A toothed wheel is described with a peripheral portion that comprises a side projection at one or more teeth, projecting with respect to one of the sides of the toothed wheel. The presence of the projection provides the necessary guide for the chain and therefore the stability of the coupling between chain and toothed wheel, without however making heavier the toothed wheel itself with an excessive thickness in all of its parts. Such a wheel also allows the lack of material in the vicinity of the projection to be exploited to make the gearshifting operations in a group of many toothed wheels easier. Then a pair of toothed wheels is also described wherein the wheel of greater diameter comprises a first specialised tooth for upward gearshifting, in which wheel the outer side of the peripheral portion comprises a substantially flat area without recesses, situated in front with respect to the specialised tooth and outside the teeth.
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