Difference between revisions of "Car parameters"

Revision as of 03:16, 18 April 2011

The units are all in MKS (meters, kilograms, seconds). It might also help to read The Physics of Racing by Brian Beckman. For unit conversion you can go to: This Site.

The .car file contains several sections. Each section will now be described, along with example values from the XS.car file. The XS has performance comparable to the Honda S2000.

Coordinate system

The .car files use the right-handed (positive) coordinate system for all parameters:

x axis: negative is left, positive is right
y axis: negative is back, positive is forward
z axis: negative is down, positive is up

Common Parameters

[section]
texture = diffuse.png, misc1.png, misc2.png
mesh = model.joe
position = 0.736, 1.14, -0.47
rotation = 
scale = -1, 1, 1
color = 0.8, 0.1, 0.1
draw = transparent

Every car section supports a set of optional parameters to describe its graphic representation. Texture is a list of textures that has to contain at least one texture, usually the diffuse color texture. Mesh defines the model mesh to be used with the texture. Position/rotation(in degrees)/scale will transform the mesh relative to parent. Color defines the color of the mesh(to be blended with the texture according to its alpha channel). Draw allows the options transparent(according to first textures alpha channel) or emissive(won't be affected by lighting, used for brake/reverse light models).

Engine

[engine]
position = 0.86, 0.0, -0.21
mass = 140.0
max-power = 1.79e5
peak-engine-rpm = 7800.0
rpm-limit = 9000.0
inertia = 0.25
idle = 0.02
start-rpm = 1000
stall-rpm = 350
fuel-consumption = 1e-9
torque-friction = 0.0003
torque-curve-00 = 1000, 140.0
torque-curve-01 = 2000, 149.14
torque-curve-02 = 2200, 145.07
torque-curve-03 = 2500, 147.78
torque-curve-04 = 3000, 169.50
torque-curve-05 = 3300, 172.19
torque-curve-06 = 4000, 169.50
torque-curve-07 = 4500, 166.77
torque-curve-08 = 5600, 172.19
torque-curve-09 = 5800, 170.83
torque-curve-10 = 6000, 168.12
torque-curve-11 = 6100, 177.61
torque-curve-12 = 6200, 186.42
torque-curve-13 = 6300, 192.53
torque-curve-14 = 6500, 195.92
torque-curve-15 = 6700, 195.92
torque-curve-16 = 7000, 195.24
torque-curve-17 = 7600, 190.49
torque-curve-18 = 8000, 184.39
torque-curve-19 = 8200, 183.04
torque-curve-20 = 8300, 146.43
torque-curve-21 = 9500, 146.43

The position and mass parameters affect the weight distribution of the car. The torque curve is calculated from max-power and peak-engine-rpm using a polynomial expression given in Motor Vehicle Dynamics, Genta (1997), where peak-engine-rpm is the engine speed at which the maximum power output (max-power) is achieved. Alternatively, the torque curve can be explicitly defined, as in the example above. A rev limit can be set with rpm-limit. The rotational inertia of the moving parts is inertia. Idle is the throttle position at idle. Starting the engine initially sets the engine speed to start-rpm. Letting the engine speed drop below stall-rpm makes the engine stall. The rate of fuel consumption is set with fuel-consumption. The actual fuel consumed each second (in units of liters) is the fuel-consumption parameter times RPM times throttle (throttle is from 0.0 to 1.0, where 1.0 is full throttle).

Clutch

[clutch]
sliding = 0.27
radius = 0.15
area = 0.75
max-pressure = 11079.26

The clutch is described by its sliding friction coefficient, radius, area and maximum applied pressure. The torque capacity(maximum transmitted torque) of the clutch is TC = sliding * radius * area * max-pressure. It should be somewhere between one and two times the maximum enine torque. TC = 1.25 * max-engine-torque is a good start value.

Transmission

[transmission]
gears = 6
gear-ratio-r = -2.8
gear-ratio-1 = 3.133
gear-ratio-2 = 2.045
gear-ratio-3 = 1.481
gear-ratio-4 = 1.161
gear-ratio-5 = 0.943
gear-ratio-6 = 0.763
shift-time = 0.2

The number of forward gears is set with the gears parameter. The gear ration for reverse and all of the forward gears is then defined. The shift-time tag tells how long it takes, in total seconds, to change gears (when autoclutch is enabled). Half the time is spent changing the gear and the other half is spent letting the clutch out. This parameter is not required and defaults to 0.2 seconds, which is a reasonable value for a manual transmission. F1 cars take about 50 ms, by comparison.

Differential

For FWD cars [differential.front] has to be defined. AWD cars require [differential.front], [differential.rear] and [differential.center].

[differential.rear]
final-drive = 4.100
anti-slip = 600.0
anti-slip-torque = 1
anti-slip-torque-deceleration-factor = 0

The final drive provides an additional gear reduction. The anti-slip parameter defines the maximum anti-slip torque. For speed-sensitive differentials, it also defines the anti-slip torque per radian per second of speed difference between the wheels. If the differential is speed-sensitive, the anti-slip-torque and anti-slip-torque-deceleration-factor parameters must be omitted or set to zero. If the differential is torque-sensitive, then anti-slip-torque defines the amount of anti-slip torque per input torque. The anti-slip-torque-deceleration-factor defines the amount of anti-slip torque per negative input torque. For a 1-way torque-sensitive LSD, set anti-slip-torque-deceleration-factor to zero, for a 2-way torque-sensitive LSD, set anti-slip-torque-deceleration-factor to 1.0, for 1.5-way, set it between 0.0 and 1.0.

Fuel tank

[fuel-tank]
position = 0.0, -1.0, -0.26
capacity = 0.0492
volume = 0.0492
fuel-density = 730.0

The fuel tank's position, the current volume of fuel and the density of the fuel affect the car's weight distribution. The capacity tag sets the maximum volume of fuel that the tank can hold. The initial volume is set with the volume tag. The density of the fuel is set with fuel-density.

Camera

[camera]
view-position = -0.35, -0.64, 0.30
hood-mounted-view-position = 0, 0.55, 0.17
view-stiffness = 0.0

The view positions define 3D coordinates for camera placement. The view-stiffness parameter defines the stiffness of the camera bounce effect, where 0.0 is a sports car and 1.0 is F1-ish.

Wing

[wing.rear]
position = 0.0, -2.14, 0.37
frontal-area = 0.05
drag-coefficient = 0.0
surface-area = 0.5
lift-coefficient = -0.7
efficiency = 0.95

Wing identifiers front, center, rear are arbitrary(can be chosen freely). A wing describes the aerodynamics(car body, front/rear wing) of the car. A car has to have at least one wing, to capture body drag. Most cars will use up to three. The frontal area and coefficient of drag, set with frontal-area and drag-coefficient, are used to calculate the drag force.

Downforce can be added with the optional parameters surface-area, lift-coefficient, efficiency. If the lift coefficient is positive, upforce is generated. This is usually undesirable for cars. The efficiency determines how much drag is added as downforce increases. The surface-area is the surface area of the wing. This value is also used in the drag calculation.

Wheel

[wheel.fl]
texture = oem_wheel.png, oem_wheel-misc1.png
mesh = oem_wheel.joe
position = -0.736, 1.14, -0.47	#track front/rear 1471/1509
camber = 0.5
caster = 6.0
toe = -0.16
ackermann = 8.46	# 50% ackermann
steering = 30

The number of wheels is fixed to four: fl, fr, rl, rr. For a FWD car the wheels fl and fr are powered, for RWD the wheels rl and rr. The wheel model has to reside in the car folder or carparts/wheel.

Coilover

[coilover-front]
spring-constant = 49131.9
spring-factor-1 = 0.052, 1.0
spring-factor-2 = 0.055, 1.2
bounce = 2600
rebound = 7900
damper-factor-1 = 0.08,1.0
damper-factor-2 = 0.1, 0.7
travel = 0.19
anti-roll = 800.0

The spring-constant is the wheel rate in N/m. The spring-factor-1 and 2 parameters define a curve for the spring response. These can be omitted if desired, in which case a factor of 1.0 will be used everywhere. Points are defined by specifying an x,y pair where x is the suspension displacement in meters and y is the factor to be applied to the spring coefficient. In this example, the spring factor will be 1.0 when the displacement is between 0 and 0.052 m, and then the spring factor will change linearly to 1.2 at 0.055 m (and beyond). The spring factor gets multiplied by the spring-constant. You can put as many spring-factor points as you want (just increase the spring-factor- number for each additional point). Note that displacement values are relative to the "zero g", "zero force" position. For best results, start VDrift with the -debug option and observe suspension displacements during maneuvering to determine where you want to put your points.

The bounce and rebound parameters are the damping coefficients for compression and expansion of the suspension, respectively, in units of N/m/s. The damper-factor-1 and 2 parameters define a curve for the damper response. These can be omitted if desired, in which case a factor of 1.0 will be used everywhere. Points are defined by specifying an x,y pair where x is an absolute value of suspension velocity in m/s and y is the factor to be applied to the damping coefficient. In this example, the damper factor will be 1.0 when the compression velocity absolute value is between 0 and 0.08 m/s, and then the damper factor will change linearly to 0.7 at 0.1 m/s (and beyond). The damper factor gets applied to the bonce or rebound damper coefficient, depending on the direction of travel. You can put as many damper-factor points as you want (just increase the damper-factor- number for each additional point).

Tire

[tire-front]
size = 215/45r17
type = touring
texture = touring

Size determines tire dimensions, weight, inertia of the tire. Tire textures are stored in carparts/tire/textures. Tire types are stored in carparts/tire. More info about tire type definition can be found here: Tire_parameters

Brake

[brake-front]
friction = 0.4
max-pressure = 4.0e6
bias = 0.65
radius = 0.14
area = 0.015
rotor = rotor_shiny_slotted_drilled

The bias parameter is the fraction of braking pressure applied to the front brakes (in the front brake section) or the rear brakes (in the rear brake section). To make sense, the rear value should equal 1.0 minus the front value. The maximum brake torque is calculated as friction * area * bias * max-pressure * radius. Some fraction of this value is applied based on the brake pedal. Brake rotor is the optional brake rotor texture. If set a brake rotor model is generated. Rotor textures are stored in carparts/brake/textures.

Suspension

[suspension-0]
coilover = coilover-front
wheel-hub = -0.736, 1.14, -0.47	#track front/rear 1471/1509
position = -0.73, 1.14, -0.03
hinge = 0,0,0
camber = -1.33
caster = 6.12
toe = 0.0
ackermann = 0
steering = 33.19

Suspension has to be defined per wheel. The referenced coilover has to be defined in the same car file. Wheel hub is the wheel position for a fully extended suspension. The position parameter is not used atm. The hinge is the center of the wheel's path as the suspension moves. The location of the hinge is determined by suspension geometry, and may be outside of the car itself.

Wheel alignment is set with the camber, caster, and toe. All angles are in degrees. For a "negative camber" the left wheel camber has to be negative, the right wheel camber positive.

Ackermann and steering are optional. Ackermann is the steering arm angle relative to wheel. Ideal ackermann(100%) is atan(0.5* track / wheelbase). For the right wheel positive ackermann is positive, for the left negative. Steering is the maximum steering angle of the wheel(for ackermann = 0). A negative steering leads to a reverted steering.

Particle

[particle-00]
mass = 30.0
position = 0.0, -1.28, -0.36

These values are used for weight distribution and rotational inertia. Up to 100 particles are supported. Most cars will use 6-10.