Every SCX24 has three gear reductions stacked together: motor pinion to spur, the transfer gear, and the axle internal gears. Change the axle internal gears and you change the truck's character without buying a new transmission. That's overdrive and underdrive in a nutshell. Here's when each is worth doing and what to expect.
What changes when you change axle gears
The final drive ratio on an SCX24 is:
final drive = (motor pinion to spur) × (transfer gear) × (axle pinion to ring)
Axle gears are the last multiplier. Swap them and you re-gear the rig without touching the transmission. Two directions to go:
- Overdrive (OD): wheels turn faster than stock for a given motor RPM. Less torque at the wheel.
- Underdrive (UD): wheels turn slower than stock. More torque at the wheel.
Stock SCX24 gearing
The stock axle is a 2T worm meshing a 16T worm gear. 2/16 ratio. Same in both axles, which means no over- or underdrive between front and rear.
Why builders run OD on the front, UD on the rear
The classic crawler gearing trick: overdrive the front axle slightly relative to the rear. The front wheels want to turn slightly faster than the rears, which pulls the truck through technical lines (the front tires “pull” rather than getting pushed). On full-scale crawlers this is a known geometry advantage.
On the SCX24 specifically:
- Stock front + stock rear: baseline. Truck pushes through corners.
- Front OD (2/14T or 2/13T worm) with stock rear: front pulls. Better at steering on technical lines, slightly less torque up front.
- Rear UD (rare on SCX24): more rear torque, even more under-rotation at the rear. Comp builders sometimes do this on certain chassis combinations.
When to overdrive (mostly the front)
OD makes sense when:
- You're running portals (which already underdrive the wheels 25%). OD partially compensates.
- You've gone brushless. The extra speed at the wheel makes the truck cover ground without losing crawl ability.
- You've added bigger tires (60mm+). Larger tire diameter effectively underdrives the truck; OD gears bring back stock-feel wheel speed.
- You want better technical-line behavior with the front pulling.
When to underdrive (rarely on SCX24)
UD makes sense when:
- You've gone brushless and don't want the truck any faster.
- You're comp-crawling at walking pace and want even slower throttle response.
- You've got smaller tires and want to bring the gearing back.
UD is more common on bigger crawler platforms. SCX24 builders tend to either keep stock gears or just do front OD.
The parts
Worm-gear options for SCX24 axles:
- LGRP 2-13T overdrive worm gear set. Around $14 to $20. The mainstream front-OD upgrade. Hardened steel, replaces the stock 2/16. Net effect: roughly 18% front overdrive.
- INJORA 2-14T overdrive worm. Around $12. Lighter overdrive (around 12%). Less aggressive change in character.
- Mofo RC bulletproof axle gears. $20 to $30. Hardened steel replacement worm in various ratios. Comp-grade durability.
- Treal hardened steel axle gear set. $15 to $20. Replaces both pinion and worm in matched hardened steel.
Almost every aftermarket portal axle kit (MEUS, LGRP, Mofo, INJORA) includes the axle gears in the housing, so the OD/UD question is whether you spec OD-equipped portals when you buy them. MEUS and LGRP both offer OD portal kits as a separate SKU.
What it actually feels like
Front OD on a brushed truck: subtle. The truck steers through tight lines slightly more easily. Top speed barely changes.
Front OD on a brushless truck with portals: more noticeable. The truck pulls through technical sections instead of pushing. Crawl behavior gets cleaner.
Heavy OD (anything past 25%): the front wheels can scrub on certain surfaces because they want to turn faster than the rears can drive them. Can feel weird on dirt and loose surfaces. Comp rock crawlers don't care; trail drivers on mixed terrain sometimes do.
Install difficulty
Easy to medium. You're replacing the worm and worm gear inside the diff. Two screws, one E-clip, pull the housing apart, swap parts, put it back together with new grease. Maybe 30 minutes per axle once you've done one. Detailed background in the gear ratios reference.
Where this sits in the overall plan: OD/UD is a tuning move, not a foundation upgrade. Do it after the servo, tires, brass, and motor are sorted. See the staged upgrade path.