Hairpin valve spring
For a brief while this simple alternative to the almost-universal helical valve spring seemed to offer real advantages for high-performance racing engines. Keith Howard explains how apiece of bent wire can raise your revs — or let you down
Excepting those very rare and generally fairly short-lived examples of desmodromic valve actuation or exotic rotary valves — like that used in the 1913 Itala – the vast majority of racing engines down the decades have all employed the conventional poppet valve system.
Although it oversimplifies its complex dynamics, particularly at high engine speeds, this is usually characterised as opening the valve ‘positively’ with the camshaft lobe, and closing it (by implication,somewhat less positively) via spring force. In most engines the valve spring comprises one or more metal coils, usually of steel or titanium; in very-high-speed engines it may be pneumatic. Either way the principle remains the same.
During the years up to around 1940 and after about 1960— prior to the introduction of pneumatic spring technology by Renault in 1986 — designers of race-car engines unhesitatingly used the helical valve spring. But during the 1940s and 1950s there was a whiff of revolution. A number of engine makers, most notably Ferrari, abandoned the helical spring for an alternative called the hairpin spring — although it looks like no hairpin I’ve ever seen.
Whereas the helical spring was positioned coaxially with the valve stem, typically under a bucket tappet, and compressed when the valve was opened, a hairpin spring lay alongside the valve stem, its axis at right angles to it. One ‘arm’ of the spring was attached to the valve stem and the other(s) to the cylinder head, so that the spring was coiled tighter as the valve opened, rather like the return spring on a garden gate.
Hairpin valve springs were first deployed in motorcycle engines. One reason, of no concern to the designer of car engines, was that they allowed better cooling of the exhaust valves in exposed valve bike engines, because it didn’t enclose the upper end of the valve stem. But two other features did appeal to car-engine designers: hairpin springs had proved more reliable than helicals, and they reduced the moving mass, and hence inertia, of the valve assembly.
Improved reliability — although this was not universal, as we’ll see — arose from the limited metallurgical knowledge of the time and the fact that helical and hairpin springs are subject to different mechanical stresses. The helical spring, though it may not look it, is a repackaged torsion bar. As the spring is compressed, its wire is subject to twisting forces which set up torsional stresses within it. The wire of a hairpin spring, by contrast, is subject to bending loads and bending stresses. Although metallurgical improvements would eventually annul the hairpin spring’s advantage, in the interim its promise of improved reliability was alluring.
So too was its reduced inertia, particularly in engines designed for high rotational speeds. As well as contributing less of its own mass to the moving valve assembly, the hairpin spring also facilitated the use of a shorter valve stem, reducing mass still further.
Although Ferrari was to become the foremost proponent of the hairpin valve spring, it was also used in car engines by Alfa Romeo during the 1930s. One of the first advocates of its use in high-performance engines was actually an Englishman: Harry (later Sir Harry) Ricardo, founder of Ricardo Consulting Engineers. Ricardo’s most famous contribution to the development of the petrol engine was his early research into detonation (knock), the development of the fuel octane concept and the importance of mixture turbulence in the combustion chamber. But he also contributed ideas and development work to a host of automotive and aircraft engines.
That Mussolini’s Italy should have approached Ricardo for help with a grand prix engine seems unlikely even so, but it did. He was asked to contribute ideas for a new GP power plant by Alfa Romeo’s Spanish engine designer Wilfredo Ricart, and one of the uncertain number of features he suggested was the use of hairpin valve springs, to help the engine rev to 9000rpm. In the event only one of the supercharged 3-litre, 135-degree V16s, intended for the Type 162, was ever built and it ran just once before Italy entered WWII, during which it disappeared. But drawings for it still exist.
Gioachino Colombo knew of the Alfa engines and, of course, of the hairpin spring’s widespread use in motorcycles. Convinced of its advantages, he incorporated it in Ferrari’s original V12, designed shortly after the war, thereby initiating what would be a consistent and distinctive marque feature, on road and track, until the close of the 1950s. How much it contributed to Ferrari engines’ reputation for reliability is impossible to quantify, but the young Maranello team notched up four driver’s championships in just seven years.
Hairpin valve springs were also specified for the ill-fated BRM V16, which was intended to rev to 12,000rpm — even higher than the Alfa. Harry Mundy and Walter Hassan at Coventry-Climax were attracted to them too and used them in the company’s first GP engine, the 2.5-litre FPE V8 designed in 1952. But Climax soon found the hairpin more promising in theory than practice. Experience proved it very difficult to manufacture, the principal problem being one of surface finish. Without a high standard of polish, minute scratches in the wire surface would act as stress raisers, leading to cracking and eventual failure. Climax quickly gave up on the hairpin spring and never used it again.
Tony Vandervell, perhaps because of his determination to beat the red cars at their own game, was less easily dissuaded. The four-pot Vanwall engine shared many features with the racing Norton motorcycles, including hairpin valve springs. Vanwall initially suffered the same surface finish problems as Coventry-Climax, but eventually George Salter and Co Ltd., the spring manufacturer, was able to make them sufficiently reliable.
When Vanwall finally clinched the constructors’ championship in 1958, the era of the hairpin valve spring had about run its course. Soon the helical valve spring would be universally re-adopted. Compared to the other changes about to sweep through F1, though, that would be a mere detail.
Our thanks to Tony Merrick Racing.