FLAT TWIN

Motorcycle Monthly - - Knowledge -

Lay­out: Two hor­i­zon­tally op­posed cylin­ders. Ben­e­fits: Low cen­tre of grav­ity, smooth and torquey na­ture.

BMW is per­haps the most renowned flat-twin (boxer) en­gine man­u­fac­turer, but the her­itage of this char­ac­ter­ful con­fig­u­ra­tion ex­tends past the Ger­man brand’s 92 years of use.

The first mo­tor­cy­cle to sport a flat-twin was a ma­chine called the Fée, later known as the Fairy, built be­tween 1905 and 1907 by the Light Mo­tors Com­pany. At the end of this pe­riod the man­u­fac­turer went bust and was bought out by Dou­glas, which went on to utilise the flat-twin tech­nol­ogy in its own prod­ucts.

As with the Fée, the ear­lier Dou­glas flat-twin mo­tor­cy­cles (such as the N3) car­ried their en­gines in-line with the frame to sim­plify the con­nec­tion be­tween the mo­tor’s out­put and the rear wheel it pow­ered. But as per­for­mance in­creased and greater cool­ing was needed, man­u­fac­tur­ers sport­ing this con­fig­u­ra­tion took to mount­ing the twins across the frame to bet­ter air the cylin­ders. An­other down­side to an en­gine be­ing in-line with the frame was the need for a longer wheel­base than might oth­er­wise have been de­sir­able.

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