Workshop: Build a Mecanum Drivetrain Chassis

The drivetrain is the base everything else sits on. If the frame is twisted or a wheel rubs, your robot will drift, waste battery and be hard to drive, and no amount of code will fully fix it. In this workshop you will build a four-wheel mecanum chassis, the most common drivetrain for rookie FTC teams because it can drive forward, turn and slide sideways (strafe). Work slowly: a careful first build saves hours of repairs later.

Tools and parts

Plan the frame layout

Most kit chassis are a rectangle: two long side rails that carry the wheels, joined by two or more cross members at the front and back. Before you tighten anything, lay all the parts on the table in their final positions. Check that the wheels will sit near the four corners, that there is room inside the frame for the battery and the Control Hub, and that the frame fits inside the robot size limit. Size limits change between seasons, so treat any number you have heard as an example and check the current official Competition Manual.

1. 1
   Assemble the side rails

   Build the left and right rails first, with the bearing holes or motor mount positions lined up exactly the same on both sides. Lay one on top of the other to compare them.
2. 2
   Add the cross members loosely

   Join the rails with the cross members and brackets. Put every screw in, but only snug them for now so the frame can still shift a little.
3. 3
   Square the frame

   On a flat table, measure corner to corner in both directions. Gently push the long corners together until both diagonals match, then tighten the screws in a cross pattern.
4. 4
   Re-check after tightening

   Measure the diagonals again, because tightening can pull the frame out of square. Press on each corner: the frame should sit flat and not rock.

Bearings, axles and motors

A bearing lets a shaft spin with very little friction while the frame holds it in place. Kits do this in different ways: some drive the wheel directly from the motor shaft through a hub, others put the wheel on its own axle supported by bearings and connect it to the motor with gears, belts or chains. Whatever your kit uses, support a wheel shaft on both sides of the wheel when you can. A shaft held on only one side (cantilevered) bends under load and wears out faster.

1. 1
   Press in the bearings

   Push flanged bearings into their holes by hand so the flange sits flat against the frame. Never hammer them; that damages the balls inside.
2. 2
   Fit axles, spacers and collars

   Slide in the shaft with spacers on each side of the wheel. Tighten shaft collars or hub set screws, with set screws pressed against the flat side of a D-shaped shaft.
3. 3
   Mount the motors

   Attach each motor to its mount using all the screws provided. Make sure the motor shaft lines up with the wheel shaft, gear or pulley so nothing is forced sideways.
4. 4
   Spin test each corner

   With the motor unplugged, turn each wheel by hand. It should spin smoothly with no clicking or tight spots. Find and fix any rubbing now.

Install mecanum wheels in an X

Mecanum wheels come in left-hand and right-hand versions, usually marked on the wheel or the packaging. Diagonally opposite wheels are the same type. After installing them, look down at the robot from directly above: the rollers on top of the four wheels must form an X across the robot. If they form a diamond instead, two wheels are swapped, and the robot will drive forward but will not strafe properly. Compare your build with the wheel maker's diagram or Game Manual 0.

Final mechanical check

1. 01Measure both diagonals one last time and record them.
2. 02Put the robot on a flat floor: all four wheels touch the ground and the frame does not rock.
3. 03Spin each wheel by hand: smooth, quiet and the same feel on all four corners.
4. 04Grab each wheel and try to wiggle it: no side-to-side play or wobble.
5. 05Go around every screw with the right tool: nothing loose, no missing nylock nuts.
6. 06Check the X pattern from above and take a top-view photo for your notebook.
7. 07Run a finger along edges and screw ends where wires will go and file down anything sharp.