Picking a drivetrain

Decide it from the game, not the video

Four questions settle this faster than any comparison table. How much of your cycle is driving? Do you need to move sideways to line up, or can you turn? Will you be pushed, and do you need to push back? And how many weeks of build time can you actually spend on a chassis before the rest of the robot stalls?

A game with long straight runs and a wide scoring target does not reward a strafing drivetrain. A game where you line up precisely against a narrow target, repeatedly, in traffic, does. Work the cycle maths first and the answer is usually obvious.

Tank

Two sides, driven wheels, turns by driving the sides at different speeds. It is the simplest thing that works and it is chronically underrated.

Good: cheap, fast to build, mechanically robust, excellent pushing power and traction, easy to program, and it keeps working when it is dirty. Fewest failure points of anything here.

Bad: cannot strafe, so lining up sideways means a turn-drive-turn sequence that costs seconds every cycle. Scrubs on turns depending on wheel choice.

Choose it when: you are a rookie team, defence matters, or your cycle does not need precise lateral alignment. A well-driven tank drive beats a badly maintained mecanum most of the season.

Mecanum

Four mecanum wheels, rollers at 45 degrees, so the robot can strafe and rotate independently. This is the default on a large fraction of FTC fields, and for good reason.

Good: holonomic movement, so alignment is one motion instead of three. Field-centric driving is straightforward with an IMU and makes a noticeable difference to a new driver. Kits exist, so build time is short.

Bad: this is the maintenance cost nobody mentions. Mecanum rollers pick up dirt, wear unevenly, and lose grip as the season goes on. A drivetrain that strafed perfectly in December drifts in March. It pushes poorly and is pushed easily, since only part of each wheel contacts at any moment. Weight distribution matters far more than on tank: an unbalanced robot strafes in a curve.

Choose it when: your cycle involves repeated precise alignment, and someone on the team will actually clean and check the wheels every few sessions.

Swerve and the exotic options

Independently steered and driven modules. It is genuinely the best drivetrain here, full holonomic movement with proper traction and pushing power, and it is the wrong choice for almost every team that builds one.

The real cost: module count multiplies your motor and encoder budget, the control code is a project of its own, and the mechanical complexity means it breaks in ways that take a pit repair rather than a bolt. Teams that build swerve typically spend six to eight weeks on the chassis. That is the entire build season for everything else.

Choose it when: you have built a competitive robot before, you have someone who can write and tune the control code independently, and your game genuinely rewards it. Otherwise the honest answer is that the same weeks spent on driver practice and a better intake will score more points.

X-drive and other omni layouts sit in between: holonomic, simpler than swerve, and with the same traction weakness as mecanum plus a less familiar control model.

The maintenance cost nobody mentions

• Every drivetrain loosens. Check and retighten motor mounts and wheel fasteners on a schedule, not when something falls off. A recurring checklist is the cheap version of this.
• Mecanum rollers need cleaning; grip loss is gradual, so nobody notices until alignment starts failing at an event.
• Chains and belts stretch. A chain that was right in December is loose in February and will skip under load.
• Encoder cables are the most-flexed wires on the robot and they fail at the connector.
• Carry a spare of whatever is single-point-of-failure. One $15 motor should not end a competition day.

The specifications that actually matter

Once the type is chosen, three numbers determine whether it is any good.

Gear ratio and free speed. Faster is not better past the point where your driver can stop accurately. A robot that overshoots every alignment has a slower cycle than a slower robot.

Weight, and where it sits. Low and centred. A tall robot with mass up high tips under acceleration and strafes unpredictably.

Motor budget. Four motors on the drivetrain is four you do not have for mechanisms. Count them before committing, since the total is capped.

Write down why you chose it

This decision is the best trade-off story most teams have, and judges reward exactly this kind of reasoning: an alternative considered, a criterion stated, a choice made against it. "We tested mecanum and chose tank because our cycle has no lateral alignment and we expected to be defended" is a complete portfolio entry.

Record it the week you decide, with the prototype photos and any timing numbers you took. Reconstructing it in February produces the version judges recognise as reconstructed. See the portfolio guide.