mass, force and tire load

[ghiboz]

hi all!
I have this question:
I made a car that the weight is almost 50% front and 50% rear:
the debug tell me that the Total mass: 1573 (kg?)

after debugging, the force in ApplyForces is almost 1573*9.81=15431 down (N?) and is right...
after I suppose that, when the car is stopped, in a steady state, the sum of the load for each wheel must give 15431 in the opposite direction of the force, but is only about 9800....

is right or not this??
thanks

[NaN]

Need more info. How did you figure out that you have 50% weight distribution? Entering specific values relative to the wheel positions median?

The most simple one is to look at the tire load obviously. You can also check if wheel position median is equal to center of mass offset.

Can you post you car setup?

Edit: median in xy-plane

[ghiboz]

thanks, here is my config:

Code:

[engine]
position = 0.0, 1.42, -0.4
mass = 180.0
max-power = 248318
peak-engine-rpm = 6750.0
rpm-limit = 7000.0
inertia = 0.25
idle = 0.02
start-rpm = 1000
stall-rpm = 400
fuel-consumption = 1e-9
torque-friction = 0.0003
torque-curve-00 = 1000, 166.26
torque-curve-01 = 2000, 175.75
torque-curve-02 = 2250, 190.66
torque-curve-03 = 2500, 192.02
torque-curve-04 = 2750, 187.95
torque-curve-05 = 3000, 196.08
torque-curve-06 = 3250, 201.51
torque-curve-07 = 3500, 208.29
torque-curve-08 = 3750, 213.71
torque-curve-09 = 4000, 220.49
torque-curve-10 = 4250, 219.13
torque-curve-11 = 4500, 215.07
torque-curve-12 = 4750, 225.91
torque-curve-13 = 5000, 238.11
torque-curve-14 = 5250, 224.56
torque-curve-15 = 5500, 223.2
torque-curve-16 = 5750, 219.13
torque-curve-17 = 6000, 216.42
torque-curve-18 = 6250, 212.35
torque-curve-19 = 6500, 196.08
torque-curve-20 = 6750, 187.95
torque-curve-21 = 7000, 186.59

[clutch]
sliding = 0.27
radius = 0.15
area = 0.75
max-pressure = 40000.0

[transmission]
gears = 5
gear-ratio-r = -4.23
gear-ratio-1 = 3.72
gear-ratio-2 = 2.40
gear-ratio-3 = 1.77
gear-ratio-4 = 1.26
gear-ratio-5 = 1.00
shift-time = 0.07

[differential-rear]
final-drive = 3.9
anti-slip = 600.0

[fuel-tank]
position = 0.0, -0.26, -0.56
capacity = 70.0
volume = 70.0
fuel-density = 0.8

[camera]
view-position = -0.35, -0.25, 0.42
hood-mounted-view-position = 0.0, 1.0, 0.45

[wing-front]
position = 0.0, 0.0, 0.2
frontal-area = 2.0
drag-coefficient = 0.3

[wing-center]
position = 0.0, 1.9, 0.60
frontal-area = 0.2
drag-coefficient = 0.0
surface-area = 0.3
lift-coefficient = -0.5
lift-efficiency = 0.95

[wing-rear]
position = 0.0, -2.14, 0.37
frontal-area = 0.05
drag-coefficient = 0.0
surface-area = 0.5
lift-coefficient = -2
lift-efficiency = 0.95

[wheel.fl.coilover]
spring-constant = 50000
bounce = 2500
rebound = 7600
travel = 0.15
anti-roll = 6000

[wheel.fr.coilover]
spring-constant = 50000
bounce = 2500
rebound = 7600
travel = 0.15
anti-roll = 6000

[wheel.rl.coilover]
spring-constant = 40000
bounce = 2250
rebound = 6800
travel = 0.15
anti-roll = 4000

[wheel.rr.coilover]
spring-constant = 40000
bounce = 2250
rebound = 6800
travel = 0.15
anti-roll = 4000

[wheel.fl.brake]
texture = rotor_worn.png
friction = 0.83
max-pressure = 2.8e6
bias = 0.7
radius = 0.1135
area = 0.015

[wheel.fr.brake]
texture = rotor_worn.png
friction = 0.83
max-pressure = 2.8e6
bias = 0.7
radius = 0.1135
area = 0.015

[wheel.rl.brake]
texture = rotor_worn.png
friction = 0.83
max-pressure = 3.5e6
bias = 0.3
radius = 0.1135
area = 0.015
handbrake = 1.5

[wheel.rr.brake]
texture = rotor_worn.png
friction = 0.83
max-pressure = 3.5e6
bias = 0.3
radius = 0.1135
area = 0.015
handbrake = 1.5

[wheel.fl.tire]
size = 195, 50, 15
type = &tire/touring

[wheel.fr.tire]
size = 195, 50, 15
type = &tire/touring

[wheel.rl.tire]
size = 195, 50, 15
type = &tire/touring

[wheel.rr.tire]
size = 195, 50, 15
type = &tire/touring

[wheel.fl.hinge]
wheel = -0.71, 1.34, -0.49
chassis = 0.66, 1.34, -0.16

[wheel.fr.hinge]
wheel = 0.71, 1.34, -0.49
chassis = -0.66, 1.34, -0.16

[wheel.rl.hinge]
wheel = -0.71, -1.21, -0.49
chassis = 0.66, -1.21, -0.16

[wheel.rr.hinge]
wheel = 0.71, -1.21, -0.49
chassis = -0.66, -1.21, -0.16

[wheel.fl]
position = -0.71, 1.34, -0.49
camber = -1.33
caster = 6.12
toe = 0.5
ackermann = 8.86    # 50% ackermann
steering = 45

[wheel.fr]
position = 0.71, 1.34, -0.49
camber = 1.33
caster = 6.12
toe = -0.5
ackermann = -8.86
steering = 45

[wheel.rl]
position = -0.71, -1.21, -0.49
camber = -0.45
caster = 0.28
toe = 0.0

[wheel.rr]
position = 0.71, -1.21, -0.49
camber = 0.45
caster = 0.28
toe = 0.0

[steering]
position = -0.37, 0.44, 0.09
rotation = 87.5, 0.0, 0.0
max-angle = 320

[driver]
position = 0.0, 0.023, 0.4
mass = 80.0

# these are used to calculate the weight distribution and balance
[particle-00]
mass = 160.0
position = 0.6, 1.34,-0.4

[particle-01]
mass = 160.0
position = -0.6, 1.34, -0.4

[particle-02]
mass = 250.0
position = 0.6, -1.2, -0.4

[particle-03]
mass = 250.0
position = -0.6, -1.2, -0.4

a question, after some navigation into code I noticed that the force of the spring depends by the displacement, but the displacement, how is calculed depending by the mass?

[NaN]

The config is not compatible with VDrift master. For what car is it? How did you determine that mass distribution is 50%, by hand?

I did the following test. Take XS.car. Set engine and driver mass to zero. Use 4 mass points at wheel positions 250kg each. I get 2620-2640 Newton load at each wheel, 50% mass distribution. It is also confirmed by mass center offset y of -0.06 = (1.14 - 1.26) / 2 equal to wheel median along y axis.

The suspension force arises from the car falling down at each simulation step. This compresses suspension, increases suspension force. The whole thing is stabilized by damping, else it would oscillate indefinitely.

[ghiboz]

thanks!
i've solved the problem, or better, found the cause:

Code:

//original VDrift code in game.cpp:
...
timestep(1/90.0),
...
//game.h
float TickPeriod() const {return timestep;}

// my code modified in game.cpp:
...
timestep(1/160.0),
...
//game.h
float TickPeriod() const {return 1/90.0;}

i've putted all to /160 and is ok... but why this thing? ok, is not a right thing change the 2 frequencies, but I don't know why only the load is change...

thanks again

[NaN]

This looks like a bug. But then considering the complexity of the GAME class...

We really need to refactor it into something more digestible, move functionality into separate modules, classes.

[NaN]

Duh, should have read your post more carefully. The problem you observed is most probably due to the fact that the GAME has a separate fixed time step logic. Whereas the physics engine has got its own. In your case the physics engine would be getting timesteps shorter than its fixed timestep, would drop simulation frames. It is interesting that it worked at all. Ideally one would feed the game logic as prestep, poststep callbacks to the physics sim, use one fixed step implementation.