Crank Length Debate, 165 vs 170 vs 175 for all mountain riding

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Crank Length Debate, 165 vs 170 vs 175 for all mountain riding
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Posted: Feb 19, 2020 at 9:47 Quote
What are your thoughts on crank length for all mountain riding? Is the trend going to shorter cranks? Standard seems to be 175 but many companies are opting for short cranks. Canyon specs the Strive with only 165 mm cranks while Transition specs 170mm across most of there bikes. The Giant Reign is size dependent for crank length. Should crank length change based on size of bike like chainstay length to maintain balance? Let me know your thoughts! Shorter cranks are stronger, lighter, better clearance, and arguably faster acceleration.

Posted: Feb 19, 2020 at 9:54 Quote
I have a strive with 165s and a couple fixed gear road bikes both with 165s. They feel much more natural pedaling to me. The extra ground clearance is a great benefit too. For those thinking of switching you'll probably want to go down in chainring size at the same time.

Posted: Feb 19, 2020 at 11:18 Quote
175 mm was the standard for many years. Lengths are trending shorter.

• Proportional to rider: Yes, but that assumes 175 (or 170) mm is the ideal reference size, which is not certain, so let's ignore the issue of proportionality.
• Efficiency: There's evidence that people adapt to whatever size they use, so it's difficult to determine what's most efficient, as it could take years of adaptation for each size, during which time the rider's physiology would change. My own experience is that I think I'm more efficient at 175 mm, but I interrupt my cadence less frequently on shorter cranks, making them the better overall.
• Stronger: The difference is trivial.
• Lighter: Even more trivial. The weight difference represents a fraction of a second over an entire ride.
• Acceleration: The tiny differences in inertia are less than the differences in efficiency and, more importantly, whether the rider can continue pedaling without pausing to avoid obstacles.

It's all about ground clearance. If pedal strikes are essentially never an issue for you, choose whatever feels best. If pedal strikes are a concern - even a little bit - I recommend 170 mm. For me, 165 mm feels less efficient; maybe I could adapt to it in time.

Posted: Feb 19, 2020 at 11:20 Quote
I’ve come to believe kinematics plays a role in this debate as my short legs prefer 175 while my taller bud prefers 170. I ponder if one were to map out our geometry would the result show the stroke as much the same for efficiency and effort.

Posted: Feb 19, 2020 at 12:03 Quote
undescended wrote:
I’ve come to believe kinematics plays a role [ ... ]

Can you expand on that?

Posted: Feb 19, 2020 at 12:33 Quote
R-M-R wrote:
undescended wrote:
I’ve come to believe kinematics plays a role [ ... ]

Can you expand on that?

Maybe assuming longer legs have more "leverage" than shorter legs thus need less leverage (shorter cranks) to accommodate?

Scientific evidence that crank length doesn't matter much.

https://www.tandfonline.com/doi/abs/10.1080/02640414.2016.1215490?journalCode=rjsp20

Posted: Feb 19, 2020 at 12:41 Quote
I was thinking undescended meant the kinematics of the bike.

There have been many studies on crank length and efficiency. The results are usually, if not always along the lines of: differences are small to negligible and riders adapt over time, causing a shift in their peak efficiency.

The idea of long legs intrinsically benefiting from short cranks doesn't make sense. If it were true, there would either be no limit, in which case infinitely tall people would need infinitely short cranks and infinitely short people would need infinitely long cranks, or there would be some local maximum for tall and short people, the biomechanical and physiological explanations for which seem impossible.

It seems plausible that flexibility and other factors would be enough for someone slightly shorter to prefer slightly longer cranks, as the differences in crank lengths and, presumably, leg lengths are modest.

Posted: Feb 19, 2020 at 12:51 Quote
I have also heard that longer cranks can cause pain in knees but that might also be due to saddle height issues. Shorter cranks do allow for taller dropper posts. Sometimes one cm makes all the difference.

Posted: Feb 19, 2020 at 13:01 Quote
rum501 wrote:
I have also heard that longer cranks can cause pain in knees but that might also be due to saddle height issues.

More generally, it's hip position. Usually too low, rarely too high, often too far back, occasionally too far forward. When the hip position is wrong, the greater range of motion from long cranks makes the problem worse.


rum501 wrote:
Shorter cranks do allow for taller dropper posts. Sometimes one cm makes all the difference.

True. The first step should be a dropper post with less stack, then a saddle with less stack. If that still isn't enough, shorter-than-ideal cranks are a reasonable solution to getting that precious drop.

Posted: Feb 19, 2020 at 13:07 Quote
R-M-R wrote:
...
It seems plausible that flexibility and other factors would be enough for someone slightly shorter to prefer slightly longer cranks, as the differences in crank lengths and, presumably, leg lengths are modest.
Yea, you got it...

Posted: Feb 19, 2020 at 22:10 Quote
Personal experience here - spent a season on 175mm and then 170mm. I like them both but prefer 175 based on my riding cadence (slower)

170 - clears rocks. Spin up to gears instead of mashing. Appears to be better for the knees. Helps if you’re sizing up a bike with a tall stack

175 - better for mashing on the pedals. For example, you’re cruising an unknown trail and a hill comes up out of no where. With 170, I’d have to downshift to get up to cadence. With 175, I can mash (more leverage)

There’s a whole science behind it backing it with data, racers, ideal cadence that builds muscle etc. etc.- don’t want to get into it but I prefer 175 for my riding ! Try em both!

Posted: Feb 19, 2020 at 22:56 Quote
https://www.sheldonbrown.com/gear-calc.html

If you play around with this gear ratio calculator, the difference between 170mm vs 175mm cranks is negligible.

I tend to agree that its basically an option exercised (in MTB) due to ground clearance.

Posted: Feb 21, 2020 at 8:26 Quote
I've been considering the whole "ground clearance" argument. 5mm is a very small amount. Even 1cm seems like too small of an amount to really make much difference in actual use. It seems like a rider would just adapt their riding and get a little lower, thereby negating that small gain. Maybe the "ground clearance" argument is just an little afterthought of an argument that people seize onto.

I could certainly see shorter cranks being an ergonomic necessity for those without the required range of motion for the extra length. A recent knee injury followed by a bike rebuild pointed out to me that seat height is actually important by it being a tiny amount too high and making my knee hurt. Dropping the height just a bit totally alleviated it. That was a first for me....it's hell getting old. As seat height and crank length are related for knee and hip range of motion, I could see shorter cranks helping as well. It does make me wonder how many of the people it helps were just running their seats a bit too high to begin with though.

Posted: Feb 21, 2020 at 15:31 Quote
Explodo wrote:
I've been considering the whole "ground clearance" argument. 5mm is a very small amount. Even 1cm seems like too small of an amount to really make much difference in actual use. It seems like a rider would just adapt their riding and get a little lower, thereby negating that small gain. Maybe the "ground clearance" argument is just an little afterthought of an argument that people seize onto.

Not in the slightest. I think you underestimate how close we come to rocks and trees. A couple of examples of experiences almost universally shared among the groups I ride with:

Cranks: A few millimeters of ground clearance makes the difference between pedals strikes being rare and being many times each ride. The only way to adapt is to interrupt the cadence, which is lame. Ground clearance is 10 mm or less on countless occasions each ride and 10 mm doubles the clearance during each of these events. The opposite is true for biomechanics. In that case, 170 mm is only 2.9% shorter; that's the scenario in which it's nearly trivial.

Handlebar width: When we all rode 720 mm bars, we could ride a couple of notoriously tight trails and have zero issues with tree clearance. At 780 mm, we all started clipping trees. A few of us now ride 800 mm and 810 mm bars and we can't even ride those trails. Multiple crashes per ride, every time, and we have completely stopped riding those trails. It's not a matter of adapting; the gaps are simply beyond what's possible - in many cases, the gaps are narrower than the bars, requiring a micro manual and quick flick of the bar to get it through on the diagonal. No one is their right mind would attempt that at speed.

Clearances are tighter than you realize.

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