Speed-Matched Locos as DPUs - How to Do It Right?

[MetroRedLine]

I model in N scale, but I suppose this issue can affect all scales.

I have worked on speed-matching all my locos and as lead MUs, they all work fine. But I model modern era and want to have DPUs on my trains, especially to justify having longer-than-usual trains, and even when I have a speed-matched loco on the rear end of the train, because it's not pulling on anything, as well as other factors (the front of the train might me on a grade and the rear of the train might be on level ground or a downward grade), the rear DPU tends to push stronger than it's supposed to. My best workaround is to "downspeed" these locos through programming, making them drag the train just enough that they make up slack but don't tax the front cars, but then that means it will no longer be speed-matched.

So how do you folks do it? Is BEMF a factor here? Would Advanced Consisting solve the problem (I haven't delved into that). My system is a Digitrax Zephyr Xtra (DCS51) and I use JMRI Decoder Pro for the hardcore programming. I also use an Accutrack II speedometer tunnel for speedmatching.

[jagged ben]

My personal experience: you have to control them separately just like real engineers do.  You have to watch the slack action and adjust the speeds to keep the slack in the right place.  It's more thrilling than any video game when hundred of dollars of freight cars are on the line!  Don't forget to blow the horn for grade crossings if you have sound.

I like to use one of the digitrax throttles with two knobs for this.  I get each consist set up and then assign one to each knob.  (I don't really like these throttles for any other reason or purpose.)

I think the Android EngineDriver app allows you to have two throttles on the screen at the same time, too, but I haven't tried that.

[MK]

Turn BEMF off when consisting.  Too much fighting happens if on.

[ednadolski]

There is the Mike Danneman trick of removing one of the drive shafts from the DPUs.  This limits the amount of pulling/pushing power that the unit can apply, thereby reducing the chances of stringlining or accordion-ing.

(Caveat, I have not tried this myself.)

Ed

[MetroRedLine]

My personal experience: you have to control them separately just like real engineers do.  You have to watch the slack action and adjust the speeds to keep the slack in the right place.  It's more thrilling than any video game when hundred of dollars of freight cars are on the line!  Don't forget to blow the horn for grade crossings if you have sound.

That's how pre-DPU helpers worked, correct? Not interested in that. I was hoping to find a way to run DPUs through DCC in a simulated way the prototype does.

There is the Mike Danneman trick of removing one of the drive shafts from the DPUs.  This limits the amount of pulling/pushing power that the unit can apply, thereby reducing the chances of stringlining or accordion-ing.

(Caveat, I have not tried this myself.)

Ed

So the best answer is to have dedicated DPU locos? If that's the best option, I can probably live with that. Maybe instead of messing with the drive shafts, I can also have sound installed in them with reduced weight (read: more frame milling) that will result in a "weaker" loco that could suit a rear DPU just fine. I mean, the advantage of that is you get to hear the prime mover wiz by at the end of the train.

[ednadolski]

Maybe instead of messing with the drive shafts, I can also have sound installed in them with reduced weight (read: more frame milling)

Possibly, tho for myself at least the drive shaft seems easier, plus you can always re-install it if you ever wanted to restore a derated loco to normal performance.

One thing I did try with dedicated DPUs was to program them to run slightly slower than head-end units.  This would tend to keep the slack a bit stretched out, as the train transitioned between grades.  Tuning gets trickier with longer trains and sharper curves.

IIRC @pbrennecke  uses derated DPUs and they work well on ~50 car coal trains, even with truck-mounted couplers.  Of note, his layout has 24" curves and max 1% grade.

Ed

[jagged ben]

That's how pre-DPU helpers worked, correct? Not interested in that. I was hoping to find a way to run DPUs through DCC in a simulated way the prototype does.

...

Nope, modern DPUs also controlled separately. Actually, it can be done either way, there are two modes. But on grades, like you mentioned in your first post oh, they would be controlled separately. When you crest a hill, you don't want the rear and front applying the same power.  Front can go into DB when rear is still pushing.

Personally I can't imagine how Danneman's solution works. I usually end up applying a little more juice to the rear units, at least on the uphills. But maybe that's because the trains I run need the rear power.

[Philip H]

Nope, modern DPUs also controlled separately. Actually, it can be done either way, there are two modes. But on grades, like you mentioned in your first post oh, they would be controlled separately. When you crest a hill, you don't want the rear and front applying the same power.  Front can go into DB when rear is still pushing.

Are they doing that from the same cab though?  None of the DPU's I see on CSX down south have an engineer in the DPU all lonely and by themselves.

[Jbub]

Are they doing that from the same cab though?  None of the DPU's I see on CSX down south have an engineer in the DPU all lonely and by themselves.

Remote control from the head cab?

[C855B]

Nope, modern DPUs also controlled separately. Actually, it can be done either way, there are two modes. But on grades, ...

Modern DPUs are in automatic/remote MU mode most of the time. What makes modern DPUs "modern" are strain gauges on the drawbars, computer inputs that detect loading, and electronic processing computes what is necessary to keep the node (point of zero drawbar pressure) between the leading and trailing motor pairs. It's this automatic function that allow for multiple DPU sets, as each pair tracks their resulting node. That, and the intermediate motors enable built-in radio repeater functions relaying control to the power that may be out of range of the front. Can you imagine manage running four sets of power all from the front manually? Talk about a high-stakes video game! No, it's highly computerized and automatic.

Not-modern DPUs ("Locotrol") were manual remotes from the front-end. However, those could be operated in a linked mode when the running was easy. It was fun watching SP's Locotrol ops over the Tehachapis in the '70s where they would have a Locotrol set two-thirds back in the train and a manned rear-end helper. As they were cutting their teeth on this newfangled system, it was fun to count the litter of broken drawbars and knuckles along the ROW.

[jdcolombo]

Turn BEMF off when consisting.  Too much fighting happens if on.

Nope.

If you have engines that use decoders with the best motor control protocols and BEMF software, there is no "fighting" or oscillation if the engines are speed matched.  I've never had to turn off BEMF or even reduce its strength (which can usually be done via some CV) using ESU LokSound and LokPilot decoders in speed-matched 3-unit sets.  The times I have seen serious oscillation problems have all been due to either MUing engines that have different decoders in them, some with less-than-good motor control/BEMF software, and/or were poorly speed matched to begin with.

And if you want to attempt DPU ops, you absolutely should NOT turn BEMF off.  BEMF is a load-compensation system, designed to keep an engine's motor running at exactly the same speed regardless of load - e.g., regardless of number of cars being pulled, upgrade, downgrade, no grade.  In theory, if you have engines that all use the same decoder, all with excellent BEMF implementation (e.g., ESU or Zimo), and EXACTLY speed match them over the entire throttle range (not using a 3-point curve, but speed matching using the 28-step curve) and also EXACTLY match momentum settings, those engines will run at exactly the same speed, whether going upgrade, downgrade, pulling 50 cars or pulling none.  That in turn should make DPU operations possible without futzing with driveshafts, etc.

@MetroRedLine
I'd like to know what decoders you are using in your engines, whether they are all the same decoders, how you have done your speed matching and momentum matching.  I have seen successful DPU operations on an admittedly-flat Ntrak layout with 200-car trains, but only when the engines used the same decoders, BEMF was fully on, and speed/momentum matching was meticulously set up.

John C.

[MK]

Nope.

If you have engines that use decoders with the best motor control protocols and BEMF software, there is no "fighting" or oscillation if the engines are speed matched.  I've never had to turn off BEMF or even reduce its strength (which can usually be done via some CV) using ESU LokSound and LokPilot decoders in speed-matched 3-unit sets.  The times I have seen serious oscillation problems have all been due to either MUing engines that have different decoders in them, some with less-than-good motor control/BEMF software, and/or were poorly speed matched to begin with.

That is absolutely not my experience with Digitrax decoders.  As you said, if you have engines that use decoders with the best motor control protocols and BEMF software...that can be accomplished.  I agree that Digitrax is not the best out there.  And most people are not on the 100% ESU camp.  Heck I would love to be those 100%'ers but the cost of an all ESU fleet is beyond my budget.

[davefoxx]

Turn BEMF off when consisting.  Too much fighting happens if on.

I can't say that this is my experience.  Do you speed-match your fleet?  That may make a difference.  I haven't turned BEMF off in any of my locomotives, but I do have them all speed-matched to the following settings: CV2 at 1 smph, CV6 at 35 smph, and CV5 at 70 smph.

DFF

[davefoxx]

I like to use one of the digitrax throttles with two knobs for this.  I get each consist set up and then assign one to each knob.  (I don't really like these throttles for any other reason or purpose.)

I agree, and that's exactly how I ran this train.  I don't have that Digitrax system anymore, but I will say that the two throttles on one controller was a big plus:

[samusi01]

I think I could get away with DPU with some of my locomotives: one of the sets of power that I take to shows is four FVM GP60s, all with ESU decoders. It's quite neat to see how closely they perform when separated by a couple of inches.

Therefore, what @jdcolombo says is spot on: one brand of decoder with good motor control, and good speed matching. I've been playing around with momentum quite a bit in the last few months, mostly on ESU and Digitrax decoders, and I've been sorely disappointed with the Digitrax decoders, as results are quite variable and they don't even play well amongst themselves, let alone with other brands. On the other hand, ESU decoders are a snap to get to play well amongst themselves.