Crawler Gearing Basics: Understanding Gear Ratios on the SCX24 and TRX4M

When I was first getting back into RC after a long break, I noticed my SCX24 would occasionally get warm during a run. Not dangerously hot, but noticeably warm to the touch. I assumed this was just what small motors did. I was wrong.

The motor was working harder than it needed to because the gearing wasn’t matched to what I was asking it to do. Once I understood gear ratios, I fixed the heat issue in about ten minutes and the truck drove noticeably better. It’s one of those things that sounds technical but is actually pretty simple once someone explains it without assuming you already know the vocabulary.

What a Gear Ratio Is

A gear ratio describes how many times the motor has to spin to turn the wheels once.

A high gear ratio (like 100:1) means the motor spins 100 times per wheel revolution. That produces a lot of torque but very slow wheel speed. A low gear ratio (like 20:1) means the motor spins 20 times per wheel revolution. Less torque, but faster wheel speed.

For crawling, you almost always want more torque and less speed. Micro crawlers are not trying to go fast. They are trying to maintain traction and precise control on rough terrain. A higher overall gear ratio is almost always the right direction for a crawler.

The confusion comes from the way the numbers work: a “higher” gear ratio has a larger number, which produces more torque and less speed. This trips people up because in everyday language, “high gear” on a bike means the hard gear you use for speed. RC crawling uses the engineering convention, not the cycling convention.

The Two Places Gearing Happens

On a micro crawler, the final gear ratio is the result of two gearing stages working together.

External gearing is the pinion gear (on the motor shaft) and the spur gear (the large plastic gear it meshes with). This is the gearing you can see and adjust without opening the transmission. Changing the pinion gear size changes the ratio between these two gears.

Internal gearing is everything inside the transmission housing and axle differentials. On most stock micro crawlers, this is fixed. You are not going to open the transmission as a beginner, and you do not need to. The internal ratio is set at the factory and stays that way.

The final drive ratio is the result of both stages multiplied together. For most purposes, you only need to think about the external gearing, because that is the part you can change.

The Pinion Gear: Small Change, Real Effect

The pinion gear is a small metal gear that presses onto the motor shaft. On the SCX24, it typically has somewhere between 12 and 17 teeth in the stock configuration depending on the version. On the TRX4M, the stock pinion is in a similar range for the motor size.

Here is the rule: fewer teeth on the pinion = lower final gear ratio = more torque and less speed. More teeth on the pinion = higher final gear ratio = less torque and more speed.

For crawling, dropping down one or two teeth on the pinion is often a noticeable improvement. The truck feels more planted on climbs. It is easier to hold a line at near-zero throttle. The motor runs cooler because it is no longer fighting its own limitations.

For the SCX24, aftermarket pinion gears in various tooth counts are available at very low cost. A set of three or four different sizes costs less than $10 and lets you experiment until you find what works on your terrain. The fit between the pinion and spur gear matters though. Mesh too tight and the gears will bind; too loose and they will skip. There is usually a small adjustment screw on the motor mount that lets you set the gap.

For the TRX4M, Traxxas sells pinion sets for their motor, and there are also aftermarket options. The platform is a little less popular for gear tuning than the SCX24 because the stock gearing is generally well-matched to casual use. But the same principles apply.

When to Go Lower (More Torque)

Lower gearing is the right call when:

• The motor is running hot after moderate use
• The truck spins tires on climbs you expect it to handle
• The truck lacks the control you want at near-zero throttle
• You have added brass weight and the motor is now working harder than before

Adding brass upgrades is one of the most common reasons to revisit gearing. Brass weight improves traction but it also makes the motor work harder to move the truck. If your motor was running fine before brass and starts running warm after, going one tooth lower on the pinion is often the fix.

When to Go Higher (More Speed)

Higher gearing is less commonly useful for crawling, but it has its place:

• If the truck feels sluggish on flat terrain and you never crawl anything technical
• If you have upgraded to a brushless system and the new motor has a very low Kv rating, higher gearing can bring back usable speed on easier terrain

For most beginners on stock brushed setups, going higher is the wrong direction. You bought a crawler, not a basher. Slower and more controllable is almost always better.

A Word on Spur Gear Wear

The spur gear is the larger plastic gear that the pinion meshes with. It wears over time, especially if the mesh isn’t set correctly or if you are pushing the motor hard in low gearing. Signs of a worn spur gear: clicking or skipping under load, visible chips in the gear teeth, or rough transmission noise that wasn’t there before.

Spur gears are inexpensive to replace and are considered a consumable on crawlers that get regular use. Keep a spare. On the SCX24, the spur gear is accessible without a full teardown. On the TRX4M, you need to get into the transmission housing, which takes a bit more work but is still manageable with basic hex keys.

If you notice skipping under load, check the spur before assuming the motor is failing. A $5 spur gear replacement is a lot better than a motor diagnosis.

The Mesh Gap

Gear mesh is how closely the pinion and spur gear contact each other. Too tight and they will bind, which loads the motor unnecessarily and generates heat. Too loose and the gears will skip under power.

The correct mesh gap on a micro crawler is roughly the thickness of a piece of paper. With the pinion seated on the motor shaft and the motor in the mount, you should feel slight resistance when rotating the spur gear by hand. Not free-spinning, not grinding. There should be a tiny amount of play.

Most motor mounts on the SCX24 have an adjustable slot that lets you slide the motor closer to or further from the spur gear. Loosen the motor mount screws, position the motor for the right mesh, and tighten. Check it again after the first run. The mesh sometimes changes slightly as the gears break in.

What to Buy

If you want to experiment with gearing on the SCX24, a basic pinion gear set in multiple tooth counts is the place to start. Look for sets of 3-4 pinions in the 13-16 tooth range from brands like Injora or RC4WD. Under $10 for a set that will give you meaningful options. This SCX24 pinion gear set is a good starting point with multiple tooth counts in the right range.

For the TRX4M, Traxxas sells replacement pinions that are confirmed fits. Aftermarket options exist but confirm the tooth profile (mod 0.4 or 0.5 depending on the generation) before ordering. This ADORLY 0.5 Mod pinion set covers 13T through 16T and is spec’d for the TRX4M motor shaft.

A spare spur gear for your platform is also worth having. Not urgent, but when one strips, you want to be able to fix it the same day rather than waiting for shipping.

---

Gearing is one of the last things most beginners think to look at, but it is one of the first things worth understanding. You do not need to obsess over it. But if your motor is running warm or the truck is not crawling the way you expect, the gear ratio is a good place to check before assuming something is wrong with the electronics.

Once gearing is sorted, the next thing to look at on the power side is whether the brushless upgrade makes sense for your use. The honest answer is usually “not yet,” but the brushless upgrade guide covers when it does and what to buy when you’re ready.

---

See also: Your First 5 Crawler Upgrades · SCX24 Brushless Upgrade · Shock and Suspension Tuning · SCX24 Platform Guide · TRX4M Platform Guide · Throttle Control Basics · Essential Tools

This page contains affiliate links. If you purchase through these links, I may earn a small commission at no additional cost to you. I only recommend gear I actually use.