Does crank length matter?

[Ajax Bay]

"Does crank length matter?"

Is crank length choice to be made in relation to a rider's height?

Nearly all bikes are fitted with 170/172.5/175mm cranks. Much easier for the industry. But think how much variation there is in bike frame sizes and the height/inside leg measurement of riders. Now why is there so little range in crank length? Have a read of Steve Hogg's article (to which I linked upthread):
SteveHoggbikefitting.com/bikefit/2011/06/crank-length-which-one/
@Racing roadkill makes a good point about increased ground clearance pedalling at speed round corners.
By the way Spa Cycles (referred to in @glasgowcyclist 's thread) offer a range of crank lengths which are otherwise very difficult to obtain.

 

[Sharky]

Gearing is a combination of crank size, chainwheel size, sprocket size and wheel size. Change one factor and the other factors will need to change to keep to the same "leverage". "Gain" calculations help to compare this, rather than using "gear inches".

Smaller cranks give the impression that cadence is increased and it is, but as it is a smaller circumference, the speed at the pedal is the same.

Smaller cranks does allow the saddle to be raised and opens up the hip angle and can make it more comfortable and potentially more aero as you can bend forward easier.

Lots of research available on the web on the merits of shorter cranks and for what it's worth, I now ride with 150mm cranks.

 

[Paulus]

Good grief, what I thought might come up with a simple answer has bought up mathematical equations. I asked the question because on a couple of my bikes I have 172.5, and on a couple of others I have 175. I even have 170 on my really old Galaxy. The only difference I can make out is a slightly increased effort with the longer crank in a higher gear, but with the wide range of gears my bikes have, it is easy to find a gear to get the cadence that suits my pedalling style.

 

[glasgowcyclist]

"Does crank length matter?"

Nearly all bikes are fitted with 170/172.5/175mm cranks. Much easier for the industry. But think how much variation there is in bike frame sizes and the height/inside leg measurement of riders. Now why is there so little range in crank length? Have a read of Steve Hogg's article (to which I linked upthread):
SteveHoggbikefitting.com/bikefit/2011/06/crank-length-which-one/
@Racing roadkill makes a good point about increased ground clearance pedalling at speed round corners.
By the way Spa Cycles (referred to in @glasgowcyclist 's thread) offer a range of crank lengths which are otherwise very difficult to obtain.

I did notice that after I'd posted, sorry. However, using the formula quoted for inseam in inches x 5.48 = crank length in mm I get a recommended crank length of 175.36mm.

 

[ColinJ]

Good grief, what I thought might come up with a simple answer has bought up mathematical equations. I asked the question because on a couple of my bikes I have 172.5, and on a couple of others I have 175. I even have 170 on my really old Galaxy. The only difference I can make out is a slightly increased effort with the longer crank in a higher gear, but with the wide range of gears my bikes have, it is easy to find a gear to get the cadence that suits my pedalling style.

Ha ha!

I have a friend who is convinced that crank length is of utmost importance. I disagree, but I don't bother telling him that I have 175mm cranks on 2 bikes, 172.5 mm on one bike, 170 mm on another, and one bike has 175 mm right/170 mm left (after I replaced a broken left crank with a shorter old one that I had lying around)! It doesn't bother me at all. The only time I did notice crank length was on an old gym bike which had stupidly short cranks. Probably no more than 150 mm and they DID feel a bit odd, though I was still able to pedal the thing.

 

[MichaelW2]

For a medium-large rider the difference of 2.5mm is so marginal it hardly matters. It is a case of small tweaks to the cadence, gearing and saddle height.

The real question about crank height is what cranks to use if you are not medium/large sized. Small riders are all using big cranks and XL riders are all using small cranks. You have to look high and low to find a well fitting crank on bikes that are not m/l size.

 

[JhnBssll]

For a medium-large rider the difference of 2.5mm is so marginal it hardly matters. It is a case of small tweaks to the cadence, gearing and saddle height.

The real question about crank height is what cranks to use if you are not medium/large sized. Small riders are all using big cranks and XL riders are all using small cranks. You have to look high and low to find a well fitting crank on bikes that are not m/l size.

Exactly that. Regarding power output I believe they did a study that showed power output was only marginally affected by crank length as longer cranks allow higher torque but lower cadence and vice versa.

 

[jowwy]

Exactly that. Regarding power output I believe they did a study that showed power output was only marginally affected by crank length as longer cranks allow higher torque but lower cadence and vice versa.

It's not about power output though, it's about stress on the knees and comfort.......having used 175 down to 165 I always feel better on 165s and my knees do feel the difference

 

[JhnBssll]

Yeah that's my argument. It's entirely about fit and makes almost no difference to power output. Bikes tend to come with the shortest cranks suitable for the frame size so they fit the widest range of riders out of the box.

 

[booze and cake]

I've got 172.5 on 4 road bikes and 175 on one. I've also got 2 singlespeeds with 170, and weirdly I don't notice the difference between 170 and 172.5 but I do notice the difference going between 172.5 and 175. (all road bikes basically the same, steel, same weight, same groupsets, wheels, cassette ratios etc)

I'm 6ft 1 inch tall and thought I'd need a 175 so that's what I started with. I find the 175's I am able to get a higher top speed, but always find long rides more fatiguing. My non science-ee, equation-free idiot brain conclusion is, it feels like with longer cranks my legs are moving in a bigger arc for the same distance cycled, so my legs are moving more and its therefore more tiring. #inyourfacebriancox #notscience #likelybollocks

So for rides of over 50 miles I always take bikes with 172.5, that seems to work best for me. YMMV etc.

 

[Ajax Bay]

You asked "Does crank length matter?"

[and] thought might come up with a simple answer

Dislocation of expectation. But I suggest the answer is "Yes, to an extent which is greater if the rider is shorter or taller (inside leg length) than the (male UK) median (say by 2 SDs)"

Regarding power output I believe they did a study that showed power output was only marginally affected by crank length

Keen to read the report of that study, if you can (be bothered to) find it or offer a reference of some kind. Part of the problem with such studies is that they try riders on different length cranks and determine only a marginal effect. But, due the the understandable constraints of such studies, they do not allow the riders to accustom themselves to the much longer and much shorter cranks. Steve Hogg discusses this in the article to which I linked, and offers his personal experience, with caveats.

using the formula quoted for inseam in inches x 5.48 = crank length in mm I get a recommended crank length of 175.36mm.

Same for me (ie 32"). This inside leg measurement is close to (male UK) median so cranks in the 'normal' range: 170-175mm will be fine (as it happens, all my bikes have 170mm cranks). I think Steve Hogg quotes the formula and then gently derides it. If you can be bothered, go short. Opens the hip angle so the rider can maintain a set power for longer/ with more comfort; and shorter circumference of pedal track encourages/facilitates higher cadence for the same foot/pedal speed - which is more efficient, up to a point (eg 110rpm).

 

[Gravity Aided]

https://www.ncbi.nlm.nih.gov/pubmed/19771448
https://www.ncbi.nlm.nih.gov/pubmed/18178104

These two studies seem to reinforce the idea that there are slight, but measurable, differences in output and joint stress by the use of differing cranks. The first has to do with the cranks themselves, and the second is more about cadence.

Then there's Bike Fit Adviser, who also has some literature to back his claims, and offers very good videos on YouTube about his topics. He knows the literature pretty well, and cites it lower down on the page.
https://bikefitadviser.com/crank-length-research/

 

[JhnBssll]

...

Keen to read the report of that study, if you can (be bothered to) find it or offer a reference of some kind. Part of the problem with such studies is that they try riders on different length cranks and determine only a marginal effect. But, due the the understandable constraints of such studies, they do not allow the riders to accustom themselves to the much longer and much shorter cranks. Steve Hogg discusses this in the article to which I linked, and offers his personal experience, with caveats.

https://www.ncbi.nlm.nih.gov/m/pubmed/11417428/

That's the abstract, I haven't read the whole study Suggests from 145 to 190mm there is minimal difference in power output. Whether the data behind it is good is another matter

 

[Ajax Bay]

https://www.ncbi.nlm.nih.gov/m/pubmed/11417428/
That's the abstract,

Thank you for the link. I too remember reading that a while ago.
From the abstract (which is just looking at what the maximum power riders can generate, presumably over a few seconds, so not really indicative of sustained riding), "maximum cycling power was significantly affected by crank length". But actually there wasn't much difference between 140-180 (I'm extrapolating) - reductions in power occurred below that (down to 120mm, and above that (up to 220mm). Among the limitations of this test and others are the ones I outlined above, particularly 'acclimatisation' to shorter cranks.
My daughter has dropped down to 165mm cranks (she's about 165 (5' 5")) with some success:
http://www.pacesetterevents.com/news/vitruvian-triathlon/vitruvian-triathlon-2018-–-provisional-results/
4:15 for a Half Ironman (resident triathlete @Tin Pot note)

 

[Deleted member 1258]

Thank you for the link. I too remember reading that a while ago.
From the abstract (which is just looking at what the maximum power riders can generate, presumably over a few seconds, so not really indicative of sustained riding), "maximum cycling power was significantly affected by crank length". But actually there wasn't much difference between 140-180 (I'm extrapolating) - reductions in power occurred below that (down to 120mm, and above that (up to 220mm). Among the limitations of this test and others are the ones I outlined above, particularly 'acclimatisation' to shorter cranks.
My daughter has dropped down to 165mm cranks (she's about 165 (5' 5")) with some success:
http://www.pacesetterevents.com/news/vitruvian-triathlon/vitruvian-triathlon-2018-–-provisional-results/
4:15 for a Half Ironman (resident triathlete @Tin Pot note)

I'm about an inch taller than your Daughter and have often wondered if I'd be more comfortable if my road bike had 165's on it, I allready have 165's on my fixed and I'm happy on them.