All About Servos: Everything You Need to Know

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Learn How Servos Work and What They Do

Servos are the muscles of a radio control model. In an RC car, servos are used to steer the front wheels, and if you’ve got a nitro-powered model, a second servo is used to open and close the carburetor and operate the brake. Servos come in different sizes to suit larger and smaller models, and some servos offer greater torque and operating speed than others.

Every RC car is equipped with at least one servo, to operate the steering system.

Traxxas Ready-To-Race^® models are all equipped with appropriately powerful servos for the models they are installed in. However, you may wish to install a more powerful or more durable servo in your model if you have upgraded to brushless power, increased the weight of your model with accessories, or frequently run your model in extreme terrain. Traxxas offers optional upgrade servos to handle the most demanding applications.

Improve your performance in high-strain applications with a high-torque servo upgrade.

What’s Inside a Servo?
The mechanics of a servo are simple. It’s basically just a gearbox; inside, a motor spins a set of reduction gears that turns the motor’s RPM into torque. Just like the transmission and power system in your RC car or truck, a servo may have more powerful motor and/or gear ratios optimized for torque (strength) or speed. “Speed,” in the case of a servo, refers to how quickly the servo can swing its output arm.

Understanding Servo Specs

Servo performance is indicated by two specs: speed and torque. For most RC modelers, torque is much more important than speed. Torque is measured in ounce-inches, which you may see abbreviated as “oz.-in.” We know what you’re thinking: “What’s an ounce-inch?”

Torque
Imagine you’re changing a flat tire on a car. You place your lug wrench on one of the wheel nuts, then stand on the end of the wrench to loosen the nut. If you weigh 180 pounds, and the lug wrench is one foot long, you are applying 180 foot-pounds of torque to the lug nut. An ounce-inch is the same concept, just with smaller increments: an inch instead of a foot, ounces instead of pounds. If you have a 400 ounce-inch servo, and you install a one-inch long arm to its output shaft, that arm will apply 400 ounces of force to the linkage you attach it to. Four hundred ounces is 25 pounds—that’s a powerful servo!

A 400 oz.-in. of torque should be capable of lifting 400 ounces with a one-inch arm.

Speed Servo speed is indicated as the time in seconds for a 60-degree movement of the output shaft. For example, if a servo’s speed is 0.15 sec/60°, that means it will sweep 60 degrees in fifteen-hundreths of a second. A skilled driver on a racetrack may benefit from a faster servo, but most drivers can’t detect a few hundredths of a second difference in servo speed.

Servo speed refers to how quickly it can sweep 60 degrees.

Check the Voltage When Comparing Specs
When comparing servo specs, be sure to note the voltage that the manufacturer says is required to achieve the stated performance numbers. For example, the Traxxas High-Torque 400 servo’s torque is stated as “400 ounce-inch @ 6.0 V.” That means the servo supplies 400 ounce-inches of torque when powered at six volts.

Suppose you’re comparing the Traxxas 400 oz.-in. servo to another brand’s servo that claims “400 oz.-in. @ 7.4 V.” That servo appears to be equally powerful since it’s also rated at 400 ounce-inches, but since it needs 7.4 volts to achieve that torque, it will offer less torque when powered at six volts. In this example, that servo’s torque would drop to about 324 oz.-in. on six volts, making it considerably less powerful than the Traxxas servo at the same voltage. Always check voltage when comparing specs.

Metal Gears, More Strength
Many servos use plastic gears, and they work well for most 1/10 scale RC car and truck applications. Most Traxxas high-torque servos use metal gears for extra strength and longer wear. When using a metal servo, you can also upgrade to a metal servo arm. Traxxas offers metal servo arms as upgrades for several models.

Metal gears for strength and long wear

Brushed, Coreless, and Brushless Construction
Just as there are brushed and brushless motors to power your RC car, the motor inside your servo may be brushed or brushless. The brushed motors come in two types: standard, and coreless. Coreless motor construction allows faster, smoother servo action. Brushless motor construction improves performance even further, and offers maximum power, control, and precision.

The 2075X and 2075R servos feature standard motors and metal gears. The X model is standard equipment in most TRX-4 models, and geared for maximum torque. The R model is geared for greater speed, and is standard equipment in the Slash 4X4 Ultimate. The 2075X and 2075R are both fully waterproof.

Traxxas 2075X servo

Traxxas 2075R servo

High-Torque 330 Blue
The High-Torque 330 Blue waterproof servo represents a big step up in steering performance, with more than double the torque of the 2075X and R servos. It features a coreless motor, aluminum heatsink mid-case, metal gears, and dual ball bearings to deliver 330 oz.-in. of torque.

High-Torque 400 Red
Traxxas’ top-performing servo is the High-Torque 400 Red. This waterproof, premium servo is equipped with a powerful brushless motor that allows it to deliver 400 oz.-in. of torque at six volts. Like the 330 Blue, it has an aluminum heatsink mid-case, metal gears, and dual ball bearings.

Servo Sizes
Servo sizes can be broken down into four main categories: large-scale, standard, micro, and sub-micro. The Traxxas X-Maxx™ and Maxx™ monster trucks are equipped with large-scale servos. Traxxas 1/16 scale models use micro servos, and the LaTrax 1/18 scale use the sub-micro servos, the smallest size. These servos are also used in the TRX-4 models to operate their T-Lock differentials and High/Low transmissions. Otherwise, all other Traxxas models use standard-size servos for steering and (in nitro models) throttle duties. Metal-gear versions of the Traxxas micro and sub-micro servos are also available.