Re: Crank Length

Postby Oli on Wed 25/Mar/09 6:35pm

cyclenutnz wrote:Does your method involve lasers?


whut? :eh:
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Re: Crank Length

Postby Spokes on Wed 25/Mar/09 6:41pm

Oli wrote:
cyclenutnz wrote:Does your method involve lasers?


whut? :eh:


There really strong pointy lights, but thats not important right now......
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Re: Crank Length

Postby Oli on Wed 25/Mar/09 6:44pm

I need a laser to tell the difference in crank lengths now? I couldn't be rigorous enough to adjust my saddle accordingly without one obviously... :huh:
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Re: Crank Length

Postby cyclenutnz on Wed 25/Mar/09 6:45pm

anything that uses lasers must be more accurate, *everyone* knows that.... :p
Light Amplification by Stimulated Emission of Radiation is the long version of "really strong pointy lights"

Spokes - I did a few calcs. Using longer cranks means that you have to use a bigger gear to get the same speed out of a given amount of physical force applied to the end of the cranks. People think that using longer cranks lets them push a bigger gear when really it's just that they have to.

So crank choice for BMX needs to be based on identifying which element is your biggest limiter - peak force application or CPV. If you have trouble maintaining cadence then CPV probably the limiter so go shorter.

Take home message - lengthening your cranks by 10mm is the same as going from a 53/16 to a 53/17. Like I said earlier - one of those options is cheap...
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Re: Crank Length

Postby chappy on Wed 25/Mar/09 8:51pm

interesting article.

couple of points of contention though...
I have no problem with people shortening their cranks but I disagree with lowering your seat with shorter cranks, and actually recommend exactly the opposite: raise the seat by the corresponding crank length difference
in the pedal stroke the point of maximum force production is at 3 o'clock. this is when the gluts and quads are both working together to provide a max force to the pedal. From 12 through to 6 (the down stroke) there is a change over from quads to quads and gluts to gluts and hamstrings producing motion at the pedal.
At 6 o'clock you want to have your knee at an angle between 140 and 150 degrees.
At 12 o'clock the main consideration is having enough range in the hips for the leg to come over the top of the stroke.

so yes if you drop crank length you can drop seat if you are worried about hip angle at top, but this would decrease the efficiency of your knee angle meaning that your knee was less straight at the bottom. a better advantage of the shorter crank would be if you were to drop further at the front end for TT's decreasing your aerodynamic drag as allowed by more room to flex your hips without your hips impinging at the top of the stroke. (this needs to be taken with care though as lower and more aero does not necessarily mean faster as power production can drop off as you go lower)

maximum crank length should be related directly to leg length, longer legs =longer cranks. there is some difference from discipline to discipline ie mtb tends to be a more upright position so they can afford to ride slightly longer cranks without running out of hip flexoin(which is why if you buy a mtb and a road bike that fits you there would tend to be a 2.5mm difference longer on the mtb)
again in discipline you will typically find that track sprinters ride smaller cranks than road or mtb or even endurance track irrespective of leg length. this is historically linked to quicker acceleration and less distance the leg has to travel ie efficiency which at max speed and power is arguably more important if you consider percentage differences in power.
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Re: Crank Length

Postby shmoodiver on Wed 25/Mar/09 9:19pm

i went from 172.5 to 175 on my colnago and my first ride on it destroyed me, i did however manage 2km/h higher on my average speed.
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Re: Crank Length

Postby Spokes on Wed 25/Mar/09 9:48pm

cyclenutnz wrote:anything that uses lasers must be more accurate, *everyone* knows that.... :p
Light Amplification by Stimulated Emission of Radiation is the long version of "really strong pointy lights"

Spokes - I did a few calcs. Using longer cranks means that you have to use a bigger gear to get the same speed out of a given amount of physical force applied to the end of the cranks. People think that using longer cranks lets them push a bigger gear when really it's just that they have to.

So crank choice for BMX needs to be based on identifying which element is your biggest limiter - peak force application or CPV. If you have trouble maintaining cadence then CPV probably the limiter so go shorter.

Take home message - lengthening your cranks by 10mm is the same as going from a 53/16 to a 53/17. Like I said earlier - one of those options is cheap...


What are your thoughts on how well it would help gate starts? EG hard sprint for 5-10 secs from standing start.
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Re: Crank Length

Postby cyclenutnz on Wed 25/Mar/09 10:08pm

My thought has always been that I'd go for a shorter crank when acceleration was required, easier to get your feet going quickly. Watching the styles of BMXers most have a pretty choppy stroke - it's all about peak force through 45-135degrees (especially given how well RMC has done without clipless pedals) and I suspect that CPV is more of a limiter than force. Some claim that shorter cranks mean less time in the dead spot but that doesn't really make sense.

I see no reason why shorter cranks would be a disadvantage in the start. Potential advantages pedalling into and out of corners. Like many things - it's all about finding what works for you - the first step is being prepared to try something that goes against conventional "wisdom"
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Re: Crank Length

Postby Spokes on Wed 25/Mar/09 10:11pm

cyclenutnz wrote:My thought has always been that I'd go for a shorter crank when acceleration was required, easier to get your feet going quickly. Watching the styles of BMXers most have a pretty choppy stroke - it's all about peak force through 45-135degrees (especially given how well RMC has done without clipless pedals) and I suspect that CPV is more of a limiter than force. Some claim that shorter cranks mean less time in the dead spot but that doesn't really make sense.

I see no reason why shorter cranks would be a disadvantage in the start. Potential advantages pedalling into and out of corners. Like many things - it's all about finding what works for you - the first step is being prepared to try something that goes against conventional "wisdom"


I weigh as much as 2 RMC's and take a lot more to get going :D

And in BMX, clips suck for starts anyway.

you need these......
pedals.jpg
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Re: Crank Length

Postby SlackBoy on Thu 26/Mar/09 12:07am

Oli wrote:I need a laser to tell the difference in crank lengths now? I couldn't be rigorous enough to adjust my saddle accordingly without one obviously... :huh:
What the bloody hell would you know about bike fit anyway chump!!
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Re: Crank Length

Postby Neub on Thu 26/Mar/09 7:49pm

I'm 5'6", my very first road bike had 175 cranks. When I got serious and bought a new bike I was told 170's definately were what I needed and I never questioned it. What I did notice immediately was how aweful my climbing felt/was on the new bike. I put this down to not being used to the bike. I don't feel that my climbing has been the same since (2 years)

On Tuesday I borrowed a friends TT bike and took a minute off my 16k TT PB. The bike was a bit big for me so I wasn't that aero. It did have 172.5 cranks and they felt great. This is why I've started to question crank length on my bike.

Does anyone have some campag 172.5's crank arms I can try? - don't have to be flash carbon ones. I'm keen to see how longer cranks might effect my general racing and especially my climbing.
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Re: Crank Length

Postby inzane on Mon 28/Sep/09 12:03pm

Ok, so after 20 years plus on 175mm cranks I put a set of Deore 170s on my 5 Spot (I picked an almost new set up for $100 just to try them out http://mountainbikingzane.wordpress.com ... er-cranks/ ). I was doing some coaching up in Vic Park Christchurch yesterday with them on. Jumping on the bike I was conscious that there was a length difference, but to be honest I really did not notice any difference in my ability to ride the bike due to the cranks.

I did test going up and over steep rocky technical uphills, where I though I would be able to feel the lack of leverage, and I really could not tell the difference!! Even starting off up a steep hill in a gear big enough that I could only just pedal it I really did not notice a significant difference in my ability to get up the hill.

As I was doing a lot of short bursts of uphill pedaling (we were practicing specific parts of tracks a number of times) I cannot yet comment on what it is like going up a big long uphill. However I will get back to you with this info soon. By others comments it is when you are spinning that you really notice the difference, and so they should feel different on a long uphill.
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Re: Crank Length

Postby Fergie on Mon 28/Sep/09 12:10pm

Think most of Jim Martin's research on crank length shows that a unless you make a huge change like 2cm in length it doesn't make a huge difference. Also Jim is a sprinter so he was looking at anaerobic performance where one is riding in as aero and tight position as possible. This is relevent for TT riders where longer cranks close up the hip angle. On a MTB you are more upright so hip angle shouldn't affect power delivery in the same way it does for a TT rider or Sprinter.

That being said I feeling like I'm pedalling huge circles when I go from 170s to 172.5mm. YMMV.
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Re: Crank Length

Postby inzane on Mon 28/Sep/09 12:25pm

Yup. I was mostly looking at the decrease in foot speed for the same cadence on shorter cranks, and was interested to see if I could feel the change in leverage while climbing steep techinical stuff on the mtb.

If a shorter crank = more efficient pedaling while not affecting my ability to climb the steep stuff then I figured it might be worth changing.

If it does not work out, then my partner should be riding on 170's anyway and will get the set of cranks.
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Re: Crank Length

Postby OAP on Mon 28/Sep/09 9:32pm

Hmm, crank length selection has always bin a brain teaser.

Follow the link for another perspective (a BMX one that is)

http://www.danscomp.com/serve.php?serve ... kchart.htm
OAP
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