RC Servo Linkage & Control Throw Calculator
Enter the servo arm, control horn, pivot spacing, neutral angles, and servo endpoint travel. The calculator solves the planar linkage to estimate both control throws, differential, pushrod length, and the servo travel needed for a target surface angle.
Positive control-surface angle (deg)30
- Calculated pushrod length (mm)
- 80
- Negative control-surface angle (deg)
- 30
- Positive trailing-edge throw (mm)
- 20
- Negative trailing-edge throw (mm)
- 20
- Throw differential
- 0%
- Neutral motion ratio
- 1
- Positive servo travel for target (deg)
- 20
- Negative servo travel for target (deg)
- 20
- Full-sweep geometry
- Reachable at 49 sampled positions from -30 deg to +30 deg.
Angles are solved in the linkage plane with rigid arms, pin joints, and a fixed pushrod length.
Positive and negative control-surface angles are shown as absolute deflection magnitudes from neutral.
How to use this calculator
- Measure the servo arm hole, control horn hole, and pivot separation in the same plane and unit.
- Enter the neutral servo arm and horn angles from the servo-to-hinge reference line.
- Enter the positive and negative servo rotations you plan to command from the radio or servo tester.
- Use the sweep status and target-travel rows to check for binding or unreachable geometry before setting endpoints.
How the linkage throw is calculated
This calculator treats the servo arm, pushrod, control horn, and fixed servo-to-hinge spacing as a planar four-bar linkage. The servo pivot is Os = (0, 0) and the hinge is Oh = (g, 0), where g is the servo-to-hinge separation. The servo arm radius is a, the horn radius is c, the neutral servo angle is theta0, and the neutral horn angle is phi0.
At neutral, the servo endpoint is A0 = (a cos theta0, a sin theta0) and the horn endpoint is B0 = (g + c cos phi0, c sin phi0). The pushrod length is the distance between those endpoints:
p = sqrt((B0x - A0x)^2 + (B0y - A0y)^2)
For any commanded servo angle theta, the new servo endpoint is A = (a cos theta, a sin theta). The horn endpoint must sit on two circles: radius p around A and radius c around the hinge. Let q be the distance from A to the hinge. The circle-intersection terms are:
x = (q^2 + p^2 - c^2) / (2q)
h = sqrt(p^2 - x^2)
The two possible intersections are the open and crossed assemblies. The calculator chooses the branch that matches the neutral linkage and then follows the closest continuous horn angle as the servo moves. The control-surface deflection is delta = phi - phi0, wrapped through angle crossings so a horn passing 0 degrees does not create a false jump.
Throw, differential, and reverse travel
The displayed positive and negative control-surface angles are absolute deflection magnitudes from neutral. Linear trailing-edge throw uses the measurement radius m that you enter:
T = m sin(|delta|)
Differential compares the two angular throws, not the servo endpoint settings:
differential = 100 * (|delta positive| - |delta negative|) / average
A positive value means the positive side has more angular throw; a negative value means the negative side has more. The neutral motion ratio comes from differentiating the pushrod-length constraint at neutral, so it reports the instantaneous horn rotation per servo rotation before endpoint nonlinearities take over.
For the target-deflection rows, the calculator searches from 0 to 90 degrees of servo rotation in each direction. It uses bisection once the target angle is bracketed. If the linkage binds or the target is not reached within that range, the result is reported as unreachable rather than extrapolated.
What changes the result most
Moving the pushrod inward on the control horn increases surface throw. Moving it outward on the servo arm also increases throw, but it can reduce mechanical advantage and may make binding more likely near the endpoints. Neutral angles matter because a linkage that is not square at center can produce different leverage on the two sides, which shows up as control differential.
What this calculator leaves out
The model assumes rigid, coplanar links with pin joints and no slop. It does not account for servo torque, spline indexing, radio curves, horn flex, hinge gaps, pushrod bending, ball-link clearance, or interference with the airframe. Use the result to set a measured starting point, then confirm the full movement on the unpowered model before operating it.
Worked example
With the default setup, the servo arm hole and control horn hole are both 15 mm from their pivots, the pivot separation is 80 mm, and both neutral arms are at 90 deg. The neutral endpoints are therefore directly above their pivots, so the pushrod length is 80 mm.
A positive servo rotation of 30 deg moves the horn by 30 deg in this parallel equal-arm case. A negative rotation of 30 deg gives the same 30 deg throw in the other direction. At a 40 mm measurement radius, trailing-edge throw is 40 * sin(30 deg) = 20 mm each way, so the differential is 0%. A target surface deflection of 20 deg needs about 20 deg of servo travel in either direction.
Common questions
How do servo-arm and control-horn holes change control throw?
A larger servo arm radius usually increases surface throw because the pushrod moves farther for the same servo rotation. A larger control horn radius usually reduces surface angle because the same pushrod movement turns a longer lever through fewer degrees. The exact result still depends on the neutral angles and whether the linkage stays reachable through the sweep.
Why do I get more control throw in one direction?
Directional throw changes when the servo arm and control horn are not in a symmetric neutral geometry. The pushrod angle changes the effective lever arm as the linkage moves, so equal servo endpoint values can create unequal surface angles. The differential row quantifies that difference from the solved horn angles.
How can I reduce linkage differential?
Start by making the servo arm and control horn close to square to the pushrod at neutral, then retest the sweep. Moving holes can help, but it also changes total throw and mechanical advantage. Radio differential can trim the final behavior, but mechanical binding should be fixed mechanically first.
How much servo rotation is needed for a specified control throw?
Enter the target surface deflection and read the positive and negative target-travel rows. The calculator searches each direction independently up to 90 degrees of servo rotation. If it says unreachable, the target is outside the solved geometry or the linkage binds before that angle.
Should the servo arm and control horn be parallel at neutral?
Parallel equal-length arms are a good sanity check because they produce nearly one-to-one angular travel in the simple layout. Real models often need offsets for packaging, clearance, or desired differential. When you use offsets, check both directions and the full-sweep status rather than assuming the radio endpoint numbers tell the whole story.
How do I convert control-surface degrees to trailing-edge millimeters?
Measure from the hinge axis to the point where you check throw, usually the trailing edge. The linear throw is that distance multiplied by the sine of the surface angle. For small angles the result is close to radius times angle in radians, but the sine calculation is more accurate as throws get larger.