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When a real train
starts and stops, it does so slowly because there is a massive amount
of weight involved. This falls into the scientific realm of "inertia"
- when a body is at rest it tends to stay at rest, when in motion
it tends to stay in motion - unless acted upon by an outside force.
The more weight involved, the more energy required to overcome inertia.
Mass times the speed equals the momentum force. The more mass there
is, the greater the momentum - i.e., the greater external force is
needed to change the inertial force of the mass. |
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With our models there is very little
inertial mass, but a great deal of external force - in comparison
to real locomotives. There is much greater energy that can be exerted
against the relatively low inertia of our model trains. This means
our trains can start and stop almost on a dime, so to speak.
Because the power-to-inertia ratio
of our models is nowhere near that of real trains, methods have been
devised that can be used to force our models to act like real trains.
Since momentum is the force we're dealing with, momentum is the term
we use to identify the feature to make model trains react slowly to
Acceleration and Deceleration - simulating the heavy mass of a loaded
train. Without momentum, a model can achieve full speed almost instantly.
Obviously, this isn't how real trains run. But with momentum activated,
the train will react more slowly, like a real one, whether the operator
likes it or not. |
Most people think of our models in one dimension.
That is, HO scale is 1 to 87, so some people automatically think
since our models weigh one pound that that would equate to the
real thing weighing only 87 pounds - making our models much
lighter than the real thing. But, the accurate equivalent would
be 87 x 87 x 87 - three dimensions of mass. A Kato Dash 9, with
decoder and details, weighs in at 1 pound 4.5 ounces. That equates
to the real thing weighting 843,707 pounds. Considering that
the BigBoy steam engine weighed in at only 772,000 pounds, well,
you get the idea. As for the motor, the Kato Dash-9 motor has
a 1.2-amp stall current at 12 volts. After doing the math (considering
all things equal, which they aren't), this equates to a real
loco having 12,711 horsepower. Yet the cars they pull are featherweight
in comparison to real ones. It's no wonder our trains can make
jack-rabbit starts and stops. |
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With DC Analog, throttles
can be purchased that provide Momentum. With some, momentum only works
with Acceleration. A set deceleration momentum is slightly dangerous.
Because most people would not start slowing the trains down soon enough,
deceleration momentum would take the train beyond the stopping point
and crash. Momentum in these units is usually not programmable. You
can turn it on and use whatever momentum the throttle manufacturer
designed it for, or you can turn it off.
While momentum is not an innovation
of DCC, it is inherent to DCC. With DCC, you not only can adjust momentum
on-the-fly to suit your needs, depending on the length of train and
how many locos are pulling it, but can also set momentum for acceleration
and deceleration differently - a train can usually stop faster than
it starts. Allowing this in our models will certainly help avoid costly
accidents.
There are several trains of thought
regarding momentum:
- those who demand it to force trains and engineers
to operate realistically,
- those who want it so they don't have to simulate
it themselves,
- those who absolutely don't want it so they can
have 100% control of their trains,
- and those who simply don't care enough to even
think about it.
Acceleration momentum is handled
in CV3, deceleration momentum in CV4. Acceleration momentum keeps
you from starting the train faster than it should start, deceleration
momentum keeps you from stopping the train faster than it should stop.
While acceleration momentum can be frustrating for some people, deceleration
momentum can be devastating for some - as previously discussed and
discussed some more below.
For people who demand full momentum to force the train to run prototypically,
they not only can set acceleration and deceleration for the train,
but can change it on-the-fly (with OPS-Mode
Programming) as they set out and pickup cars during switching.
The values programmed into these CVs depend on your assessment of
how the train should operate with the amount of cars and load on the
train. This will require a certain amount of experimentation to develop
some rules of thumb for your operation. In both CVs a value of zero
(0) turns momentum off, which is how decoders come from the factory.
For people who want to use it simply
to keep from having to simulate a loaded train manually, it's best
if they don't program too much deceleration momentum in. Most people,
without some practice, simply can't gauge when to start decelerating
to make the train stop at the appointed spot. If coming to the end
of a spur or up to a cut of cars for coupling, this can be devastating.
You can turn the throttle to zero and the train continues to go, slowing
down only as fast as the programmed momentum allows it to. By not
programming as much momentum into deceleration as you have in acceleration
momentum, it gives you a little more leeway in that judgement - at
least until you get enough practice to avoid accidents. |
Personally, I don't use momentum
any more. Before 128 speed steps with DCC, momentum was absolutely
necessary to keep locos from "jumping" from one speed step
to another. But with 128-speed-step control, along with Digitrax's
digital encoder knobs, you can easily simulate a loaded train by clicking
through the speed steps one by one. |
With all things model, doing everything to scale
just isn't feasible. For example, I'm sure you've heard that
it takes a loaded train up to a mile to stop. In HO scale, that's
equivalent to 60 feet. I doubt seriously if most people would
want their locos to take 60 feet to stop after setting the throttle
to zero. |
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128 speed steps,
controlled by digital encoders, provides control so fine that you
can't tell the speed difference from one speed step to the next unless
you have something to gauge it by - such as another train running
at the same speed on a parallel track. Call me a control freak, but
I enjoy having this kind of control - and momentum takes part of that
away. |
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Momentum is strictly a decoder
function. Once programmed it works on all DCC layouts. Not only that,
it is also useful for stopping blocks as outlined on the Braking
Section page.
So far as I know, all decoders offer
acceleration and deceleration momentum. It's one of the basics of
DCC decoders. Even though they all offer it, they don't all implement
it the same - check the documentation for your decoder.
Momentum is not intrusive. It comes
turned off and you don't have to use it unless you want to. |
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