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.
the part worth keeping
Key takeaways
- Tank/skid-steer drives the left and right sides independently; West Coast Drive cantilevers the wheels and uses a center drop for clean turning
- The AndyMark AM14U6 KitBot is a ready-made WCD base offered through the FRC Kit of Parts
- WCD is simple, rugged, and great at pushing, but cannot strafe sideways
Mechanical, Build & PneumaticsDrivetrains: Moving the Robotlesson 1 of 4
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where this came from
Sources and corrections
This lesson is AI-assisted: drafted from primary sources, then reviewed and edited by hand. Errors still get through. When one is reported we fix it and write down what changed, in public, in the corrections log.
sources and further reading
- docs.wcproducts.comWCP FRC Build System – Motion Systems
- andymark.comAndyMark AM14U6 KitBot Drive Base
clipped to this lesson
Articles that go further on this
The lesson gets you through the topic. These go wider on it, and they read in one sitting.
- 4 min readFRC Drivetrain Types: Tank, Swerve, and MecanumA beginner-friendly guide to FRC drivetrains: how tank, swerve, and mecanum drives work, their tradeoffs, and how to pick one for your robot./blogread it
- 18 min readFRC Wheels and Traction: Tread, Durometer, and Choosing the Right WheelHow to choose FRC drivetrain wheels: coefficient of friction, Shore A durometer, tread compounds, Colson vs pneumatic vs traction, diameter, and pushing power./blogread it
- 13 min readFRC Swerve Module Offsets: Calibration & Backwards WheelsZero your FRC swerve module offsets correctly, and fix wheels that spin backwards, modules that fight each other, and field-relative drive that feels rotated./blogread it
answer sheet
Lesson quiz
All 3 right completes the lesson. Miss one and only that question comes back, anything you already answered correctly stays banked.
0 of 3 answered
01What structural feature distinguishes a West Coast Drive (WCD) from a traditional tank drive?
02Both tank drive and West Coast Drive steer the robot using which method?
03Which is a commonly cited advantage of a West Coast Drive over a traditional tank drive?
Answer every question to submit.
All 47 lessons in Mechanical, Build & Pneumaticsopenclose
01 / prerequisites
02 / drivetrains
03 / power-transmission
04 / structure-materials-fasteners
05 / mechanisms-fabrication-assembly
06 / pneumatics-fundamentals
07 / pneumatic-components
08 / build-wire-program
09 / safety-rules-testing
10 / worked-examples-mini-projects
- Not read yet:Mini-Project 1: A Single-Jointed Arm From Math to Motion
- Not read yet:Mini-Project 2: A Two-Stage Cascade Elevator
- Not read yet:Mini-Project 3: A Velocity-Controlled Flywheel Shooter
- Not read yet:Mini-Project 4: A Pivoting Roller Intake
- Not read yet:Mini-Project 5: Integrating a COTS Swerve Module
11 / common-mistakes-troubleshooting
- Not read yet:Pneumatics Won't Fire: A Full Diagnostic Tree
- Not read yet:The Robot Won't Drive Straight (and Other Drivetrain Sins)
- Not read yet:Gearboxes That Grenade and Fasteners That Vibrate Loose
- Not read yet:Closed-Loop Mechanisms That Oscillate, Sag, or Stall
- Not read yet:Field-Ready Reliability: Inspection, Spares, and the Pit Checklist
12 / advanced-techniques-case-studies
- Not read yet:Characterizing Any Mechanism with SysId
- Not read yet:Simulation-Driven Design with WPILib Physics Models
- Not read yet:Motion Profiling and Superstructure Coordination
- Not read yet:Designing for Weight, Stiffness, and Manufacturability
- Not read yet:Case Studies: Learning From Open Alliance Robots