godot-vehicle-sandbox

A playground for developing a semi-realistic vehicle simulation for games in the Godot Engine. The goal of the project is to develop vehicles that behave as close as possible to real vehicles while keeping the code as simple and fast as possible. Vehicles are to be made up of different configurable building blocks to allow the creation of various types of vehicles, e.g. FWD/RWD/AWD combustion engine/electric cars, trucks, trailers, trikes, bikes, etc.

Building blocks

Vehicles are created from a set of nodes and resources. In general, the nodes are supposed to connect the various components, while the resources are supposed to be independent, so as to be potentially reusable in multiple contexts. For example, Axle depends on Wheel, but Transmission does not depend on Motor or any of the other components.

Nodes

• Vehicle
  • Root node of your vehicle
  • Must contain at least one Axle
  • Is RigidBody3D and requires collision shape(s), realistic mass and center of mass
  • Can be assigned a Motor to drive the vehicle
  • Can be assigned a Transmission to support multiple gears
  • Can be assigned a Differential to split torque between front and rear axle
    • Will be ignored if vehicle does not have exactly two axles
• Axle
  • Distributes torque input to a set of wheels
  • Must be a child of Vehicle and contain at least one Wheel
  • Can be assigned a Differential to split torque between left and right wheel
    • Will be ignored if axle does not have exactly two wheels
  • Can be assinged an AntiRollBar to reduce vehicle roll
• Wheel
  • Provides traction when in contact with the ground
  • Must be a child of Axle
  • Must be assigned a Tire for determining traction forces
  • Can be assigned a Suspension for independent wheel suspension
    • Consider always using a suspension; wheels without suspension will currently sink into the ground unless prevented in some other way
  • Can be assigned a Brake for braking and traction control

Axle and Wheel could also be used outside of the intended hierarchy, but would require manual handling that is normally provided by their intended parent node.

Resources

Note that some resources are stateless and single instances can be reused while others are stateful cannot be reused. Resources that would commonly be reused tend to be stateless, including Tire, Suspension and Brake. Here, state refers to dynamic input at runtime, e.g. current suspension compression, rather than properties that are considered static configuration, e.g. suspension stiffness. Configuration parameters can still be changed at runtime and will affect all dependents, e.g. all wheels using a common suspension resource.

• Motor (stateful)
  • Generates torque based on throttle input, RPM and a user-defined torque curve
• Transmission (stateful)
  • Transforms input torque and RPM based on use-defined gear ratios (manual or automated)
• Differential (stateful)
  • Splits input torque between two outputs at a fixed ratio and based on their feedback
• AntiRollBar (stateful)
  • Reduces vehicle roll by reducing the difference in suspension compression of an axle's wheels
• Tire (stateless)
  • Determines traction forces based on user-defined longitudinal and lateral friction curves
• Suspension (stateless)
  • Per-wheel spring-damper suspension keeping the vehicle upright and determining wheel load
• Brake (stateless)
  • Defines the maximum torque values for stopping a wheel via normal brake or handbrake

Example

A typical four-wheeled car should have the following the structure.

Vehicle (Motor, Transmission, Differential)
├ Collision Shapes
├ Meshes
├ Axle (Differential, AntiRollBar)
│ ├ Wheel (Tire, Suspension, Brake)
│ │ └ Mesh
│ └ Wheel (Tire, Suspension, Brake)
│   └ Mesh
└ Axle (Differential, AntiRollBar)
  ├ Wheel (Tire, Suspension, Brake)
  │ └ Mesh
  └ Wheel (Tire, Suspension, Brake)
    └ Mesh

For concrete examples see the /vehicles directory.

Input

See Project Settings > Input Map for all input mappings, but here is a short summary.

Note that holding brake does not automatically reverse. You need to manually shift into reverse and then accelerate as usual. Shifting is possible even if the transmission is automated, but gear changes between forward gears will quickly be overriden by the automation algorithm.

Keyboard

• W/A/S/D or arrow keys - acceleration, brake and steering
• Space - handbrake
• E/Q - shift up/down
• C - change camera

Game controller

• Triggers - acceleration and brake
• Shoulders - shift up/down
• Left stick - steering
• Bottom action button - handbrake
• Top action button - change camera

Known issues

Wheels do not collide with and glide over obstacles in an unrealistic manner.

This is a result of the ray/shape-cast approach used to simulate the wheels. They do not have their own collision shapes and cannot be handled by the physics engine. All interactions with the environment, such as the traction forces, must be coded manually. While wheel collisions could be handled to a certain degree, doing so is difficult and might not even make a big difference in some cases. For now, it's out of the scope of this project.

The automated transmission is scuffed.

The automation algorithm for the transmission was hacked together quickly, just to have some kind of automatic transmission for easier testing. It attempts to always select the gear that will provide optimal acceleration. Unfortunately, sometimes it will shift to unnecessarily high gears. I have not looked into the issue, since it's quite low-priority for me.

Vehicle struggles to move when motor provides extremely high torque.

Unfortunately, I have not yet looked into the issue yet, but I suspect it might be a case of the engine immediately hitting the RPM limit, cutting off, hitting the limit, cutting off... you get the point. There shouldn't be a problem with realistic torque values. The example vehicle with a real-life engine peak torque of 700 Nm seems to work properly even with a peak torque of 2700 Nm.

There is no explicit clutch.

A clutch is currently not being simulated. The engine is either fully engaged or fully disengaged from the drivetrain at any point in time. In addition, the simulated engine RPM are artificially restricted to not fall below the defined idle RPM, even if the wheel RPM are lower. Overall, this acts like a clutch when accelerating from a standstill, but is not physically correct. As a side-effect, the engine cannot be stalled.

Braking keeps locking up the wheels.

Yes, there is no ABS yet.

Attribution

The provided example vehicle uses the following third-party assets.

• "Mercedes Benz G-class W263" (https://skfb.ly/6SHrP) by Lexyc16 is licensed under Creative Commons Attribution (http://creativecommons.org/licenses/by/4.0/).
• "Car tire squeal skid loop" (https://opengameart.org/content/car-tire-squeal-skid-loop) by qubodup is licensed under Creative Commons Attribution (CC-BY 3.0) (http://creativecommons.org/licenses/by/3.0/).
• "Motor Loop 2" (https://pixabay.com/sound-effects/motor-loop-2-103532/) by Pixabay is licensed under Pixabay Content License (https://pixabay.com/service/license-summary/).

Specs for the example vehicle have been retrieved from the following sources.

• https://www.automobile-catalog.com/car/2021/2968940/mercedes-benz_g_400_d.html