Changing Cassette For Lower Gearing

[Seight_Returns]

I have a Spec Sirrus as my commuting bike - couple of big hills that I struggle with on my commute, so would like to change the cassette to one of those with the big "granny ring" gear to give a very low gear for the hills.

Current drivetrain is 8 Speed Shimano Sora with long cage derailleur and a 12-25 Casette.

Thinking about changing to an HG-40 11-30 or 11-32 MTB casette. I've read that Shimano road and MTB casettes are interchangeable, but am a bit concerned that the deraiileur will extend sufficiently to accomodate the bigger gearring.

Any reason why this wouldn't be a good idea ? Anything I need to do apart from fit the new cassette ?

[Parrot of Doom]

What's on the front? I know where you're coming from, my old road bike had a 12-21 on the back with a 42-52 on the front. It was fine for everything but there was a 1 in 3 incline I always struggled with (Rawson's Rake in Ramsbottom).

Maybe you might want to just slow down a little and take things easier?

Edited by Parrot of Doom on Wednesday 22 October 18:09

[Seight_Returns]

Parrot of Doom said:

What's on the front?

Sorry, should have said, got a 48-38-28 triple on the front.

Starting to think about a 11-34 Megarange Cassette. The highest 7 ratios are 11-26 and reasonably close ratioed, so shouldn't feel too much different to my current setup, but with an ultra low cog for the hill from hell.

[pdV6]

You can change the cassette no problem (note that you'll need a cassette tool to undo the lockring). The long cage rear mech will be fine - it's the front chainrings that dictate what size rear mech you need and there's nothing longer than a "long" one anyway.

However, you'll probably need a new chain in order to lengthen it to accommodate the increase in size of the granny gear. To determine the crrect length, I think the rule of thumb is to run the chain around the largest chainring and largest sproket (without threading it through the rear mech) and then add 2 complete links.

Hopefully your front chainrings won't be so worn that they'll need changing too due to the new (unworn) chain.

Should have added: You'll obviously need to find an appropriate 8sp cassette and chain to avoid having to change the shifter as well.

Edited by pdV6 on Wednesday 22 October 18:28

[Parrot of Doom]

How long have you been commuting? Those ratios sound pretty easy to me. Is it possible your saddle is too low, or tyres underinflated, or something similar? Or perhaps you just need to eat a bit more chicken