Crank Length Debate

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Posted: May 7, 2022 at 8:49 Quote
Should crankarms be proportionally sizes on trail bikes?

Are 165mm cranks just as efficient on the way up as a 175mm?

Are we missing out in mtb not using 172.5 and 167.5mm lengths?

Do you even care about the crank length on your bike?

Posted: May 7, 2022 at 10:33 Quote
I don’t see an issue with proportionally sizing cranks, but “ideal” crank length is highly dependent on multiple variables and ultimately rider comfort and efficiency boils down to preference and feel as much as any concrete factors.

Steve Hogg, Lennard Zinn, and many others have beaten this conversation to death as far as efficiency goes, and the answer seems to always be “it depends.”

I’m an ex-roadie of the extremely fit variety, and I have gotten on just fine with everything from 170mm-180mm over the years. I’ve even raced a stage race on 175r/172.5l when a power meter crapped out. By the end I couldn’t tell difference.

On the road I prefer 172.5mm even though I’m 188cm. Mostly because I ride in a very low position and they give me just a touch more room for hip articulation and keeping my chest open. This is on a 56cm frame with a -17x130mm stem.

My gravel cx bike is a 58cm and I’m currently running a longer 175mm crankset simply because it came with the bike. In a more neutral, balanced position it works just fine. I run the same on my trainer bike because aero is irrelevant and I’m not in an extreme hinged position during hard efforts. And, most importantly, it’s what I had lying around.

My trail bike is currently set up with 170mm but I’ll probably go 165 or even 160 in the future. My frame has a relatively low bb and I wouldn’t mind the extra clearance in certain situations.

On the road I’m a bit picky, but I’m also spinning for hours on end and often at 300+W. Everything needs to be dialed or discomfort and fatigue set in rather quickly hindering performance or, even worse, causing injury.

Tldr: Pedaling feel and efficiency is obviously of importance in mtb, but with the on/off nature of trail riding I can’t much more than the most marginal gains within the 10-15mm of variance for the vast majority of riders aside from those with extreme physiologies or bike setups.

Posted: May 8, 2022 at 22:37 Quote
I ran 165mm on my trail bike for a week, felt lime I had less leverage, put my 170s back on when I couldn't stomach the idea of a particular hard ride on the 165.

Posted: May 9, 2022 at 12:03 Quote
It is, unfortunately, a personal preference. I put 165mm cranks on my bike because I have a low bottom bracket. I'm also spastic and tend to mash on my peddles when going up steep hills; which causes the wheel to spin out. The short cranks helped that a bit.

Posted: May 9, 2022 at 14:20 Quote
I've got different crank lengths on each of my bikes (road, CX, MTB, and track) and I literally cannot tell them apart

Posted: May 9, 2022 at 18:33 Quote
If you have bad knee or hip problems you'll find that shorter cranks are a lot easier on your joints.
I personally ride a 60 cm road bike, roughly the same size mountain bike and I find 170mm to be the longest I can tolerate. Of course, I've undergone 14 knee surgeries and I have one artificial knee.

Your mileage may vary.

Posted: May 9, 2022 at 19:25 Quote
Personally I'm a big fan of 165. Much less pedal strikes (very low BB) with the added benefit of much less knee and hip pain, so that was a bonus.
Do I feel like i have less leverage.... Maybe?? But then I just change gears.

Posted: May 11, 2022 at 14:33 Quote
There's quite a bit of info out there on this topic (including a recent article from Seb on this site ) and it can be a bit of an interesting blackhole to fall down.

The very high level summary is that the crank length doesn't make too much of a difference until you get to the extremes (120mm and 200mm for an average height person). You will pretty naturally adjust your cadence and have a similar/same output power. With shorter cranks* you will have more clearance to avoid rock strikes so thats a benefit. You might need to downsize your chainring at the same time if you are already using your lowest gear a lot.

*some brands just move the hole on the crank up and leave the actual crank length the same. You wont the same benefit of extra clearance (and unnecessary weight) if this is the case so do a bit of comparing before purchasing.

Posted: May 12, 2022 at 5:32 Quote
Swapped from 175 to 170 last week. My bb is obscenely low (27.5+ in a 29'er frame) so I needed more pedal clearance.

Couldn't really notice the change in leverage. Did notice that I was spinning smaller circles, but barely. Definitely noticed the pedal clearance. First thought was, "should have gone with 165's"

Posted: May 16, 2022 at 13:54 Quote
I can't tell much difference pedaling going from 175-170. It does absolutely have fewer rock strikes on the 170, but one other thing is that you end up changing your seat position a little higher for your extension to be the same. So maybe I should have a longer dropper? but I think my bar was slightly low for the seat before but close enough that I didn't think much about it, but I ended up getting a bar with more rise.

I don't know, it's fine- not the best money I ever spent on a bike and not the worst.

Posted: May 17, 2022 at 15:38 Quote
I went from 175mm that were too long to 165mm (that were really hard to find). I have not regretted it for a second. I did not feel I lost any efficiency uphill, and in fact I can now climb out of the saddle way easier than I could before, and the reduction of pedal strikes was shocking.

This was a helpful article for me:

As was Sheldon Brown's discussion on it.

Posted: May 18, 2022 at 6:41 Quote
just went from 175 to 165. Can't really tell the difference expect for way less rock strikes.

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