How do I achieve a lower gear on a Norco Bigfoot with a 9 speed cassette? I have 2015 Norco Bigfoot fat bike which has a Shimano HG-20 11-34T 9 sp cassette a with a  Shimano Alivio RDM 4000 SGS 9 sp Rear Derailleur and a  Shimano EZ-Fire EF 51 9 sp Rear Shifter. As I live in a particularly hilly area and intent to most ride this bike off road along animal pads in the bush I would like to reduce the gearing by having a 42t cog on the cassette. From reading I understand this can be done by
- removing a small cog and installing an 42T extender cog in the cassette,
- by replacing the current cassette with a wide range 9 sp cassette (hard to find),or by
- changing to a wide range 10 speed cassette with a change of derailleur, chain and shifter.

Please provide advise as to the best, most economical option. Could I install a 10 speed cassette and just use 9 cogs?

  From https://www.norco.com/bike-archives/2015/bigfoot-62/
Looking at the spec, your bike has a double front chainring.  Are you shifting into the smallest front ring when climbing?  If not, then do so.   

You can possibly buy a smaller front chainring too, that will leave the higher gears alone for faster/road usage, like downhills.



The single cheapest solution is to pace yourself by slowing down, harden up your muscles and climb at a rate that you can sustain.   This is the most economic option.

Is the grade steep enough that you physically can't ride it?  Consider taking an alternate route, or changing a straight climb into a series of switchbacks.



A 9 speed cassette is still available to buy - simply look for one with the biggest big cog and see how it goes.  You will need a new chain too.  Converting your existing cassette won't work if its a one-piece unit, or if the smallest cog is the lockring.

Example http://www.wiggle.com/shimano-alivio-9-speed-cassette/ is a 11-34 tooth for quite cheap.