max-accel measurement unit


Does anyone know the measurement unit of max-accel and max-decel? The wiki only says it is a force but does not give the unit of measurement. Searching the forum, I could also not find an explanation. Is it measured in Newton?

Many thanks,
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In Trainz these values (max-accel and max-decel) define a force accelerating the object, not the rate of acceleration (See HERE).

I will be delighted if someone from N3V can shed some light about the "max-accel" tag and - generally - about the whole DCC physics. After all, they should know how it works...

This graph is the result of a series of tests I performed using a 120-ton locomotive hauling a consist of twelve 50 ton coaches fitted with the "default-wagon" enginespec. "Max-accel" for the locomotive was set at 14000, 21000, 28000 and 42000.

The measurements were obtained by pausing the game and taking note of the elapsed time: as a consequence, they are subject to some degree of inaccuracy. Using a script would undoubtedly provide more accurate results, but my attempts at scripting always end in a "parse error" :(.


The vertical axis represents acceleration in metres per second squared, the horizontal axis represents speed in tens of kilometres per hour (14 = 140 kph).

As you can see from the graph:
max-accel = 14000 -> acceleration at 10 kph = ~ 0.16 m/s2
max-accel = 21000 -> acceleration at 10 kph = ~ 0.2 m/s2
max-accel = 28000 -> acceleration at 10 kph = ~ 0.22 m/s2
max-accel = 42000 -> acceleration at 10 kph = ~ 0.26 m/s2

If someone can deduce a formula from this graph, I will be eternally grateful to him :D.

I suppose DCC physics somehow takes into account the effect of friction and air drag, since acceleration decreases as speed increases. The graph looks quite similar to the one that I obtained in Cab Mode with constant tractive effort. It is interesting how acceleration for the two highest settings of "max-accel" actually increases until 20 kph, then decreases while remaining parallel with the other graphs.

Currently, I use this empirical formula to calculate the "max-accel" value:

max-accel = 0.85 x MaxTE [kN] x 100
P-dehnert gave this link: (See HERE).
On this official reference page the resistance calculations are given, and there is stated that the total resistance R is measured in lb.
Strictly taken this is impossible, as lb is a mass unit, not a force unit. Using lbf (pound-force) would be better, but it is often done wrongly. Using N would be even better, but Trainz doesn't.

As max-decel acts in the same way as R, and max-accel as -R, we can assume that they are also measured in lb. Then they can simply be added and subtracted from R in the train deceleration formula. To be sure this formula should be inspected, but I don't know where to find it. 1 lbf is about 4.45 N.

Inspecting the graph learns that the distance between the top and bottom line is about 0.14 m/s2. (The value at low speed should not be taken, as it is affected by wheel slip.)
The train mass is 720 ton, so the difference in max-accel is 731,520kg*0.14m/s2=102,400 N, assuming that long ton is ment.
The difference in max-accel is also 42,000-14,000=28,000 units, so the unit calculated from the graph is 102,400/28,000=3,7 N.

As there are several inaccuracies, the unit could very well be the supposed lbf, because that is not far from the value calculated from the graph.
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You've opened a can of worms here. As a long term user of the US system of units (69+ years) I have to say I was taught that in the FPS (foot pound sec) system of units the pound (lb) is a unit of force. I have taken to use lbf to emphasize it is a force. In this system the slug is the unit of mass. 1 slug weights aprox 32.2 lbs at the surface of the earth. Or stated in a more physically correct way: a force of 1 lbf will accelerate a mass of 1 slug at a rate of 1 f/s2.

Here's what wiki has to say about it:
Pound-force as force unit

The technical or gravitational FPS system, is a coherent variant of the FPS system that is most common among engineers in the United States. It takes the pound-force as a fundamental unit of force instead of the pound as a fundamental unit of mass.

In this sub-system, the unit of mass is a derived unit known as the slug.

1 slug = 1 lbF ⋅ 1 s^2/ft

In the context of the gravitational FPS system, the pound-force (lbF) is almost universally simply referred to as the pound (lb).
The resistance forces specified in the Trainz documentation (I'm talking both the technical manual last issued in TRS04 maybe and the Trainzwiki enhanced docs linked to above) are based on US engineering units. I verified long ago that this is true within the accuracy of measurements taken in the game. In my own work on resistance forces in Trainz I've always used the following units which have been (and still are) used by US railway engineers for resistance calculations:

lb = lbf = pound force
ton = 2000 lbf (so called US short ton)
g = acceleration of gravity = 32.14 f/s^2 at sea level.

At sea level a mass of 1 kg will weigh approx 2.205 lbf

Having gotten that out of the way the max-accel tag was never given units (by Auran or N3V) since it was a larger makes it accelerate faster type of value in the old technical docs. Since trs06/tc1-3 or maybe even TS9 train mass has had a greater effect in dcc control mode. I've never really investigated dcc control. It's not real physics simulation as far as I'm concerned. There has never been any discussion that I'm aware of concerning exactly how this value is used in the game.

If you want to run some additional tests in Trainz I'd suggest you set up a straight and level test track. Make up a new espec to use for the locomotive you're testing using the following for the motor container:
resistance 0
maxvoltage 0
maxspeed 30
max-accel 10000
max-decel 10000
axle-count 0
surface-area 0
moving-friction-coefficient 0
air-drag-coefficient 0
This will ensure that a default value other than 0 is not used. The only remaining resistance will be a constant force (at least nearly so at moderate to high speeds) of say 1.3 lbf per ton (2000 lbf) of total weight of the railcar(s). Prior to TC1 I think it was nearly a constant force with just a moderate increase at starting from a standing start - like I'd expect to see with roller bearings used universally since the late steam era. However sometime around TC1-TC3 the force is much larger at low speed and decreases as speed increase to something approximating the value specified. It now seems to behave like the old journal bearings used in the early - mid steam era.

Anyhow, I digressed a bit there, the new espec used on level and straight track will limit the resistance force to what is hard coded in the game and we can't change at present. I'd use just the loco with the new espec and vary the max-accel in the espec and vary the mass of the loco (mass tag in the loco's config). Use an old one with no fuel or sand or other queues to worry about - the game only reports to nearest tonne in the details box. If you use a large enough mass (don't just use the loco's actual mass you'll need something approaching a consist value) the smaller residual axle force won't have too much effect on the results.

I don't know if the dcc control dial makes a difference or not or if just dials in a speed limit that the loco will accelerate to.

Just a thought.

Bob Pearson
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