Crank arm length...

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Re: Crank arm length...

Postby Nobody » Mon Sep 16, 2013 11:18 pm

warthog1 wrote:
Nobody wrote:True, but even lowly, overweight, middle-aged me with a heavy steel bike and no real sprinting ability is still happy to run a 50 which is almost 90% (89.2) of a 56. That's because it's mainly flat where I ride. Somewhat like a TT race really...


Are you regularly finding yourself in the 50-11 though? It is about a straight chainline where you spend the bulk of your time. Unless you are tting at 50kmh a 53 is more than adequate to achieve that.
But the center of the cassette (5th of 9) is supposed to match between the two chainwheels for center of chainline. So one right of middle, 6th out of 9 should be fairly straight. Straighter than 9th anyway. I run a 14-25, so 0% chance of me using a 50-11. :D
I'm a wimpy spinner, so probably spend most of my time in 3rd or 4th out of 9, which is probably a 21 or 19, so 63.8 or 70.5 inches. So if I got the maths right, then me running 4th, or 19 with a 50 is (approx) the same as a 21 (21.2) with a 56. Can't see a big difference really. :mrgreen:
Admittedly though, out of 11 speed rear is 6th being the middle. So that means probably 7th or 8th on an 11 speed for a straight chainline is going to be a 15 or 14 tooth, so you'd have to move a lot faster than me, not that that is a difficult thing to do. :oops: But if you don't need a straight chainline then 56 is not a problem, just run it in 3rd instead for 70.5 inches.
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by BNA » Mon Sep 16, 2013 11:41 pm

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Re: Crank arm length...

Postby Nobody » Mon Sep 16, 2013 11:41 pm

warthog1 wrote:
Nobody wrote:
Also you never seem to be able to find a list for the track riders, why is that? :roll: :P

Actually I just noticed on by the description which looks to be also the shortest:
Marty Northstein 167.5mm in Keirin (170mm in kilo)


I have never ridden track but try this. Chris Boardman mght have had osteoarthritic knees like yourself but the rest weren't hobbling old cripples :mrgreen:
Thanks. It appears plenty of 170s for the shorter people. I'm shorter than all the males with a listed crank length so 165mm still seems OK for me. :D
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Re: Crank arm length...

Postby DoubleSpeeded » Mon Sep 16, 2013 11:51 pm

i stuffed up guys

apologies

i meant 54 as large chainring... lol not 56.
:oops:

well 54 feels hard enough for me.

its 170mm.. im not 100% sure of my inside leg but at about 175cm it should be about right.

seems like some research at universities have looked into it.

however, 172.5 the stroke of the leg to me feels so much better..(i should've got it in that size, shame you cant really test a crankset until you buy and fit it)
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Re: Crank arm length...

Postby Nobody » Mon Sep 16, 2013 11:58 pm

DoubleSpeeded wrote:i meant 54 as large chainring... lol not 56.
:oops:
All those "discussions" and calculations for NOTHING??? :(








:wink: :D
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Re: Crank arm length...

Postby thearthurdog » Tue Sep 17, 2013 5:38 am

I'm quite tall at 193cm. I had always used 175 mm on my road bike and 180 mm on my time trial bike. I recently upgraded to Quarq Riken (power) cranksets and couldn't get 180 mm ones. My coach had been telling me to go back to 175 mm anyway. I now have 175 mm on both of my bikes and can barely notice the difference. The only time I felt the 180 mm may have been 'slightly' better was in those rare moments in a time trial when you have a slight downhill and a tail wind and you are REALLY FLYING (I run a 55 x 11 on my TT bike). But even that difference (as stated) is extremely minimal.
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Re: Crank arm length...

Postby thearthurdog » Tue Sep 17, 2013 5:42 am

warthog1 wrote:
Nobody wrote:Probably not where you live, but on the flat with a TT bike that can go 10% faster (at least) then 53 tooth X 1.1 is 58.3 teeth. So I would say 56 is fine for most. Depends on what crank length you're running too, because that is a form of gearing as well.


Tony Martin runs a 58 tooth but then he tts at over 50 kmh. No one on here comes close to that, 56 is well over geared for most.
Unless we have Cancellara or Wiggins posting on here that is.

I find the 55 just rolls a little more nicely that a 53 when you are flying along. Also, if you have a tail wind / downhill it really does make a difference to have a 55 vs a 53. Nothing worse than running out of gear in a time trial, that's just losing seconds.
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Re: Crank arm length...

Postby warthog1 » Tue Sep 17, 2013 7:13 am

Nobody wrote:All those "discussions" and calculations for NOTHING??? :(



:lol:
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Re: Crank arm length...

Postby warthog1 » Tue Sep 17, 2013 7:36 am

thearthurdog wrote:I'm quite tall at 193cm. I had always used 175 mm on my road bike and 180 mm on my time trial bike. I recently upgraded to Quarq Riken (power) cranksets and couldn't get 180 mm ones. My coach had been telling me to go back to 175 mm anyway. I now have 175 mm on both of my bikes and can barely notice the difference. The only time I felt the 180 mm may have been 'slightly' better was in those rare moments in a time trial when you have a slight downhill and a tail wind and you are REALLY FLYING (I run a 55 x 11 on my TT bike). But even that difference (as stated) is extremely minimal.


I used to tt over a set course of different numbers of laps. 177.5 were slightly faster than 175. Not much in it though. Given I don't get to tt where I live I want to try the 177.5 on the roadie and see if I climb any better.
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Re: Crank arm length...

Postby Mulger bill » Tue Sep 17, 2013 8:26 am

toolonglegs wrote:Meh... run what works for you.

[/thread] :wink:
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Re: Crank arm length...

Postby DoubleSpeeded » Tue Sep 17, 2013 9:37 am

Nobody wrote:
DoubleSpeeded wrote:i meant 54 as large chainring... lol not 56.
:oops:
All those "discussions" and calculations for NOTHING??? :(








:wink: :D

more or less, yes! sorry :mrgreen:


Mulger bill wrote:
toolonglegs wrote:Meh... run what works for you.

[/thread] :wink:


true but you can only experiment by buying one that you assume will suit and then when its time for an upgrade... go one up but then theres a chance that we wont adapt to it after all the time riding the shorter crank arm.

Ive purchased not long ago The Vision/FSA Trimax Carbon TT crankset with Ceramic bearings (btw, ceramic bearings don't make much noticeable difference either)
all up approx under $900 including the BB30 bearing... 54t and 170mm....

i often have the thought on the back of my head "I probably should've got the 172.5mm one....
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Re: Crank arm length...

Postby mikesbytes » Tue Sep 17, 2013 9:47 am

What's missing from this discussion is foot velocity. A 170mm crank with a 53 ring is roughly equivalent to a 180mm crank with a 55 ring.

Unless you go to extreme crank lengths, all that happens with different cranks is that you end up riding a different gear in the same situation
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Re: Crank arm length...

Postby Xplora » Tue Sep 17, 2013 11:04 am

I was horsing around pushing my 50/11 down the M7 last week, with around a cadence of often 40rpm, 50 on the downhills LOL I use 175 cranks. I'm pretty much convinced it comes down to optimal physiological differences for "you". Whoever that is. Some people will generate power from calves, or hip flexors, or adductors, or glutes, or whatever, better than another person. The different length crank is going to tap into those specialisations. If you are a gutless wonder, but can explode with high cadences and maintain them while your pedal stroke remains "small", then a short crank is going to work well for you because you can turn the pedals faster for each stroke on the bike. If your body wouldn't know what fast twitch was if it was slapped in the face, then longer cranks to encourage slower efforts will be helpful. I'm looking at power meter stuff and I'm so close to going to 172.5 just because it is more common that it isn't funny LOL
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Re: Crank arm length...

Postby Comedian » Tue Sep 17, 2013 11:25 am

Interesting. I'm building a bike for SWAMBO who is 175 cm tall. I was thinking 172.5..

I'm 186 cm and had a bike fit and he recommended 172.5. I had another one when I got a bike made and he looked at me and asked "are you a spinner or a grinder"... To which I answered a grinder. :o. So now I've got one bike with 172.5 and one with 175.

Is be struggling to pick a difference. If I did I'd say the bike with 172.5 is marginally easier to spin on.. But the other bike is faster. But then I don't have a cadence sensor on the 175 bike...
Once you can climb hills on a bike it's all downhill. :mrgreen:

Hopefully I'll know what that's like..... one day. :shock: :lol:

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Re: Crank arm length...

Postby DoubleSpeeded » Tue Sep 17, 2013 11:29 am

Comedian wrote:Interesting. I'm building a bike for SWAMBO who is 175 cm tall. I was thinking 172.5..

I'm 186 cm and had a bike fit and he recommended 172.5. I had another one when I got a bike made and he looked at me and asked "are you a spinner or a grinder"... To which I answered a grinder. :o. So now I've got one bike with 172.5 and one with 175.

Is be struggling to pick a difference. If I did I'd say the bike with 172.5 is marginally easier to spin on.. But the other bike is faster. But then I don't have a cadence sensor on the 175 bike...


are you talking, the same crank size and cassette sizes & Rims weight?
because im sure they would make a big difference in the feeling of easier or harder to turn.
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Re: Crank arm length...

Postby Comedian » Tue Sep 17, 2013 11:31 am

DoubleSpeeded wrote:
Comedian wrote:Interesting. I'm building a bike for SWAMBO who is 175 cm tall. I was thinking 172.5..

I'm 186 cm and had a bike fit and he recommended 172.5. I had another one when I got a bike made and he looked at me and asked "are you a spinner or a grinder"... To which I answered a grinder. :o. So now I've got one bike with 172.5 and one with 175.

Is be struggling to pick a difference. If I did I'd say the bike with 172.5 is marginally easier to spin on.. But the other bike is faster. But then I don't have a cadence sensor on the 175 bike...


are you talking, the same crank size and cassette sizes & Rims weight?
because im sure they would make a big difference in the feeling of easier or harder to turn.

Absolutely not! While the cranks and gearing is the same the wheels and bike are quite different.

2.5mm is a really small distance!
Once you can climb hills on a bike it's all downhill. :mrgreen:

Hopefully I'll know what that's like..... one day. :shock: :lol:

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Re: Crank arm length...

Postby zero » Tue Sep 17, 2013 1:51 pm

Comedian wrote:
2.5mm is a really small distance!


Its worth considering that the sprocket height is well up the crankarm and therefore the effective leverage change might well be much more than 2.5mm / 175mm.
it is very noticeable to me how much harder it is to spin 177.5s vs 175mms.
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Re: Crank arm length...

Postby scotto » Tue Sep 17, 2013 2:42 pm

Xplora wrote:I was horsing around pushing my 50/11 down the M7 last week, with around a cadence of often 40rpm, 50 on the downhills LOL I use 175 cranks. I'm pretty much convinced it comes down to optimal physiological differences for "you". Whoever that is. Some people will generate power from calves, or hip flexors, or adductors, or glutes, or whatever, better than another person. The different length crank is going to tap into those specialisations. If you are a gutless wonder, but can explode with high cadences and maintain them while your pedal stroke remains "small", then a short crank is going to work well for you because you can turn the pedals faster for each stroke on the bike. If your body wouldn't know what fast twitch was if it was slapped in the face, then longer cranks to encourage slower efforts will be helpful. I'm looking at power meter stuff and I'm so close to going to 172.5 just because it is more common that it isn't funny LOL

i bet you didnt realise those 175 cranks you sold me off youre other bike were actually 172.5. I didnt either until a while back. I cant tell a difference. but i did go speedplays to regain the right leg length/ knee angles - damn you intergrated seat post at it highest ! (ps - speedplays are so good)
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Re: Crank arm length...

Postby warthog1 » Tue Sep 17, 2013 3:18 pm

zero wrote:
Its worth considering that the sprocket height is well up the crankarm and therefore the effective leverage change might well be much more than 2.5mm / 175mm.
it is very noticeable to me how much harder it is to spin 177.5s vs 175mms.


I can pick the difference immediately I jump back on the tt bike. You also adapt really quickly too.
I don't find it limits my ability to spin but I can stay on top of a big gear a touch easier. The difference isn't huge but for me it's there. I went down to 172.5 also and found it harder to stay on top of a gear.
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Re: Crank arm length...

Postby Xplora » Tue Sep 17, 2013 3:33 pm

I promised that crank was unused. Unfortunately it was not inspected we'll enough either. At least it was not FSA lol!!!
Steve Hogg mentioned that you can raise your seat with smaller cranks, because you don't have to reach down quite as far. That's why I say everyone has different strengths.

Very worth remembering that you will create a style based on your equipment. I ride different with a 23 cassette vs 28.
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Re: Crank arm length...

Postby human909 » Tue Sep 17, 2013 5:02 pm

zero wrote:Its worth considering that the sprocket height is well up the crankarm and therefore the effective leverage change might well be much more than 2.5mm / 175mm.
it is very noticeable to me how much harder it is to spin 177.5s vs 175mms.


It is 1.4% difference no matter how big or small the sprocket is.
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Re: Crank arm length...

Postby DoubleSpeeded » Tue Sep 17, 2013 6:31 pm

i have tried a 175mm crank arm today on a MTB...

boy does that feel good.

feels i could lift my leg higher to pound the pedal... unlike my 170mm TT... feels like spinning in small circles ... =( $900 later...
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Re: Crank arm length...

Postby zero » Tue Sep 17, 2013 7:17 pm

human909 wrote:
zero wrote:Its worth considering that the sprocket height is well up the crankarm and therefore the effective leverage change might well be much more than 2.5mm / 175mm.
it is very noticeable to me how much harder it is to spin 177.5s vs 175mms.


It is 1.4% difference no matter how big or small the sprocket is.


If you have a sprocket that is the same height as your crank, you derive no leverage or footspeed change from the crank - the chain is moved at foot speed with exactly the raw force produced by your foot. ie no mechanical torque gain or loss, and no speed gain or loss.

A sprocket halfway down the crank has halved chain speed with respect to foot speed, and doubled force with respect to foot force.
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Re: Crank arm length...

Postby il padrone » Tue Sep 17, 2013 7:37 pm

zero wrote:
human909 wrote:It is 1.4% difference no matter how big or small the sprocket is.


If you have a sprocket that is the same height as your crank, you derive no leverage or footspeed change from the crank - the chain is moved at foot speed with exactly the raw force produced by your foot. ie no mechanical torque gain or loss, and no speed gain or loss.

A sprocket halfway down the crank has halved chain speed with respect to foot speed, and doubled force with respect to foot force.

I thought the discussion was about different crank lengths, not about the gearing (lever) effect of a crank upon different sized chainrings.

Apples with oranges methinks. :? :o



Chainring size does not enter into it. Think of the 'chainring effect' of pedalling a 60t ring verss a 42t ring. Which is harder?

But what if it's a 60 x 28 (57") versus a 42 x 11 (103")
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Re: Crank arm length...

Postby zero » Tue Sep 17, 2013 8:14 pm

il padrone wrote:
zero wrote:
human909 wrote:It is 1.4% difference no matter how big or small the sprocket is.


If you have a sprocket that is the same height as your crank, you derive no leverage or footspeed change from the crank - the chain is moved at foot speed with exactly the raw force produced by your foot. ie no mechanical torque gain or loss, and no speed gain or loss.

A sprocket halfway down the crank has halved chain speed with respect to foot speed, and doubled force with respect to foot force.

I thought the discussion was about different crank lengths, not about the gearing (lever) effect of a crank upon different sized chainrings.

Apples with oranges methinks. :? :o



Chainring size does not enter into it. Think of the 'chainring effect' of pedalling a 60t ring verss a 42t ring. Which is harder?

But what if it's a 60 x 28 (57") versus a 42 x 11 (103")


The number of teeth don't affect the force on or speed of the chain beyond the fact that we all use the same size teeth so number of teeth implies the radius of the sprocket.

ie its equally possible to build a 53T sprocket and chain system that is only half the height of the 53T sprocket on my bike. Just requires different hub design to accomodate the smallest sprockets to preserve the ratio effects. Note that such a design would also be harder to construct because under all circumstances the chain would be expected to have twice the force on it. If that wasn't a problem, one imagines that aero weenies would have long since clamoured for mini sprockets in an aero case, and mountain bike riders would have long since clamoured for mini sprockets that don't grind on logs.

all I'm saying is that when you just up and change cranks 2.5mm, you get more changes to your effective gearing than you may expect, and that for some bikes it may go a bit out of scope if one of the end ratios is in regular use. ie my road bike which sheared a bottom bracket I've removed the 177.5 cranks, and will use 175s from now on, and I will be interested to see how hard the minimum ratio of 39-21 becomes as a result of that change. It will be a larger change than 1.4%
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Re: Crank arm length...

Postby human909 » Tue Sep 17, 2013 9:14 pm

zero wrote:all I'm saying is that when you just up and change cranks 2.5mm, you get more changes to your effective gearing than you may expect.

And I and il padrone have pointed out, that this is not the case.

zero wrote:my road bike which sheared a bottom bracket I've removed the 177.5 cranks, and will use 175s from now on, and I will be interested to see how hard the minimum ratio of 39-21 becomes as a result of that change. It will be a larger change than 1.4%


No that is not correct. For the change you suggested you will require 1.4% more force on the pedal to achieve same result at the wheel. (If the only change is the crank length. Crank cadence will be the same, velocity of the foot 1.4% less, Velocity * Force = Power, so power is the same as expected.)

Further explanation:
For a given set of conditions and only varying the crank size all we require is identical cadence and identical torque. Torque is defined by F x D, where D is distance from axis (aka the crank length). Thus a reduction in crank length by 1.4% means a corresponding increase in force.
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