can only speed match at one predetermined throttle setting.

Digital throttles indicate either speed steps, 0 to 28, or 0 to 128, or percentages, 0 to 100%. Other throttles have a dial, which require a mark of some sort to consistently return it to the same spot in its rotation. We'll deal with this in a little while.

We'll assume the decoders in your locomotives are NMRA compliant and the CV's we'll be adjusting, (#'s 67 through 94), have values ranging from 0 to 256.

To give a visual indication of what happens when programming CV's 2, 5 and 6, the chart below shows an imaginary speed profile for two GP38's but from different manufacturers. The value of CV5 on both of them has been adjusted so they run at the same top speed. CV5 on the red locomotive has been set to about 230 while the blue is about 180. (The settings may be much lower on your locomotive.) What's important to understand is that the two locomotives have different CV values but run at the same top speed.

Chart 1

Now, keeping in mind that you can only speed match at either full throttle or one other predetermined throttle setting, we have to adjust the value of CV6 at that predetermined throttle setting so the two locomotives run at the same speed.

For the sake of discussion, we'll assume you're going to speed match at 40% throttle. Using the red locomotive as our 'master' (the locomotive to which all others are going to be matched) we can adjust CV6 on the blue locomotive so it runs at the same speed as the red one at 40% throttle. In Chart 2, we've set CV6 on the blue locomotive to a value of about 220 while the value of CV6 on the red locomotive is about 190.

Chart 2

What's important to note here is that although the locomotives are running at identical speeds at 40%, the values of CV6 are different.

At every other throttle setting the lines are either closer or further apart, which means the speeds of the locomotives are different, too. This is why you cannot match speeds through the whole throttle range by CV's 2, 5 and 6 alone, but only at the one, predetermined throttle setting. The only exception is when you have two identical locomotives from the same manufacturer with the same decoder installed. Theoretically, they should run in unison from 0 to 100% throttle.

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The throttle setting doesn't have to be 40%. It can be anything you want and more advanced throttles have a digital readout showing the setting. Others, however, have just a dial. The Digitrax Zephyr is one of these and it is, for some unknown reason, numbered 1 to 6. This writer made up a circular disk numbered 1 to 100, plasticized it then secured it to the dial face with double-sided tape. In this picture the throttle is set at 50%. But if you can, borrow a throttle with a digital readout.

Zephyr Throttle

Hopefully, the foregoing has explained why, if using CV's 2, 5 and 6, you can only match speeds at the one predetermined throttle setting.

To match through the whole throttle range, each of 28 CV's, #67 to 94 have to be adjusted on the 'slave' locomotive so the speed of the master and slave stay the same even though the CV values will probably be different.

If you make a mess when programming you can always reset the decoder to its factory defaults. And if you don't know who manufactured the decoder, read the value of CV8 which is a decimal code for the manufacturer. The NMRA provides a list that identifies these codes. It can be found at http://www.nmra.org/sites/default/files/appendix_a2c_s-9.2.2.pdf When you know who made the decoder you should be able to download the guide from the manufacturers website and that should tell you how to reset to the default settings.

We'll use percentages when talking about throttle settings/speed steps, but some throttles have 28 speed steps and others 128. These are displayed across the top of the charts.

Set the value of CV's 3 and 4 (acceleration and deceleration momentum) to zero.

Add 16 to the value of CV29. This tells the decoder to use CV's 67-94 for speed rather than CV5 and CV6.

You may want to start by adjusting the acceleration curve of the master locomotive. This is good practice and it's recommended you record the CV values as you enter them. A table to record them has been appended at the end of this page, as well as a table you can use as a paper record of your final CV settings.

No matter what values you program, the value of successive CV's MUST be the same or higher than the CV preceding. In other words, the value of CV68 must be the same or higher than that of 67. The value of CV69 must be the same or higher than that of 68, and so on. In Chart 5, you'll see that the values of the helper's CV's, 76 through 94, stay the same.

Assuming a GP38 is the master locomotive, it will probably have a fairly fast acceleration during the early stages then the rate of acceleration slows the more the throttle is advanced, much like a car. This could be represented by a curve that looks like this:

Chart 3

In the example above, the value of CV94 (maximum speed) is 180. To get the curve, you have to experiment starting with CV68, the value of which is about 15. CV69 is about 30, CV70 is 50, increasing by about 20 points through each CV up to CV76, but the incremental value of theCV's then decreases thereafter. CV77 is about 135; CV78 is 140, CV79 145. The difference in values between successive CV's gets smaller and smaller the more the throttle is advance.

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Of course, you may want a slow starting but fast finishing locomotive. Chart 4 shows how that may look.

Chart 4

Here, there are small differences in the values of the 'slower' CV's and they increase as the throttle is advanced.

Once you've got your master accelerating to how you like it, you can program the slave locomotive(s) to match.

On the slave:

They may not run at the same speed throughout the whole throttle range. In this case, make a note of the throttle setting(s) where the locomotive ran significantly run faster or slower. Adjust the CV that's closest to the noted throttle setting(s) up or down as necessary.

Keep doing this until they're running together from zero mph to top speed.

You may have a locomotive that you'll only use as a helper. It won't go as fast as a GP38, but could help one up a grade, and their speed profiles could look like this:

Chart 5
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If you want to program this profile, set up the GP38 first, adjust the helper so it matches speed to 'X' % throttle (you choose what 'X' will be), then keep the value of the remaining CV's the same. In this case the two locomotives match speeds to 35% throttle. The helper's CV's 75 to 94 stay the same, while those of the GP increase at varying rates to top speed.

Although this chart shows the values of CV's 67 to 76 as being the same for both locomotives, in practice they will likely be different. Our aim is to match speed, not CV values.

The last chart (#6) shows how the throttle and CV settings could look for three locomotives that all run at the same speed through the whole throttle range. The different CV values are the consequence of having different decoders, different motors, different gearing and different wheel sizes.

Chart 6

The reason the curves are so, well . . . curvy, is because the DC output voltage of one decoder at, say 20% throttle, may be slightly different to that from another decoder, or the motor may turn faster/slower or the gear ratios and/or wheel sizes are different on each locomotive.

Given the variables in decoders, motors, gearing and wheel sizes it's highly unlikely you'll get perfect speed matching. The locomotives may push and pull against one another at various times, but this is normal. One thing in particular to be aware of is that track resistance will cause any locomotive to slow down as it goes into a curve and speed up again on the straight. This is especially so with steam locomotives with long fixed wheel bases.

Locomotive decoders should be set up with the same number of speed steps and the reason why is quite simple. A throttle's output is in 128 steps. If the decoder is set to 128 speed steps, it can march in unison, so to speak, with the throttle. But if a decoder is set to 28 speed steps, it will take 128/28 = 4.57 throttle speed steps before the decoder accepts the next increase/decrease in speed. So if a decoder in one locomotive is set up with just 28 speed steps and others are at 128, the chances of them running at the same speed at any time other than flat out are remote.

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CV Record

The CV Record can be downloaded here

Once you've matched speeds, record the value of the CV’s. This next form may help and you can download it here

DCC Decoder Sheet
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