Paging Zakeen re: Crank Length

Discuss light weight issues concerning road bikes & parts.
Nefarious86
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by Nefarious86

When I built my Epic I didnt realize that I was sent 180mm cranks until after they were installed. Turns out with the cadence I ride at on dirt they work perfectly and I've also had mates try it and they agree they work.

Basically there is no magic bullet formula for this stuff, proper trial and error is the only real way to get it right for your needs.
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NiFTY
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by NiFTY

Great to have the great hanseeno drop by the forum again. Good luck this year Adam ya bloody legend.
Evo 5.02kg SL3 6.77 Slice RS 8.89 viewtopic.php?f=10&t=110579" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

by Weenie


Johnny Rad
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by Johnny Rad

Knowledge bomb.

Mind blown that Zakeen / Adam would spend a few moments to educate us neophytes.

If I had the time / money, I'd love to try different can arms to see if I could feel a difference...
Last edited by Johnny Rad on Sun Mar 12, 2017 5:49 am, edited 1 time in total.

TheKaiser
Posts: 480
Joined: Thu Sep 05, 2013 3:29 pm

by TheKaiser

First of all Zakeen, a huge thank you for replying to this thread, that's really above and beyond man. Much respect.
zakeen wrote:
TheKaiser wrote:So, Zakeen, can you pull yourself away from your long hours of being a Protour hardman and take a minute or two to fill us in? And if you have any extra time, how about a step-by-step how-to to help guide us would-be DIY carbon shoe builders?


I dont like to say to much, so you are right in that way :D

But to say a few things I disagree of, which does not mean I am correct, I watched your video Cyclespeed and your points on why its better to have shorter cranks were:

Leg doesn't come up so high so you can get your torso lower just by having shorter cranks.

This has nothing to do with crank length, I run super long. Then I ride a frame a little smaller then I should, but I must do that so I can have a shorter steering tube, meaning my stem, without spacing is as low as possible and then I still have a negative angled stem. If I ran the a bigger frame then I wouldnt be able to get my handle bars so low(bigger frame bigger steering tube). So having said all this. I really have an extreme low bars and still my legs dont hit my torso. I did however move my saddle more forwards which opens up my hip flexors and tilled down my saddle which helped even more.


So my takeaway from this is that torso angle is not strictly a product of crank length, which I completely agree with, although I don't know that it has "nothing to do with crank length". It seems for Zakeen on his personal setup, the forward saddle position opened the hip angle sufficiently in a manner similar to a TT bike, so that thigh/torso interference is not a problem, despite his long crankarms. This meant that the mechanical aspect of how to affix the bars lower was the big barrier to getting the torso angle low enough, not some biomechanical issue of the thighs restricting expansion of the diaphragm, fouling on the ribs, or reaching an excessively acute angle that resulted in impingement. Femur length could play a role here as well, but we don't have figures on that for comparison.

zakeen wrote:This gets more interesting. Power output is Watts equals RPM X Torque. So having a longer crank arm requires less Torque/strain( which I believe is better for your joints ). Doing smaller circles to maintain higher speed I disagree a little. Legs are going up and down, like pistons in a engine. Every time they go up, stop and down they lose energy and the muscle has to re-fire again. I feel its easier to maintain a larger circle.

If you are at the same place by simply changing up the gear, then yes you are right, the amount of force would be the same for the smaller crank to do that 12 to 6oclock rotation in a smaller gear compare to the longer crank in a bigger gear. Force might be the same(depending on gearing differences). However, the wheel rotates far longer in a bigger gear then in a smaller gear. So you yourself say right there, that there is a difference. Same force, different crank length, different gearing's. But one gearing has a longer rollout would mean that difference in rollout is the advantage of longer cranks. You would rider further with the same force applied on the crank.


So my takeaway from this is that, while a shorter crank/lower gear would allow an equivalent linear foot speed and torque output as a longer crank/higher gear, the shorter crank will require one to spin a higher RPM to maintain the same riding speed. Zakeen is saying that he thinks that there are cyclical losses in the pedaling motion and so it is advantageous to reduce the required number of up/down cycles of the leg while maintaining a low force requirement. Long cranks allow you to do this. For example you would need to do approximately 6 rotations of a 170mm crank for every 5 rotations of a 180 crank if you were trying to achieve an equivalent force/linear foot speed.

zakeen wrote: How I got to my crank length were pretty simple. There is no real way of measuring the workload of a body(yes we can throw on SRMs and guess the body is about 25% efficient and work out the total amount of calories spent through an SRM is multiplied by 4 to give what out bodies used in energy to produce those watts, but that's still guessing, they do say its very accurate). But after speaking with a mechanical engineer they set up my bike to be efficient. Then I went and spoke with then tested with Bio mechanic and asked them how do I become as efficient as possible to spin this position. I was able to get my position based on that. I dont want to say to much but some food for thought:


This is an excellent point and seems to tie into the previous point about up/down cycles of the legs. It is conceivable that there are losses or inefficiencies that go along with an increased number of up/down cycles of the leg, and they would be difficult to measure. It seems like Vo2 could be a way of trying to assess efficiency, although I don't know if it would be sufficiently sensitive for this type of thing. This study suggests (https://www.ncbi.nlm.nih.gov/pubmed/9356762) suggests that Vo2 does vary with crank length, however it doesn't correlate to leg length and the abstract doesn't address if there is a consistent trend toward longer or shorter being most efficient. From my anecdotal experience there is an accommodation period with a lot of positional changes so that can also skew results, and if one was going to test this fairly you would need to do something like swap crank lengths every few days while constantly testing on an indoor trainer to give each one a fair shake, and avoid simply having your most familiar win, or having a change in fitness lead to a phantom effect that could be mistakenly attributed to the crank change.

zakeen wrote: Your muscles are at most powerful as operate with greatest active tension when they are close to ideal length (normally their resting length). So to get the maximum out of the quad its best to be at maximum effort from 2:30 oclock to 6 oclock or while standing even 1 oclock and imagine the leverage on a longer crank while standing :shock: .

Cycling races are long... super long. I could hit the gym and hit hard weights or I can hit slightly less hard weights and be there longer....


Both of those points seem like they could just as easily be used to advocate a shorter crank/lower gear combo, as a shorter crank will keep your muscles closer to their ideal length for more of the pedaling cycle, and the lower gear will provide the reduced loads to reduce fatigue in long races, spin to win, etc...

zakeen wrote: Im not saying longer is better, I personally tried 172.5, 175, 177.5, 180 and 185mm. I ride with what I feel best. You should ride what feels best for you

You're inspiring me to do some more experimentation with position. Out of curiosity, do you have some specific tests that you have used that help quantify the effects of those different lengths, or is a more of a qualitative "feel" sort of thing?

Thanks again for taking the time to stop by, and much good mojo going out to you for this season.

TheKaiser
Posts: 480
Joined: Thu Sep 05, 2013 3:29 pm

by TheKaiser

TheKaiser wrote:
zakeen wrote:This gets more interesting. Power output is Watts equals RPM X Torque. So having a longer crank arm requires less Torque/strain( which I believe is better for your joints ). Doing smaller circles to maintain higher speed I disagree a little. Legs are going up and down, like pistons in a engine. Every time they go up, stop and down they lose energy and the muscle has to re-fire again. I feel its easier to maintain a larger circle.

If you are at the same place by simply changing up the gear, then yes you are right, the amount of force would be the same for the smaller crank to do that 12 to 6oclock rotation in a smaller gear compare to the longer crank in a bigger gear. Force might be the same(depending on gearing differences). However, the wheel rotates far longer in a bigger gear then in a smaller gear. So you yourself say right there, that there is a difference. Same force, different crank length, different gearing's. But one gearing has a longer rollout would mean that difference in rollout is the advantage of longer cranks. You would rider further with the same force applied on the crank.


So my takeaway from this is that, while a shorter crank/lower gear would allow an equivalent linear foot speed and torque output as a longer crank/higher gear, the shorter crank will require one to spin a higher RPM to maintain the same riding speed. Zakeen is saying that he thinks that there are cyclical losses in the pedaling motion and so it is advantageous to reduce the required number of up/down cycles of the leg while maintaining a low force requirement. Long cranks allow you to do this. For example you would need to do approximately 6 rotations of a 170mm crank for every 5 rotations of a 180 crank if you were trying to achieve an equivalent force/linear foot speed.


After trying to wrap my head around and sum up Zakeen's explaination I was looking around for studies having to do with crank length, cadence, and metabolic cost and I came across this site (http://www.nettally.com/palmk/crankset.html) by Kirby Palm discussing his formula for crank length based on inseam. On his references page, he critiques many studies on crank length, and then had the following interesting passage:

"Perhaps the most interesting feature of the Martin/Brown/Anderson/Spirduso article mentioned above is a reference to another work:

Caiozzo V.J. and Baldwin K.M., "Determinants of work produced by skeletal muscle: potential limitations of activation and relaxation", American Journal of Physiology 1997, 273:C1049-1056.

Apparently this research showed that the amount of work generated by muscles in cyclic activity decreases with increasing frequency of the cycle because the relaxation kinetics between muscle contractions hampers full excitation of the muscle. In other words, a higher cadence results in a lower force on the pedal, regardless of crank length. Sorta makes sense, doesn't it? You know how hard you can push on a pedal at a standstill; imagine how difficult it would be to apply that same amount of force at 100 strokes/minute. This is a powerful argument that a cyclist should be using the longest crank that he can comfortably ride, since this will permit the lowest possible cadence at a given pedal force and power output and hence permit the highest excitation state in the muscles."


I found that to be an interesting point that jibes with Zakeen's suggestion that the lowered force requirements of a longer crank are not totally interchangeable with a shorter crank/lower gear/higher cadence, due to the difference in the number of firing cycles of the muscle, with the fewer cycles of the longer crank being advantageous.

gewichtweenie
Posts: 152
Joined: Fri Sep 25, 2015 3:12 pm

by gewichtweenie

Forum buffoon checking in.
TheKaiser wrote:In it, he basically says its simply because longer cranks give you more leverage, ignoring the fact that you can alter the drivetrain system's leverage with gearing changes as well

At some point of you run out of gears. Or, in the same gear, youre losing ground speed. Gearing - i.e. mechanical leverage - helps adapt the source output to the application. Gearing allows a a Fiat panda motor producing 65Nm of torque to scoot the car down the road. But gearing wont give that Fiat a Ferrari acceleration or 300kph top speed.


zakeen wrote:This gets more interesting. Power output is Watts equals RPM X Torque. So having a longer crank arm requires less Torque/strain( which I believe is better for your joints ). Doing smaller circles to maintain higher speed I disagree a little. Legs are going up and down, like pistons in a engine. Every time they go up, stop and down they lose energy and the muscle has to re-fire again. I feel its easier to maintain a larger circle.


Smaller circles are easier to spin. Less displacement. Less energy. Like pistons in an engine... when engineers want to increase the rev limit, they decrease the piston stroke.

Im not saying longer is better, I personally tried 172.5, 175, 177.5, 180 and 185mm. I ride with what I feel best. You should ride what feels best for you

But the thing you are right on... it's a personal thing. Lower stroke reduces torque. It's a compromise. Same thing with crank leverage vs ability to maintain spin.

LeDuke wrote:That's a WorldTour pro dropping knowledge.


1 pro rider chooses abnormally long cranks. and hundreds of others dont. truly, different strokes for different folks.

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