Cen­tre Hub Steer­ing, Lead­ing Link Forks & Twin Head Steer

Old Bike Australasia - - QUASARS -

Un­ortho­dox steer­ing ar­range­ments are a re­cur­ring theme through­out the his­tory of Feet For­ward mo­tor­cy­cles for two main rea­sons. The en­gi­neers in­volved could see the tor­tu­ous load path, from the front wheel to the forks and through the head­stock be­fore be­ing fed into the frame. They also saw the in­her­ent lim­i­ta­tions of tele­scopic forks; com­bin­ing sus­pen­sion and steer­ing loads in one de­vice (once de­scribed as “a tri­umph of en­gi­neer­ing over com­mon sense”).

Cen­tre hub steer­ing uses an in­clined king pin in the cen­tre of a large di­am­e­ter in­ner hub and the wheel sits on thin large in­ter­nal di­am­e­ter bearings on the in­ner hub. The king­pin is on a shaft that is mounted on a swing­ing arm con­nected di­rectly to the frame. Although an ex­cel­lent ar­range­ment, there are a cou­ple of draw­backs. Firstly the size of the bearings and the arm that hold the king­pin limit the steer­ing lock, brakes were dif­fi­cult to ar­range (un­til the ad­vent of disc brakes) and they are ex­pen­sive to make. Jack Di­fazio de­vel­oped a work­able ar­range­ment which he fit­ted to a num­ber of ma­chines in the 60s and 70s. Royce Creasey fur­ther re­fined the cen­tre hub steer­ing de­sign, con­sid­er­ably im­prov­ing the steer­ing lock.

Lead­ing link forks of­fer some of the ad­van­tages of cen­tre hub steer­ing at a frac­tion of the price. How­ever the main draw­backs are that the load path into the frame is still tor­tu­ous and the higher ro­ta­tional mass that can af­fect steer­ing.

Twin head steer ma­chines used the tele­scopic forks of the donor ma­chine but the head­stock was heav­ily mod­i­fied so it piv­oted back­wards and for­wards. Steer­ing was con­trolled by drag links con­nect­ing the front wheel spin­dle to a steer­ing arm mounted on the lower part of mo­tor­cy­cle frame.

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