The fork springs are simply given a coat of oil to protect them until installation. Titanium isn’t prone to rusting and can be left raw. Epoxy-based powder coating can survive the flexing. A steel rear spring needs a coating to prevent corrosion. Fork springs are polished to reduce friction inside the fork legs (rear shocks are not polished). Next, the spring is pressed (or pre-set) so that it does not change its free length during use. To further reduce tension, the spring is shot-peened. Next, both ends of the spring are ground flat. The spring is heat-treated to reduce the tension and cooled slowly so that it doesn’t become hard and brittle. (3) Common bike springs are cold-coiled, but other processes are used to lessen some of that built-in tension. Hot-coiling springs help keep the material in a natural state, but the additional equipment and processing time relegate hot-coiled springs to products that can justify the high cost. Bending wire into a spiral does this throughout the coil, creating a built-in tension that hurts the performance of a shock spring. At the outside of the curve the material is stretched, and at the inside it is compressed. (2) Imagine bending a rubber eraser into a U-shape. These machines can pop out springs surprisingly fast. The machine feeds wire around a curved mandrel so that the coils rise up like a snake. Generally, spring manufacturers buy various diameter wires from a supplier and coil their own springs with a CNC machine. Stronger material allows for fewer coils. Material with a better flex property makes a livelier, more responsive spring. Material quality is a huge factor in performance. (1) Most shock springs are made of silicon-chromate steel (and occasionally titanium).
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