How to change a servo on an RC car?
What is a servomotor?
In automobile modeling, the servo motor is essential for controlling the car by translating the driver's orders into mechanical movements. On a thermal model we use a servo for the throttle and another for the steering whereas on an electric model, generally only one servo is enough. When it is broken or when you want to improve it, it is essential to know how to change the servo .
When should I change my servo?
If your servo is damaged, the simplest solution is to buy a new one of the same model. If you want to change it for an upgraded model , you need to know what type of servo is compatible with your car and what level of performance you need. 1/10 and 1/8 scale cars normally use a standard size servo. If you are not sure about the size, you can measure your old servo and choose a new one with the same dimensions.
How to choose a servo?
The choice of servo
The next thing you need to do is choose a servo that is powerful and fast enough for your car. There is no servo motor power chart, so it is best to check what the manufacturer's recommendations are. It is important that the servo is fast and powerful enough to steer the car. Especially four-wheel drive cars that demand dynamic torque .
Most 1/10 and 1/8 scale models use standard servos, with dimensions of approximately 40x36x20mm and mounting holes spaced 47mm apart. Most manufacturers make your choice easier by choosing a standard size or listing compatible models. If you are replacing a servo on a micro model or 1/5 scale car, you may need a smaller or larger servo than the standard servo. Measure your old servo and compare the dimensions with the new one before purchasing it.
Servo characteristics
- The torque (in oz.in among the Anglo-Saxons or in kg.cm) designates the maximum power of the servo, indicating the weight that a lever arm is capable of lifting by 1 cm at a certain tension.
- The rotation speed refers to the time it takes for the lifter to travel 60° at a certain tension.
- The material of the gears must be metal for leisure and titanium for competitions because plastic gears are fragile.
- The operating voltage is usually between 4.8V and 6V. The more we increase the voltage, the more we increase the torque.
- The servo can be digital or analog . Digital servos are slightly more precise and faster but they consume a little more.
The choice of rudders
When replacing a servo , it is important to check if the horns of the old servo are compatible with the new one and to purchase new ones if not. The most common spline number is 23T and 25T and it should be the same as the servo motor. Aluminum spreaders are more durable than plastic spreaders.
How to install the new steering servo?
1/ Disconnect the servo from the receiver
If you have a receiver model with a case, you may need to open it to access the connections. Check your vehicle manual for details on how to access the receiver jacks. If you have lost it, most manufacturers offer a PDF version online. When equipped with two servos, the steering servo is located in plug 1 (which may be labeled "CH1", "ST" or "STR") and the throttle servo is in plug 2 ("CH2", "TH" or "THR").
2/ Remove the rudders
Depending on your car's configuration, you may need to remove the servo to gain access to the rudder pedals. If not, go straight to step 3 otherwise, simply remove the screw in the center of the lifter to remove it. If you are removing a servo, you may have to remove a servo saver instead of the rudder. If this is the case, proceed as with the rudders.
3/ Remove the servo
Most servos are held in place by four screws, two on each side, but some vehicles only have one per side. Remove the screws to release the servo. If the screws are different lengths, note which screws go where.
4/ Connect the new servo and center the output shaft
Turn on your transmitter and center the tuning dial on the channel you are using ("ST" = direction, "TH" = throttle). Plug the new servo into the correct port on the receiver and power on the receiver. Operate the transmitter to ensure the servo responds. If it doesn't respond, you may have inserted the plug backwards or into the wrong outlet.
5/ Install the new servo
If you cannot access the rudder screws when the servo is installed in the car then skip to step 6. Replace the screws you removed in step 3. If your new servo is slightly larger or more small than the old one and it doesn't quite fit, you need to use a properly sized servo mount . Some chassis can adapt to the size of the servo , then you just need to loosen a few screws to be able to place the bracket.
6/ Install the rudders
Turn your radio system and receiver back on if you turned them off. Screw the lifter onto the output shaft. In most cases, the rudder should be installed parallel to the servomotor housing with TRIM set to 0 on the radio. It may not be completely parallel due to the number of splines but this can be corrected later using the radio
7/ Reassemble the receiver box (if there is a box)
After making sure the servo is working as it should, turn it on by pressing the button. If the box uses a gasket to protect the receiver from water, make sure it is properly installed.
8/ Adjust the servomotor
All you have to do now is set the maximum wheel angle (the wheel end position or EPA) and the wheel center position (TRIM) using your radio. We recommend starting the EPA setting at 50% of its maximum capacity and then gradually reaching 100% so as not to damage the servomotor.
