Edumacate me on crank length, please

[Tickdoc]

Every bike I've ever owned has been a 172.5 crank armed bike.

So that is the average, right?

What happens if I ride a 170 cranked bike?

What happens if I ride a 175?

Is there a pronounced effective range of gearing change, or just easier to turn small, harder to turn long effect that shows up over long rides?

Fwiw, I have a 31" inseam.

I'm curious if anyone here has experimented with crank arm length to any great degree and if it ever did anything to sway them from the norm.

[Louis]

For some crazy reason (swapped out by the LBS?) the first bike I ever bought new years ago, a 62 cm Trek 410, came with 170mm crank arms. At the time I was too much of a noob to realize that for that size frame 170 was most likely way too short.

After a few years of riding it I put some 172.5 cranks on and didn't really notice much difference. After a year or so I put 175's on and at the point definitely noticed a change. Not really on the flats, but on rolling hills it was easier to "make it over the top" without having to downshift. I've stuck with 175's ever since.

Good luck.

[makoti]

I think it's mostly pedaling style. If you spin more, the shorter cranks are better. More of a masher, longer. This assumes that, like above, you are not riding a very large frame with very short cranks. I ride 172.5 because everything about me is medium, which is why I can never get any good deals on clothing. ;-)

[Drmojo]

170 for fixed gear
Better to spin
172.5 for road
175 for my mountain bike
I am med also

[jmal]

For your inseam, it makes no difference at all.

[ceolwulf]

GCN has had a couple interesting videos on this lately.

https://youtu.be/eMAxH_Ud8YE

https://youtu.be/vUygkHlcVMQ

I'm 6' (182cm) tall and the conventional wisdom would probably have me on 175s. I'm used to 170s now and won't go back to a longer crank. I tried when I built my TT bike, I put 175s on it and the ache in my knees after a hard effort told me to change ASAP, which I did, and no more knee trouble.

The extra leverage in the mid stroke of a longer crank is something the gearing should compensate for. Of course the distance the foot travels will be more so cadence will be slower.

My thinking has always been, when working out on the leg press sled or doing squats, the less range of motion from the top that you go through, the more weight you can do; so it stands to reason the less you bend your legs, the more force on average you can exert. So shorter cranks should be faster. Feel free to shoot this down but it works for me ;) And of course the differences in leverage, gearing, everything could all cancel each other out. At least that seems to be what the test findings, as much as there are any, support - there isn't much, if any, difference in power between crank lengths.

Bottom line I think is to choose crank length for comfort and for what works with your position.

[rustychisel]

Road bikes: 172.5mm, 170mm, 170mm compact cranks

Fixed gear bikes: 170mm, 172.5mm, 167.5mm

I notice the difference and adjust to it in about 5 minutes of riding.

Conclusion: ride what ya got

[spartanKid]

Here is a relevant recent thread: http://forums.thepaceline.net/showthread.php?t=146904

[cachagua]

From that other conversation:

If these cranks are for a TT bike, consider shorter cranks as they will allow you to get your torso lower.

Is this right? I think a shorter crank will put your torso higher. If we assume your leg should straighten out to the same length at the bottom of the pedal stroke, no matter what crank length, then the seat height required to accommodate that length (and hence, the height of your torso) will be higher with a shorter crank, not lower.

I.e., if you switch from a longer crank to a shorter one you have to raise your seat by the difference to achieve the same leg extension. (Or if there's a piece of this I'm missing completely, set me straight.)

Interestingly, the differences we're talking about in crank length are in the same range as the differences in BB drop that we're quite sensitive about in frame design. Does a different crank raise or lower your center of gravity and affect the bike's handling like a different BB drop does? If all other factors remain the same do lower BBs and shorter cranks offset each other, w/r/t this effect?

Another thing I've wondered: pedal clearance is not what it used to be. I haven't measured precisely, but there's no doubt you can lean your bike over considerably farther before you hit a Speedplay X on the ground than you could with a Campy Nuovo Record pedal. What does that do to questions of crank length (and for that matter, BB drop)?

Then, you got your fatter tires... bikes is complicated.

[rustychisel]

From that other conversation:



Is this right? I think a shorter crank will put your torso higher. ....

That there thinking is predicated on longer cranks forcing your legs into a greater range of motion, effectively rotating your hips more and pushing the thighs up into the diaphragm.

It's one of the reasons TTers rotate the hips down and forward. some can do it, others not so. Effectively, much is made lately of keeping the power yield high by NOT adopting such a position, keeping the torso higher but presenting a more slender wind profile (elbows tucked, hands in front of face)...

[Peter P.]

I have a 32" inseam and size 10 feet for my 5'6" height.

I've ridden 170mm cranks forever.

When I bought my first mountain bike back in 1984, the standard for mountain bikes was 175mm cranks. They drove me nuts. I do a lot of road riding on my mountain bike, especially to and from the trails, and the acceleration was painfully slow.

So when I bought my second mountain bike I asked for 170mm cranks, and I've been happy ever since.

The general rule of thumb is the longer the legs, the longer the cranks. Lennard Zinn suggests crank length to be 21-21.6% of leg length. I think experimentation might be the best way to find out what works for you.

[cachagua]

No, not talking about your leg in motion, only in one position. What I was trying to say was this: picture your leg when the pedal's at its lowest point -- you're leg's "straightened" out. Now picture varying your crank length -- when you shorten the crank, your leg moves up; lengthen the crank and your leg moves down. And your seat moves up and down to match, and your pelvis, and so on...

That clarify it? Changing crank length in isolation raises the rider when you go to a shorter crank, and a longer crank lowers the rider.

Or??

[rustychisel]

Ah, I see what underlies your question now, so, I agree with you. Shorter cranks means compensating for saddle position, but bear in mind it's a matter of 2.5mm or 5mm... and that measure is about how much lower the saddle on one of my fixed gear bikes is to assist a slightly differing riding style.

It is, incidentally, why I hold that saddle height should always be measured from centre spindle of pedal to top of saddle plane, parallel to seat tube, based on the 109% method from errr, back in the days of Hinault and Lemond.

[cachagua]

Right, and thanks for the reminder, now I'm wondering if the last couple times I measured, I cheated and went from the BB spindle center to the seat... shouldn't make a diff, though, since all my cranks are the same length.

But re-reading now in the light of your comment, I might have misunderstood the post I quoted. If your chest -- your upper torso -- is low enough that your knees hit it at the upper part of the pedal stroke, then yes, totally, a shorter crank will reduce that interference.

That is how long since I've ridden a time trial -- I'd forgotten the annoyance of kicking my ribs. (Ha ha, that's how long since I had ribs.)

[CampyorBust]

I can feel it. 170mm only for me, it seems to line up the ball of my foot with the pedal axel the best. I have one bike with 172.5 cranks it doesn’t feel right.

[oldpotatoe]

Every bike I've ever owned has been a 172.5 crank armed bike.

So that is the average, right?

What happens if I ride a 170 cranked bike?

What happens if I ride a 175?

Is there a pronounced effective range of gearing change, or just easier to turn small, harder to turn long effect that shows up over long rides?

Fwiw, I have a 31" inseam.

I'm curious if anyone here has experimented with crank arm length to any great degree and if it ever did anything to sway them from the norm.

Few years ago a lady brought in a DeRosa for service, really nice 8s Campagnolo...right crank arm was 170, left was 172.5..she didn't even know it(been riding it for years).



Put 3 people in a room and ask about crank arm length, get 4 opinions..It's like saddle comfort, gear ratios, handlebar shape preference..

Remember talking about 2.5mm..a circle that is 16mm larger..not much.

First bike had 170mm, I 'upgraded' to 172.5...didn't really feel any difference. I use 172.5 now..

[Hilltopperny]

I use 172.5 on all of my road bikes and my gravel/adventure around town bike is 170. I can feel differences but they are also very different bikes. I ride lots of rolling hills on the roadies and the gravel bike is usually ridden in flatter areas with more terrain changes but not many climbs or decents.

[Tickdoc]

and what do we have here?
https://41.media.tumblr.com/743623ea2e359b11404af3a40fcebaef/tumblr_o0nlfn22Bd1qgwe7to1_1280.jpg

[Ti Designs]

and what do we have here?
https://41.media.tumblr.com/743623ea2e359b11404af3a40fcebaef/tumblr_o0nlfn22Bd1qgwe7to1_1280.jpg

A crank with a pivot along it's length. I've made something similar for someone who's knee replacement left them with very limited range of motion. The pivot subtracts from the needed range of motion going over the top, sadly it doesn't add any length to the lever are until it's at the bottom where it does no good.


On the subject of optimal crank length, Pioneer in their force vector power meter has come up with the perfect tool for finding the right crank length, but it costs $1600 per guess, and you would need at least two for comparison.

Some shop should have a program where someone can sit in with an experienced fitter for a while, like they do at teaching hospitals. Most of fitting is simply understanding ranges of motion which so many people are more than happy to ignore or overlook. If your body is manipulating something it's called exercise - that's good. If something is manipulating your body, and your body is fighting it, it's called injury (over time) - that's bad. The crank can exceed a riders range of motion in any direction. Too far back can exceed range of motion at the knee, too far forward will tug on the foot causing a pull reflex of the anterior tibialis. Too high and it'll exceed range of motion of the knee or hip, pushing the hip up and abusing the SI joint... The thing is, I see this stuff all the time. People come in for a fitting 'cause someone told them to, not because they know there's something not right. A little tiny tug here or there isn't even noticable before I point it out, how could it be a problem? I'm quick to point out that it's a little time tug, 80 times/minute for a few hours. A jack hammer with a 1" piston only puts 90 pounds of force on the cutting tip. It's the 8,000 times per minute that's doing the damage.

[tommyrod74]

GCN has had a couple interesting videos on this lately.

https://youtu.be/eMAxH_Ud8YE

https://youtu.be/vUygkHlcVMQ

I'm 6' (182cm) tall and the conventional wisdom would probably have me on 175s. I'm used to 170s now and won't go back to a longer crank. I tried when I built my TT bike, I put 175s on it and the ache in my knees after a hard effort told me to change ASAP, which I did, and no more knee trouble.

The extra leverage in the mid stroke of a longer crank is something the gearing should compensate for. Of course the distance the foot travels will be more so cadence will be slower.

My thinking has always been, when working out on the leg press sled or doing squats, the less range of motion from the top that you go through, the more weight you can do; so it stands to reason the less you bend your legs, the more force on average you can exert. So shorter cranks should be faster. Feel free to shoot this down but it works for me ;) And of course the differences in leverage, gearing, everything could all cancel each other out. At least that seems to be what the test findings, as much as there are any, support - there isn't much, if any, difference in power between crank lengths.

Bottom line I think is to choose crank length for comfort and for what works with your position.

I'll take the opposite tack, based on personal experience as well.

I think it should be based on preferred cadence and range of motion/flexibility.

I went from 175 on all bikes to 170 to try and open up hip angle, lower front end, etc - based on studies I'd read.

I found that, personally, I can't hang out at 90-100 RPM comfortably. I gravitate to 75-85 RPM and do very well in that cadence range. I also have a long-ish stride when running and a relatively slow cadence - but am a decent runner (for a cyclist).

The 170 crankarms forced me to either spin a lower gear at a higher cadence than I was comfortable with, or put in more force per pedal stroke in the same gear as before (causing knee issues).

Standing climbing (which I do a lot at 62-ish KG) was awful - awkward, felt as if the pedal was dropping out from under me too quickly to use my body weight to drive the pedals.

Might also be related to less fast-twitch (definitely a TT/climber type) vs. fast twitch - less overall muscle mass, meaning I need a longer crankarm (more leverage, higher pedal speed) for a given gear and cadence?

Anyway - for me, at 5'10", 32" inseam, 170mm was too short and 175mm feels awesome. YMMV.

I can still run >150mm of drop and hang out in the bar drops for an entire crit, so the longer arms don't impair that.

[carpediemracing]

I have a sub 29" inseam. I started out on 170s briefly, got 167.5s as soon as I could (1990?), had to go to 170s due to 167.5s wearing out (1997), then went to 175s in 2003 after an A-B-A experiment (170-175-170).

Based on conventional wisdom I'm losing speed so I tried to go back to 170s in 2009 and 2011. I moved back to 175s and immediately found improvement in results. SRM from 2008-present and power seems very similar so it's not peak power, it's something to do with the cadence/etc. I spent a lot of time adapting to whatever new crank length (typically change length in Oct-Nov for the following season).

For what it's worth my best years since 1992 (on 167.5s) was 2010 (175s) and 2015 (175s). I upgraded to Cat 2 end of 2010. I had similar results in 2015 but didn't earn enough points for an upgrade. I earned every point I ever earned in field sprints.

Part of it is I think that I'm losing some pedal speed as I approach 50. Amazing that I'll be 50, but yeah, I will be. I'm a solid 20% slower in top speeds compared to 25 years ago.

I compared 170 vs 175 for top speed, 175 is better for me every time I've tested from 2003 on, even with acclimatization to the 170s for many months (10 months or more). I'm consistently 2-3 mph faster on 175s. Since I have Cannondale SI cranks I can change just the arms and test. It's not blind to me but I want the 170s to be faster.

I'm still trying to follow conventional wisdom and just recently put 170s back on the bike. I noticed a couple things right off, on the trainer.

1. I can't go many more pedal revolutions in a sprint. 175s give me more leverage so I can use a bigger gear and therefore go farther. 170s force me to use a slightly smaller gear (1 cog) and I blow up a bit earlier, mainly due to not being able to turn the same gear.
1A. Related, I can't rock the bike on the trainer. I think if I could I'd be better on the 170s. I sprint out of the saddle, and on my trainer I can't replicate the motion.

2. My peak power is a bit higher. I think this is because I can punch down on the pedal a bit faster, due to shorter lever. It's the same reason I like narrower bars (38 cm equivalent of Zipp SL80 or something), because I can input a much sharper motion. I immediately saw about 100w peak higher with the 170s (1200w vs 1100w right now). My sustained power is so much lower than 175s that the 170s averages out to be about 50w lower for 23 seconds (my standard sprint reference since I started on Zwift; before it used to be 18 seconds). Right now I'm doing 800w for 23s instead of 850w for 23s on 175s (and 950-980w in the summer, on 175s). It may be that I'm still losing fitness but a 50w drop is pretty big for just a few days, whereas it took me months to drop from 900 to 850w.

I'm trying the 170s because higher saddle position, less forward. I baseline off the bottom and front position of cranks, so for 175->170 it means raising saddle 5 mm and moving it back 5 mm (due to 5mm less forward pedal axle at 3 o'clock position). High saddle, less forward, should be more aero for my back (which is not flat, points up a bit, and it allows me to breathe better) and it also gets me within range of having a UCI legal saddle position (not planning on doing UCI races, just want to have a bike that isn't several centimeters outside of UCI spec).

I also went to the Adamo saddle (twin tusk) to push the legal nose back over 5 cm. I'm currently about 5-10 mm too far forward for a UCI saddle (used to be 70? mm too far forward), and my bars are about 10mm too low for UCI likes. I think the rest of the bike is UCI okay.

[carpediemracing]

What I would do is get two sets of otherwise same cranks in different lengths. 175, 170. Same BB would help so you can just switch the cranks without messing with derailleurs etc.

Then do some runs on each crank.

If you have a powermeter you'll have some concrete data to analyze. Otherwise do some Strava segments (even if you make them yourself), different lengths, different speeds (fast, medium, slow).

I had, at the time, Daytona group on my bike. In a big effort I went faster on 175 cranks on my mountain bike with 2" knobbies than my 170s on my road bike.

I bought 175 Daytonas, stuck them on my road bike, and immediately saw a huge jump in speed.

That was late 2002. Stayed with them until 2008, when I tried for a year to use 170s ("because I was fit again and therefore should be able to turn a 170 faster"). Total mistake. 175s in 2010 and I was flying again. 2011 I tried 170s again, thinking I'd made mistakes in 2009. Again, horrible. 2012 175s, fine.