Q How does my gear­box work?

Motorcycle News (UK) - - MCN Garage -

Sean Whit­taker, Nova Rac­ing It’s easy to take your gear­box for granted through thou­sands of changes. Sim­ply flick the lever with your foot to se­lect two dif­fer­ent, rapidly spin­ning gear ra­tios and the bike leaps for­ward as its road speed is matched to the en­gine’s sweet spot in its rev range.

But every slick gearchange re­lies on a whole se­quence of events from pre­cisely en­gi­neered com­po­nents. As you move the lever you turn a shift­ing arm or spin­dle that is at­tached to a se­lec­tor drum. This has grooves or tracks ma­chined into it to guide the se­lec­tor forks, which then move the tough­ened steel gear ra­tios into a dif­fer­ent pair­ing. Those ra­tios have match­ing ‘dogs’ (from dog col­lar) ma­chined into them that look like a ring of 4-5mm pegs.

As the drum ro­tates, its grooves or tracks pull one fork out then an­other in to shuf­fle the ra­tios, while a de­tent spring gets the gears to mesh cleanly by stop­ping the drum over or un­der-ro­tat­ing. A re­turn spring brings the lever back to its start po­si­tion for the next change.

To get a good solid shift the dogs may be ma­chined with an ‘un­der­cut’ or 3-5-de­gree slope on their re­verse side that pulls the two ra­tios to­gether.

Honda’s Mo­togp bikes have a seam­less gear­box de­sign, ru­moured to cost £300,000, which uses com­plex elec­tron­ics to elim­i­nate the pitch­ing back and forth which re­duces grip. But we think the next sports­bike devel­op­ment is dog ring de­signs, where the spin­ning ra­tios are hol­lowed out and there­fore lighter and eas­ier to shift, sav­ing a tenth of a sec­ond each time, with the dog ring within the ra­tio it­self.

Prob­lems start when poorly ex­e­cuted changes mean the dogs and forks be­gin to wear, or the var­i­ous springs start to lose their ten­sion. The gear­box feels slacker as you have to keep pres­sure on the lever to get the drum to move, or wait mo­men­tar­ily for the worn dogs to mesh fully each time. With 40 to 50 gearchanges per lap at a typ­i­cal cir­cuit, lost time soon racks up.

Every gearchange re­lies on finely en­gi­neered com­po­nents

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