What Is Sub Trim on an RC Transmitter?
Sub trim on an RC transmitter is a setup adjustment that shifts the neutral point of a channel without changing the trim lever position.
It is used to center a servo or control surface precisely, especially during model setup and calibration.
If you have ever trimmed a model repeatedly and still could not get a true center, sub trim is likely the tool that solves the problem.
Understanding how it works can improve servo alignment, control feel, and overall radio setup.
Sub Trim Explained
On a typical RC transmitter, each channel controls one function such as throttle, aileron, elevator, rudder, steering, or a winch.
Sub trim changes the electrical midpoint of that channel, telling the receiver that the new neutral position is slightly offset from the default center.
In practical terms, sub trim makes the servo stop at a different center point while the transmitter stick remains physically centered.
That matters because many servos do not install perfectly aligned with linkages, horns, or control arms unless you make a fine adjustment in the radio.
How Sub Trim Differs From Normal Trim
Normal trim and sub trim are often confused, but they serve different purposes.
- Normal trim: A user-facing adjustment on the transmitter, usually a small switch or slider, used to correct slight drift during flight or driving.
- Sub trim: A setup-level adjustment that changes the channel’s center point before or after mechanical linkage alignment.
The key difference is location and intent.
Normal trim is meant for operational correction, while sub trim is meant for configuration.
If you use normal trim to fix a badly centered servo, you may run out of adjustment later and reduce control resolution.
Why Sub Trim Matters in RC Models
Sub trim helps you build a better mechanical and electronic baseline for the model.
When the servo center is correct, the linkages can be set more symmetrically, which reduces binding and makes the controls more predictable.
It is especially useful in these situations:
- Installing a new servo with a slightly off-center output shaft
- Aligning control surfaces after setting servo horns and linkages
- Centering steering on RC cars and trucks
- Setting neutral on helicopters, airplanes, drones, and boats
- Compensating for small manufacturing tolerances in servos and linkages
Proper sub trim use can also improve the performance of electronic stabilization systems, because the transmitter’s center point is closer to the true mechanical neutral.
When Should You Use Sub Trim?
Use sub trim when the servo or control surface is slightly off center after the linkage is installed and the model is otherwise mechanically set up correctly.
It is the right choice when the problem is small and you want a clean neutral point in the transmitter.
Examples include a rudder that sits a few degrees left at neutral or an RC car steering servo that points the wheels slightly off-center when the wheel is straight.
In those cases, sub trim can bring the neutral point back into alignment without forcing the linkage to sit at an awkward angle.
However, if the offset is large, it usually means the mechanical linkage should be corrected first.
Excessive sub trim is often a sign that the servo horn, pushrod length, or control arm installation needs attention.
How to Set Sub Trim Correctly
Although menus vary across brands such as Futaba, Spektrum, Flysky, FrSky, Radiomaster, and Sanwa, the general process is similar.
- Power on the transmitter and receiver.
- Center all sticks, wheels, and switches needed for the channel.
- Enter the sub trim or servo center menu.
- Adjust the value until the servo arm or control surface is centered.
- Install the servo horn as close to 90 degrees as possible.
- Fine-tune the linkage mechanically if needed.
The best practice is to center the servo first with sub trim, then use the linkage to achieve proper geometry.
That keeps the servo operating around its natural midpoint, which improves servo throw and consistency.
What Is the Difference Between Sub Trim and End Point Adjustment?
End point adjustment, also called EPA, ATV, or travel adjustment, changes how far a channel moves at each end of its range.
Sub trim changes the center point only.
These two settings solve different problems.
- Sub trim: Adjusts neutral or center.
- End point: Adjusts maximum travel in each direction.
If a servo centers correctly but does not move far enough, endpoint adjustment is the tool to use.
If a servo moves properly but sits off-center at neutral, sub trim is the correct setting.
Can You Use Too Much Sub Trim?
Yes.
Too much sub trim can create several issues, including reduced servo range, uneven travel, and added stress on the servo or linkage.
In extreme cases, it can move the servo away from its most efficient operating range.
Large sub trim values may also hide a mechanical problem.
If the servo horn is installed far from center or the linkage length is incorrect, the fix should usually be mechanical rather than electronic.
A well-built setup typically needs only small sub trim corrections.
Best Practices for RC Transmitter Setup
To get the most from sub trim, start with proper mechanical alignment before making radio adjustments.
This approach gives you the cleanest neutral point and the best control response.
- Install the servo horn as close to 90 degrees as possible at neutral.
- Use sub trim only for small center corrections.
- Reserve normal trim for temporary in-use adjustments.
- Check that linkages do not bind through the full range of motion.
- Verify endpoint settings after sub trim is finalized.
- Recheck the setup after changing servos, horns, or receivers.
For aircraft, this method helps maintain consistent control authority.
For surface models, it keeps steering and throttle/brake behavior predictable.
In both cases, a properly centered transmitter channel makes the model easier to tune and more reliable to operate.
Common Questions About Sub Trim
Does sub trim affect every channel?
Most modern transmitters allow sub trim on individual channels, but the available options depend on the radio system.
Some radios let you set separate sub trim values for throttle, rudder, elevator, aileron, and auxiliary channels.
Should sub trim be used during flight or driving?
No.
Sub trim is primarily a setup tool.
If the model drifts during use, normal trim is usually the better short-term correction because it does not alter the stored neutral point in the same way.
Is sub trim the same as servo reverse?
No.
Servo reverse changes the direction of servo movement.
Sub trim shifts the center position without reversing motion.
They solve different problems and are often used together during initial setup.
Why Sub Trim Improves Precision
RC systems depend on accurate center points.
A transmitter with well-set sub trim helps the receiver and servo work around a stable neutral position, which improves handling and consistency.
That is especially important in models with digital servos, gyro stabilization, or precise control surfaces where small errors can become noticeable.
When you understand what sub trim on an RC transmitter does, you can build cleaner setups, reduce mechanical stress, and avoid overusing normal trim.
The result is a model that responds more accurately and requires less correction during use.