why does my mecanum drift when driving straight

The most likely cause of a mecanum drivetrain drifting when trying to drive straight is incorrect wheel orientation. According to Game Manual 0, the rollers on the wheels must form an "X" shape when viewed from above, meaning they angle inward toward the center of the robot . To check this, look down at your robot from above; if the rollers form a rhombus instead of an "X", the wheels are mounted in the wrong positions, which prevents the force vectors from canceling out correctly to drive straight .

The next most likely cause is incorrect motor direction configuration in your software. Game Manual 0 explains that most motors spin counterclockwise by default, meaning you typically need to reverse the left-side motors to drive forward . However, if your drivetrain uses external gearing between the motor and the wheel—such as on the goBILDA Strafer or REV Mecanum kits—this behavior may be reversed . To diagnose this, prop the robot off the ground and run a forward command; if any wheels spin backward or at mismatched directions, you must correct their direction in your code .

Another common cause is wheel slippage due to high acceleration or poor roller quality. Game Manual 0 notes that mecanum drive encoder localization can drift during high acceleration due to a general lack of traction . Additionally, wheels that use bushings instead of ball bearings, such as TETRIX or AndyMark Standard Duty wheels, suffer from high roller friction and poor performance . To check this, limit your robot's acceleration in software to see if the drift stops, and spin the rollers by hand to ensure they rotate freely on ball bearings rather than sticking on bushings .

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Mecanum TeleOp > Deriving Mecanum Control Equations

Game Manual 0·CC BY-NC 4.0

Now that we have a functioning mecanum driving program, there are a few things that can be done to clean it up. The first of these would be multiplying the left X value by something to counteract imperfect strafing. Doing this will make the drive feel more accurate on non axis aligned directions, and make field centric driving more accurate. In this tutorial, we will use 1.1, but it's really up to driver preference.

Java

double y = -gamepad1.left_stick_y; // Remember, Y stick is reversed!
double x = gamepad1.left_stick_x * 1.1; // Counteract imperfect strafing
double rx = gamepad1.right_stick_x;

The other improvement we can make is scale the values into the range of -1 to 1.

Since the SDK simply clips (limits) the powers to that range, we can lose the ratio we are looking for unless we proactively put all the numbers back in that range while still maintaining our calculated ratio. For example, if we calculate values of 0.4, 0.1, 1.1, and 1.4, they will be clipped to 0.4, 0.1, 1.0, and 1.0, which is not the same ratio. Instead, we need to divide all of them by the largest power's absolute value when it exceeds 1:

Java

// Denominator is the largest motor power (absolute value) or 1
// This ensures all the powers maintain the same ratio, but only when
// at least one is out of the range [-1, 1]
double denominator = Math.max(Math.abs(y) + Math.abs(x) + Math.abs(rx), 1);
double frontLeftPower = (y + x + rx) / denominator;
double backLeftPower = (y - x + rx) / denominator;
double frontRightPower = (y - x - rx) / denominator;
double backRightPower = (y + x - rx) / denominator;

Make sure to set the powers on your motor and update this every loop in an opmode!