How To Change a steering servo on an RC Car

When your servo breaks or if you want to upgrade from a RTR-series servo, you need to know how to change it. When you’re getting a new servo, you also need to consider a few things before making the purchase and when installing the new servo.

If your servo is damaged, the easiest fix is to buy a new one that is the same model. If you want to upgrade, you need to find out what kind of servo is compatible with your car and what level of performance you need. The cars in the 1:10 – 1:8 size range normally use a Standard-sized servo. If you’re not sure about the size, you can measure your old servo and choose a new one with the same measures.

The next thing you need to do is choose a servo that is powerful and fast enough for your car. There is no chart for how powerful a servo should be, so it is good to check what the manufacturers’ recommendations are. It’s important that the servo is fast and powerful enough to steer the car. Especially the 4WD cars require dynamic torque for proper on power steering. Here’s an approximate chart of the car types and examples of suitable servos.

1:10 2WD Buggy / Short Course
Torque: 6-12kg Speed: max. 0.12s
Highest DS400

1:10 Touring Usually Low-Profile
Torque: 6-15kg Speed: Max.0.1S
Highest DLP650 for racing and serious lapping and Savöx SC-1251MG for hobby use

1:10 4WD Buggy
Torque:
10-12kg Speed: max. 0.15s
Highest DT450

1:10 4WD Short Course
Torque: 15-25kg Speed: max. 0.18s
Highest DS700

1:8 Cars
Torque: +20kg Speed: max. 0.2s
Highest DT750

The servo price range is quite wide. You could say the better the servo the higher the price. The servos of the biggest manufacturers like Highest, Savöx and Sanwa are more expensive, but they are stronger and more trustworthy compared to the servos of smaller names, even though they claim to have the same performance. Over the lifespan of one high quality servo, you may have a couple of small-name servos break down, even though the claimed performance is the same.

When replacing a servo, it’s important to check if the old servo horn is compatible with the new servo and buy a new servo horn if it’s not. Servo manufacturers use different bores – the most common are 23T (Sanwa, Acoms, Spektrum) and 25T (Futuba, Highest, Savöx). There are two different types of servo horns: short (generally Touring and 2WD buggies) and long (generally 1:8 Buggies). For cars with internal combustion engines there are double-horned servo horns (throttle and brake arm). Some servo horns are the offset-type, that bring the servo horn outwards in relation to the servo axle. It is possible that the above-mentioned servo horn don’t fit your car, so you have to use the original servo horn.

Installing the servo

You start the process by removing the old servo from the car.

• First you take off the servo cable from the receiver.
• Next, if necessary take off the servo mount from the bottom of the car. Now you can better access the mounting screw of the servo horn.
• When you’ve removed the servo horn, you can start installing the new servo.

• Start by centering the servo:

1. In the transmitter settings, set the End Point, Travel, Dual Rate to 100% (EPA, TRAVEL, ST D/R)
2. Set Trim and Sub Trim to 0 (center)
3. Connect the servo to the receiver’s steering channel (steering ch1, throttle/brake ch2)
4. Switch on the power on the transmitter and the car so you can see the servo center itself. Turn the wheel to make sure the servo turns and works normally.
5. Switch off the power on the car and the transmitter.

-> the servo is now centered.

• Next you install the servo horn. The horn should be installed with a 90° angle in relation to the servo case, so the servo horn movement to both directions is the same.

• The steps when installing a servo are in reverse order compared to the removal.

Seting up the Transmitter

After this you set the turning direction, maximum travel range, and center point in the transmitter settings. It’s good to do the settings with the car wheels in air so you can better observe their movement.

• Make sure the servo turns correctly, that is, when turning left the wheels turn left and so on. If they turn in the wrong direction, change the ST REV/NOR setting in the transmitter.
• First you set the Trim in the center. If the adjustment range is not enough, then check the length of the steering link and make sure that the servo arm was installed in the right position. If the servo arm is in right angle in relation to the case and when the Trim in the transmitter is in the center, you can adjust the length of the steering link (if possible) so that the wheels are straight.
• Next you adjust the EPA (End Point Adjust) / Steering Travel.
  EPA and Travel: First, turn the wheels in one direction and increase or decrease the turning until you see the steering links turn to the most extreme position. This way you can stop the servo from turning more than the steering structure allows. Incorrectly adjusted end points overload the servo and in the worst case they might break it. Next do the same adjustment by turning to the other direction.
  ST D/R (Dual Rate): This setting adjusts turning both left and right at the same time. When you adjust the Dual Rate, it’s important to check both directions, so that the end points don’t exceed those of the car’s. The steering is not totally symmetrical in all cases, and for some cases the dual rate is adjusted according to the shorter end-point.

• Next you put the car on the ground and test drive to see how the servo functions and make the final adjustment to the Trim setting to make the car move straight forward.