Which servo suits my RC car? Torque, speed and size explained

Hardly any part changes the way a car drives as much as the steering servo. If it is too weak, it cannot turn the wheels properly off-road and the model ploughs straight on. If it is too slow, the model reacts sluggishly. A stronger servo is therefore one of the most popular upgrades, especially when a ready-to-run model gets bigger tyres, more weight or spends a lot of time off-road.

The values in the data sheet

• Torque (kg·cm): the actuating force. 20 kg·cm means the servo can hold 20 kg on a 1 cm lever. More torque helps with heavy vehicles, big tyres and off-road driving.

• Speed (s/60°): the smaller the value, the faster the servo. It states how long the servo needs to turn by 60°: 0.10 s/60° is quick, values around 0.06 s/60° are very fast.

• Voltage: torque and speed always apply to a specific voltage. When comparing, look at the figure after the @: a servo rated “25 kg·cm / 0.14 s @ 7.4 V” is weaker and slower at 6.0 V.

• HV (high voltage): HV servos are designed for higher voltages, usually up to 7.4 V or 8.4 V; the exact value is in the data sheet. Servos without an HV rating work at 4.8 to 6.0 V. Never connect such a servo to a 7.4 V BEC.

• Motor: servos with an iron-core motor are inexpensive, coreless motors react faster and more finely, brushless servos are particularly strong, efficient and durable.

• Gears: metal gears withstand impacts and crashes better than plastic and are the first choice for off-road models and crawlers.

• Digital or analogue: digital servos hold their position more precisely and firmly but draw more current. A quiet hum at standstill is normal for digital servos.

• Water resistance: water-resistant servos (WP) are the right choice for snow, puddles and wet grass.

Servo sizes: standard, low profile, mini, big scale

• Standard: footprint of about 40 × 20 mm, height about 36 to 40 mm. The usual size in 1:10 and 1:8.

• Low profile: the same footprint but only about 25 to 30 mm high. For low-slung touring car, drift and racing chassis.

• Mini and micro: much smaller, for models from 1:16 to 1:12. Be sure to compare dimensions and mounting with your old servo.

• Big scale: large servos for 1:6 and 1:5 with torque from about 60 kg·cm.

Also check the spline of the output shaft: the servo horn or servo saver has to match it. Many standard servos have 25 teeth.

How much torque does my model need?

The following figures are guide values for the steering servo. Heavy vehicles, big tyres and off-road use call for the upper end.

Leave some headroom: a servo with more torque than your model needs at minimum works further from its limit and lasts longer. For off-road models, metal gears and water resistance are the most useful extras. Suitable servos are listed under standard servos, mini servos and big scale servos.

Power supply: HV servo and BEC

A strong servo only helps if it gets enough power. It is supplied by the ESC’s BEC, and two values count:

• Voltage: an HV servo only delivers its full performance at 7.4 V or more. Many ESCs can be switched to 7.4 V. Check beforehand that your receiver also tolerates this voltage.

• Current: powerful servos briefly draw high currents when steering under load. If the BEC is too weak, the voltage drops: the servo jitters or the receiver briefly drops out. For comparison: the Hobbywing QuicRun WP10BL60 G2 supplies 6.0 V at 3 A, the QuicRun Fusion Pro either 6.0 V or 7.4 V at 6 A. If the BEC is not enough, an ESC with a stronger BEC or an external BEC helps.

Read more about ESCs and BECs in the guide Brushless motor and ESC.

Installing and setting up a servo

1. Check that the steering moves freely: without the servo, the steering must move easily and without binding from stop to stop.

2. Find the centre: switch on transmitter and receiver, set steering trim to 0. Only then fit the servo horn, ideally at a right angle to the steering link. Correct small deviations with the sub-trim on the transmitter.

3. Limit the end points: set the steering travel (EPA) on the transmitter so that the servo does not push against the steering stops at full lock. Otherwise it buzzes, gets hot and draws unnecessary current.

4. Servo saver: the spring-loaded servo saver protects the servo gears in a crash. Crawlers often use a solid aluminium servo horn so that the full force reaches the wheels.

The servo jitters or buzzes: the most common causes

A servo tester, such as the one built into the ToolkitRC MC8, helps with troubleshooting: it lets you check the servo independently of receiver and ESC.