▁production ▁units ▁became ▁available ▁in ▁April ▁ 2 0 0 8 ▁and ▁were ▁used ▁for ▁the ▁measurement ▁of ▁the ▁London ▁Mar athon , ▁which ▁took ▁place ▁on ▁April ▁ 1 3 , ▁ 2 0 0 8 . ▁ ▁The ▁g ear ing ▁( 2 6 0 / 1 1 ▁counts ▁per ▁wheel ▁revolution ) ▁is ▁identical ▁to ▁that ▁of ▁the ▁Jones - O er th ▁model . ▁ ▁Use ▁for ▁meas uring ▁road - race ▁courses ▁To ▁measure ▁road - race ▁courses , ▁the ▁counter ▁is ▁fitted ▁to ▁a ▁b icy cle ▁between ▁the ▁left ▁fork ▁leg ▁and ▁the ▁front ▁wheel . ▁The ▁tab ▁or ▁tabs ▁on ▁the ▁large ▁ring ▁g ear ▁eng age ▁with ▁the ▁sp okes , ▁thus ▁providing ▁drive ▁to ▁a ▁Ve eder - Root ▁counter . ▁ ▁Before ▁the ▁counter ▁is ▁used , ▁the ▁b icy cle ▁must ▁first ▁be ▁cal ibr ated ▁by ▁being ▁r idden ▁on ▁a ▁straight ▁section ▁of ▁road ▁between ▁marks ▁whose ▁separation ▁has ▁been ▁accur ately ▁measured ▁by ▁steel ▁t ape . ▁A ▁cal ibration ▁can ▁then ▁be ▁calculated ▁in ▁terms ▁of ▁counts ▁per ▁kilometer . ▁Next , ▁the ▁b icy cle ▁is ▁r idden ▁over ▁the ▁race ▁course ▁to ▁determine ▁its ▁length . ▁Finally , ▁the ▁b icy cle ▁is ▁re cal ibr ated ▁by ▁r iding ▁again ▁over ▁the ▁cal ibration ▁distance . ▁This ▁is ▁done ▁to ▁check ▁for ▁changes ▁in ▁b icy cle - w heel ▁diameter ▁due ▁to ▁temperature ▁changes , ▁air ▁leak age , ▁and ▁other ▁causes .