Rocker arms are a pivot point for pushrods to push upward against, so the valve may be pushed down. They are also found in over-head cam engines, but they can be avoided with the over-head camshaft design.
In the picture, the rocker arm is the red, square object. It pivots (or "rocks") on the blue (larger) circle. The yellow valve spring is pushed down by the rocker arm. The spring is seated in the thin green component, which creates a pivoting flat surface for the spring, and the small purple circle is what it pivots on.
Due to a leverage effect, the longer a rocker arm is, the further it presses down on the valve stem and spring. This will open the valve more, but takes more energy to push it open.
High Performance Rocker Arms
Most of the time, performance rocker arms are designed to improve two qualities: durability and leverage for improved valve lift. When the valvetrain is upgraded, stock valve springs are often replaced with stiffer ones, which usually means the rocker arms need to be stronger to prevent breaking, bending, and other wear. Improved leverage is only possible to achive in certain engines, or when the engine block's head(s) are redesigned.
- Valve Arms
- Pushrod Arms