⚙️ Differential
A gear or fluid-filled unit that lets the left and right (or front and rear) wheels turn at different speeds through a corner.
Thicker diff fluid = smoother power delivery and more on-power steering/stability.
What it actually does
A differential lets the two wheels on an axle turn at different speeds, which is what a car needs to corner without dragging a tyre. Without one, the inside wheel is forced to travel the same distance as the outside wheel and the car fights every turn.
In practice
RC cars use gear diffs filled with silicone oil, ball diffs adjusted by a spring and thrust bearing, or solid spools with no differential action at all. Four-wheel-drive cars usually have three: front, centre and rear, each changing the handling in a different way.
Worth knowing
Diffs are a tuning tool, not just a mechanical necessity. Thicker oil or a tighter setting makes an end of the car behave more like a spool — more drive, less rotation.