In nearly all engines manufactured prior to 20+ years, timing belts were manufactured from thick rubber straps. To this day, hardwearing rubber materials remain a functionally ideal solution for maintaining the tension between vital moving parts in the short-to-mid-term.
However, over longer periods, rubber compounds are prone to degrading through constant exposure to excessive heat from the engine and contact with aggressive substances like motor oil. The result of this prolonged exposure is a loss of optimal tension. This negatively impacts the precise opening and closing times of engine valves. This dictates the need for the eventual replacement of a cambelt to restore faltering engine performance back to its previous peak.
Some modern variants of timing belts can be constructed from a variety of more high-tech materials, each bringing a range of slightly different properties and features for consideration when planning future replacements and upgrades.