Understanding gearing: FDR, rollout and motor temperature

Gearing trades acceleration against top speed, and both against heat. Get it wrong in one direction and the car is slow down the straight; wrong in the other and you cook a motor.

The two numbers

Final drive ratio (FDR) is how many turns the motor makes for one turn of the wheels:

FDR = (spur ÷ pinion) × internal ratio

The internal ratio is fixed by your transmission and diffs — the kit manual has it. So the only things you change are the spur and pinion.

• Bigger spur or smaller pinion → higher FDR → more acceleration, lower top speed, cooler motor.
• Smaller spur or bigger pinion → lower FDR → more top speed, slower off the corner, hotter motor.

Rollout is how far the car travels for one motor revolution:

Rollout (mm) = (tyre diameter × π) ÷ FDR

Rollout is the more useful number, because it accounts for tyre size. Two cars with the same FDR but different tyre diameters are not geared the same. Rollout also lets you carry a known-good figure between tracks and between cars — and it is why worn tyres effectively gear the car up as the day goes on.

RaceBase's gearing tools will work both out for you.

Using temperature to find the limit

Temperature is the honest feedback. Check the motor and ESC immediately after a run, with an infrared thermometer rather than a fingertip.

• Motor under about 75 °C is a reasonable working target for most brushless setups.
• ESC under about 70 °C, though check what your manufacturer specifies.
• Above those, gear down — smaller pinion or larger spur — or the magnets and bearings will suffer.

Ambient temperature matters more than people expect. Gearing that was fine on a cool spring morning can be too tall on a hot afternoon, so recheck when the weather turns.

How to change it

One tooth on the pinion at a time. A single tooth is a noticeable change; two is a big one. Change one thing, run it, note the temperature and the lap time.

Then reset the mesh. With the pinion against the spur, you want a slip of paper's worth of backlash — a faint amount of play, not a tight bind. Too tight strips the spur and drags the motor; too loose chews teeth. Check it at several points around the spur, since spurs are rarely perfectly round.

Where the gearing actually pays

Match the gearing to the track, not to the spec sheet. A tight, technical infield rewards acceleration out of slow corners. A track with a long straight rewards top speed. If the car is still accelerating hard as it reaches braking, you have room to gear up; if it stops pulling well before the end of the straight, you are geared too tall.

Record what you run in the setup sheet for that track. Next visit starts from a known point instead of guesswork.