Building a drivetrain that drives straight

Push the stick straight forward and the robot drifts left. Every rookie team meets this robot. The tempting fix is in code — and code can hide it — but the cause is almost always mechanical, and a mechanical problem that is hidden in October becomes a broken robot in January.

This lesson is the checklist for a drivetrain that drives straight because it is built straight.

Mecanum Wheels Tips & Tricks· goBILDAWatch on YouTube

Why robots curve

A robot drives straight only when the left and right sides push equally. Anything that makes one side push less turns the robot towards that side:

A wheel or shaft that binds — too-tight bearings, a rubbing collar, an over-tensioned chain.
A wheel that is not touching the floor properly because the frame is twisted.
Wheels that are not parallel, so they fight each other.
Weight sitting over one side, so those wheels grip more than the others.
A motor or gearbox that is different from, or more worn than, the others.

Square: measure the diagonals

Square (frame): All four corners at 90°. On a frame whose opposite sides are already the same length, the two diagonals are equal only when it is square.

Measure corner to opposite corner, both ways, from the same kind of point on each corner (a bolt centre or wheel shaft is ideal). If the two numbers match, the frame is square. If one diagonal is longer, the frame has been pushed into a parallelogram, and the wheels on one side sit slightly ahead of the other.

Assemble on a flat surface — a table or a sheet of plywood, not the carpet.
Leave the frame bolts snug but not tight so the frame can still move.
Measure both diagonals and gently push the long diagonal's corners together until the two numbers match.
Tighten the bolts a little at a time, working across the frame, re-checking the diagonals as you go. Cranking one corner fully tight first pulls the frame out of square.

Stiff: a frame that does not twist

A chassis that twists acts like a table with one short leg. On a four-wheel drivetrain, especially mecanum, one wheel ends up carrying very little weight. It spins without pushing, and the robot veers.

You can test it with paper. Put the robot on a flat floor, slide a strip of paper under each wheel, and pull. Every strip should be firmly pinched. If one pulls out easily, that wheel is barely touching the ground.

Make the frame stiffer with cross members between the two sides, and avoid long unsupported rails. A single cross member near the middle of a tall rectangle does a lot less than two near the ends.

Some drivetrains want an uneven floor contact on purpose

A drop-centre six-wheel drive deliberately lowers its middle wheels slightly so only four wheels touch at once. That is a deliberate design, with a specific small drop. On a four-wheel or mecanum drivetrain, all four wheels should carry weight.

Aligned: wheels parallel and upright

Every wheel shaft supported in two bearings where you can, spaced apart. A shaft hanging off one bearing lets the wheel tilt under load.
Spin each wheel and watch its edge. A wobble means a bent shaft or a hub that is not seated square.
Matching wheels on each side. A worn wheel has a smaller diameter, so that corner covers less distance every revolution.
Mecanum rollers in an X from above. Covered in the previous lesson; it is still the most common setup mistake.

Free: every wheel takes the same effort

Lift the robot onto a box so the wheels are off the ground and turn each wheel by hand. They should all feel alike. A wheel that is noticeably stiffer is binding — look for an over-tight chain or belt, a collar pressing on a bearing, or a bearing pushed into its hole crooked.

For a more objective version, run all four motors at the same power with the wheels off the ground and display each motor's encoder velocity in telemetry. A wheel that is consistently slower than the others has extra friction somewhere.

Chain or belt tension on both sides

On a chain or belt drivetrain, an over-tightened run on one side is one of the most common causes of a curve. Set both sides to the same light tension, and re-check chain every few practice sessions — it stretches.

Balanced: where the weight goes

Centre of mass: The single point where the robot's weight effectively acts. Where it sits decides how weight is shared between the wheels.

Keep it near the middle of the wheelbase. Weight shifted towards one side or corner means those wheels grip more than the others. On a mecanum drivetrain this is the classic cause of a robot that rotates while it strafes.
Keep it low. The battery and other dense parts belong low in the chassis. A high centre of mass tips over when the robot stops hard or when a lift extends.
Remember it moves. A lift or arm that extends forward shifts the centre of mass forward and up. Test driving with the mechanism extended, not just stowed.

A repeatable straight-line test

Fixing a curve means changing one thing and checking whether it got better. You need a test you can repeat.

Lay a straight line of tape a few metres long on the practice floor.
Start the robot square on the line, same spot every time.
Drive straight forward at a fixed power for a fixed time, with no steering input.
Measure how far sideways the robot ended up from the line, and which way. Write it down.
Change one thing, then repeat. If the drift shrinks, keep the change. Do the same driving backwards — some problems only show in one direction.

Only once the mechanical drift is as small as you can get it should software take over. Heading correction with the hub's IMU, and the odometry covered in the programming track, clean up the last few centimetres — they should not be fighting a bent shaft.

FTC Friday Episode 6: Battle-Proofing Your Strafer· goBILDAWatch on YouTube

Check yourself

Your tank drive robot curves to the right whenever it drives forward at full power. With the wheels off the ground, the right-side wheels are noticeably harder to turn by hand. What is going on?