Layout: Two pairs of two cylinders in a square configuration. Benefits: Narrow in profile, big torque, affordable to build.
Ariel took the decision to employ designer Edward Turner in the late 20s, having seen his illustration for a square-four motorcycle engine. Turned away by BSA, Ariel quickly got Turner’s innovative 500cc configuration into production and showcased a motorcycle aptly dubbed the Ariel Square Four at the Olympia Show in 1930.
Between 1931 and 1958, a total of five different versions had come and gone, with over 15,000 examples of the popular machines being sold during that period.
Despite its engine capacity increasing from 500cc to 997cc, the square-four layout was to prove incapable of revving high enough or producing sufficient power to compete with the latest and greatest options coming to light by the end of the 50s and was consequently dropped. Brothers George and Tim Healey – fans of the square-fours – bought-up all remaining square-four components from Ariel and built just 28 Healey 1000/4s between 1971 and 1977, having tuned the engines from 45bhp to 52bhp, before they eventually packed up.
Suzuki then took up the square four design in the 80s, when it released the two-stroke RG500 Gamma race bike in 1984, going on to inspire the production of the RG500 Gamma road bike between 1985 and 1987. Despite achieving a respectable output of 94bhp, no further advancements to the square-four were made.