Double Valve Spring: Benefits & Reasons

[anhtho]

In some motorbike engines, I notice there are two springs (inner and outer) for a valve. Why do they have to make it two ?
Thanks

 

[Doug Huffman]

Avoiding extended operation at resonant frequency valve float.

 

[Kishan Majethia]

In some motorbike engines, I notice there are two springs (inner and outer) for a valve. Why do they have to make it two ?
Thanks

In engine valves? Can you specify which bikes too?

 

[gmax137]

which bikes?

Many "classic" British motors have double valve springs. Triumph, Norton, Vincent, etc. I don't know firsthand about newer / Japanese designs. I was always told the springs rub slightly creating a damping effect, preventing valve float (as Doug says above). They don't rub much though, once they kind of polish each other. So maybe the two spring constants have an effect on the resonance other than the purported rubbing/damping.

 

[Randy Beikmann]

What gmax said is true, but I've wondered about the damping between the springs myself. How long does that last, and does it damage the springs? It could certainly be to have two different "surge frequencies" so that instead of the whole spring mass surging to cause float, only one spring at a time does. But that means one would have to be lower than the other - why not make them both high - and then why not just have one spring?
I have seen "beehive" springs that are pretty cool, because they minimize the mass at the moving end of the spring, which delays surge.

 

[gmax137]

After 1945 there was a change back to coils, largely because of the difficulty of enclosing hairpins without serious interruption to air-flow through the head fins, particularly on a parallel twin. Coil-spring fracture is mainly caused through "surging" or a state of rapid vibration of the centre coils; all modern coil springs are duplex with not more than six free coils in the outer. The inner spring often has one or two more coils of lighter gauge wire and it is an advantage for it to be a push fit within the outer one as this provides a certain amount of friction damping and so helps to prevent surging.

This written in 1948, I don't know if today's modern engines use the duplex springs. Probably metallurgy has moved along in the intervening decades and modern springs don't fracture the way they did in the 1940s. Maybe someone can comment on the current valve springs?

By the way if you are not familiar with Phil Irving you're missing out. Velocette, Vincent, Brabham F1; take a look here:

http://en.wikipedia.org/wiki/Phil_Irving

 

[jerromyjon]

Most newer engines (~10 years old) that I repair contain a shorter single coil spring with fewer turns than older designs. I think valve mass and travel distance has been reduced to the extent that valve spring issues are virtually unheard of in consumer engines.

 

[jerromyjon]

I've wondered about the damping between the springs myself

The first engine that I ever worked on (1970's small block Chevy V8) had a flat strip of steel wrapped around in a tube shape type of inner spring. I tried taking the inner spring out once and it was quite difficult to twist the inner spring in order to get it to move from within the outer spring. The strip was about twice as wide and half as thick as the outer round coil wrapped the opposite direction. I estimate that adds up to quite a lot of dampening! The last one I built about 10 years ago had larger valve diameter with a smaller stem and guide with high performance single coils, and a roller cam and rockers of course.