Choosing a drivetrain
Tank, mecanum, and the rest — what each one is good at, which wheels they use, and what a rookie team should build.
By the end you can
- Describe how tank (skid-steer) and mecanum drivetrains move and turn
- Tell traction, omni and mecanum wheels apart and say what each is for
- Choose a drivetrain for a rookie season and justify the trade-offs
The drivetrain is the one mechanism every robot needs, every match, for the whole match. If it fails, nothing else on the robot matters. That makes it the place to be boring and reliable rather than clever.
Start with the wheels
A drivetrain is mostly defined by its wheels.
- Traction wheel
- A normal wheel with a grippy rubber tread. Lots of grip forwards and backwards, and lots of grip sideways too — which is great for pushing and terrible for turning on the spot.
- Omni wheel
- A wheel with small rollers around its rim, at 90° to the wheel. It drives forwards and backwards normally but slides freely sideways. Often used at the corners of a tank drivetrain so it can turn easily.
- Mecanum wheel
- A wheel whose rollers sit at 45° to the wheel. Four of them, each with its own motor, let the robot drive in any direction — including straight sideways — without turning.
Compliant wheels are not drive wheels
The soft, squishy "compliant" wheels you will see on intakes are designed to grip game pieces, not carry a robot. Do not build a drivetrain on them.
Tank (skid-steer) drivetrains
- Skid-steer
- Steering by running the left and right sides at different speeds. Run them in opposite directions and the robot spins in place.
A tank drivetrain has a left side and a right side. It cannot move sideways; to change direction it turns first.
The variations you will meet:
- 2-wheel drive ("pushbot") — two powered wheels at the back, unpowered wheels at the front. Good for learning to build with a kit. Game Manual 0 recommends moving on from it for your competition robot: it is slow to turn and underpowered with only two motors.
- 4-wheel drive — all four wheels powered. Using four traction wheels makes turning sluggish, because the wheels have to scrub sideways across the tiles. Most teams use two traction wheels and two omni wheels as a balance, and keep the chassis close to square.
- 6-wheel drive — six wheels with either omni wheels at the corners or a drop centre, where the middle wheels sit slightly lower so the robot only touches the floor on four wheels at a time. Great traction and turning, and the classic "defence" drivetrain.
Tank strengths: traction, pushing power, simple to build and program, the most forgiving if a wheel is slightly misaligned. Tank weaknesses: it has to turn before it can go somewhere new, which costs time in a game full of small adjustments.
Mecanum
Four mecanum wheels, each driven by its own motor. Spin all four forwards and the robot drives forwards. Spin the two on one diagonal opposite to the two on the other diagonal and the angled rollers push the robot sideways — this is called strafing. Mix the two and you can move in any direction while turning, or while not turning at all.
That freedom is why mecanum is one of the most popular drivetrains in FTC, from rookie teams to world-championship robots. Lining up with a scoring position becomes a small sideways nudge instead of a three-point turn.
The trade-offs are real, though:
- Less traction and pushing power. The rollers grip less than a rubber tread, so a heavy tank drivetrain can push a mecanum robot around.
- Strafing is slower than driving forwards, because the rollers slip more when strafing.
- No redundancy. Each wheel has its own motor; lose one and the robot can no longer drive properly.
- Wheels slip, so counting wheel rotations is a poor way to know where you are in autonomous. Many teams add separate tracking wheels — see understanding odometry.
The X rule
Mecanum wheels come in left-handed and right-handed versions, and the order you put them on matters.
The rollers must form an X from above
Look straight down at the robot. The rollers on the top of the four wheels should form an X pointing in towards the centre. If they form a diamond instead, the robot will turn badly and strafe wrongly, and no amount of code will fix it. Check this the moment the wheels go on.
Holonomic options you will hear about
Mecanum is one kind of holonomic drivetrain — one that can move in any direction without turning first. A few others exist; know what they are, but do not build them as a rookie.
- X-drive — four omni wheels mounted at 45° in the corners. Strafes as fast as it drives forwards, but it is awkward to package and usually ends up direct-driven.
- H-drive — tank-style omni wheels plus a sideways "strafer" wheel in the middle.
- Swerve — every wheel module can rotate to point in any direction while it drives. Fast and strong, but it needs extra actuators to steer each module, it is expensive, and it is genuinely hard to build and program well.
So what should a rookie build?
Game Manual 0's advice is to stick to the simpler drivetrains — 4- or 6-wheel tank, or mecanum — because complexity costs reliability, and reliability wins matches.
| Tank (4WD/6WD) | Mecanum | |
|---|---|---|
| Motors needed | 2–4 (4 recommended) | 4, one per wheel |
| Strafe sideways | No | Yes |
| Pushing power | High | Lower |
| Build difficulty | Easiest | Easy with a kit |
| Programming | Simplest | A little more maths |
| Gets pushed around | Rarely | More often |
For most rookie teams, a mecanum chassis built from a kit (both REV and goBILDA sell them) is the right answer: it goes together from instructions, it is proven, and strafing makes driving forgiving while your drivers are still learning. Pick tank if you expect to need pushing power or the game has terrain to climb, or if motors are scarce.
Read the game first
Drivetrain choice should come from the game. Is there a barrier to drive over? Is it a game where robots crowd into the same spot? Will you need every motor port for mechanisms? Answer those before you order wheels. The motor count and size limits are in the current season's Competition Manual — check them before you design.
If you are going to model your drivetrain before building it, the CAD track's designing a drivetrain lesson walks through laying one out in Onshape.
Check yourself
This season's game rewards lots of small sideways adjustments in front of a scoring area, and there is no terrain to drive over. Your team is new and has a tight budget. Which drivetrain is the best fit?
Check yourself
Your new mecanum robot drives forwards fine but turns slowly and strafes at an odd angle. Code has been checked twice. What should you check first?
Next
You have picked a drivetrain. Now pick its speed: motor, gearing and wheel size.
Sources and further reading
- Game Manual 0 — Drivetrains (selection priorities and glossary)
- Game Manual 0 — Tank (Skid-Steer) Drivetrains
- Game Manual 0 — Holonomic Drivetrains (mecanum, X-drive, H-drive, and the X rule)
- FIRST Tech Challenge Competition Manual — robot rules change every season
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