⚙️ Spur
The large main gear the pinion drives. Set the pinion-to-spur mesh with the paper trick for quiet, long-lasting gears.
What it actually does
The spur is the large gear driven by the pinion, usually plastic, and it is deliberately the softer of the two. If something in the drivetrain is going to give up under a big shock load, far better it is a cheap spur gear than a motor shaft or a transmission.
In practice
Changing spur size shifts the final drive ratio the opposite way to the pinion: a larger spur raises the FDR, giving more acceleration and cooler running. Many people keep the spur fixed and tune with pinions, simply because pinions are quicker to swap.
Worth knowing
A stripped spur usually means the mesh was too loose or the slipper was too tight. Replacing the gear without fixing that will strip the next one too.