Help settle an argument about crank length.

[thom]

Maybe, maybe not. At the same power output & rpm 175mm cranks deliver a 3% reduction in require pedal pressure compared to 170mm cranks. That's about 1/2-1/3 of a 1t gear step. When pushing big gears riders can want sub-1t gearing steps.

Yeah but... ok so the force you need to apply is less but the work you put into the crank is the same if the cadence remains constant :
Work = Force x distance
The outer radius being greater, so the distance moved is longer.

And since your power output is the rate of doing work, that will be the same. I'm guessing that there is some kind of physiological sweet spot that suit ones individual body that is a pay off between pedal pressure and crank length. As a few others have said, I very much doubt people can really tell (on a road bike at least) without accurate machine measurement though.

To be honest, I'm rather confused by this and have been since I saw a comment about how Wiggo improved his TT performance this year over last year. Although Wiggo does have specific preferences about crank length this is more about cadence and gearing (which is related to gearing as previously mentioned).

I think the story is that last year, SKY spent a lot of time analysing TT performance for Wiggo, going with the perceived wisdom that high cadence was good. Remember, there's a physiological aspect to muscle this, in that with blood flow, your blood flow is removing lactic from muscles when they aren't in tension ( I think...). So a higher cadence helps flush out the lactic more frequently... ? I dunno but I thought that was part of LA's high cadence climbing logic although clearly now, all bets are off on that one ;-)

Anyway, SKY essentially couldn't really understand how Tony Martin managed to outperform Wiggo throughout the season in TTs because they did pretty much everything they could - TM was not close but a lot better than Wiggo.
So last winter, they experimented with lower cadences and higher gearing like TM, and they found that in the mechanics of the bike/body that Wiggo could then produce a higher torque for the same power output during TTs...
So that's what they went with this year and I think Wiggo had to work on changing his TT style quite hard.

This is my honest recollection of something Wiggo said but I don't quite understand it because I think it has to be explained by there being some power leak when at a higher cadence - perhaps the power transfer in the pedal stroke is more efficient at a slower cadence because the angle you apply pressure at varies more slowly ? perhaps because you don't do as much work to move the body/legs as much that helps ? perhaps Wiggo's body is more efficient at
doing work with a certain pedal pressure ? I'm speculating because I think the mathematical mechanics kind of says, for the same cadence, whatever the crank length, the work you do to move the pedal is the same and varying the crank length is like others have mentioned, just the same as varying the gearing.

 

[GrasB]

Yeah but... ok so the force you need to apply is less but the work you put into the crank is the same if the cadence remains constant :
Work = Force x distance
The outer radius being greater, so the distance moved is longer.

Biometrically it's far more complicated than that. For some sets of muscles & tendons the foot speed that matters with regard to maximum exertion at a given cadence & time. For others it's actual cadence regardless of the leg travel that defines the maximal exertion thresholds. What works for you is a blend of these factors. Also what may apply for your 2 min maximal effort may not apply for your FTP...

 

[thom]

Biometrically it's far more complicated than that. For some sets of muscles & tendons the foot speed that matters with regard to maximum exertion at a given cadence & time. For others it's actual cadence regardless of the leg travel that defines the maximal exertion thresholds. What works for you is a blend of these factors. Also what may apply for your 2 min maximal effort may not apply for your FTP...

I can well believe that - I guess it's part of the game where switched on pro-teams eke out their competitive advantages these days. But not the average commuter.

 

[tyred]

About 100 years ago, there was a theory that long cranks (8" or more) with a high gear ratio reduced rider fatigue as some felt that it meant less crank revolutions per distance travelled. It quickly fell out of favour for various reasons - variable gears became more common, the trend towards smaller wheels and smaller frames with more "sporting" geometry lowered bottom bracket height but mostly because for most people of normal size and build, it is easier (less tiring, less chance of injury) to spin a lower gear with short cranks at a higher revolution than it is to apply a huge amount torque to the long crank/high gear set up.

 

[amaferanga]

I've ridden with 170mm, 172.5mm and 175mm cranks with a power meter and the crank length made no noticeable difference to power output. Nor can I feel any difference between the crank lengths. I've also tried elliptical rings and they didn't increase my power output either.

You just don't get Watts for free in cycling.

 

[SKoob]

I've got 170s on my hybrid and 175s on my steel road bike and I find it easyer to get a higher cadence on my hyrbid. I've got quite short legs so maybe thats why I notice the difference in crank length?

 

[Smokin Joe]

I've got 170s on my hybrid and 175s on my steel road bike and I find it easyer to get a higher cadence on my hyrbid. I've got quite short legs so maybe thats why I notice the difference in crank length?

Or perhaps you're comparing your riding on two completely different type of bike?

 

[GrasB]

Or perhaps you're comparing your riding on two completely different type of bike?

Your cadence should be inversely proportional to crank length. Every rider I've seen who's given it enough time to normalise to new crank lengths change their cadence so that their nominal foot speed is about the same. Riders who aren't that in-tune with what their legs are actually doing tend not to feel a difference, riders who are more aware of their biometrics often can feel the change but not be worried by it.

 

[GrasB]

I've ridden with 170mm, 172.5mm and 175mm cranks with a power meter and the crank length made no noticeable difference to power output. Nor can I feel any difference between the crank lengths.

You shouldn't see power gains, what may or may not change is fatigue over long rides, especially multi-day hard riding.

I've also tried elliptical rings and they didn't increase my power output either.

You tend to need a period of time to train into elliptical chainrings & it's fairly specific training as well. I've been told that it takes about 3000 miles to fully train into ellipticals.

 

[SKoob]

Or perhaps you're comparing your riding on two completely different type of bike?

Could be, I just tend to find I can get my feet spinning faster at the same percieved effort on my hyrbid which surely would be more difficult being heavyer and with wider tyres. Could well be down to the gearing though.

 

[steve52]

Yup, a longer crank is a longer lever so easier. There was some genius or other who said something like ''Give me a lever long enough and I will move the world.'' Probably longer than 175cm in his case.

but the cadence is so low

 

[asterix]

I don't have a normal cadence. In fact, over decades of cycling, I have never bothered to count cadence.

What I like to do is vary my pedalling, and thanks to trying to hurtle down hills with a rather low top gear I can spin those cranks till my legs are simply a blur. I'm indifferent to 5 millimetres of crank more or less.

On climbing, I am content to grind up the hill if need be, and usually like to have a cog or two in reserve rather than spin. Again, I can't see that small differences in crank length would help or hinder.

My custom audax machine that fits me perfectly has 170 cranks. Those who claim that crank length is important would say I need 175 or more with my leg length. Roberts didn't give me the extra 5mm and I've no reason to think they were wrong after riding the Raid Pyrenean on it.

Of course I am not a competitive cyclist and accept that the infinitesimal margin between winning and losing might be affected by crank length, but for mere mortals like me it doesn't really matter.

 

[amaferanga]

You shouldn't see power gains, what may or may not change is fatigue over long rides, especially multi-day hard riding.


You tend to need a period of time to train into elliptical chainrings & it's fairly specific training as well. I've been told that it takes about 3000 miles to fully train into ellipticals.

I didn't expect to see power gains. My comment was aimed at those who seemed to think their power output would increase by just changing crank lengths! I doubt you'd see a perceptible change in post-ride fatigue in changing crank length by 5mm either. Of course you could convince yourself that it makes a difference, but that's not the same.

As for elliptical rings, I rode a whole winter and early spring races on elliptical rings and I managed to convince myself that for road racing with changes in pace, etc. they actually had a negative impact. So I changed back to round rings and while I didn't suddenly get better, I sure didn't get worse either. And the fact that front shifting was faultless again with the round rings (compared to mostly ok just with elliptical rings even after weeks of tinkering) has me convinced that elliptical rings are just pointless.

 

[Night Train]

I think (and I have no direct proof or evidence of this) that crank length will have a minimal effect on useful mechanical efficiency or torque transmitted by the cyclist.
However, it probably has a greater effect on rider comfort and bio-mechanical efficiency if the crank length is adjusted relative to leg length.
The longer your legs the easier it is to spin longer cranks and have a longer stroke length. If you have shorter legs then you will need shorter cranks and have a shorter stroke length. This would be because of the amount of flex, or the maximum bend angle, in the knee when the pedal is at its highest point, relative to the lowest point where the leg is approximately straight.

The legs have an easier time extending, pushing on the pedals, when it does not have too great an angle at the knee. Therefore, to avoid too great a knee angle short legs should have short cranks and a shorter stroke then long legs.

 

[GrasB]

I doubt you'd see a perceptible change in post-ride fatigue in changing crank length by 5mm either. Of course you could convince yourself that it makes a difference, but that's not the same.

Recovery from short cranks will depend on the rider. A low foot speed rider will be more likely to benefit from shorter cranks than someone who will sit at 110rpm on 175mm cranks al day quite naturally.

As for elliptical rings, I rode a whole winter and early spring races on elliptical rings and I managed to convince myself that for road racing with changes in pace, etc. they actually had a negative impact.

This is the opposite to what I found with a brief ride on a bike with 12.5% ellipticals. It was easier to accelerate especially under high loads like when climbing or sprinting, where they tended to smooth out the more choppy pedalling motion but it really messed up my pedal rhythm when in >110rpm full flow on the flat.