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get the acro bike by ridel and hold the B-button
While on your bike simultaneously press B and the desired direction.
just use b left/right at the same time ------------------------------------------------------------ press up and B at the same time to go up and down and B to jump down.
the only way to jump on bike is to jump of a cliff!
if your talking about the thin white poles in safari zone and near fortree city, i can help. first you have to go to mauville city and go to the bike shop (if you need to) and swap you mach bike for an acro. go back to the white bridges and ride you acro bike on them. it will work! just ride the bridges on your bike down them and if there are spaces simly hop using b and left or right at the same time to jump your bike to the next bridge. hope this helps.