A connecting rod links a piston pin to a crankpin. In the illustrated split-big-end arrangement, a detachable cap and bearing shells surround the crankpin; a closed small-end eye surrounds the piston pin. The animation shows these parts moving together while the calculator evaluates piston position, travel, rod inclination and instantaneous speed.
Related actuator design tools
This calculator describes a rotating crank connected to a sliding piston. If you are designing a separate actuator-driven linkage, calculate its mounting geometry and load through the full travel before selecting an actuator.
For structural members, the actuator load should be applied through brackets that do not introduce unintended bending. Check both static load and the extra load created when the mechanism starts, stops, or binds.
A crank or cam may tolerate continuous rotation through a difficult part of the cycle, while a linear actuator has defined end positions and internal limits. That difference matters when the mechanism needs repeatability or must not drive into a hard stop.
- Map actuator stroke to the actual output travel before choosing a model.
- Check for binding, changing leverage, and peak load near the ends of travel.
- Decide whether timed motion is acceptable or position feedback is required.
For a FIRGELLI design review, calculate the load case with the FIRGELLI actuator force calculator and plan the switching or controller arrangement with the linear actuator wiring diagram generator. If an actuator or cylinder is already installed, the linear actuator replacement finder helps keep the comparison tied to real dimensions.
Connecting Rod End Interactive Calculator
Explore the motion of an inline slider-crank. Change the crank throw, rod length and speed, or pause at a selected crank angle. The piston pin, connecting rod ends and crankpin follow the calculated geometry.
Equation Used
- Rigid connecting rod, fixed crankshaft centre, no piston-pin or cylinder offset.
- Rod length is pin-centre distance, greater than crank throw throughout the selectable range.
- Uniform crankshaft angular speed; no combustion, inertia force, bearing stress or rod-bolt preload calculation.
- Bearing shells and cap show the arrangement of a split big end. Their sizes and fasteners are schematic.
Pin-centre dimensions are to scale. The piston, cylinder, cap and bearing details are illustrative, not a manufacturing drawing. Speed is instantaneous, not mean piston speed. Playback is 300 times slower than the entered RPM.
Two rod ends with different motions
The crankshaft turns about a fixed centre. Its offset crankpin follows a circle, while the piston pin stays on the cylinder centreline. The rigid connecting rod joins those two moving pins and swings from side to side.
The big-end cap belongs to the rod and stays attached to it throughout the cycle. The cap moves as part of the rod around the crankpin. The crankpin rotates inside the big-end bearing. At the other end, the small-end eye permits the rod to change angle relative to the translating piston.
The cutaway exposes the piston pin, rod beam, bearing shell, split cap and two cap-fastener positions. An OEM piston-and-connecting-rod overview is the construction reference. Piston size, bore, cap thickness and fastener details are illustrative; only the crank throw and rod pin-centre length are dimensional inputs.
At top dead centre, the crank and rod lie along the cylinder axis with the piston at its highest position. Half a revolution later it reaches bottom dead centre. The full piston stroke is twice the crank throw.
Reading a slider-crank mechanism
This model represents an inline piston-and-crank arrangement found in reciprocating engines and machines. It demonstrates how a rotating crank and a constrained slider are connected by a rod; it does not simulate combustion or decide whether a particular engine component is suitable.
Changing the rod length changes rod inclination and the intermediate piston motion. Changing crank throw changes stroke. Engine speed changes instantaneous piston speed and playback rate, while the geometric positions at a selected crank angle remain the same.
Position and instantaneous speed
Let r be crank throw, l be the distance between rod pin centres, and theta be crank angle from top dead centre. The crankpin is r sin(theta) sideways from the cylinder axis and r cos(theta) above the crankshaft centre. The rod’s axial projection is sqrt(l² − r² sin²(theta)).
Consequently, piston-pin height is x = r cos(theta) + sqrt(l² − r² sin²(theta)). Travel down from top dead centre is l + r − x. Rod inclination is asin(r sin(theta)/l), with its sign indicating the side of the cylinder axis occupied by the crankpin.
For angular speed omega = 2 pi N/60, piston velocity is omega times dx/dtheta, where dx/dtheta = −r sin(theta) − r² sin(theta) cos(theta)/sqrt(l² − r² sin²(theta)). The calculator reports its absolute value multiplied by five to convert inches per second to feet per minute.
The speed is zero at both dead centres. It is not constant during the revolution, and its maximum is generally not exactly at a quarter turn. The calculation uses a rigid, centred mechanism with constant crank speed.
Example: quarter-turn position
At the default 1.75 in crank throw, 5.70 in rod length and 90 degree crank angle, the piston-pin height is sqrt(5.70² − 1.75²), approximately 5.425 in. Travel from top dead centre is approximately 2.025 in, and rod inclination is approximately 17.9 degrees.
At 3,000 RPM, the instantaneous speed magnitude at this quarter-turn position is 1.75 × (2 pi × 3000/60) × 5, approximately 2,749 ft/min. At zero degrees, travel and speed are both zero. At 180 degrees, travel is the full 3.50 in stroke and speed is again zero.
Pause the animation to hold a value, or use the crank-angle slider. Autoplay updates the angle control and all four results to the displayed position.
What this calculator establishes
The results describe kinematics: where the parts are and how quickly the piston moves. They do not establish rod strength, bolt stretch, bearing clearance, oil-film capacity, fatigue life or a permissible engine speed. Those questions require component-specific geometry, materials, loading and manufacturer information.
The rod remains longer than the crank throw throughout this calculator’s input range, so the square-root closure has a real value at every angle. A shorter rod would not permit the same full-revolution centred geometry.
The visual uses a cutaway and enlarged bearing-end details to make the mechanism readable. These are explanatory features rather than component dimensions to manufacture.
Connecting-rod motion questions
Does the big-end cap spin around independently?
No. It remains attached to the connecting rod. The crankpin rotates relative to the bearing carried by the rod and cap.
What does rod length measure?
The distance between the piston-pin and crankpin centres, not the overall length of the forging.
Why does piston speed change with crank angle?
A constant-speed crank produces nonlinear slider motion through the angled rod. The derivative of piston position therefore varies during the turn.
Is the speed result an average?
No. It is the instantaneous speed magnitude at the displayed crank angle, converted to feet per minute.
Can these results select a rod or its bolts?
No. The model calculates motion, not component strength or fastener preload.
Construction and kinematic references
Volkswagen Jetta and GLI Quick Reference Specification Book, February 2014, printed page 33 (PDF page 47), provides an OEM exploded overview of a piston, piston pin, connecting rod, bearing shells, cap and fasteners. It is used here to identify the component arrangement, not to prescribe dimensions or tightening specifications for another engine.
X-engineer: Kinematic analysis of the piston derives slider-crank piston position and its derivatives. The equations above state the exact centred geometry used by this calculator.
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