> For the complete documentation index, see [llms.txt](https://yagsl.yassrobotics.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://yagsl.yassrobotics.com/explanation/chassis-control.md).

# Chassis Control

Beyond individual module behavior, `SwerveDrive` and `SwerveDriveConfig` control a few chassis-wide concerns: how fast the robot is allowed to go, how commanded velocities get corrected for a subtle simulation-of-motion error, and how to make the robot maximally hard to push.

## Max Speed

YAGSL/YAMS store a maximum chassis linear and angular speed and use it for:

* `SwerveDriveKinematics.desaturateWheelSpeeds` — scaling down all module speeds proportionally if any one module would need to exceed the physical maximum.
* Telemetry (`swerve/maxSpeed`).
* Deriving a default drive feedforward from the motor's free speed, when `pidfproperties.json` doesn't set one explicitly.

The maximum speed represents the physical maximum speed of the robot — it is not a "slow mode" throttle (use `SwerveInputStream.withScaleTranslation`/`withScaleRotation`, or scale your controller input directly, for that).

It's set once via `SwerveDriveConfig.withMaximumChassisSpeed(LinearVelocity, AngularVelocity)`:

```java
var cfg = new SwerveDriveConfig()
    .withMaximumChassisSpeed(MetersPerSecond.of(4.5), DegreesPerSecond.of(360));
```

{% hint style="info" %}
This is a constructor-time setting on `SwerveDriveConfig`, not a JSON field — set it in the code that builds your `SwerveDriveConfig` before passing it to `SwerveParser.createSwerveDrive(...)`.
{% endhint %}

## Chassis Speed Discretization

When you command a `ChassisSpeeds` with both translation and rotation, WPILib's `SwerveDriveKinematics` computes module states assuming that velocity is held constant for the whole next timestep. In reality your loop runs at a discrete rate (typically 20ms), and the true path curves slightly within that window — this shows up as unwanted skew/drift, especially at higher rotation rates.

`ChassisSpeeds.discretize(speeds, dt)` corrects for this by working out what constant-velocity command over `dt` would actually produce the desired end pose. YAMS applies this automatically once you set a discretization time:

```java
var cfg = new SwerveDriveConfig()
    .withDiscretizationTime(Seconds.of(0.02))
    .withSimDiscretizationTime(Seconds.of(0.02)); // optionally different for simulation
```

If you don't set a discretization time, no discretization correction is applied. `0.02` (matching the default robot loop period) is a good starting point for most teams.

## Lock Pose

Lock Pose is a special stance where all the wheels point inward into an X formation, making the robot extremely difficult to push. It's most useful for holding position during defense, or at the end of a match.

```java
drive.lockPose();
```

{% hint style="warning" %}
Lock Pose should only be used when no other drive input is given, or you may get undefined/fighting behavior between the lock and your normal drive command.
{% endhint %}

A typical binding calls it repeatedly while a button is held:

```java
driverXbox.x().whileTrue(Commands.runOnce(drive::lockPose, driveSubsystem).repeatedly());
```


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