## Crank arm length...

zero
Posts: 3056
Joined: Tue Jan 05, 2010 2:54 pm

### Re: Crank arm length...

My 39T inner ring is a 80mm lever. There cannot be F x D on the chain because of that lever.

If I hold the chain speed constant (ie your identical cadence), then my footspeed won't change by 1.4% by changing the cranks to 175 from 177.5. I expect its 2.6% or that my lowest gear will "feel" half a tooth higher as a result of this change.

human909
Posts: 8371
Joined: Tue Dec 01, 2009 11:48 am

### Re: Crank arm length...

zero wrote:My 39T inner ring is a 80mm lever. There cannot be F x D on the chain because of that lever.

Torque about the axis from the pedalling is still F x D as I stated. Sure you can use the diameter of the ring to calculated chain tension but that is hardly relevant. Chain tension won't change if all you are changing is the cranks.

zero wrote:If I hold the chain speed constant (ie your identical cadence), then my footspeed won't change by 1.4% by changing the cranks to 175 from 177.5.

Yes it will. Your cadence is constant as you stated but your foot is now moving in a circle that is larger by 1.4%. Therefore your footspeed is 1.4% faster on 177.5.

zero wrote:I expect its 2.6% or that my lowest gear will "feel" half a tooth higher as a result of this change.

Your expectations do not correlate with the calculated reality.

eeksll
Posts: 2366
Joined: Tue Oct 20, 2009 10:36 pm

### Re: Crank arm length...

Nobody wrote:...
In the real world shorter cranks have many advantages:
Less knee problems.
...

trying to understand all thats going on here, I think I do.

Wouldn't a longer crank arm require less force on the pedal to create the same power at the wheel/chainring ?
Requiring less force would be better for my knee? so longer would be better?

side note: I sometimes wonder if this slight measurement differences make that much difference (crank arm length, saddle position, seat height ... we are talking less than 10mm mostly, surely our bodies can cope). Not that its not good to discuss though.

warthog1
Posts: 6997
Joined: Wed Jul 25, 2012 4:40 pm

### Re: Crank arm length...

eeksll wrote:
trying to understand all thats going on here, I think I do.

Wouldn't a longer crank arm require less force on the pedal to create the same power at the wheel/chainring ?
Requiring less force would be better for my knee? so longer would be better?

side note: I sometimes wonder if this slight measurement differences make that much difference (crank arm length, saddle position, seat height ... we are talking less than 10mm mostly, surely our bodies can cope). Not that its not good to discuss though.

Given the leg is lifted slightly higher with increased crank length, longer cranks increase leg flexion at the knee so that force is applied to the knee whilst it is hinged at a more acute angle. Can cause some people knee problems.

zero
Posts: 3056
Joined: Tue Jan 05, 2010 2:54 pm

### Re: Crank arm length...

human909 wrote:
zero wrote:My 39T inner ring is a 80mm lever. There cannot be F x D on the chain because of that lever.

Torque about the axis from the pedalling is still F x D as I stated. Sure you can use the diameter of the ring to calculated chain tension but that is hardly relevant. Chain tension won't change if all you are changing is the cranks.

zero wrote:If I hold the chain speed constant (ie your identical cadence), then my footspeed won't change by 1.4% by changing the cranks to 175 from 177.5.

Yes it will. Your cadence is constant as you stated but your foot is now moving in a circle that is larger by 1.4%. Therefore your footspeed is 1.4% faster on 177.5.

Alright I see where I've gone wrong, my bad. Thanks for correcting me.

Nobody
Posts: 8599
Joined: Thu Sep 18, 2008 12:10 pm
Location: Sydney

### Re: Crank arm length...

eeksll wrote:side note: I sometimes wonder if this slight measurement differences make that much difference (crank arm length, saddle position, seat height ... we are talking less than 10mm mostly, surely our bodies can cope). Not that its not good to discuss though.

You must be young. I didn't care too much about it either until I got to middle age. Now I'm fighting off various joint problems just to stay on the bike.

eeksll
Posts: 2366
Joined: Tue Oct 20, 2009 10:36 pm

### Re: Crank arm length...

warthog1 wrote:Given the leg is lifted slightly higher with increased crank length, longer cranks increase leg flexion at the knee so that force is applied to the knee whilst it is hinged at a more acute angle. Can cause some people knee problems.

cheers.

Nobody wrote:
eeksll wrote:side note: I sometimes wonder if this slight measurement differences make that much difference (crank arm length, saddle position, seat height ... we are talking less than 10mm mostly, surely our bodies can cope). Not that its not good to discuss though.

You must be young. I didn't care too much about it either until I got to middle age. Now I'm fighting off various joint problems just to stay on the bike.

hmmm maybe thats why I have a dodgy knee

actually pretty sure its because I refuse to stop running .... might be about time though

human909
Posts: 8371
Joined: Tue Dec 01, 2009 11:48 am

### Re: Crank arm length...

zero wrote:Alright I see where I've gone wrong, my bad. Thanks for correcting me.

No problems. I'm an engineering/physics geek. So I will argue the analysis fairly doggardly.

DoubleSpeeded
Posts: 441
Joined: Sat Jul 13, 2013 6:18 pm

### Re: Crank arm length...

human909 wrote:
zero wrote:Alright I see where I've gone wrong, my bad. Thanks for correcting me.

No problems. I'm an engineering/physics geek. So I will argue the analysis fairly doggardly.

Hmmm thinking of perhaps downgrading a crank...

ok well now that we know your a brainiac...you may help us out

what would a good comparison be... for e.g

170mm length 53T and a 175mm length 53T?

toolonglegs
Posts: 15451
Joined: Fri Jun 29, 2007 7:49 pm
Location: Somewhere with padded walls and really big hills!

### Re: Crank arm length...

Just get an SRM multi-length crank ... you need a power meter anyway if you want to beat Tony Martin!.

human909
Posts: 8371
Joined: Tue Dec 01, 2009 11:48 am

### Re: Crank arm length...

DoubleSpeeded wrote:Hmmm thinking of perhaps downgrading a crank...

ok well now that we know your a brainiac...you may help us out

what would a good comparison be... for e.g

170mm length 53T and a 175mm length 53T?

I'm not sure what your exact question is. A 170mm is ~2.9% smaller, therefore:

-For a given cadence you will need ~2.9% more force to produce the same power
OR
-For a given force you will need ~2.9% faster cadence to produce the same power

Though really the cadence/force trade off is best made intelligent gear selection, we all do this while riding. Crank choice should be determined based on the optimal crank circle size for an individuals biomechanics combined with practical considerations like pedal strike etc.

(In other words my calculations tell me narzzhing! Though one would expect that individuals with longer legs would in general prefer longer cranks. One would also expect that longer cranks are superior for out of the saddle riding.)

Xplora
Posts: 8272
Joined: Sat Dec 11, 2010 12:33 am
Location: TL;DR

### Re: Crank arm length...

^^ They would human - longer cranks will definitely favour out of saddle work, but out of saddle is brutally inefficient, and you might get some better results by keeping it tighter and losing the larger moment from the longer crank.

DoubleSpeeded
Posts: 441
Joined: Sat Jul 13, 2013 6:18 pm

### Re: Crank arm length...

toolonglegs wrote:Just get an SRM multi-length crank ... you need a power meter anyway if you want to beat Tony Martin!.

NO mate

id need an electric motor to beat tony martin..
and a good surgeon to beat his collagen enhanced lips

toolonglegs
Posts: 15451
Joined: Fri Jun 29, 2007 7:49 pm
Location: Somewhere with padded walls and really big hills!

### Re: Crank arm length...

DoubleSpeeded wrote:
toolonglegs wrote:Just get an SRM multi-length crank ... you need a power meter anyway if you want to beat Tony Martin!.

NO mate

id need an electric motor to beat tony martin..
and a good surgeon to beat his collagen enhanced lips

But you do need crank based power meter ( unless you get a Powertap Zipp ) ... all this talk of 2.5mm here or there is fine ( more or less if wearing new knicks, warmer socks ), it might make minimal improvements to your TT pace ( but how would you actually know without a PM! )... a power meter WILL make major differences to your TT'ing.

winstonw
Posts: 1793
Joined: Sun Apr 10, 2011 7:18 pm
Location: Brisbane

### Re: Crank arm length...

warthog1 wrote:Given the leg is lifted slightly higher with increased crank length, longer cranks increase leg flexion at the knee so that force is applied to the knee whilst it is hinged at a more acute angle. Can cause some people knee problems.

I occasionally correct bike fits for cyclists experiencing pain. Here's some general rules I follow:

- For cyclists whose primary issue is knee pain, I include trialing a raised seat so that the knee does not bend as much. This is usually a knee angle of 25 degrees with leg extended. (spinning tends to be easier with a knee angle of 30-35 degrees). I do this a lot for masters cyclists.

- masters cyclists with bodyfat over 15% +/- stiff spines have more difficulty getting a more efficient aero position via a larger hip angle, due to visceral fat pushing up on the diaphragm (which restricts breathing) and pushing on the femoral vein and inferior vena cava (which restricts venous return). To assist getting lower on the bars, and remaining comfortable there for longer, I'll recommend a shorter crank. I've done this for 4 people this year and it has made a significant difference. Their 57-58cm frames came with 175cm cranks, 3 of them are now on 170, and one on 172.5.

barefoot
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Joined: Tue Jan 29, 2008 11:05 am
Location: Ballarat

### Re: Crank arm length...

If I put longer cranks on a bike, I should drop the saddle (and, to be anal, bars as well) by the same amount to get the same leg extension at the bottom of the stroke.

But then I have to lift my foot higher at the top of stroke. So the difference in leg flex is actually double the difference in crank length.

I think there's an upper limit to the amount of leg flex each person finds comfortable. If your cranks are short enough to not exceed this limit, there's not much to be gained by adjusting crank length slightly. As above, slightly different foot velocity, leverage, effective gearing etc.. But if you're riding cranks that are too long, making you flex your leg more than is comfortable, then there's big benefits to be had in going to a shorter crank.

I have fairly short legs. I've had 175 and 170mm cranks - the same model canks on the same bike - and the difference is significant. 175mm is just not comfortable. I don't think I would get anywhere near the same difference going to 165mm cranks, because 170mm is short enough.

Other people often tell us that they can hardly notice a difference between 170 and 175mm cranks. That's fine - 175mm must be short enough for them. My theory is that if they kept increasing the length of their cranks... 180... 185... 190... eventually they'd find a length that is simply too long, requiring them to bend their leg too much. That being the case, the next smallest crank would probably be best for them.

I've never ridden 172.5mm cranks. I'm not sure whether they'd be too long like 175s, or short enough like 170s. I don't feel any great compulsion to find out either way. If my legs are happy to bend that far, I might get a bee's appendage better performance from the longer crank. If my legs aren't happy to bend that far, it would be significantly detrimental. I know 170s work for me, and I'm happy to stick with them.

tim

Nobody
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Location: Sydney

### Re: Crank arm length...

barefoot wrote:My theory is that if they kept increasing the length of their cranks... 180... 185... 190... eventually they'd find a length that is simply too long, requiring them to bend their leg too much. That being the case, the next smallest crank would probably be best for them.
That is if they can actually prove to themselves that the longest crank that isn't uncomfortable is actually the fastest for them, without doing any injury (which is probably more important). The science doesn't appear to say that the longest crank you can handle is necessarily better or worse than other lengths of cranks. When I've changed crank lengths I've noticed that my performance didn't change.

So I'd argue the opposite, that if performance isn't affected, that the shorter cranks should be easier on your knees, so allowing you to ride more as you get older. Riding more usually equates to better performance.

barefoot
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Location: Ballarat

### Re: Crank arm length...

Nobody wrote:
barefoot wrote:the next smallest crank would probably be best for them.
That is if they can actually prove to themselves that the longest crank that isn't uncomfortable is actually the fastest for them

Very true. That's my unstated assumption, which may or may not be true.

My main point, though, was that different length cranks probably don't feel all that different to one another, until you find your own limit of too long. So people who aren't bothered by the difference between 170 and 175 probably don't have much to contribute to discussions between people who really are bothered by the difference.

tim

toolonglegs
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Location: Somewhere with padded walls and really big hills!

### Re: Crank arm length...

You wouldn't actually know if your performance had improved without some pretty good testing including a power meter ... Adaption time could take time if you make big changes. OP is talking TT bike here. We are only talking 1 or 2% improvements here... So " feeling " better doesn't actually prove performing better.

warthog1
Posts: 6997
Joined: Wed Jul 25, 2012 4:40 pm

### Re: Crank arm length...

toolonglegs wrote:You wouldn't actually know if your performance had improved without some pretty good testing including a power meter ... Adaption time could take time if you make big changes. OP is talking TT bike here. We are only talking 1 or 2% improvements here... So " feeling " better doesn't actually prove performing better.

If you are tt iing regularly over a set course was enough for me. 8 km laps the more laps the greater the time gains for longer cranks. Ie as the distance increased my performance fell away less with the longer cranks.
I have a regular 19 min climb segment I do on a bunch ride here every tues when work allows. It is recorded on strava and I plan to run the 177.5 cranks on there and see if the times are better. I suspect there will be a marginal gain given my experience on the tt bike. I have no sprint anyway so care not how that is affected.

warthog1
Posts: 6997
Joined: Wed Jul 25, 2012 4:40 pm

### Re: Crank arm length...

Nobody wrote:That is if they can actually prove to themselves that the longest crank that isn't uncomfortable is actually the fastest for them, without doing any injury (which is probably more important). The science doesn't appear to say that the longest crank you can handle is necessarily better or worse than other lengths of cranks. When I've changed crank lengths I've noticed that my performance didn't change.

So I'd argue the opposite, that if performance isn't affected, that the shorter cranks should be easier on your knees, so allowing you to ride more as you get older. Riding more usually equates to better performance.

Were you racing when you measured your performance?
How did you measure it?
I would argue given that list I posted earlier and the fact that tt ers tend toward longer cranks, maximal aerobic performance tends to be slightly better on longer cranks. Obviously not the case if the are causing pain.

twizzle
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### Re: Crank arm length...

I swapped from 175mm on my road bikes and 172.5mm on my TT bike to 180mm on all bikes. Cadence dropped slightly, it felt a bit easier. No measurable difference in average power for TT's. But... there was quite a gap between TT's, so I was quite surprised to get good average power (~350W) with a CTL of around 40.

I'm waiting to see how I go on the next short hill TT (December) as last year I ran out of gearing (39/28), so the extra torque (~5%) could be useful to keep me at my chosen power output (~420W for ~ 12 minutes).
I ride, therefore I am.
...real cyclists don't have squeaky chains...

toolonglegs
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### Re: Crank arm length...

warthog1 wrote:
toolonglegs wrote:You wouldn't actually know if your performance had improved without some pretty good testing including a power meter ... Adaption time could take time if you make big changes. OP is talking TT bike here. We are only talking 1 or 2% improvements here... So " feeling " better doesn't actually prove performing better.

If you are tt iing regularly over a set course was enough for me. 8 km laps the more laps the greater the time gains for longer cranks. Ie as the distance increased my performance fell away less with the longer cranks.
I have a regular 19 min climb segment I do on a bunch ride here every tues when work allows. It is recorded on strava and I plan to run the 177.5 cranks on there and see if the times are better. I suspect there will be a marginal gain given my experience on the tt bike. I have no sprint anyway so care not how that is affected.

Without average watts over a segment it is still guess work, yes it may be good guess work. 1 or 2 % improvement could come from any very slight enviromental condition.
But yes I agree that regular tests can give you a pretty close idea... but even on a climb a very slight tailwind can make 1 or 2 % difference.
btw ... have a look at the difference in distances on a strava segment on the Alt leader board on velo veiwer ... can be a couple of hundred meters between first and last over some segments.
Edit ... just realized the alt leader board doesn't exist anymore

Nobody
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Location: Sydney

### Re: Crank arm length...

warthog1 wrote:Were you racing when you measured your performance?
C'mon, you know the answer to that one. If I was racing, would it make my answer more valuable? Are you racing? I think you told us the answer to that one too.
warthog1 wrote:How did you measure it?
Average speed. Just as fallible as most other people's methodology.
warthog1 wrote:I would argue given that list I posted earlier and the fact that tt ers tend toward longer cranks, maximal aerobic performance tends to be slightly better on longer cranks. Obviously not the case if the are causing pain.
I much prefer RC's post on this. I'll re-post it here for everyone's entertainment:

Jacques Anquetil doper
Lance Armstrong doper
Magnus Backstedt 177.5mm
Chris Boardman 170mm
Santiago Botero doper
Angel Casero 175mm
Mario Cipollini doper (?)
Fausto Coppi 171mm
Malcolm Elliott 172.5mm
Tyler Hamilton doper
Bernard Hinault poss doper
Miguel Indurian doper (?)
Laurent Jalabert poss doper
Greg Lemond 175mm
Robbie McEwen 175mm
Nobody ACF 165mm
Eddy Merckx doper
David Millar doper
Francesco Moser doper
Marty Northstein 167.5mm in Keirin (170mm in kilo)
Graham Obree 175mm
Marco Pantani doper
David Rebellin doper
Roger Riviere 175mm
Jean Robic prob doper
Tony Rominger doper
Oscar Sevilla 175mm
Jan Ullrich doper
Rik Verbrugghe 175mm
Erik Zabel doper
Alex Zulle doper (?)

And yes, a possible gain might be there (although the science says no so far) but there are plenty of known advantages of shorter cranks (also listed previously). Since knees are one of the most common problems that cyclists have, I thought that this might be a bigger factor in crank length selection. But knowing the general culture of cycling in AU, it probably won't factor much compared to a possible small gain in efficiency.

warthog1
Posts: 6997
Joined: Wed Jul 25, 2012 4:40 pm

### Re: Crank arm length...

Nobody wrote:If I was racing, would it make my answer more valuable?

Well if you were tt ing over a set course a number of times and had a consistent trend in your times developing then yes, I think your results would be more valuable.
Nobody wrote:Are you racing? I think you told us the answer to that one too.

Not much at the moment. However the climb I am going to time the longer cranks on I attack at or near my max every week. It is recorded on strava and if you look at the HR graph it is an almost straight horizontal line at around 195. I will soon get an idea over time which length of crank is faster for me. I dont expect a big change btw.

Nobody wrote:Average speed. Just as fallible as most other people's methodology.

Combine it with maximal effort and if your training load is relatively consistent you will establish atrend IMO

Nobody wrote:I much prefer RC's post on this. I'll re-post it here for everyone's entertainment:

Jacques Anquetil doper
Lance Armstrong doper
Magnus Backstedt 177.5mm
Chris Boardman 170mm
Santiago Botero doper
Angel Casero 175mm
Mario Cipollini doper (?)
Fausto Coppi 171mm
Malcolm Elliott 172.5mm
Tyler Hamilton doper
Bernard Hinault poss doper
Miguel Indurian doper (?)
Laurent Jalabert poss doper
Greg Lemond 175mm
Robbie McEwen 175mm
Nobody ACF 165mm
Eddy Merckx doper
David Millar doper
Francesco Moser doper
Marty Northstein 167.5mm in Keirin (170mm in kilo)
Graham Obree 175mm
Marco Pantani doper
David Rebellin doper
Roger Riviere 175mm
Jean Robic prob doper
Tony Rominger doper
Oscar Sevilla 175mm
Jan Ullrich doper
Rik Verbrugghe 175mm
Erik Zabel doper
Alex Zulle doper (?)

I don't see how doping or not affects the results. Would not doping make the same athlete choose shorter cranks to maximise their speed?

Nobody wrote:And yes, a possible gain might be there (although the science says no so far) but there are plenty of known advantages of shorter cranks (also listed previously).

The studies I have seen relate to short term maximal anaerobic (sprinting) power.
There is significant incentive for competitive cyclists at the highest level to win, evidence the amount who have doped . They tend to use longer cranks. Why?

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