Valve spring shimming?
03-27-2013, 12:13 PM
#1
Valve spring shimming? Came across a cam thread from a while back by Pat G. this morning while browsing around. He was running AFR heads and shimming the 8019 duals to within 0.050" of coil bind (@ peak lift I'm assuming?). By decreasing its open height you are preloading the spring following F=kx. Advantages are increased spring pressure for handling aggressive lobe profiles and reduction of "spring surge"...not really sure what that's referring to unless they have a progressive rate or something...but it supposedly increases the longevity of the springs.
At what point does this become advantageous? IIRC he was using LSK and X-ER lobes on a lot of the cams.
Are there any tradeoffs associated with doing this? Lifter life?
Anyway, anyone who knows anything about this please chime in.
At what point does this become advantageous? IIRC he was using LSK and X-ER lobes on a lot of the cams.
Are there any tradeoffs associated with doing this? Lifter life?
Anyway, anyone who knows anything about this please chime in.
Last edited by ckpitt55; 03-27-2013 at 12:25 PM.
03-27-2013, 01:27 PM
#2
It does increase spring pressure a la Hook's Law, which may or may not be a good thing depending on your spring selection, but it helps suppress harmonics in the spring by causing more "dead coils", which is when the coil contacts an adjacent coil and cannot vibrate as much. At coilbind, all the coils are touching each other, essentially making the spring solid, so you're trying to get as many "dead coils" as possible without making them go into coilbind.
I hope that makes sense.
I hope that makes sense.
03-27-2013, 02:16 PM
#3
It does increase spring pressure a la Hook's Law, which may or may not be a good thing depending on your spring selection, but it helps suppress harmonics in the spring by causing more "dead coils", which is when the coil contacts an adjacent coil and cannot vibrate as much. At coilbind, all the coils are touching each other, essentially making the spring solid, so you're trying to get as many "dead coils" as possible without making them go into coilbind.
I hope that makes sense.
I hope that makes sense.
changing the active length of the spring shifts the resonance away from the excitation frequency - in this case you want the first harmonic to ideally show up outside of the operating range of the motor right? or at the very least be out of phase with input from the rocker arm. not having any numbers in front of me I don't know if that's realistic or not
the increased pressure may or may not be beneficial for everyone, but from the harmonics perspective (and assuming the springs can handle it) shimming would be beneficial, wouldn't it? Along with using a high spring rate, I could see that really helping to cut down on the potential for high rpm valve float
sorry for rambling, this is interesting stuff to me
Last edited by ckpitt55; 03-27-2013 at 02:27 PM.
03-28-2013, 04:09 AM
#4
makes sense. a means of affecting a "damper" where one is not available.
changing the active length of the spring shifts the resonance away from the excitation frequency - in this case you want the first harmonic to ideally show up outside of the operating range of the motor right? or at the very least be out of phase with input from the rocker arm. not having any numbers in front of me I don't know if that's realistic or not
the increased pressure may or may not be beneficial for everyone, but from the harmonics perspective (and assuming the springs can handle it) shimming would be beneficial, wouldn't it? Along with using a high spring rate, I could see that really helping to cut down on the potential for high rpm valve float
sorry for rambling, this is interesting stuff to me
changing the active length of the spring shifts the resonance away from the excitation frequency - in this case you want the first harmonic to ideally show up outside of the operating range of the motor right? or at the very least be out of phase with input from the rocker arm. not having any numbers in front of me I don't know if that's realistic or not
the increased pressure may or may not be beneficial for everyone, but from the harmonics perspective (and assuming the springs can handle it) shimming would be beneficial, wouldn't it? Along with using a high spring rate, I could see that really helping to cut down on the potential for high rpm valve float
sorry for rambling, this is interesting stuff to me
The issue I see when shimming close to bind is you can get bit by tolerance stack-ups within the valve train. In other words, setting cylinder #1 to within 0.050" of coil bind may not result in similar coil bind on the next cylinder with the same shim unless you measure and shim separately. I usually measure each and set up a spreadsheet of the measurements to ensure they are consistent and I know the values in advance.
03-28-2013, 07:39 AM
#5
You really won't change the harmonics because they are a function of the spring stiffness. You can change the valve bounce, etc. that will minimize other responses within the spring. The valve springs should have thier first harmonic well above the RPM range by design, although that was not the case with the Crane 832's. It's first harmonic was just above 6,000 RPM and was seen as a dip in power after which it would recover if you pushed through it.
The issue I see when shimming close to bind is you can get bit by tolerance stack-ups within the valve train. In other words, setting cylinder #1 to within 0.050" of coil bind may not result in similar coil bind on the next cylinder with the same shim unless you measure and shim separately. I usually measure each and set up a spreadsheet of the measurements to ensure they are consistent and I know the values in advance.
The issue I see when shimming close to bind is you can get bit by tolerance stack-ups within the valve train. In other words, setting cylinder #1 to within 0.050" of coil bind may not result in similar coil bind on the next cylinder with the same shim unless you measure and shim separately. I usually measure each and set up a spreadsheet of the measurements to ensure they are consistent and I know the values in advance.
f = 1/2 * sqrt(k*g/W) -- k=spring rate, g=gravity, W=weight of active turns
W = (pi^2*d^2*D*Na*y)/4 -- d=wire diam, D=spring diam, Na=num active turns, y=specific weight
picking some values and running the numbers, you get a ~10% increase in bandwidth by taking even 1/2 of an active coil out of the mix. more gains can be had when you start decreasing the specific weight along with that.
Agreed on the tolerance stackups. Tony will be setting up my heads though and addresses this so I'm not worried about it.
03-28-2013, 11:50 AM
#6
Sounds like there is some confusion of spring harmonics and spring natural frequency.
You cannot change active coils by shimming or running X amount from bind. Only way to change active coils is with a different spring.
Basically what we are doing when shimming X amount from bind is decreasing the amount room the coils have to "clash". It is basically the same principle of a guitar string. The further the points of load are the string area it moves is further than if you were to shorten distance by 50%. By shortening this distance coil "clash" movement is reduced. When the valve shuts you already have less coil movement lessening the time the coils stabilize.
Now, 0.050" from bind is NOT a rule of thumb on every engine. We recommend anywhere from 0.040" to 0.120" from bind depending on application.
You cannot change active coils by shimming or running X amount from bind. Only way to change active coils is with a different spring.
Basically what we are doing when shimming X amount from bind is decreasing the amount room the coils have to "clash". It is basically the same principle of a guitar string. The further the points of load are the string area it moves is further than if you were to shorten distance by 50%. By shortening this distance coil "clash" movement is reduced. When the valve shuts you already have less coil movement lessening the time the coils stabilize.
Now, 0.050" from bind is NOT a rule of thumb on every engine. We recommend anywhere from 0.040" to 0.120" from bind depending on application.
03-28-2013, 12:31 PM
#7
Sounds like there is some confusion of spring harmonics and spring natural frequency.
You cannot change active coils by shimming or running X amount from bind. Only way to change active coils is with a different spring.
Basically what we are doing when shimming X amount from bind is decreasing the amount room the coils have to "clash". It is basically the same principle of a guitar string. The further the points of load are the string area it moves is further than if you were to shorten distance by 50%. By shortening this distance coil "clash" movement is reduced. When the valve shuts you already have less coil movement lessening the time the coils stabilize.
Now, 0.050" from bind is NOT a rule of thumb on every engine. We recommend anywhere from 0.040" to 0.120" from bind depending on application.
You cannot change active coils by shimming or running X amount from bind. Only way to change active coils is with a different spring.
Basically what we are doing when shimming X amount from bind is decreasing the amount room the coils have to "clash". It is basically the same principle of a guitar string. The further the points of load are the string area it moves is further than if you were to shorten distance by 50%. By shortening this distance coil "clash" movement is reduced. When the valve shuts you already have less coil movement lessening the time the coils stabilize.
Now, 0.050" from bind is NOT a rule of thumb on every engine. We recommend anywhere from 0.040" to 0.120" from bind depending on application.
I guess the disconnect I'm having is how the spring surge relates to spring harmonics and input from the rocker arm - if you think of an engine operating at constant speed, your input is going to occur at some uniform frequency. every time the rocker moves the spring, it has a complex response containing all of its resonances at varying magnitudes. these are fixed based on the geometry of the spring. since you say that shimming does not affect the # of active coils, does this mean that the surge is occurring independently of spring harmonics? Is the frequency at which it travels end to end dependent on the input frequency (i.e. if the ramp hits it harder the wave travels faster)?
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03-28-2013, 01:09 PM
#8
The guitar string analogy made a lot of sense, thanks.
I guess the disconnect I'm having is how the spring surge relates to spring harmonics and input from the rocker arm - if you think of an engine operating at constant speed, your input is going to occur at some uniform frequency. every time the rocker moves the spring, it has a complex response containing all of its resonances at varying magnitudes. these are fixed based on the geometry of the spring. since you say that shimming does not affect the # of active coils, does this mean that the surge is occurring independently of spring harmonics? Is the frequency at which it travels end to end dependent on the input frequency (i.e. if the ramp hits it harder the wave travels faster)?
I guess the disconnect I'm having is how the spring surge relates to spring harmonics and input from the rocker arm - if you think of an engine operating at constant speed, your input is going to occur at some uniform frequency. every time the rocker moves the spring, it has a complex response containing all of its resonances at varying magnitudes. these are fixed based on the geometry of the spring. since you say that shimming does not affect the # of active coils, does this mean that the surge is occurring independently of spring harmonics? Is the frequency at which it travels end to end dependent on the input frequency (i.e. if the ramp hits it harder the wave travels faster)?
In a vehicle we have a multitude of objects emiting frequencies. The engine, drive line ect. When your chassis or engine is emitting the same frequency the valve spring creates, you get a situation where the spring will go out of control. This is what is commonly called spring harmonics.
To answer your question, yes coil surge and harmonics are two different things but sometimes related.
