The rack bars are machine cut from a special quality of steel : the profile is not symmetrical and the bars must be installed the right way round. Bars tend to fracture between the fixings. When spotted these breaks are marked and then supported with wedges until the bar can be replaced. The pinions consist of an outer ring that is easily replaced. This ring is mounted onto a centre disc, and springs between the two reduce shock loads and allow the small movement needed to accommodate joints and curves. The pinion rings have symmetrical teeth and are turned round to double their working life.