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Shorter cranks on MTB

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Shorter cranks on MTB

Old 01-07-19, 08:12 PM
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u235
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Shorter cranks on MTB

What are any negatives to a shorter crank like a 165 crank on an MTB? I know there is a mathematical and logical difference but is there really a real world noticeable difference in torque capability compared to maybe just using a lower gear on an occasional steep sloppy climb? I switched to 165 on my gravel bike a while ago for some knee problems and with a higher cadence worked out all positive. I still have a 175 on my XC bike and it now feels awkward. Short of retraining myself back to a variable speed masher which eventually failed with me before or getting used to spinning a 175 like my 165, seems like an equal 165 MTB crank is the way to go provided the torque difference is tolerable. It is an XC and I mainly use it like they are marketed, 30+ mile trail and mixed technical rides with climbs.

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Old 01-07-19, 09:48 PM
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Do you have shorter legs? Shorter cranks may make sense. Another advantage on a MTB would be less pedal strikes.

Crank length like most items on the cockpit have to be personal choice. My cockpit won't suit another rider whose cockpit I may hate. ... If it is something you are used to on one bike, it will likely work on another.
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Old 01-07-19, 11:49 PM
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Most MTB's I've dealt with have 170's. I would not hesitate to put 165's on any of those bikes. I just don't have any ...

The stroke length will be less, so a bit less torque. But if you go down a tooth or two and go to Oval chainrings you should get most of it back

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Old 01-08-19, 03:40 AM
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IMO, it's down to the character of the trails you're riding.
Where I'm at, and the riding that I do, it's very common with short, sharp "humps" along the track.
Too small to be called climbs, too big to be jumps. Longer cranks makes it easier to simply muscle past these instead of downshifting.
It's not ride-stoppingly important, but there is a difference.
I too have gone to shorter cranks for a more knee-friendly pedalling style, and I manage.
There are other ways around. If you're familiar with the track, you can either accept to shift a little more frequently or try to carry a little more momentum into the hump to get over.
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Old 01-08-19, 04:54 AM
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No drawbacks whatsoever. And any "positives" would likeluy be purely personal and not quantifiable ... you might "feel" a little different and whether you "fee;" better or worse ... it wouldn't be measurable. petty much, try and see. It could not possibly harm you in any way i could imagine, and it could not possibly harm your performance.

In a lot of ways, crank length is (I believe, I do not Know) mostly about comfort. I like 175s on my bikes, but I have abnormally long legs. I ride 165s on one bike and it is fine. after a minute I cannot tell, if at all. (When I had 165s on All my bikes except one, i could tell .. . for about 45-75 seconds.)

I have a very hard time believing there is a dramatic increase or decrease in torque based on the half-inch difference.
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Old 01-08-19, 07:23 AM
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Originally Posted by dabac View Post
IMO, it's down to the character of the trails you're riding.
Where I'm at, and the riding that I do, it's very common with short, sharp "humps" along the track.
Too small to be called climbs, too big to be jumps. Longer cranks makes it easier to simply muscle past these instead of downshifting.
It's not ride-stoppingly important, but there is a difference.
I too have gone to shorter cranks for a more knee-friendly pedalling style, and I manage.
There are other ways around. If you're familiar with the track, you can either accept to shift a little more frequently or try to carry a little more momentum into the hump to get over.
This is my dilemma and why I asked. Thanks for the experience. Long stetches of uneventful trail between singletracks. I have Deore XT 8000 2x and I see bare arm 8000-1 realtively cheap and I can transfer over my rings.
When I went from 170 to 165 on my gravel/road bike I was surprised how different it was at first. Now that I got the short crank pedalling concept down, this should go well too. I've ridden my gravel bike on some of the same routes as my XC when it's dry with some go arounds.

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Old 01-08-19, 11:09 AM
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How much spare seat post length do you have. If you go from a 175 mm crank arm to a 165 mm crank arm you will probably need to raise your seat by roughly an inch.

You can put me down as somebody who has benefitted from shorter cranks and less leg range of motion following a leg injury and surgical repair.
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Old 01-16-19, 02:38 PM
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New cranks installed. no rides yet but in the house spitball adjustments put my seat 2cm higher as noted by Retro Grouch and now I have a feeling I'm going to need a shorter stem but we'll see after a few rides. I have a 6 degree 100 on there now so I have a lot room to go shorter if needed.

Note about the cranks..
I have Deore XT 8000 2x crankset. Shimano sells a 8000-1 "1x" crankset with no chainrings and no BB and what I bought, saved me a lot of money. It is the same exact crank as the 8000-2 2x cranks that comes with rings. Just moved my rings over. I saw references to this but some people said they were different and some said they were the same. They are the same.

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Old 01-16-19, 05:50 PM
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Basically you don't know if you don't try. The wife is short at 5' 3-1/2". She had always been on 170s and never thought about it. Got her 165 and she did not notice a thing. Got her some real shorties as an experiment and to deal with a toe clearance issue. 150mm. She did a 30 second test ride and came back said we needed little cranks on her other bike too. Then she disappeared for an hour or so and confirmed she wanted shorter all around. Hills that were walking hills for her are now ridden and ridden in higher gears. She doesn't need tiny gears any longer. No way we would have known this if the experiment were not made.
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Old 01-16-19, 10:22 PM
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I made an interesting discovery not long ago when I was playing with some configurations in Sheldon Brown's gear calculator. Changing crank length by 5mm has no effect on the final number.
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Old 01-19-19, 01:29 PM
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Originally Posted by Retro Grouch View Post
How much spare seat post length do you have. If you go from a 175 mm crank arm to a 165 mm crank arm you will probably need to raise your seat by roughly an inch.

You can put me down as somebody who has benefitted from shorter cranks and less leg range of motion following a leg injury and surgical repair.
Hi, why wouldn't the change in seat height be equal to the crank arm change itself, 10mm or so, rather than 25mm you suggest? When I set my saddle heights on bikes with different length cranks I adjust by the change in the crank, which then keeps the measurement from the seat top to the pedal axle the same. I may be missing your point, however.
Thanks
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Old 01-21-19, 05:18 PM
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Brent Emery, fabled cyclist of 70s to some of us, pushes very short cranks, mostly to a triathlete clientele. And sells quite a few and has been doing so for years. He suggests putting saddle up 2,3,4mm. Before we read that my wife had done the same exact thing by seat of her pants after switching from 165 to 150. Ron Boi, also a well known coach, suggested up 3mm after he heard we had done this. YMMV but try a small adjustment before a big one.
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Old 01-21-19, 09:32 PM
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Shorter cracks mean knee friendly pedaling! So don't worry!
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Old 01-27-19, 06:56 PM
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Follow-up with the new cranks.
All positive. Did a 35 miles ride through thick and thin and no problems at all and no knee pain. Huge improvement powering along on the long and boring which I expected and even made my climbing and slogging up single track feel better, the opposite of what I expected. I didn't like riding that bike for more than 30-40 miles and used my gravel bike instead but I think it's an option now.

With the seat, eventually ended up maybe 7mm higher .

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Old 01-27-19, 10:32 PM
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There you go, win-win
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Old 01-28-19, 03:45 AM
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Originally Posted by u235 View Post
With the seat, eventually ended up maybe 7mm higher .
Exactly! If you shorten the cranks by 5mm then you need to raise the seat by 5mm, simple as that.
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