A trunk piston connects directly to a connecting rod through a piston pin. Rod inclination can produce a transverse force component between piston and liner; this calculator provides a simplified entered-force decomposition.
Trunk Piston Rod Interactive Calculator
Enter crank radius, rod length, an assumed axial force and an allocation percentage. Results are static arithmetic—not cylinder-pressure, dynamic side-load, contact-pressure, wear or engine-design predictions.
Equation Used
- An ideal zero-offset in-line slider-crank supplies the angle relation when L is at least r.
- Axial force is a user-entered static assumption, not derived from pressure, inertia or friction.
- The entered allocation percentage simply multiplies the side-force result.
- The 0.98 limit caps the reported angle. It does not make a rod shorter than the crank radius capable of a complete revolution.
- Pressure history, crank angle of peak force, inertia, friction, lubrication, clearances, stiffness, contact area, stress, wear, fatigue and safety are omitted.
Piston Pin, Connecting Rod and Liner
Trunk-piston construction places the connecting-rod small end on a pin within the piston skirt. Patent literature describes the piston reciprocating in the liner while the connecting rod links the piston pin to the crankshaft.
Because the rod is inclined during much of the cycle, a transverse component can act between piston and liner. The calculator isolates a single static decomposition at a geometric maximum.
What This Calculator Represents
The tool computes a clamped angle from r/L, multiplies an entered axial force by tan(angle), applies an entered percentage to that result and reports L/r.
The percentage is an allocation input. It is not derived from engine direction, pressure history, skirt contact or measured loading.
Clamped Geometry and Force Arithmetic
clamped maximum angle = asin(x)
side-force arithmetic = entered axial force × tan(angle)
allocated amount = entered share × side result / 100
entered ratio = L/r
Worked Arithmetic Example
For r=100 mm and L=400 mm, L/r=4 and the clamped maximum angle is about 14.48°. With entered axial force 100 kN, the side-force arithmetic is about 25.82 kN. A 75% entered share allocates about 19.36 kN.
Model Boundary
- Calculated: a clamped angle surrogate, static force decomposition, entered allocation and length/radius quotient.
- Not calculated: cylinder pressure, gas force, crank-angle history, reciprocating inertia, friction, offset, real thrust/anti-thrust distribution, skirt contact pressure, liner deformation, lubrication, temperature, scuffing, wear, fatigue, dimensions, materials or safe design.
Frequently Asked Questions
Is axial force calculated from pressure?
No. It is entered.
Is the allocation percentage predicted?
No. It is a user-entered multiplier.
What if rod length is less than crank radius?
The rod is too short for a complete crank revolution; outputs remain capped arithmetic only.
Is maximum side force necessarily at maximum rod angle?
No; real axial force varies with crank angle.
Are inertia and friction included?
No.
Can this size a piston skirt or liner?
No.
References
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