Comp Cams Valve Spring Information - The Verdict On Valvesprings

HPP: What is "natural frequency" and how does it relate to valvesprings?
TG: The natural frequency of any object is the harmonic frequency at which it cyclically vibrates when subjected to external input. There are two basic examples: free vibration and forced vibration.

An example of free vibration is a guitar string that's been plucked. It vibrates back and forth after the initial external force is removed for a certain number of times per second, and that number is its natural frequency, in Hertz (Hz). It continues until the vibration decays, the energy completely dissipates, and the string stops moving.

A car's wheel and tire that's out of balance is an example of forced vibration. The wheel and tire will vibrate up and down (somewhat violently), shaking the car. Because the car is constantly moving, the external force is making the wheel spin at its natural frequency.

Valvesprings are exposed to both types of vibration during normal operation. As the valve opens and closes, the spring is subjected to forced vibration. The spring is then subjected to free vibration while the valve is closed and the lifter rides on the camshaft's base circle. This results in a harmonic frequency that's independent of engine speed. As engine speed climbs, cam-cycle duration gets increasingly shorter and the number of vibration cycles decreases, but the natural frequency remains the same.

HPP: How does natural frequency relate to "critical speed?"
TG: As the valvespring coils vibrate at the spring's natural frequency, the spring oscillates more vigorously at certain engine speeds. When that particular engine speed is reached, it's known as "critical speed," and several things happen simultaneously, which compromise valve control. The result is a lack of spring force to hold the valve closed at that speed, which can lead to a condition known as "valve bounce" or worse, broken valvetrain parts.

HPP: How can valve bounce be combatted?
TG: We can tune a valvespring by tweaking a few different aspects of its geometry or the material from which it's constructed to control valve bounce. This includes changing the spring's overall diameter, wire diameter, the number of active coils, or the space between the spring coils. We refer to this as "pitch layout," and it will affect either the spring's stiffness or mass, or both.

While there are no steadfast rules when tuning a spring, we often try to design a spring with a higher natural frequency, if possible. It isn't as easy as changing numbers, however. The dilemma that's presented to the spring designer is juggling the stresses around desired loads or frequencies while constrained to a limited space.

Beehive springs have damping coils designed into them. This consists of a couple of tightly wound coils at the bottom of the spring, and because these coils are spaced more closely, the resonance (or surge) wave bounces among them. Much of the resonance energy is trapped in this "sink" area and dissipates before it can rebound back up the spring.

HPP: Does this relate to the damper found in some cylindrical spring packages?
TG: Yes. As the valve opens very fast at high engine speed, a surge wave is generated by the motion that travels down the valvespring. The damper uses the friction that exists between it and a cylindrical valvespring to "damp" or minimize spring surge, which can cause any number of issues, including those mentioned before, but some cylindrical dual- and triple-springs don't require dampers. Instead, they use their interference fit to perform the same task, while beehive springs contain damping coils.