How RC pinion and spur gears trade speed for acceleration, the final drive ratio math, setting mesh with the paper trick, and using motor temps to gear right.
Gearing is the one setup change that costs almost nothing, takes ten minutes, and completely changes how your car drives. It's also the thing most people mess up first. I've stripped spurs, cooked motors, and blown up an ESC or two learning this, so let me save you the parts money.
The short version: two gears do the work. A small pinion on the motor shaft, and a big spur bolted to the transmission. Swap either one and you're trading acceleration for top speed, or the other way around. That's the whole game. Everything else is just detail.
The pinion is the little metal gear pressed onto your motor shaft. Counted in teeth. A 21T pinion, a 23T, whatever. The spur is the big plastic (usually) gear it drives. Small pinion, big spur is the general layout.
Here's the part that trips people up. Going up on the pinion — say 21T to 23T — gives you more top speed but slower punch off the line, and it makes the motor work harder, so it runs hotter. Going up on the spur does the opposite: more acceleration, less top end, cooler running. Most of the time you'll leave the spur alone and just play with pinion teeth, because pinions are cheap and easy to swap.
Think of it like a bike. Big gear up front is a hard pedal — fast if you can push it, but murder on the hills. Same thing here. Overgear it and the motor's grinding uphill the whole run. That heat has to go somewhere, and it goes into your magnets and your ESC.
Two numbers matter. Spur teeth divided by pinion teeth gives you the gear ratio. So an 87T spur on a 21T pinion is 87 ÷ 21 = 4.14. That's your transmission ratio.
But that's not the real number, because there's more gearing downstream in the diffs. The number you actually care about is the final drive ratio (FDR):
FDR = (spur ÷ pinion) × internal drive ratio
The internal ratio is a fixed number for your chassis — it's in the manual. A lot of 1/10 buggies sit around 2.6 internal. So that 4.14 transmission ratio times 2.6 gives an FDR of about 10.8. Higher FDR means more torque and acceleration, lower top speed. Lower FDR means faster but lazier off the line and hotter. Write your FDR on a piece of tape inside the lid so you're not doing arithmetic at the track.
If you're running a stock/spec class with a fixed motor, the rules usually cap you somewhere and you tune FDR to the track. Big open track, gear taller (lower FDR). Tight technical track, gear shorter so you've got punch out of the corners.
Mesh is how tightly the two gears engage, and getting it wrong is how spurs get eaten. Too tight and the gears bind, drag, and heat up. Too loose and the teeth barely catch — that's the loud grinding you hear right before a spur strips its teeth off.
The paper trick has saved me more spurs than anything. Loosen the motor screws. Push the pinion into the spur. Slip a strip of regular printer paper between the two gears, then push the motor so the pinion pinches the paper against the spur. Tighten the motor screws down while it's pinched. Pull the paper out — it'll tear, that's fine. That gap the paper left is just about perfect.
Any time you change pinion size you have to re-set mesh. Bigger pinion sits differently, so the motor has to move. Don't skip this. I've watched people bolt on a bigger pinion, forget the mesh, and grenade an $18 spur on the first pull.
You can theorize about ratios all day, but the motor tells you the real answer. After a proper run — a full pack at pace, not a lap around the driveway — check the motor can with an infrared thermometer. A cheap $12 laser one is fine.
Keep it under about 160°F (roughly 71°C). That's my rule of thumb for staying safe. Up around 180°F you're cooking magnets on a brushless motor and the smell of hot brushed motor is its own warning. If you're over, you're overgeared — drop a pinion tooth (or go up on the spur) and run it again. Cool and lazy? You've got room to gear up for more speed.
Check it right when you pull off the track, not five minutes later once it's cooled. And check the ESC too while you're at it — an overgeared setup bakes the speed control as much as the motor. Hobbywing and Castle units will thermal-shut-down to protect themselves, which feels like a dead throttle mid-run. That's the ESC telling you the same thing the thermometer would.
Pitch is tooth size, and it has to match between pinion and spur. Mixing them is a fast way to shred both. Three common ones:
You don't get to choose in a vacuum. Your chassis and class decide it. Stay in one system and buy your pinions and spurs to match. If you're not sure what your car runs, the manual spells it out, and honestly Traxxas keeps their manuals easy to find over at traxxas.com if you run their stuff.
Overgearing for a speed run is number one. Somebody wants a big GPS number, throws a huge pinion on, and holds full throttle down a parking lot. Motor hits 200°F in one pass and the magnets never recover. If you're chasing top speed, do short pulls and check temps between every single one. Speed runs are hard on hardware, full stop.
Number two is bad mesh, which we covered — too loose strips the spur, too tight cooks everything and robs runtime. Number three is people forgetting that a fresh, grippy track adds load. More traction means the motor works harder to spin the same gearing, so a setup that ran cool on a dusty morning track can run hot by afternoon once it's rubbered in. Same gears, hotter motor. Gear down a tooth when grip comes up.
One more: gearing interacts with your motor choice. If you're still sorting out what motor to run, read my take on brushed versus brushless motors first, because the right gearing depends entirely on the can you're spinning. And whatever you land on, checking temps is part of your post-run routine — do it every time, not just when something feels off.
Start simple. Pull the pinion your car came with, note the size, and change one tooth at a time — never two. Run a pack, check the temp, adjust. Once you find the number that keeps you under 160°F with the punch you want, write it down. A pack of pinions runs ten bucks; if you need spares or a spur to match your chassis, the marketplace listings are the fastest place to grab used ones cheap, and it's worth saving your dialed numbers in your setup sheets so you can put it right back after a rebuild.