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Gearing is all linear. 165mm cranks are 5% shorter, dropping 2t is 7% lower and 4t is 13% lower. If your current chainring works on the new cranks, I'd try it out first and see if you even notice that you need lower gearing. If you do notice and want to change, then a 28t will be the ticket. This is assuming you already have the "ideal" gearing for your riding already.
 

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Discussion Starter · #3 ·
Gearing is all linear. 165mm cranks are 5% shorter, dropping 2t is 7% lower and 4t is 13% lower. If your current chainring works on the new cranks, I'd try it out first and see if you even notice that you need lower gearing. If you do notice and want to change, then a 28t will be the ticket. This is assuming you already have the "ideal" gearing for your riding already.
Thanks! How does one know if they have the ideal gearing?

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If on your climbs, you are in the lowest gear and can't push hard enough to keep spinning the pedals at a normal speed, you need lower gearing. If you are on your fastest section that you pedal (maybe a slight downhill, flat hard-pack section, etc) and you are spinning so fast that you can't spin faster (like you're spinning too fast, not that it's too hard to pedal) then you need higher gearing. If both is true, you need wider range. Just feel it out on your normal rides and see whether you'd want higher or lower gearing.
 

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Thanks! How does one know if they have the ideal gearing?

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You use the whole cassette and are (reasonably) satisfied with the 'ends'. If you're not using the biggest cog, you can bump up the front ring 2 teeth, and Vice-versa. If you stall out on the big cog, drop 2 teeth
 

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Then you're probably good.

I found myself on a 32x50, which was a little too spinny for anything but the most "im literally dying" recovery spin, however the next cog down was a 42, which wasn't always enough for steep climbs... going to a 34t front was the magic middle-step. 34x50 is good for most climbs here in PHX, and I actually use all 12 gears now.
 

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My personal experience was I didn't need to drop ring size as I dropped crank size. The loss of leverage in the crank is made up for in the biomechanical advantage/efficiency with the smaller leg angles. That was my experience dropping from 175 to 165. I did drop chainring size when I went from 165 to 155 moving from a 34t to a 32t oval but that was because of chainring selection more than because I thought I needed to.
 

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Mechanically speaking, a shorter crank arm makes for a taller gear, somewhere in the neighborhood of 2t per 5mm length. Your max possible MPH at your max cadence will be higher at a smaller crank size.

If you don't feel out of gears in the granny gear, and didn't need to sprint faster, then no worries on changing crank size up or down.

 

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Always in the wrong gear
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If you don't feel out of gears in the granny gear, and didn't need to sprint faster, then no worries on changing crank size up or down.
Did you mean chainring size?
Nobody (except the occasional singlespeeder) changes crank length for difference pedal cadence on an MTB. Every case I've ever heard of is due to knee pain and/or pedal-to-ground clearance as new bikes seem to be designed around a very low BB.
 

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Like the post says I'm going from 175mm to 165mm cranks. Was running a 30T chainring before. Should I go down to 26 or 28T now?

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On one bike I went from 175 to 165 and 32 normal to 32 oval at the same time. Felt and still feels perfect.
Second bike was 175 to 170 and stayed with 32 round. It's perfectly fine but I find that I like the oval on the other bike and will try that next on #2.
 

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Did you mean chainring size?
Nobody (except the occasional singlespeeder) changes crank length for difference pedal cadence on an MTB. Every case I've ever heard of is due to knee pain and/or pedal-to-ground clearance as new bikes seem to be designed around a very low BB.
Add hip pain to reasons why for shorter cranks.
 

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Add hip pain to reasons why for shorter cranks.
For sure. any joint pain is reason enough.

I've read a few things that seem well founded in science that suggest we're ALL on cranks that are too long, biomechanically, when you take into account MTB geometry and suspension things. That there's some good reason to suggest 155-160mm cranks are appropriate for anyone under 6-feet tall.

Personally, I'm on 170's, and would happily go 155 but there's some (complex explanation) weirdness with it requiring my dropper post to be pushed up too tall. My bike was designed with a dumb bend in the seat tube, and forces me to use a OneUp V2 180mm due to its very short max insertion. Since I can't buy a longer post, short cranks means I raise the post up for leg extension, but my seat is 15mm higher ad full drop.
 
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