Arduino Mega2560 Manual page 8

Step 3) Connect wheels to the motors
The Mecanum wheel has some sub-wheels on the main wheel. The four Mecanum wheels have
two types by the sub-wheel directions. Check the type A wheel and type Wheel by following
picture 8 left side . You must 100% sure that correct type of wheel is installed into
correct position as per picture 8 right side, the rolling direction of each wheel
will determine the whole car moving directions showed in the picture.
Picture 8: Two types of Mecanum wheel and their installation position in the car