Enter the distance from the center of the valve stem to the pivot point and the center of the pushrod to the center of the pivot point to determine the rocker arm ratio.

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.

Rocker Arm Ratio Definition

What is a rocker arm?

A rocker arm is a level, 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 rock arm ratio?

The ratio of a rocker arm is a measure of the leverage of mechanical advantage that is achieved by the rock arm due to it’s specific geometry.

In general rocker arm ratios are designed to be in the range of 1.5:1 to 1.8:1.

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