Traxxas TRX-4 Brushed Gearing Guide — FDR, Pinion & Spur Chart

Traxxas TRX-4 Brushed RC Gearing & Optimization Guide

Optimizing your gear ratio is one of the most effective ways to balance speed, torque, and electronics longevity in your Traxxas TRX-4 (Titan 21T Brushed / RTR). The relationship between your pinion gear (attached to the motor) and your spur gear (attached to the transmission) dictates how hard your motor has to work — and on a 1/10 rock crawler & scale trail truck platform with a 40.66 internal transmission ratio, even a single-tooth pinion change shifts your final drive ratio by 3-5%.

The brushed TRX-4 RTR ships with the Titan 21T 550-size brushed motor and a Ni-MH-tuned XL-5HV ESC. Portal axles produce an effective final drive well above 40:1, which is why the low-RPM brushed motor feels perfectly matched to slow rock work. Because brushed motors deliver peak torque at stall, obstacle-heavy driving on the TRX-4 wears brushes and glazes the commutator faster than any other failure mode on the truck.

Rock Crawler & Trail Truck Drivetrain Notes for the Traxxas TRX-4 Brushed

Ultra-low speed crawling on a Traxxas TRX-4 Brushed demands heavy grease packed into the portal or straight axles to damp shock loading and keep the ring-and-pinion quiet under sustained torque. Whether you are running a precisely tuned slipper clutch or a fully locked differential, the goal is to make the drivetrain deliver torque smoothly instead of shock-loading it into a rock face. High-torque binds against a ledge can spike amp draw well past continuous ratings and overheat the motor even at walking pace, so a small in-line wattmeter or an ESC with current logging is worth more than another temperature reading — heat shows up long after the damage is already done.

🛠️ Essential Tools Required for Gearing Changes

Hex drivers (1.5mm, 2.0mm, or 2.5mm depending on the Traxxas TRX-4 Brushed variant)
Paper strip (for setting precise gear mesh)
Infrared temperature gun (crucial for monitoring motor heat after each run)
Threadlock (for the pinion gear grub screw)
Pinion gear puller (recommended when swapping gears on a hot motor shaft)

📋 Comprehensive Gearing & Temperature Guide

1. Understanding Pinion vs. Spur Gear Adjustment

Changing your gears alters your final drive ratio. Installing a larger pinion gear or a smaller spur gear increases top-end speed but increases the load on the motor, causing it to run hotter. Conversely, a smaller pinion or larger spur increases torque and acceleration while lowering top speeds and keeping your motor cool. On the Traxxas TRX-4 Brushed, this trade-off is amplified by the fixed 40.66 internal ratio — small external changes have a direct thermal consequence.

2. How to Set a Perfect Gear Mesh

Improper gear mesh will quickly strip your spur gear or bind your drivetrain.

Loosen the motor mount screws slightly.
Place a small strip of standard notebook paper between the pinion and spur gear teeth.
Press the gears tightly together and tighten the motor mount screws.
Roll the paper out. The paper should have clean, crisp crinkles without ripping.

Recommended Pinion & Spur Chart

All combinations use a 45T 48-pitch spur. FDR is calculated as (spur ÷ pinion) × 40.66 internal ratio.

Pinion	Spur	FDR	Typical Use
11T	45T	166.34	High-bite carpet / tight indoor
13T	45T	140.75	High-bite carpet / tight indoor
15T	45T	121.98	High-bite carpet / tight indoor
17T	45T	107.63	High-bite carpet / tight indoor
19T	45T	96.3	High-bite carpet / tight indoor

Understanding Pinion & Spur Gears

Stock brushed TRX-4 uses a 16T pinion / 45T spur (mod-0.8). A drop to 14T is a common brush-preservation tweak for hard-obstacle drivers — less peak stall current, less brush heat, longer motor life.

Rollout Targets

Because of the portal reduction, rollout on the brushed TRX-4 is measured in fractions of an inch per motor revolution — closer to 0.4 inches, which is what makes precise throttle control possible on a rock face.

Motor Temperature Management

Brushed TRX-4 motors rarely climb past 150F (65C) even on hard trails because sustained RPM is so low. What DOES climb is brush and comm temperature at stall — inspect brush length every 8-10 packs and clean the comm with motor spray whenever you notice a sudden drop in low-speed pulling power.

⚠️ Critical Safety & Temperature Warning

Always use an infrared thermometer to check your motor and ESC temperatures during a run. RC electric brushless motors should never exceed 160°F (71°C). Exceeding 180°F (82°C) risks permanently demagnetizing your motor rotor and frying your Electronic Speed Controller. If your Traxxas TRX-4 Brushed is running above these thresholds, you must "gear down" by installing a smaller pinion gear immediately, improve airflow with a larger motor fan, and inspect the drivetrain for binding.

Calculate a custom FDR for your Traxxas TRX-4 Brushed

The link below opens the calculator with Custom / Other Chassis pre-selected, the Traxxas TRX-4 Brushed's internal ratio of 40.66 and its recommended battery of 7-Cell Ni-MH (8.4V) already set — just plug in your pinion, spur, motor, and tire to see top speed, runtime, and FDR for your exact setup.

Open calculator with 40.66 ratio + 7-Cell Ni-MH (8.4V) preset →Brushed / 7-Cell Ni-MH (8.4V) stock setup →

🔧 Brushed Motor Maintenance for the Traxxas TRX-4 Brushed

Brushed motors on the Traxxas TRX-4 Brushed are wear items — the carbon brushes, springs, and copper commutator all consume themselves as the truck runs, and 90% of "my truck feels slower" complaints on a brushed RTR trace back to worn brushes or a glazed comm rather than a bad pack or the wrong gearing. Build a five-minute inspection into every 4-5 pack cycles and the stock Titan-class motor will outlast most bearings on the truck.

1. Inspect the carbon brushes

Pop the brush hoods off the endbell and pull the brushes out with a pair of tweezers. New brushes on a 540-size motor are roughly 6-7mm long. Once they wear below 2mm the spring can no longer keep firm contact with the commutator — you get arcing, hot brushes, and a sudden drop in top-end speed at the same gearing. Replace brushes as a pair, never just one, and always with the motor cold.

2. Clean the commutator

A glazed or dirty commutator shows up as reduced pulling power at low speed and a burnt-electronics smell after a hard pack. Remove the brushes, blast the commutator surface with a can of dedicated motor spray (never brake cleaner — it attacks bearing seals), then run a comm stick lightly across the copper segments while spinning the shaft by hand. The copper should come back to a bright, uniform pink; any dark spots or grooves mean the comm needs a true-up on a comm lathe or a motor replacement.

3. Check brush spring tension

Weak springs cause the brushes to bounce at high RPM, which shows up as an intermittent power cut on straights. Compare the spring compression side by side with a spare — if one is noticeably shorter or softer, replace the pair.

4. Break in new brushes properly

After a brush or motor swap, run the truck on a stand (wheels off the ground) on a 6-cell Ni-MH pack for 2-3 minutes at low-to-moderate throttle. This seats the new brush face to the commutator profile before you subject it to full stall torque on a rock face or bash pack.

Rule of thumb on the Traxxas TRX-4 Brushed: if top-end speed drops noticeably at the same gearing and pack, inspect the brushes and commutator BEFORE you assume the motor is dying — 8 out of 10 times it is a $6 brush set, not a $35 motor.

Ni-MH vs. LiPo on the Traxxas TRX-4 Brushed

Running a traditional 7-Cell Ni-MH (8.4V) nickel pack on the Traxxas TRX-4 Brushed behaves very differently from a modern LiPo of similar nominal voltage. Ni-MH cells sag noticeably under load — a fresh 7-cell 8.4V stick pack measures closer to 7.2-7.6V once the trigger is pinned, which lowers real-world motor RPM and top speed by 10-15% compared to a 2S LiPo bench number. This voltage drop also means the ESC pulls higher current to hold speed, so brushed Titan-class motors on Ni-MH actually run hotter per lap than the same truck geared identically on a 2S LiPo, even though the pack voltage looks lower on paper.

Weight distribution changes too: a 6-8 cell Ni-MH stick or hump pack weighs roughly 380-460 g, versus 220-290 g for a comparable 2S/3S hard-case LiPo. On a short course truck or stadium truck like the Traxxas TRX-4 Brushed, that extra ~150 g sits low and rearward, softening the chassis' pitch response and loading the rear tires harder on corner exit. Bashers converting from Ni-MH to LiPo almost always need to add a tooth to the pinion (to compensate for the lighter, punchier LiPo delivery) and re-check motor temps after the first pack, because the same FDR that was cool on nickel can heat-soak on a high-C LiPo.

Rule of thumb: on Ni-MH, target motor temps 10-15°F cooler than the LiPo maximum listed above — the sustained current draw is higher, and nickel packs are less forgiving of over-gearing before they thermally cut out.

Traxxas TRX-4 (Titan 21T Brushed / RTR) Gearing Guide