How do I achieve a lower gear on a Norco Bigfoot with a 9 speed cassette?

Question

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/

Answer 1

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.

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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.

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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.

Answer 2

Using only 9 cogs of a 10 speed cassette will not work as the shifter throw (amount of cable pulled/released) is different between 9 and 10 speed systems.

Considerations: Derailleur Cage Length. To go to 42 tooth set up you need to verify you have the long cage version of your Alivio derailleur. Medium cage Shimano derailleurs typically can accommodate 37 tooth cogs so you described system may have a medium cage derailleur and not work with a 42 tooth cog.

MBTR has a thread dedicated to 8/9 speed 42 tooth setups Here: http://forums.mtbr.com/drivetrain-shifters-derailleurs-cranks/11-42-drivetrains-8-9-speed-finally-943575.html Essentially, they use a One-Up 42 tooth cog (designed for 10 Speed Systems on a 9 Speed cassette and add the 9 speed spacer behind the cassette on the hub body and indicate it works with vintage XT Thumbshifters. Caution: The thread indicates they use the "ghost" position (click beyond the 8th gear of their 8 speed thumb shifter) so it may not work with your Alivio shifter

Another Caution: https://www.oneupcomponents.com/pages/compatibility indicates Shimano Alivio (CS-HG50-10), Deore (CS-HG62-10) and SRAM XX (XG-1099) cassettes are not compatible with the OneUp adapter sprocket. In these cases the purchase of a compatible cassette will also be necessary.

So to be frugal and mitigate risk, I would buy a new 9 speed cassette. 9 Speed 11-36 cassettes are readily available which are guaranteed to work for about $50 USD and although it may not give your the gear inch reduction of a 42 you can rest assure there will not be any compatibility issues.

Another route would be to evaluate your front chain ring sizes. Can they be reduced by a tooth or two?

Answer 3

There are two things that occur to me. One is easy, which is buy one of the 11-40 9spd cassettes made by Sunrace or IRD. Then, after making sure your RD has the total capacity (it's 45t for RD-M4000, so you're seeing if the [large cog - small cog]+[large ring - small ring] number is equal or less than 45) number required, you'll need to drop it down a little bit by either jacking in the b-tension screw, installing it in reverse and/or installing a longer one, or if needed using a derailer hanger extender, but I'm not exactly sure which one because none of them are made with this exact purpose in mind. (Usually the longer screw is enough in my experience when playing these kinds of tricks, but only usually.)

The other idea is you could use one of the 42t cogs made for 10 speed and attempt to make the spacing play nice with 9 speed by using .4mm worth of shim spacers to adapt the 3.95 10spd pitch to 4.35mm 9spd. I haven't done this but I don't see a reason it wouldn't work.

Using 9 cogs off a 10-speed cassette isn't how this works, because the pitch will be wrong.

What else may be possible, and this is speculative, is starting with a fully non-spidered 10-speed 11-42 cassette, meaning where all the cogs are individual and there are spacers in between each cog, and replacing those spacers with appropriate width ones. A bunch of different companies have made various cassette conversion spacers that work on this principle. You may even be able to harvest some off a spent 9spd cassette. As long as you can get the cog to cog spacing to be 4.35, it should index.