cassette ratios

svboden

Sorry to be a real novice but my son has asked to upgrade his Road bike cassette - can i swap a 8 speed cassette 12-25 ratio for a 8 speed 11-30 cassette.
Will i need to change anything else?

Wooliferkins

Yes as long as your rear derailleur has the capacity to take it. Major manufacturers SRAM/Shimano/Campag have their user manuals online and they will tell you the mechs capacity.

markos1963

If its a triple chainset on the bike then the deraillier should have the capacity to take it. If not then Shimano medium mechs are specced for 27 teeth but can be adjusted with the B screw to take 28 teeth. I think Campag ones are ok for 29 teeth so might stretch to 30 if careful. How old is your son? If he is young why would he want an 11 tooth top gear, this is useless for most riding situations.

Mccaria

Also need to check chain length given the bigger sprocket on the back (or avoid using big ring on the front and bigger rings at the back)

keef66

Also make sure he gets one that fits the freehub. In simple terms Shimano and SRAM are compatible / interchangeable, if he's got Campagnolo then he needs to stick to Campag.

Berk Bonebonce

30 t is probably just overstretching the rear mech. Use a max sprocket of 27/28. You will also need a new chain.