The crank length thread

[FrankDay]

Here is an interesting comment related to this thread that can be found here

Frank Day you have that right. When I was at Cervélo and working with CSC, we showed Sastre how reducing his TT crank arm length would put him in a better position and make him faster. He resisted at first, but then embraced the advice. He was never a particularly good time trialer but his 5th place showing in the final TT in 2008 TdF, an impressive result, came shortly after that change.

My guess he didn't go any shorter than 165 as his sponsor probably didn't make any cranks shorter than that.

 

[Night Rider]

Here is an interesting comment related to this thread that can be found

Frank Day you have that right. When I was at Cervélo and working with CSC, we showed Sastre how reducing his TT crank arm length would put him in a better position and make him faster. He resisted at first, but then embraced the advice. He was never a particularly good time trialer but his 5th place showing in the final TT in 2008 TdF, an impressive result, came shortly after that change.

My guess he didn't go any shorter than 165 as his sponsor probably didn't make any cranks shorter than that.

Sastre was 12th in the final time trial, not 5th. He was 28th in the first time trial. Is your source implying he changed length between the two time trials?

What happened the following year when he was mid 60's in the giro time trial?

 

[CoachFergie]

Sastre was 12th in the final time trial, not 5th. He was 28th in the first time trial. Is your source implying he changed length between the two time trials?

What happened the following year when he was mid 60's in the giro time trial?

Anecdotal based practice: FTL!!!

 

[coapman]

I was thinking about this and trying to come up with a test people could do that might lead them to considering they need to go shorter. So I came up with this test. Stand against a wall with your knees bent about the same as they are when you pedal is at the bottom of the stroke. Mark your height. Then, lower yourself until your knees start to feel the strain, and mark this height. Your pushing down force didn't change (half your weight) but the effort needed to do so certainly did. So, this seems to be a good indicator of the ROM of the knee for effective force application. If you can't push as hard the advantages of longer cranks becomes smaller and smaller or, eventually, negative.

I don't understand this test.

 

[FrankDay]

I don't understand this test.

It is basically doing a half squat to discover the point where you can feel the knee is losing leverage such that one needs to use a lot more muscle effort to generate the same downward force. If your knee is bending a lot more than this when you ride your bicycle you are probably losing leverage, power, and efficiency.

 

[FrankDay]

Sastre was 12th in the final time trial, not 5th. He was 28th in the first time trial. Is your source implying he changed length between the two time trials?

What happened the following year when he was mid 60's in the giro time trial?

I have no clue. I only reproduced someone else's post on another web page that went to this thread. I did link to that page so why don't you go there and ask your question to him and if you get an answer you can come back here and tell us all.

 

[FrankDay]

From Chris Hopkinson on FB

Just done a test of the set up on one of my 2 RAAM bikes - the one with PowerCranks on. I have shortened the cranks to 155mm and raised the saddle height on this bike. Tested it out on the Richmond CC 11.5mile hilly tt course and came within 3secs of my fastest time ever - did 27mins 26secs. Got a new chain on but not changed cassette, need to do that now as I'm sure without the chainslip I could have gone 15-20secs faster!

I had been pushing him to do a crank length test. He liked it so much he is going to make the change even this close to RAAM. I hope to get him to try 145 before then as I suspect he would like that also. My guess is he will stay with his long cranks in the mountains as he isn't geared properly for climbing on short cranks.

 

[TeoSheva]

This question would probably be best asked on the crank length thread but since you asked it here I will answer it here.

The answer to this is not particularly simple. The simple answer is the athlete should choose the crank length that optimizes him or her. For climbing this is primarily a power consideration as aerodynamics plays little role, especially on steeper slopes and if the rider doesn't also have to go downhill also.

That having been said there are two considerations to generating power, force on the pedal and pedal speed. It is easier to put force on the pedals if they are moving slowly but high power also involves high pedal speed. Longer cranks have an advantage because they allow a higher pedal speed at any given cadence. Short cranks have an advantage because it is easier to apply force to the pedal because the knee has better leverage if it isn't bent so much.

So, in my opinion, it is all about gearing. If you find that 110 mm cranks is best for your power can you find a gear that will allow you to keep your cadence up to something reasonable for the highest slope you might encounter. This will be a lot different for a 6% slope than a 20% slope. Let's say you can climb at a sustained wattage of 250 watts. What gearing would you need? Well, analyticcycling.com says you would be riding at 4.92 m/s on a 6% slope and 1.66 m/s on a 20% slope. On 700c tires the wheel would be turning 2.24 rps at 4.92 m/s and 0.75 rps at 1.66 m/s.

Now, let's calculate the gearing gain necessary to not go below 60 rpm. 60 rpm is 1 rps. So, to keep rpm above 60 we would need a minimum "gain" of 1/2.24, or 0.446, at 4.92 m/s and 1/0.75, or 1.33, at 1.66 m/s. What does this mean for gearing choices? On the 6% slope the rear cog can be no smaller than 0.446 of the front chain ring on the 6% slope and the rear cog can be no smaller than 1.33 larger than the front chain ring.

Now, lets look at crank length. Lots of people think longer crank lengths give more leverage but this is wrong. The leverage is really dictated by the ratio of the crank length and the radius of the front chain ring. If the crank length and the chain ring have the same diameter the leverage advantage is zero because the ratio is 1. So 175 cranks and a 52 tooth chain ring provides a leverage advantage of 1.75/105 or 1.66 which means that the pedal speed is increased and pedal force is decreased by this amount for the same power and cadence. To get the identical leverage advantage on 145 mm cranks one would need a 43 tooth chain ring and on 110 cranks one would need a 33 tooth chain ring.

So, what does this mean? On 700C tires and generating 250 watts on a 6% slope and keeping cadence above 60 rpm one could ride that using a 52/23 combination on 175 cranks, 42/19 on 145 cranks and 34/15 on 110 cranks. So, for ordinary climbs (6% slope) it is possible to be easily fit your bike with the gearing necessary to climb. If you find that one crank length gives you a power advantage over the others then you should choose that one. However, when the slopes get very steep the situation changes. To maintain cadence the front chain ring must be smaller than the rear chain ring and the smaller the cranks the smaller the chain ring. Let's put a 36 tooth cog on back and see what is needed. So, to climb a 20% slope at a cadence or 60 at 250 watts requires a 27 tooth front chain ring on 175 cranks, a 22 tooth front chain ring on 145 cranks, and a 17 tooth chain ring on 110 cranks. Such a set-up is almost impossible to attain (although we have made a set of PowerCranks with a 23 tooth front chain ring for someone racing Mount Washington).

So, in conclusion, when the slopes get really steep, the only option is longer cranks. When the slopes are moderately steep shorter cranks are probably just fine, as long as you are properly geared properly for them, so then choose the crank that is best for you.

Hope my math was correct. I am sure some of you will correct me if I made a mistake. Either way, the correct math won't lie and this will give you the general considerations for making this choice.

wow, that's impressive!
Thanks a lot for the moment, I will give your answer to my friend!

 

[FrankDay]

wow, that's impressive!
Thanks a lot for the moment, I will give your answer to my friend!

The short answer is:
1. It depends.
2. You gotta be smarter than your equipment (i.e., make your equipment fit your needs rather than try to fit yourself to your equipment).

 

[CoachFergie]

Interesting blog post on crank length after a long and often amusing debate on Wattage forum...

http://bikeblather.blogspot.fr/2014/07/crank-length-whateverwithin-reason.html

 

[FrankDay]

Interesting blog post on crank length after a long and often amusing debate on Wattage forum...

http://bikeblather.blogspot.fr/2014/07/crank-length-whateverwithin-reason.html

Interesting? Hardly.
From the article

There's a trend in TT/Tri circles to use shorter than "normal" crank lengths. This is done to allow folks to either open up the distance between their torso and leg at the top of the pedal stroke (to be better able to produce power) and/or allow them to lower the angle of their torso relative to the ground in an attempt to get more aerodynamic.

Then he offers his "pepsi challenge"

1: Go TT up your favorite 15 minute or longer hill at LT or greater power (by HILL I mean something with average grade 6% or greater) with your current setup and time it a few times.
2: Then put on crank arms that are only 87.5% in length of your current setup. That would be 148.75mm if you are currently riding 170s. You now have my proportional experience setup of me riding 175mm cranks.
Take your own medicine and re-gear appropriately. Now ride that setup for a few weeks and go TT up your favorite hill a few more times and time it.

If you can show me you can do the same time with your medicinal gearing going full tilt, then I'll happily eat some humble pie.

So, the main reason given for people going for shorter cranks is that they might be able to achieve a better aero position while maintaining (or increasing power) then he offers a "Pepsi challenge" that has essentially nothing to do with power in the aero position and further specifies people changing to a crank length that would be essentially impossible to obtain (148.75mm) and then, for going to all that trouble, the author would be willing to eat some "humble pie". LOL Fergie, do you actually understand what you read and post links to?

It is inconceivable to me that bike frames vary from 48 to 62 cm (30% variation) whereas standard crank length vary from 170 to 175 (3% variation) but that such a variation in crank length would be actually found to be optimal for the majority of riders.

My contention remains, if you haven't experimented with crank length (really experimented) you can't possibly know if the crank length you are riding is optimum FOR YOU for the kind of riding you do.

 

[tanhalt]

I see your reading comprehension hasn't gotten any better over the years. The "challenge" was someone else's. I took it, and demonstrably proved him wrong.

I would think with the way you use my quote (unattributed, of course :-/ ) on cranks being "just one lever in a series of levers between your foot and the ground" that at least you might read a bit more carefully.

 

[Captain Serious]

I decided to think out loud on here:

Due to having some busted cranks and a bit of a sore knee, I dusted off a few sets of my 165mm cranks a couple of months ago.

I'm about 182cm tall with long legs (~89cm inside leg).

The 165s feel odd at first, but after a week I didn't notice the difference at all, and now my 172.5s feel like windmills. And while I don't have a powermeter, I don't feel at all slower.

So there

Useless anecdote? Probably. Cathartic? Maybe, but I don't see how.
Ha

 

[FrankDay]

I see your reading comprehension hasn't gotten any better over the years. The "challenge" was someone else's. I took it, and demonstrably proved him wrong.

My bad. Apparently this part of your post

cheers,
-kieran"

ended up on the bottom of my page and I didn't notice that there was more. No wonder it didn't seem to make much sense.

I would think with the way you use my quote (unattributed, of course :-/ ) on cranks being "just one lever in a series of levers between your foot and the ground" that at least you might read a bit more carefully.

I do have a couple of observations now that I have read the entire thing. First, your "experiment" of 1 simply reiterated the findings of Martin, who found essentially no difference in power between 170 and 145. And, you geared essentially perfectly because to keep the same pedal speed for the two crank lengths you needed to increase to 85 on the 150's and you rode 84.

But, I disagree with you that the two QA plots are essentially the same. The lower right quadrant is much different in that pedal speed never gets much beyond 1.6 m/s whereas with the longer cranks pedal speed is sometimes seen near 1.8 m/s. Pedal speeds stays in a much narrower range with shorter cranks it would seem from your analysis. Not sure how important that is but I think it is a reasonable observation to make.

Regarding not attributing the comments it seems to me that providing a link satisfies the attribution "requirement."

 

[FrankDay]

This guy (link) isn't very aggressive but at least he seems to get the concept.

 

[FrankDay]

Just got a call from a researcher setting up a new lab. He told me his last lab did a study looking at crank length and efficiency in triathletes in the aero position comparing 175, 165, 155, 145. Apparently 145 was most efficient. Will have to wait for the details but the paper is apparently being prepared.

 

[M Sport]

A win for Adam Hansen at the Vuelta last night. 180mm cranks. Paper to follow.

 

[Tapeworm]

^ it was actually his 38cm bars and -30 degree stem that did it. And the handmade (his hand actually) custom carbon ~$2k shoes.

 

[Captain Serious]

It says on various sites that Hansen is 6ft (183); is this true?
I thought he was taller

 

[FrankDay]

Today I received a report of a trial comparing 145 and 175 mm cranks. The trial tried to make the crank length the only variable. The ride was done on the Expresso bike system which allows a rider to ride repeated courses against "ghosts" that represent previous rides. Here is the result of two rides over the same course using a "fixed" gear following the same ghost. The rides lasted about 25 minutes and were within 0.2 seconds of each other so it seems they would be pretty comparable from a power output perspective. Note the the ride on the 145 cranks the average and maximum heart rate was 10 bpm lower than when done on 175 cranks. Since HR corresponds reasonably well to oxygen consumption this would suggest that in this instance the 145 cranks improved rider efficiency about 7-8%. Such that if his efficiency on 175 cranks was 20% his efficiency on 145 cranks would be about 21.5%. Needless to say the rider was pretty excited. I believe another rider did the same thing with a similar result Here is the graph of this one effort. Discuss.

 

[ray j willings]

I switched from 172 to 170 about 3years ago. I found climbing a bit easier " I climb on my 53t " It seemed like my legs turned the cranks more easy.
I don't feel quite as fast when flat road riding.
The Bike I am building now I am putting on a 172 crank to see how that feels now.

The test for hairless legs in the wind tunnel seems more advantageous
https://www.youtube.com/watch?v=DZnrE17Jg3I

 

[FrankDay]

Re:

Today I received a report of a trial comparing 145 and 175 mm cranks. The trial tried to make the crank length the only variable. The ride was done on the Expresso bike system which allows a rider to ride repeated courses against "ghosts" that represent previous rides. Here is the result of two rides over the same course using a "fixed" gear following the same ghost. The rides lasted about 25 minutes and were within 0.2 seconds of each other so it seems they would be pretty comparable from a power output perspective. Note the the ride on the 145 cranks the average and maximum heart rate was 10 bpm lower than when done on 175 cranks. Since HR corresponds reasonably well to oxygen consumption this would suggest that in this instance the 145 cranks improved rider efficiency about 7-8%. Such that if his efficiency on 175 cranks was 20% his efficiency on 145 cranks would be about 21.5%. Needless to say the rider was pretty excited. I believe another rider did the same thing with a similar result Here is the graph of this one effort. Discuss.

I am quite impressed (negatively) that it is now about 4 days since this was posted and we now have the first post in regards to this - a simple personal experience relating a change from 172.5 to 170. As a basis of discussion I can think of at least 4 different explanations for the data, not all of which involve crank length as the basis for the difference seen. Come on folks!

 

[ray j willings]

Re: Re:

Today I received a report of a trial comparing 145 and 175 mm cranks. The trial tried to make the crank length the only variable. The ride was done on the Expresso bike system which allows a rider to ride repeated courses against "ghosts" that represent previous rides. Here is the result of two rides over the same course using a "fixed" gear following the same ghost. The rides lasted about 25 minutes and were within 0.2 seconds of each other so it seems they would be pretty comparable from a power output perspective. Note the the ride on the 145 cranks the average and maximum heart rate was 10 bpm lower than when done on 175 cranks. Since HR corresponds reasonably well to oxygen consumption this would suggest that in this instance the 145 cranks improved rider efficiency about 7-8%. Such that if his efficiency on 175 cranks was 20% his efficiency on 145 cranks would be about 21.5%. Needless to say the rider was pretty excited. I believe another rider did the same thing with a similar result Here is the graph of this one effort. Discuss.

I am quite impressed (negatively) that it is now about 4 days since this was posted and we now have the first post in regards to this - a simple personal experience relating a change from 172.5 to 170. As a basis of discussion I can think of at least 4 different explanations for the data, not all of which involve crank length as the basis for the difference seen. Come on folks!

Can you tell more about the 172 - 170? be interesting to hear your view/experience.

 

[FrankDay]

Re: Re:

Can you tell more about the 172 - 170? be interesting to hear your view/experience.

Nope. That wasn't my report. Having said that, let's assume there is a difference since differences can be uncovered in larger changes (145-175) and we can presume that any change might be a smooth line. But, I believe the difference 2.5 mm would make is so small that it would be impossible for any one individual to measure an actual difference. It would also be essentially impossible to tell a difference in the University setting as I suspect the difference 2.5 mm would make would be so small that it would take huge numbers of experimental subjects to uncover a statistical significance making such a study difficult to do for pretty much zero purpose. I think it is enough to demonstrate that crank length (over a much wider range than most are used to) can make a difference such that each rider ought to experiment to find out what is the best crank length for them.

 

[JamesCun]

You're baiting and then provoking people is rather lame. If it isn't of interest, such is life.