Air-Cooled Aircraft Engine Cylinders
The hairpin valve springs pictured to the left were possibly pioneered by Salmson in 1911, and later used not only on British single-cylinder racing motorcycle engines, but also by Ferrari and others into the 1950s. This use was a response to the same problem that led to desmodromic valves at Ducati, Norton (test only) and Mercedes - namely the fatigue of coil springs from “ringing”. Hairpin springs ran cool because they were exposed, and they were less subject to fatigue. They could also be changed without engine disassembly.
Around 1964 cleaner steels produced by vacuum remelting became available in quantity, making possible the manufacture of highly fatigue-resistant spring wire. Previous wire was made from electric furnace steel - then the cleanest available. Vacuum remelted steel wire made desmo and hairpins redundant. Today Ducati engineers respond to the question “Why still desmo?” much as Bosch engineers did to the 1945 question “why direct injection when carbs were so much simpler?”. They said that once they’d started down that road, it was simpler to continue rather than start over with another technology.
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