Tank and West Coast Drive

What it is

A tank drive (also called skid steer or differential drive) has wheels on the left side and wheels on the right side. The two sides are driven independently: drive both forward to go straight, drive them at different speeds to turn, and drive them in opposite directions to spin in place. Because the robot "skids" sideways when turning, it is called skid steer.

The most popular variant is West Coast Drive (WCD). Its defining features are:

• Wheels cantilevered (supported on one side) off the outside of the side rails, so they can be serviced without disassembling the frame.
• A center drop (6-wheel drop): the middle wheel sits roughly 1/8" lower than the four corner wheels. This reduces the contact patch during turns so the robot pivots cleanly instead of fighting four-corner traction.
• Belt or chain running inside the rails to link the wheels on each side to one gearbox.

Why teams use it

• Simple and rugged. Far fewer moving parts than swerve, so it is forgiving for rookies.
• Pushing power. With grippy wheels and several motors, a tank drive is excellent at defense and shoving.
• Cheap entry. The AndyMark AM14U6 "KitBot" chassis offered through the FRC Kit of Parts is a ready-made 6-wheel drop-center skid-steer base. Its wheels run on live axles between the side rails rather than cantilevered, so it is not a true West Coast Drive, but it teaches the same drop-center principles. It needs at least two CIM-class motors per the user guide, with four recommended for competitive play; recent kits also support brushless motors such as the REV NEO.

Components

WCD is usually built from 2x1 aluminum tubing (e.g., REV MAXTube 2x1 or WCP/VEXpro versa-frame) bolted together with gussets. Each side typically uses traction wheels in the corners and may use omni wheels to ease turning. A pair of gearboxes (one per side) couples the motors to the wheels.

Limitations

• The robot can only drive in the direction it is facing; to go sideways it must turn first.
• Hard turning scrubs the wheels and wastes energy.

WCD remains a strong, legitimate choice. Many teams that cannot reliably support swerve win matches with a well-built, well-driven tank drive. The deciding factor is usually whether your software and electrical sub-teams can take on swerve's workload (see the next lesson), not whether swerve is "better" in the abstract.