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.
You can see this wave take place on a child’s Slinky toy spring (remember them?). Hold one end fixed, and then quickly move the other end and you’ll see the wave of movement of each coil pass up and down the length of the spring. The continual back and forth movement of the compression wave creates a vibration in the spring that is referred to as “surge”.
Surge becomes a problem if it gets close to an order of the springs natural frequency. This can lead to the compressive wave going out of control, which will cause loss of valve control and will also result in extreme stresses in the spring. Either of these effects can end up with engine failure.
One way to tackle the problem of surge is to use nested twin springs. A nested spring is made up of two or more springs running in parallel. Typically, the nested twin spring will be a small spring contained within a larger spring. The two springs will have different natural frequencies, which can make the nested twin spring less susceptible or even immune to the problem of spring surge.
The outer diameter of the inner spring and the inner diameter of the outer spring are usually chosen so as to create a small amount of interference between the two springs. If this is the case then it is essential that the direction of windings is different between the springs, otherwise they will get caught up in one another. The interference will provide a means of damping, allowing unwanted energy to be converted to heat from the friction between the two springs.
When run for extended periods, this interference will of course wear the surfaces of the springs and reduce the life of the nested spring assembly. Note that titanium nested springs cannot be run with interference. That’s because titanium has an inherent nature to gall (the macroscopic transfer of material between metallic surfaces) when in contact with other titanium surfaces.
Cosworth Valve Springs
We currently supply two types of valve spring, and they are both of the nested twin spring variety. Our PR8121 valve spring is a non-interference spring, whilst our DR4601 valve spring has interference between the inner and outer springs.
You can see more details of these two springs here: PR8121 VALVE SPRING (INNER & OUTER)
and here: DR4601 VALVE SPRING (INNER & OUTER)