Valve Springs

Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more

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Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.Cycle World

In the early days, the springs whose force caused motorcycle valve trains to actually follow the cam lobes that operated them broke often. A common hedge against such failure was to nest three helical springs around each valve, and more than one maker tried to overcome the problem by pure statistics—by arranging as many as eight valve springs around a large pie-plate-shaped retainer atop each valve. The reasoning? They can’t all break at once.

The reason valve springs failed is because, in order to do their work, they had to be stressed to a fair percentage of the material’s ultimate tensile strength. That level of stress soon caused any surface or interior defects of the spring wire to propagate as a crack, bringing many failures.

The obvious path to improvement was to create extremely “clean” spring steels remarkably free of internal defects and then to draw them into wire and manufacture springs using optimum techniques such as surface polishing and shot-peening.

Yet even with such materials and care, life expectancies of valve springs continue to be defined by the conditions of their use. Springs in OEM engines are made to last hundreds of millions of stress cycles by designing them conservatively to operate at low-to-moderate peak stress levels of 30 to 40 percent of the material’s ultimate strength.

As more vigorous cam action requires more from springs, design must push the material harder. For racing engines whose lifetime is measured in hours, lifetimes of mere millions of cycles allow use of peak stress of 70 percent of the material’s limit. In Pro Stock Auto drag racing, springs may have to be changed after every sub-seven-second run and stress is pushed even nearer to the spring wire’s tensile strength.

When in the single-digit years of this century MotoGP teams were having to change metal valve springs every day to avoid failures, it seemed prudent to consider something better—"springs" whose elastic medium is not a solid but a gas—loosely given the name "pneumatic valves." It was Renault's F1 team that developed this system in the 1980s.

Many people assume this means some exotic system of opening and closing valves with gas pressure, but the systems are conventional, opening the valves with a cam. The “spring” is just a moderate gas pressure (more or less 20 atmospheres) acting against the underside of what looks very much like an inverted-bucket-type tappet. The only thing exotic in such systems is how the tappets are sealed to their bores, the valve stems are sealed to their guides, and oil is supplied where needed but without accumulating where it’s not.

On pneumatic-valve engines that must run for an hour or so at a time, a small pressure bottle and regulator on the vehicle is charged before use to maintain system pressure. In endurance racing, a small pump serves the same purpose.

Over many years futurists have been hailing one or another system for driving valves through electromagnets, solenoids, or hydraulics, but it's not clear that any of these systems can rise to the task of operating at racing rpm before societal/governmental pressures sweep away the internal combustion engine altogether. With the present emphasis on sending ever-smaller and more economical engines to power production autos (Honda have just developed a 10-speed automatic for such use) the ability of a variable timing-and-lift valve drive to give Harley valve action at low revs and GSX-R numbers as engines rev up has strong attraction.

Two things have to happen to send all of the above to the museum: 1) electric cars must at last become fully practical, that is, able to carry family and friends while towing the ski boat up to the lake 400 miles away, and 2) generation of the required electricity must shift rapidly and substantially away from its present 86 percent dependency on things that come out of the ground (coal, natural gas, uranium). Otherwise, the complex internal combustion engine and the extensive systems that support it must soldier on, likely for decades to come.