Double Valve Spring: Benefits & Reasons

In summary: There was no strip or tube around the inner spring.In summary, the two springs are there to prevent valve float. They don't rub much, and modern engines don't use the duplex springs.
  • #1
anhtho
9
0
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
 
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  • #2
Avoiding extended operation at resonant frequency valve float.
 
  • #3
anhtho said:
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?
 
  • #4
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.
 
  • #5
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.
 
  • #6
P.E. Irving in Tuning for Speed said:
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
 
  • #7
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.
 
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Likes gmax137
  • #8
Randy Beikmann said:
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.
 

1. What is a double valve spring?

A double valve spring is a type of valve spring used in internal combustion engines. It consists of two springs nested together, with one inner spring and one outer spring, to provide extra support and strength to the valve.

2. What are the benefits of using a double valve spring?

The main benefit of using a double valve spring is that it provides increased durability and reliability compared to a single spring. It also allows for higher engine speeds and more aggressive cam profiles, resulting in improved engine performance.

3. What are the reasons for using a double valve spring?

A double valve spring is typically used in high-performance engines that require extra support for their valves. This includes race cars, high-performance street cars, and heavily modified engines. It is also commonly used in engines with higher than stock lift camshafts.

4. How do double valve springs differ from single valve springs?

Double valve springs differ from single valve springs in that they have two springs nested together, whereas single valve springs only have one spring. This provides increased strength and support for the valve, allowing for higher engine speeds and more aggressive cam profiles.

5. Can any engine use a double valve spring?

No, not all engines are compatible with double valve springs. They are typically used in high-performance engines that have been modified for increased power and speed. It is important to consult with a professional mechanic or engine builder before installing double valve springs in your engine.

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