My GF is having various knee ops and cannot put any kind of effort when she hits hills, i was trying to find out about getting some really super low ratio gears by changing her current MTB setup of the following F / R cogs

Shimano FC-M311 48x38x28T
Shimano CS-HG30 11-30T ( 8-Speed )

the rear mech is a Shimano Alivio (RD-M410) Top / Normal and front is bog standard Shimano FD-M310 Top Swing…

I was going to get a 24T or maybe 26T for the front – will i have to muck around with the rear der to get proper rise/tension or change the chain tension?

thanks everyone

Have you thought about a HG-40 “Megarange” cassette? 11-34 gives a low gear and shouldn’t need any changes (as long as she is using the gears correctly).

48-38-28 are all pretty high so even if you swapped out the the 28t to a 24 you would then have a big leap to 38! so youd be better off changing them all down to something like 24-32-44 (she might not even use the big ring)
if you reduce all the ring down in size you may need to take out a couple of chain links
nowt wrong with 8 speed but i’d personally get a 11-32 sram pg850 much lighter and also very cheap from bike24

If you can change the chainrings on these cranks (a quick google suggests they might be riveted rather than bolted on) then you could even have a 22 tooth small ring, but you would have to get smaller middle and outer chainrings to match as the front derailleur will not be likely to manage pulling the chain from a 22 to 38 tooth chainring in one shift. Rear derailleur should be fine as long as she avoids the smallest 3 or 4 rear cogs when she’s in the smallest front ring: if its not, take a link out of the chain.

If you can afford it, you might be as well off to buy a whole new square taper crankset with 22/32/44 rings (most common for mountain bikes nowadays) and you could also get a rear cassette with a 32 tooth biggest cog. 22 teeth chainring plus 32 tooth rear sprocket is well spinny, hard to get beyong a brisk walking pace in that ratio! If you do change both you would do well to spend £6.50 on ebay on a new kmc 8 speed chain as well so that they all wear out together. Old chain on new cogs is often skippy and wears your new cogs out faster.

Hope that helps.

Actually druidh’s 11-34 idea is best: probably try that one first, if you have the tools at home for this, have a look on ebay for one as it will likely be a few quid less than if you walk into halfords for one.

thanks for the advice, i think druids way is easier, but not going to give her the range she needs, i suspect what everyone has said would be right, in that i would have to change 2 chainrings up front so its easier to go from through the range – i just hope its bolted on not riveted – and hopefully that means i dont have to change the chain either….

DAMMMMMMMMMMMmm its riveted, cannot remove the chain rings 1 by 1, new front crank and therefore a new chain too ……..

Alivio / non series Shimano chainsets cost less than a couple of rings. Just get the cheapest chainset around (and potentially a square taper BB to go with), and you’ll have 22/34/42 (or 44) on the front.

34 tooth rear cassettes are common enough and ought to be within the capacity of the Alivio mech – if not, a 32 tooth gives you a bit more than you currently have.

TBH it sounds like her bike could be a trekking / city bike setup with gearing that high.