Resistance Setting

Aim - this section describes the settings that can be applied to Open Rails WAG and ENG files for "optimal" resistance settings.

If you wish to provide any feedback or suggest corrections, please use the Contact page. Please provide appropriate references.

To calculate some of the standard resistance settings it is recommended that FCalc is used. This will make the calculations easier.

Index

Introduction to Resistance

Key Resistance Parameters for inclusion in Wagon files

Sample Code for inclusion in Wagon files

Useful References


Introduction to Resistance

In steam days train resistance was expressed in pounds per ton (be aware of whether it is US or UK tons). The train resistance then needed to be overcome by the tractive effort produced by the locomotive to move the train.

It was made up of the following elements:

  • General resistance (typically on a straight level track)
  • Grade resisitance
  • Curve resistance
  • Acceleration resistance
  • Tunnel resistance
General resistance (on straight and level track)

General resistance is made up of the following elements:

a). Bearing resistance - which in more modern stock, typically post 1970 was roller bearing, whereas older stock (varied from country to country) typically had journal bearings (named resistance bearings by the roller bearing companies). Typically roller bearings had a lot lower values of resistance then journal type bearings, but were more expensive.

b). Air Resistance - was the resistance force that the train needed to exert to overcome the resistance of the air.

c). Miscellaneous - resistance due to concussion, oscillation, flange resistance and the rolling of wheels on rails.

W. J. Davis, Jr. became famous for correlating a large mass of data and deriving a series of formulas to model the effects of resistance.

These formulas take the following form:

Train resistance = A + B * V + C * V^2

Where the ABC values are known as the Davis co-efficients, and the V values are the speed of the train.

For a more detailed description refer to this page.

By default Open Rails accepts Davis values in the metric units of Newtons, and is based upon speed values in metres/second. FCalc will produce values that conform with this standard, and can be entered directly into the ENG or WAG file. Therefore, unless you have access to a published ABC values, it is recommended that FCalc be used as the default calculation.

Great care should be taken when studying resistance formula from different sources, as they may have different units of measure. Some formula might be based upon speeds in mph, kmh or m/s. Similarly the resistance value calculated can also be in lbs/ton, kg/tonne, or N/tonne. Therefore the ABC values may need to be converted to values that will be accepted by Open Rails (ie N, Nm/s, N(m/s)2).

To customise Open Rails the following parameters may be entered in the Wagon section of the WAG or ENG file.

ORTSBearingType ( x )
ORTSDavis_A ( y )
ORTSDavis_B ( y )
ORTSDavis_B ( y )

Where x is the relevant bearing type, and y is the value calculated by the FCalc tool or from published information, as described on the Resistance Calculation page.

Grade resistance

This was the additional resistance encountered by a train climbing a grade. It was typically related to the weight of the train.

Open Rails automatically calculates this based upon the weight of the wagon.

For a more detailed description refer to this page.

Curve resistance

Curve resistance was the additional resistance that a train experienced as it negotiated a curve.

Curve resistance was impacted by a number of factors, including the sharpness of the curve, and the wheelbase of the wagon. The sharper the curve and the longer the rigid wheelbase of the rolling stock the higher the frictrional force.

For a more detailed description refer to this page.

Open Rails models curve resistance, and for maximum accuracy, wagon parameters need to be entered.

Application in OR

Open Rails models this function, and the user may elect to specify the known wheelbase parameters, or the above "standard" default values will be used.

OR calculates the equilibrium speed in the speed curve module, however it is not necessary to select both of these functions in the simulator options Menu. Only select the function desired.

By studying the "Forces Information" table in the HUD, you will be able to observe the change in curve resistance as the speed, curve radius, etc vary.

OR Parameters

To customise Open Rails the following parameters may be entered in the Wagon section of the WAG or ENG file.

ORTSRigidWheelBase ( x y )
ORTSTrackGauge ( x y) (also used in curve speed module)
CentreOfGravity ( x y z ) (also used in curve speed module)

Where x, y & z are distance parameters in valid OR distance values.

Example use is as below.

ORTSRigidWheelBase ( 0.0ft 3.0in )
ORTSTrackGauge ( 4.0ft 8.5in)
CentreOfGravity ( 0m 2.28m 0m )

Open Rails uses 'standard' track superelevation design values as a basic standard default. It is possible to specify values of of SuperElevation for different track curve radii within the Route. For more detailed information on how to do this in OR refer to Track SuperElevation.

To see how each of these parameters impacts upon the maximum allowable speed around a curve refer to the calculators shown on the Curve Speed Test page.

OR Default

The above values can be entered into the relevant files, or alternatively if they are not present, then OR will use default values as described below.

Rigid Wheelbase - as a default OR uses the figures shown above in the "Typical Rigid Wheelbase Values" section.

Starting curve resistance value has been assumed to be 200%, and has been built into the speed impact curves.

OR calculates the curve resistance based upon actual wheelbases provided by the player or the appropriate defaults. It will use this as the value at "Equilibrium Speed", and then depending upon the actual calculated equilibrium speed (from the speed limit module) it will factor the resistance up as appropriate to the current train speed.

Steam locomotive wheelbase approximation - the following approximation is used to determine the default value for the fixed wheelbase of a steam locomotive.

WheelBase = 1.25 * (axles - 1) * DrvWheelDiameter
Acceleration resistance

Acceleration resistance is equal and opposite to the force necessary to produce acceleration from one speed to another over a known distance or time.

Tunnel resistance

When a train goes through a tunnel, it meets resistance as it "pushes" a column of air through the tunnel. This effect is more pronounced for high speed trains, and will be impacted by the size of the tunnel, and the aerodynamics of the train.

For a more detailed description of tunnel resistance refer to this page.

Application in OR

To enable this capability it is necessary to select the "Tunnel Resistance" option on the Open Rails Menu. The implication of tunnel resistance is designed to model the relative impact, and does not take into account multiple trains in the tunnel at the same time.

The default tunnel profile is determined by the route speed recorded in the TRK file.

OR Route Parameters

Open Rails has basic default tunnel design parameters included as standard. However if desired, the route modeller may override these by including specifc tunnel parameters for the route in question.

To insert these values in the Route see the Tunnel Resistance settings for more details and the test route for an example implementation.

OR Defaults

OR uses the following standard defaults

i) Tunnel Perimeter

Route Speed

Single Track

Double Track

< 160 km/h

21.3 m

31.0 m

160 < 200 km/h

25.0 m

34.5 m

200 < 250 km/h

28.0 m

35.0 m

250 < 350 km/h

32.0 m

37.5 m

ii) Tunnel Cross Sectional Area

Route Speed

Single Track

Double Track

< 120 km/h

27.0 m2

45.0 m2

< 160 km/h

42.0 m2

76.0 m2

200 km/h

50.0 m2

80.0 m2

250 km/h

58.0 m2

90.0 m2

350 km/h

70.0 m2

100.0 m2

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For more detailed information refer to the links below.

Key Resistance Parameters for inclusion in Wagon files

The key parameters that impact upon the resistance and resistance performance of a train are described on the following web page.

Standard Resistance Parameters for ENG and WAG Files (updated Aug 2015)

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Sample Code for inclusion in Wagon files

Typically the lines shown in red text are the only ones that would need to be changed on individual wagons.

Comment ( *** Resistance *** )
ORTSBearingType ( Roller )

Comment (Type: Steam - Standard, Speed: 100km/h, Axles: 6, Bearings; Roller, Area: 10m2, Weight: 65.0t tons metric, DrvWeight: 42.0 tons metric, Drag: 1 )
ORTSDavis_A ( 8084.8 )
ORTSDavis_B ( 23.0335 )
ORTSDavis_C ( 5.796 )

Comment ( *** Curve Resistance and SuperElevation *** )
CentreOfGravity ( 0m 2m 0m )
ORTSTrackGauge ( 4ft 8.5in )
ORTSRigidWheelbase ( 0.0ft 56in )
ORTSUnbalancedSuperelevation ( 6in )

Note: There are some subtle differences between wagons and locomotives.

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Useful References

Locomotive Data - Baldwin Company - 1944

Locomotive handbook - American Locomotive Company (Alco)

FCalc is available for download from TrainSim and then by typing in "fcalc.zip " into the File Name field of the search page.