Servos Explained: Torque, Speed, Splines and the Voltage Trap
What the two numbers on the box actually mean, why they're worthless without a third, the spline table nobody publishes clearly, and why a servo that only misbehaves under load is usually a power problem.
Updated Jul 22, 2026 · RC Crash Crew
The servo is the part that actually does something physical when you move the wheel or the stick. Everything else in the radio chain — transmitter, receiver, ESC — exists to tell it what to do.
It is also the part most often bought on the wrong numbers, fitted with the wrong horn, and fed the wrong voltage. This page covers what the numbers on the box mean, how to read them against your car, and the spline and voltage traps that end servos early.
The two numbers on the box
Every servo is sold on two figures, and both are meaningless without a third.
Torque — how hard it can push. Quoted in kg·cm (kilogram-centimeters) or oz·in (ounce-inches). The conversion is fixed:
- 1 kg·cm = 13.9 oz·in
- 1 oz·in = 0.072 kg·cm
So a "20 kg" servo and a "278 oz-in" servo are the same servo with a different sticker.
Speed — how fast it moves, quoted in seconds per 60 degrees (written `0.12 sec/60°`). Lower is faster. A 0.10 servo is quick; a 0.20 servo is slow enough that you can feel the delay in steering.
And the number that makes both of those real: voltage.
The voltage catch — read this before you compare anything
Torque and speed are always quoted at a specific voltage, and the same servo produces very different numbers at different voltages. A spec sheet will typically list something like:
| Supply | Torque | Speed |
|---|---|---|
| 4.8 V | lowest | slowest |
| 6.0 V | middle | middle |
| 7.4 V (HV only) | highest | fastest |
Two consequences follow, and both cost people servos:
- Comparing two servos means comparing them at the same voltage. A 15 kg figure at 7.4 V and a 15 kg figure at 6.0 V are not the same servo. Manufacturers know which number looks better on the box.
- Feeding a servo more voltage than it is rated for will destroy it. A standard 6.0 V servo on a 7.4 V supply runs hot, strips gears and eventually cooks its electronics. HV (high voltage) servos are built for 7.4 V and up; standard ones are not.
Where the voltage comes from is the ESC's BEC — the circuit that steps battery voltage down to feed the receiver and servo. Many modern ESCs let you select 6.0 V or 7.4 V. Check what yours is set to before plugging in a new servo. This is a settings menu, and settings menus are how good servos die.
Analog or digital
The difference is how often the servo checks and corrects its own position.
Analog servos update roughly 50 times a second. Cheaper, lower current draw, and noticeably softer — they give a little when something pushes back.
Digital servos update several hundred times a second. That gives:
- Much stronger holding torque — it resists being pushed off position
- Faster, more precise response to small inputs
- Higher current draw and more heat, because it is correcting constantly
- An audible buzz at rest, which is normal and not a fault
For steering on anything that hits things — which is most RC — the holding torque of a digital servo is the real advantage. It is also why a digital servo needs a BEC that can actually supply the current.
Gears: the part that breaks
- Plastic (nylon) gears — quiet, light, cheap, and they strip. On the upside, they strip *before* the servo case or your steering linkage does, which is sometimes the point.
- Metal gears — far stronger, slightly noisier, more expensive. Standard on anything meant to survive being crashed.
- Titanium / hardened steel — for high-torque, high-abuse use. Diminishing returns unless you are actually stripping metal gears.
A coreless or brushless motor inside the servo means faster response and better efficiency, and is worth paying for on steering. It says nothing about the gears — check both.
"Metal gear" does not always mean every gear is metal. Some servos use a metal output gear and plastic for the rest. If a spec sheet says "MG", it is worth reading further.
The servo saver, and why you should not remove it
Between the servo and the steering is usually a sprung part called a servo saver. When a wheel hits a curb, the impact travels back down the steering linkage into the servo's output gear. The saver is a spring that lets the linkage move under a hard hit and absorbs that shock.
People remove or lock them out because a soft saver makes steering feel vague. Then they strip a servo, because the shock now has nowhere to go except the gear train.
If your steering feels vague, the fix is usually a stiffer servo saver, not no servo saver. Many are adjustable — a nut on top sets the spring preload. Tighten it a little at a time until the vagueness is gone and stop there.
Horns and splines — the compatibility problem
The spline is the toothed shaft the horn clamps onto. Spline counts are not standardized across brands, which is why a horn from one servo will not fit another that looks identical.
The commonly encountered counts:
| Spline | Commonly associated with |
|---|---|
| 25T | Futaba, Savox, and most servos sold as "Futaba-compatible" |
| 23T | Sanwa / Airtronics, JR / Spektrum |
| 24T | Hitec |
| 21T / 15T | Older and micro servos |
⚠️ Treat this table as a starting point, not a guarantee. Manufacturers have changed splines between product generations, and "compatible" on a horn's packaging means compatible with a *named* spline, not with your servo. Check the spline count in your servo's own documentation before buying horns or a steering saver. The cost of getting it wrong is a horn that appears to fit, then rounds out the spline under load and ruins the servo's output shaft.
Wire colors
Three wires: signal, positive, negative. Two conventions are in use.
| Convention | Signal | Positive | Negative |
|---|---|---|---|
| Futaba / JR style | White | Red | Black |
| Airtronics / older style | Orange (or white) | Red | Brown |
The red is always positive and always in the middle. That is the useful constant — if you are unsure which way round a plug goes, the middle wire being red is your anchor. Reversing a servo plug will usually not destroy it immediately, but it will not work, and on some receivers it will.
Center, trim and sub-trim — three different things
- Servo center is where the servo sits with no input. It is a property of the servo.
- Trim on the transmitter shifts the neutral point of your input. Use it for small corrections while driving.
- Sub-trim shifts the servo's own center in the radio's setup menu. Use it to correct a mechanically off-center installation.
Do the mechanical part first. Fit the horn as close to straight as the spline allows, then adjust the linkage length, and only then use sub-trim for what is left. Using a large amount of sub-trim to fix a badly fitted horn eats into your available travel on one side — you end up with more steering lock one way than the other, and no obvious reason why.
End points (EPA) limit how far the servo travels. Set these so the servo never drives the steering into its mechanical stops. A servo straining against a stop draws heavy current, gets hot, and eventually strips — and it will do this silently, while the car is just sitting there.
Symptoms and what they mean
| What it does | Likely cause |
|---|---|
| Buzzes or jitters at rest, constantly | Normal for digital servos. If it is new, a poor connection or an under-supplied BEC |
| Twitches randomly | Interference, a damaged signal wire, or a failing receiver |
| Moves one way but not the other | Stripped gear tooth, or end points set wrong |
| Grinding noise when steering | Stripped or chipped gears |
| Gets very hot with the car standing still | Straining against a mechanical stop — check end points and linkage |
| Weak, slow, or resets under hard steering | BEC cannot supply enough current; the receiver is browning out |
| Works on the bench, fails under load | Same as above — an under-specified BEC is the usual answer |
That last pair is worth dwelling on. A servo that misbehaves only when the car is working hard is usually a power problem, not a servo problem. Swapping in a bigger servo makes it worse, because a bigger servo draws more current from the same overloaded BEC.
Waterproofing
"Waterproof" and "water resistant" are marketing words with no shared definition in RC. Some servos carry a real IP rating; most say "waterproof" and mean sealed seams and a gasket.
What actually matters:
- The case seams and the wire entry are where water gets in.
- The output shaft seal is the weak point on a servo that gets submerged rather than splashed.
- Salt water kills electronics even after it dries. Anything that has been in salt water needs a fresh-water rinse of the *outside* and then to be dried thoroughly.
There is a fuller guide on this site about what waterproofing really means across the whole car — the short version is that a waterproof servo on a car with an unsealed receiver box has not solved anything.
Choosing one, practically
- Find what your car came with, and what the manual or the manufacturer recommends. That is your baseline.
- Match or beat that torque at the voltage you will actually run, not at the headline voltage.
- Check your ESC's BEC — both its voltage setting and its current rating. This is the step everyone skips.
- Confirm the spline against the horns and servo saver you already own.
- Metal gears for anything that crashes.
- Then worry about speed. Torque failures break parts; speed differences change how it feels.
More torque than you need costs money and draws more current. Less torque than you need costs servos.