An introduction to vehicle physics
A physics vehicle is a collision object used to simulate a vehicle driving on (or gliding over) a smooth surface.
Bullet’s physics vehicle makes many simplifying assumptions:
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A rigid chassis, supported by wheels.
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Acceleration, steering, and braking forces are applied to the chassis, not the wheels.
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The wheels have no momentum, and as long as they touch the ground, they don’t lose traction or "drift". Tire friction is calculated using a basic anisotropic friction model.
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Engine power is unlimited. There is no transmission and no gearing.
The chassis is simulated by a dynamic rigid body, with all the features thereof. During each simulation step, the vehicle controller casts physics rays to calculate the height of each wheel’s axis above the ground. Those heights are used to calculate the forces exerted on the chassis by the suspension.
Instead of applying forces or directly updating the vehicle’s velocities, an app using a physics vehicle should specify when and how the vehicle accelerates, steers, and brakes.
Creation
You can create a vehicle without wheels by invoking the
PhysicsVehicle constructor.
This allows you to specify the shape and mass of the chassis.
Before adding it to a physics space, you should add wheels to it.
For stability, a vehicle needs at least 3 wheels, and the chassis center of mass
should be located between the wheels, roughly speaking.
HelloVehicle is a SPORT app that demonstrates the direct creation of a vehicle, followed by automated steering and acceleration. Things to notice while running the app:
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The vehicle has a wedge-shaped chassis.
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The vehicle has 4 wheels, arranged in a rectangle.
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The vehicle circles to the left, accelerating steadily until it "wipes out" and flips over.
Configuration and tuning
Bullet’s vehicle model includes many details that can affect stability and/or realism. The complexity of the model is often an obstacle to achieving the desired behavior. Kester Maddock has published a 16-page document on this subject.
Chassis shape
Because the chassis is a dynamic rigid body, it requires a suitable collision shape. In particular:
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it should be 3-dimensional, and
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its center and local axes should be plausible.
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A symmetrical collision shape will place the center of gravity rather high, resulting in a vehicle prone to tipping. To avoid this pitfall,
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By default, the local axes of the chassis are assigned as follows:
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+X points left,
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+Y points up, and
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+Z points forward.
However, the local-axis assignments can be customized to match any 3-D model. For instance:
vehicle.setPhysicsSpace(physicsSpace);
VehicleController controller = vehicle.getController();
Vector3f right = new Vector3f(0f, -1f, 0f);
Vector3f up = new Vector3f(0f, 0f, -1f);
Vector3f forward = new Vector3f(-1f, 0f, 0f);
controller.setCoordinateSystem(right, up, forward);Maximum suspension force
A vehicle’s suspension connects its wheels to its chassis and supports the weight of the chassis. If the maximum suspension force is set too low, the suspension will collapse, causing the chassis to scrape the ground.
The default maximum suspension force is 6000 per wheel. For a vehicle with 4 wheels and a mass of 3000, the default is inadequate for a gravitational acceleration of 8 or more. If the weight of the chassis is distributed unevenly among the wheels, some of the maxima might need to be increased accordingly.
Suspension rest length
Rest length is the length of a spring when no force is applied to it. If the suspension’s rest lengths are too large, the chassis will seem to be jacked up on stilts and the vehicle will be prone to tipping, even when not moving.
Suspension stiffness
Stiffness is the force exerted by a spring divided by its change in length. If the suspension is too stiff, a small bump could cause the vehicle to bounce violently. If it isn’t stiff enough, a large bump could cause the chassis to "bottom out".
Suspension damping
Each wheel has 2 suspension damping parameters, one for expansion and one for compression. The range of plausible values depends on the suspension stiffness, according to the formula in the javadoc:
damping = 2f * k * FastMath.sqrt(stiffness);where k is the suspension’s damping ratio:
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k = 0: undamped and bouncy.
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k = 1: critically damped.
Good values of k are between 0.1 and 0.3.
The default damping parameters of 0.83 and 0.88 are suitable for a chassis with the default stiffness of 5.88 (k=0.171 and 0.181, respectively). If you override the default stiffness, you should override the damping parameters as well.
Friction slip
The friction slip parameter quantifies how much traction a tire has. Its effect is most noticeable when the vehicle is braking.
Too much traction could cause a vehicle to flip over if it braked hard. Too little traction would make braking ineffective, as if the tires were bald or the supporting surface were icy.