CVs, Speed Table, or Throttle Control...

Model Railroader Electronics and DCC
1

Hello All,

Many model locomotives run at unrealistically high speeds at “full throttle” and don’t respond well at factory-set lower-speed steps.

The majority of my motive fleet has Digitrax decoders; both sound and non-sound.

In the literature provided with Digitrax decoders, there is a chart labeled; “Speed Tables-How the Loco Responds to the Throttle.”

It lists the “Loco Type”; Switcher, Road Switcher, and Mainline Loco, with suggested CV values for V Start (CV02), V Mid (CV06) and V Max (CV05) for how each one responds to the throttle steps be it 14, 28 or 126 (128).

According to Digittax, adjusting these three (3) CVs changes the throttle response in the following manner:

  • “Switcher- -Concentrated low speed. Limited top speed.”
  • “Road Switcher- -Prototypical top speed w/evenly distributed curve from 0 to top speed.”
  • “Mainline Loco- -Quick increase to cursing speed then levels of to prototypical top speed.”

When setting up locomotives, other than speed-matching MU’ed locomotives, do you adjust the speed(s) depending on locomotive type?

If you do adjust the speed(s) do you use CV 02, 06, and 05?

Do you use the speed table function; JMRI or another CV programmer?

Or, do you just limit the top speed manually with your throttle no matter the speed steps; 14, 28, or 126 (128) and top speed of the loco?

Some decoders support a “Switching” mode, enabled by a function command.

Does anyone use this function?

Thank you in advance for your thoughtful and respectful responses.

2

I have a couple of Bachman engines that came with their somewhat proprietary Sound Value decoders, a Mikado and a GG1. Both have a problem with jackrabbit starts, and the normal CV fixes don’t work.

I don’t run my engines very fast, so I adjusted Vhigh so that the total range of speeds would be lower. That got rid of the jackrabbit starts.

3

I use TCS decoders for motor-only installations so I rarely have to adjust CVs 2, 5, and 6 (if availale) to achieve excellent slow starts. For me, top speed is far less a concern as slow speed & slow starts. My goal is 1sMPH or <.

I don’t bother with speed tables, as the 3 CVs work just fine for me. And, since it’s only 2 or 3 CV changes, I do it via my Power Cab throttle in programming mode.

I rarely operate any locomotive at speed step 126. My Power Cab and CAB-04p throttles can be setup to switching mode. It’s a cool feature but I can just as easily press the reverse button to accomplish the same thing.

Tom

4

I’ve been learning about setting the speed controls and have done it to a couple Bachman’s steam and Athearn diesel with excellent results using only a couple settings . I’ve just got JMRI setup and when I’m not doing something else on the RR I will try to learn the ins and outs with the JMRI features . I’ve found the following cv changes worked well for me …

Cv 2 - start voltage 00-255 -01

Cv 3- acceleration voltage 00-31 -25

Cv 4 - deceleration rate 00-31 -25

Cv5-voltage Max 00-255-64 main line Loki’s set higher for higher speeds

Cv6-voltage Mid 00-255-38

And just in case ,

Cv8 = 8 is for factory reset

5

I have a couple of diesel consists.
First consist is easy, two Athearn blue box SW7’s back to back.

Second consist, Athearn blue box F7A, Bachmann Genesis F7B.
I used speed table to lower Bachmann top speed to match Athearn.

I used speed table to raise bottom speed to just start moving on both.

Used programmer to draw straight line in speed table.

Done