⚙️ Gear Mesh
How tightly the pinion and spur gear teeth engage with one another.
Set it with a strip of paper: press the pinion into the spur with the paper trapped between them, tighten the motor screws, then roll the paper out — it should come out crimped but not torn. Check at several points around the spur, since spur gears are rarely perfectly round.
What it actually does
Gear mesh is how tightly the pinion on the motor engages the spur gear. Get it right and the drivetrain is quiet and lasts. Get it wrong in either direction and you will be buying gears — too tight binds and overheats, too loose lets the teeth skip and strip.
In practice
The classic method is to set the mesh with a strip of paper: slip a piece of ordinary paper between the gears, push the motor in until the teeth close on it, tighten the motor screws, then remove the paper. That leaves roughly the right small amount of backlash. Spin the wheels by hand afterwards — it should turn freely with a barely perceptible amount of play.
Worth knowing
Re-check the mesh whenever you change pinion size, and after a heavy crash. Listen for a whine that rises with speed: that usually means too tight.