Calculate rocker arm ratio from valve-stem and pushrod pivot distances, or find valve lift from cam lobe lift and rocker ratio in multiple units.

Distance ratios are an ideal lever approximation. Valve lift is theoretical gross lift at zero lash; actual ratio can vary through the motion.

Rocker Arm Ratio Formula

The following formula is used to calculate a rocker arm ratio:

RR = Y / X
  • Where RR is the rocker arm ratio
  • Y is the distance from the center of the valve stem to the center of the pivot point
  • X is the distance from the center of the pushrod to the center of the pivot point.

To calculate a rocker arm ratio, divide the distance from the valve stem to the pivot point by the distance from the pushrod to the pivot point.

Rocker Arm Ratio Definition

What is a rocker arm?

A rocker arm is a lever, typically an oscillating lever in a combustion engine, that converts radial movement from the cam into linear movement at the poppet valve.

What is a rocker arm ratio?

The rocker ratio describes valve-side motion relative to pushrod-side motion. The pivot-distance formula is an ideal lever approximation; actual effective ratio can change through the lift cycle.

The calculator includes 1.5:1 to 1.8:1 presets for convenience; these are not limits or recommendations. Gross valve lift equals cam lobe lift multiplied by rocker ratio under the constant-ratio, zero-lash approximation.

Example Problem

How to calculate a rocker arm ratio?

The following example problem outlines the steps required to calculate a rocker arm ratio.

First, determine the distance from the center of the valve stem to the pivot point of the rocker arm. In this case, this is measured to be 3.25 inches.

Next, determine the distance from the pushrod to the pivot point. For this example, this is measured to be 2.5 inches.

Finally, calculate the rocker arm ratio using the formula above:

RR = Y/X

RR = 3.25/2.5

RR = 1.3 : 1