Cassette adjustments & bike repair course

5milestogo

Hi, I have an Shimano 12 - 23 cassette on at the moment and to help getting up the hills am thinking of taking off the 12 and adding a separate 25 ring onto the other end to give me a 13-25.

Do you reckon I will need to re-tune/ align the gears or if its running fine at the moment should I expect it to just 'drop in place'.

I am looking to avoid any cost & a trip to the LBS who will have to do the gears as my DIY skills are more like GSI - get someone in!

On another note would be grateful if anyone can recommend a good bike repair course that's worth attending in London area so I can learn a bit more without breaking things!

Thanks in anticipation

keef66

I don't think your Shimano cassette will be amenable to the kind of tinkering you have in mind. Usually the 3 biggest sprockets are mounted together on a carrier. Simplest solution is to buy a replacement cassette with the range of gears you want; 12-25 or 12-27. And replace the chain at the same time.

You'll need a cassette lockring tool, an adjustable spanner to hold it, and a chain whip. You'll need a chain tool to split the old chain and join the new one (or use a SRAM or KMC joining link)

John.T

It depends on which model cassette you have. You would need a new 13 tooth top sprocket as the top one is different. You may also need a new 14 for some cassettes. You also need a new 25 sprocket and a spacer. You would also find the change a little impaired as the new sprockets may not match the cassette. You may have difficulty getting hold of parts.
Just get a different cassette. Much less fuss. Providing the chain is long enough it should just go straight on. If the chain has many miles on it it should be changed as well or the new cassette will wear prematurely.