In contrast to cassette sprockets, which are mounted on the freewheel body and fastened with an end ring, helical sprockets are screwed on using special tools. The freewheel mechanism is located in the sprocket and not, as with the cassette, in the hub. In combination with Shimano HYPERGLIDE (HG) chains, the HG sprockets provide a fast, precise indexing function. The special, computer-designed sprockets feature particular tooth profiles and specifically positioned climbing aids that optimise the chain guide during shifting. Conventional sprockets let the chain ride on the tooth tips before it sits on the sprocket. HG sprockets guide the chain directly and without riding onto the next sprocket. This results in fast, smooth and noiseless gear changes.Compatible with Shimano Tourney and Altus systems. For trekking with comfort.