The valve spring has one simple job to do – it just has to provide a force that will keep the inlet or exhaust valve under control. But as we’ll see in this technical feature, it’s a bit more complicated then that. And when things go wrong with valve springs, things really go wrong. So it pays in the long run to use the best valve spring available. Nested twin valve springs (made up of one spring inside another) are extremely popular on race engines, and in this technical feature, we’ll discover why.
Valve Spring Behaviour
Before we look in more detail at nested twin valve springs, let’s look at how a valve spring behaves when its loaded. When compressed, the spring provides a reactive force which mainly occurs from the twisting motion of the coiled wire. If the load applied is constant, then the load in each coil will be equal. But when the engine is running, the continuous reciprocating movement of the valve compresses the spring and then lets it expand again.
When the valve begins to open, the spring starts to become compressed and the coils will accelerate. The coils nearest the camshaft will see the entire inertia of the spring. As we move along the length of the spring, successive coils have less inertia loads as there is less mass below them. This means that the acceleration of each coil diminishes down the length of the spring. Hence the first coil will close up most and each subsequent coil less, starting off a compressive wave down the spring.
When the valve gets to around its mid-position, its acceleration will drop to zero, and at this point the coils will all be travelling at the same speed. But as the valve starts to decelerate, the opposite now happens – the coil at the other end of the spring closes up more than the ones above it, and the compressive wave reverses direction.