Our motorcycle push-rod engines require a spring strong enough to return the valve and all of its operating gear to its closed position. To cancel out some of this reciprocating weight, early engines were fitted with return springs to the rockers or push-rods to assist the valve spring.
As development of valve springs progressed the need for assister springs diminished, but the design of the spring now came into question. Although coil springs have direct loadings with minimal side thrust on the valve stem, the exhaust valve in particular suffered the effects of localized heat transfer around the base of the spring, which caused the spring coils to settle (sag) and lose their efficiency.
The late '20s saw some innovative designs, and none more so than the 1925 TT JAP multi spring arrangement, which incorporated no less than eight coil valve springs per valve. Although reliable, the concept had limitations, mainly the massive amount of weight, of which about half is reciprocating. During this time, when engine revs started to increase appreciably, particularly racing engines running high revs for long periods, coil springs suffered fatigue from the effect of surging. This is a state of rapid vibration that affect the center coils often resulting in total fracture of the wire. To overcome these problems, hairpin valve springs found favor.
Early hairpin springs of open design featured small diameter coils and long legs but with constraints on available space, later springs incorporated larger coils and shorter legs, which curved and overlapped. The main benefit of these resulted in the coil end of the spring being cooled by the airflow and positioned away from the localized heat source. An additional benefit of this design was that the air flow around the exposed valve stem was unrestricted.By the mid '40s conventional coil springs had undergone extensive development and were back in fashion. Improved wire materials, heat treatment, revised winding techniques to reduce surging and minimize localized heat transfer from the cylinder hear to the valve spring played a vital part in improved performance and reliability. By now most valve springs were of the enclosed coil type, oil returning form the engine was used to provide adequate cooling and lubrication of the related components.
Lower valve spring cups were machined to provide the smallest contact area, to minimize transfer of heat to the uppermost coils and reduce the effect of spring sag. There washers (cups) normally have a taper on the lower edge to allow oil to run under the cup and provide another source of cooling. Later designs incorporated heat resistant insulating washers (Tufnol) between the cylinder head and valve spring seat.
By the mid '50s sports engines benefited from smaller and lighter springs, thus saving reciprocating weight, and the use of chrome vanadium alloy steel with progressively wound coils (the distance between each coil considerably reduced at one end), provided surge control and gave a progressive rate of compression – the rate being the pressure in kilograms exerted per centimeter of compression.
With this type of spring the rate increases progressively until full lift when the poundage would be about the same as a conventionally wound spring. Progressive springs normally run with a lower closed valve seat poundage. The configuration of the spring reduces lost motion by minimizing deflection of the pushrods and valve gear, therefore improving performance. It's very important that progressively wound springs are fitted with the closed end towards the cylinder head.
Most springs have outer coils wound in a right-hand direction, while the inner is wound left ways along with one or two more coils than the outer, which also helps to reduce surge. Another improvement was the use of specially ground wire to ensure freedom from draw marks, which was ultimately shot-peened to improve reliability. Quite often, with dual valve springs, the inner is a push fit within the outer. This provides a level of damping and minimizes surge, although some modern high performance springs have drifted away from this design.