When the TR4 came to market, one of the few critics about the model that succeeded the famous TR3 was its harsh ride. In order to make the ride more confortable, Triumph presented the new IRS (Independent rear suspension) model in 1965. The IRS had a new semi-trailing arm system with coil springs and lever-arm shocks; in order to fit this new system the chassis had to be redesigned, resulting in a slightly wider and heavier car. Production of the IRS lasted until 1967 when the TR5 was introduced.