Resistance Setting
Aim - this section describes the settings that can be applied to Open Rails
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.
General resistance (on straight and level track)
Impact of Wheel Bearing Temperature Rise on Resistance
Trailing Locomotive resistance
Key Resistance Parameters for inclusion in Wagon files
Sample Code for inclusion in Wagon files
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.
Application in ORThe correct application of resistance in Open Rails (OR) is critical to ensure a high degree of accuracy in the performance of the train being modelled.
If test values are available from the relevant railway company in regard to the stock being configured, then they should always be used in preference to the default values suggested below.
When relevant values are not readily available, then the table below can be used to get a good approximation. It should be noted that the categorises identified in the table below are indicative only, and should only be used as a guide.
Operating Speed |
Track Type (Condition) |
Vehicle Type |
Period |
Suggested Formula |
|---|---|---|---|---|
Freight Wagons |
||||
40mph to 50mph |
Track flexible (wooden sleepers), light weight track, numerous track joints |
Journal bearing, older style design |
Pre 1950 |
|
40mph to 50mph |
Track semi-rigid, heavier weight track, longer rail sections |
Roller bearing, older design |
Post 1950 |
|
> 75mph |
Track rigid (concrete sleepers), welded rail |
Roller bearing, modern design |
Post 1990 |
|
Passenger Stock |
||||
< 60mph |
Track flexible (wooden sleepers), light weight track, numerous track joints |
Journal bearing, older style design, light weight, minimal streamlining |
Pre 1960 |
|
60mph to 125mph |
"Track rigid (concrete sleepers), welded rail" |
"Roller bearing, modern design, significant streamlining" |
Post 1960 |
|
> 125mph |
High Speed Design |
Streamlined |
All eras |
As per manufacturers figures |
Locomotives |
||||
All speeds |
Track flexible (wooden sleepers), light weight track, numerous track joints |
Standard Design - Steam Locomotive (journal bearing) |
All eras |
Original Davis Formula (plus mechanical resistance) |
60mph to 125mph |
Track semi-rigid, heavier weight track, longer rail sections |
Standard design (early) |
Pre 1970 |
|
> 125mph |
Track rigid (concrete sleepers), welded rail |
Standard design (modern) |
Post 1970 |
|
For higher speed stock, ie over 50mph, you may want to read this section on air drag, but be wary adjusting these figures.
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
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).
Note: For steam locomotives, it is suggested that the combined locomotive and tender resistance be calculated, as if one unit, and then proportion the Davis values across the two files.
To customise Open Rails the following parameters may be entered in the Wagon section of the
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.
Starting Resistance
Starting resistance is automatically calculated in OR, based upon the
As part of its calculations Open Rails determines the axle loading for each wagon, therefore it is important to ensure that the number of axles entered in the
OR uses information described in the Starting Resistance section to calculate the relevant values.
Impact of Wheel Bearing Temperature Rise on Resistance
The temperature of the wheel bearing is influenced by many factors, including the type of bearing, ambient temperature, type of lubrication, etc. To accurately model all the different factors involved is extremely complex, and therefore Open Rails (OR) has instead used a representative bearing heat model to simulate the typical outcomes for bearing temperature heating or cooling effects.
Features provided by the OR wheel bearing heating model include:
- Bearing heats up and cools down as the train moves and stops.
- Bearing resistance in cold weather is significantly higher then when the bearing is at its 'normal' operating temperature. Typically railway companies elected to reduce loads for trains in cold conditions. The OR model will reduce the car resistance as the bearing heats up, and it will increase resistance as the bearing cools down.
- OR has a built in temperature model to determine the ambient temperature. The ambient temperature is calculated based upon a world model of the average temperatures at various latitudes. OR will use the latitude of the route to calculate the ambient temperature. As ambient temperature also decreases with height above sea level, OR takes this into account as well, and varies the temperature accordingly.
- Depending upon the
ActivityRandomizationLevel setting in the Option menu, an overheating bearing (hotbox) may be randomly initialized on any trailing car in the train (locomotives and tenders are excepted from overheating bearings). The Hotbox will be activated randomly within the first 66% of the activity duration. So for example, in an activity with a 20 minute duration, a hotbox will only be activiated in the first 12 minutes of the activity, if it has been initialised.
OR will provide a warning message if the bearing starts to overheat.
A special smoke effect can be added to the adjacent to the wagon hot box. This will be triggered if the bearing overheats. See information on the
For a more detailed description refer to this page.
Wind resistance
This is the additional resistance encountered by a train when a wind is blowing.
Open Rails automatically calculates this resistance. Application in OROpen rails has a built in wind generation model which develops winds with different directions and wind speeds. The model is randomly initialised when OR first starts, and currently the wind speed and direction are limited to prevent excessive variation of the wind. The current wind conditions will be displayed on the FORCES HUD in OR. Calculation of the wind resistance is based upon the cross sectional area of the car found by calculation from the Size statement, and the standard default Drag constants used in the original Davis formula.
OR ParametersTo customise Open Rails the following parameters may be entered in the Wagon section of the
Where x is either an area parameter or constant respectively (Note: This constant needs to be the imperial constant, rather then the one used with a metric implementation). Ideally these should be the same values that were used in calculating the Drag value in the Davis formula from the previous section.
For a more detailed description refer to this 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 OROpen 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 ParametersTo customise Open Rails the following parameters may be entered in the Wagon section of the
Where x, y & z are distance parameters in valid OR distance values.
Example use is as below.
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.
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 ORTo 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 ParametersOpen 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 DefaultsOR 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 |
Trailing Locomotive resistance
The Davis formulas for level track allocate the majority of drag resistance to the leading locomotive, as it is to be expected that this unit will present the greatest resistance to the movement through STILL air. Following cars or locomotives will not present as great a resistance, and hence it can be expected that the drag coefficient applied in the calculation of the R3 value will be less.
As an
Where x is a constant value.
Depending upon how the resistance has been calculated for a tender, it may be necessary to add this parameter to the tender
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 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. There are some subtle differences between wagons and locomotives.
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 ( *** Wind Resistance *** )
ORTSWagonFrontalArea ( 120.0ft^2 )
ORTSDavisDragConstant ( 0.0024 )
ORTSTrailLocomotiveResistanceFactor ( 0.20833 )
Comment ( *** Curve Resistance and SuperElevation *** )
CentreOfGravity ( 0m 2m 0m )
ORTSTrackGauge ( 4ft 8.5in )
ORTSRigidWheelbase ( 0.0ft 56in )
ORTSUnbalancedSuperelevation ( 6in )
Notes:
These parameters are optional, and not specifically required in the
Useful References
FCalc is available for download from TrainSim and then by typing in "fcalc.zip " into the File Name field of the search page.