Choosing the wrong servo for your RC car can lead to a quick replacement when it overheats. But a quality servo can last decades if it’s properly matched up to your car.
Determine the physical size, standard, mini, or low-profile. Choose the torque you need based on the size and weight of the car. Heavy, off-road RCs need aluminum gears and housing. For fast steering, get a servo with a 0.1s reaction time. For wet conditions, make sure the servo is waterproof.
I’ll go through these points in detail further below, as there’s a lot more to think about than that. But first, what does a servo do in an RC car?
The function of a servo in an RC car is to turn the wheels. In nitro cars, servos are used to adjust how much fuel flows through for throttle control. But in this article, we’ll be looking at servos for electric RC cars.
These types of servos are called positional rotation servos, and they’re used in almost all RC cars. This means that the arm of the servo can move accurately within a 180° radius, to turn the wheels fully for sharp turns or to move them slightly for small driving corrections.
How To Choose The Right Servo
Servo Size
You have to get a servo that will fit into the chassis you’re using. There isn’t any way to adjust the space to accommodate a bigger or smaller servo, and even if there were, it wouldn’t make sense to put massive servos into tiny cars or vice versa. So check to see which size servo your RC car accommodates.
There are only about three different servo sizes. The standard size is 40 mm long, 38 mm high, and 20 mm wide. This is the size of most servos in 1/8 to 1/10 scale RC cars.
If you have a car with a lower-profile body, you might need a low-profile servo. If the servo is under 38 mm in height, it’s considered a low-profile servo.
Small RC cars need small servos, so 1/12 to 1/24 scale cars use mini servos that are about half the size of a standard servo.
Check which servo will fit your car and then evaluate how much torque, speed, and other specifications you require.
How much torque does an RC servo need?
Small 1/12 to 1/24 scale RC cars need roughly 100oz/in to 175oz/in torque if they’re only used on smooth surfaces.
Small 1/10 to 1/8 scale off-road RC cars need roughly 150oz/in to 220oz/in to handle the extreme terrain, ramps, and obstacles.
Bigger 1/8 to 1/10 scale RC trucks and buggies need at least 250oz/in to run reliably because of the extra weight of the vehicle. Even at the same scale, trucks and buggies are physically bigger than cars and therefore need stronger servos.
Crawlers also need servos with lots of torque because they go through a lot of stress when slowly climbing obstacles. You’ll also need around 250oz/in or more to get a good lifespan out of it.
Choosing a servo speed
The servo speed for an RC car should be under 0,2 seconds. For big hobby-grade RC cars, it should be 0,1 seconds or fewer. A slow servo causes a delay when turning and leads to a lack of handling.
If you would like to know more about how RC Cars turn, please check out my post: How Do RC Cars Turn? A Complete Guide
Motor types
Servo motor options include cored brushed, coreless brushed, and brushless. Brushed cored and coreless motors are cheaper and get the job done well in most cases.
Coreless servos perform better than cored brushed motors because they have much quicker response times, are smoother to control, and are more energy efficient. But coreless motors can overheat quickly because of how fast and consistently they spin and because there’s still some friction inside.
The best RC servo motors are brushless because of their simple and highly efficient design. They last almost forever with almost no maintenance if you don’t abuse them. For instance, if you overload the motor by using it in a heavy RC car, you might overheat it and ruin the bearings inside.
Not only do brushless servo motors last longer, but they are also far more reliable and provide more torque than brushed servo motors. The biggest downfall is that they’re very expensive compared to brushed options.
Voltage
For as long as I can remember, RC servos have run on 4.8V to 6.0V because of nitro and gas RC cars requiring a receiver battery. 4 X AA batteries or a 5-cell NIMH battery pack provide 6V of power to the servo and this is what was used to power the servos years ago.
Things have changed substantially since those days and now most RC cars are battery-powered options. This is good because batteries provide high power and voltage to get a highly reactive driving performance.
So now servos also run on 7.4V (nominal) and 8.4V (charged) voltages for better performance. You can use these servos in lower voltage situations, but they will be underpowered and won’t perform as well as they should. For the most part, the torque and speed will be reduced.
Waterproof RC servos
There are some waterproof servos available, but in most cases, you should avoid water completely, unless the servo is labeled waterproof. Ideally, you need an IP rating of 65 (IP65), which means that the servo can be exposed to splashes of water.
IP67 means that the servo can be submerged in water and still operate, but no matter the waterproof rating, there are always limitations. For example, if the servo goes a few feet under the water, the water pressure will cause leaks.
For more on waterproofing, please take a look at my article: How To Waterproof An RC Truck: Step By Step
Servo housing options
Most RC servos have a plastic casing to protect the insides, some are made of aluminum, and others are a combination of the two. Obviously plastic is the cheaper option, but it also has one other benefit, plastic is light so your RC car will have a better power-to-weight ratio.
But if you can afford a servo with an aluminum casing, it will last longer and help to dissolve heat far better than plastic. Besides that, metal servos are more secure in the chassis and even help to make the entire car stronger by making the chassis more rigid.
Very powerful servos need to have metal housing to avoid damage around the mounting points inside. The obvious downside to aluminum servos is that they cost a lot more than their plastic counterparts, but they will last longer and save you money in the long run.
Servo Gears
Just like with the servo housing, servo gears are also available in plastic or metal. Plastic gears work fine for lower-end hobby-grade and toy RC cars, but if you have a high-performance model, you’ll need metal gears to survive.
Metal options include aluminum, titanium, and steel, and get more expensive in that order.
Are All RC Servos Compatible?
Most standard RC servos use a standard 3-pin plug and will work in any RC car if it fits. But you must make sure to get a servo with specifications high enough to run the car. A weak servo in an off-road truck will be destroyed within minutes while a high-end fully metal servo costs more than most RC cars on the market and would be a waste of money for a 1/12 scale drift car.
How Long RC Servos Last
Servos last anywhere from a few months to many years, depending on the material they’re made of and how you use them. Plastic servos don’t last long unless you’re careful not to overload them with obstacles or steep uphills.
Metal servos usually last many years, but can also be broken quickly if pushed too hard.
Differences Between Digital and Analog RC Servos
The main difference between digital and analog servos is how the servo receives the data and processes it. The reaction time of digital servos is generally higher than analog servos and they provide more torque. Digital servos also have better holding power, so they can move from a fixed position easier than analog servos.
However, digital servos are more expensive than analog, and sometimes the price difference isn’t worth it. For example, a light 1/10 scale RC car will do just fine with an analog servo and will be a lot cheaper. But if you’re running a 1/10 off-road truck, you might need a digital servo to get over obstacles, through grass, and up ramps.
What Causes RC Servos To Fail?
Cheap servos fail because they’re made with low-grade cables and aren’t wound properly. Sometimes the insulation is insufficient to keep dust and moisture out. Other causes of servo failure include bad oil, which causes the bearings to fail. Overloading a servo will cause it to break as well.
Conclusion
Besides making sure you have the right size and strength (torque) servo, choosing the right one for your car depends on your budget. It’s almost always better to go for a faster, more powerful servo if you can afford to. Metal gears last longer but aren’t needed for slow-paced driving on smooth surfaces. Likewise, a metal servo case is much stronger than a plastic one but isn’t required for lightweight driving.
If you’re replacing an existing servo because the gears broke, choose a servo with similar specs that include aluminum gears. If the servo broke due to liquid damage, upgrade to a waterproof model. Choose a servo that has a better part in place of the part that broke.
