Reference Arm Node
This is to help modders, as I have seen a few asking about what this does.
What it does
The reference arm node in the .truck file serves an important purpose. That purpose is to determine where the torque reaction in a chassis will be. If that node is placed in front of the specified wheel, then it will provide more traction, as the wheel pushes the chassis down into the ground. This is good for vehicles such as off-roaders and high-grip racecars. If the arm node is behind the specified wheel, then grip will be reduced as the wheel will help lift the chassis off the ground. This is useful for drifters and other cars and trucks that need less grip.
How it is Implemented
The arm node is implemented through the wheels section of a .truck file.
A good example vehicle that shows how the arm node works is box5diesel's Baja Trophy Truck. When in the air, if you accelerate in 6th gear, the nose of the truck will be lifted up. If you brake while in the air, the nose of the truck will be pushed down. This is due to the arm nodes of the rear wheels being located in front of the rear axle, and the arm nodes for the front wheels, located behind the front axle.
This is a rough diagram I made to show how it works. In the top diagram, the arm node is behind the wheel, so as the wheel spins counterclockwise, it applies upward force to the node(red) in the blue circle, therefore imparting less traction as the chassis is pushed upward.In the bottom diagram, the arm node is in front of the wheel, so as the wheel spins counterclockwise, it applies downward force to the circled node, therefore imparting more traction as the chassis is pushed downward.