Crank length
- Thread starter rich p
- Start date
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HLaB
Marie Attoinette Fan
U
User482
Guest
I've had 170, 172.5 and 175 mm cranks on various bikes over the years, and I've never been able to tell the difference.
Edge705
Well-Known Member
- Location
- Thornton Lancashire
Does it not relate to clearance? I.e smaller bikes don't have the clearance for longer cranks?
Scilly Suffolk
Über Member
I think I'm right in saying that 170mm is the "standard" length crank that bikes are sold with.
Having both 170mm and 172.5mm cranksets, I did a fair bit of research recently to help me decide which bike should get which cranks: one bike is for distance and the other for speed.
The "common sense" opinion is that longer legs need longer cranks (and vice versa), which at first seems obvious. However, the more I thought about it, the less convinced I was.
When pedalling your leg, basically, moves up and down and you describe a circle with your foot. This size of the circle is dependant on the length of the crank but is unaffected by the length of the leg: a long leg will describe the same circle a short one.
Various formulas are offered in support of the "long leg, long crank" theory, but if you already know the answer you want then devising a formula to support it is straight forward.
However, you only need a basic understanding of physics to know that the length of a lever (in this case the crank) will affect the force required to achieve a set outcome (in this case turn the chainwheel once): a shorter lever will require more force than a long one as it is closer to the fulcrum.
Looking at it from another angle: a longer lever will achieve more from one rotation than a short one.
Other considerations are ground clearance (which is why shorter cranks are usual on track bikes) and the stress on your knees (a longer crank will flex your knee through a greater range).
So what did I decide? Because it will produce the maximum output and because I ride it less often (being aware of the greater stress on my knees) I put the longer cranks on the "fast" bike and the shorter on the "distance" bike.
Having said all that, I didn't set out to buy different length cranks (the 172.5mm set were just a good price) and wouldn't go out of my way to do so in the future: the affect is marginal at best.
170mm cranks will suit 90% of people 90% of the time [N.B. these figures are plucked out of the air and have no basis in fact].
Having both 170mm and 172.5mm cranksets, I did a fair bit of research recently to help me decide which bike should get which cranks: one bike is for distance and the other for speed.
The "common sense" opinion is that longer legs need longer cranks (and vice versa), which at first seems obvious. However, the more I thought about it, the less convinced I was.
When pedalling your leg, basically, moves up and down and you describe a circle with your foot. This size of the circle is dependant on the length of the crank but is unaffected by the length of the leg: a long leg will describe the same circle a short one.
Various formulas are offered in support of the "long leg, long crank" theory, but if you already know the answer you want then devising a formula to support it is straight forward.
However, you only need a basic understanding of physics to know that the length of a lever (in this case the crank) will affect the force required to achieve a set outcome (in this case turn the chainwheel once): a shorter lever will require more force than a long one as it is closer to the fulcrum.
Looking at it from another angle: a longer lever will achieve more from one rotation than a short one.
Other considerations are ground clearance (which is why shorter cranks are usual on track bikes) and the stress on your knees (a longer crank will flex your knee through a greater range).
So what did I decide? Because it will produce the maximum output and because I ride it less often (being aware of the greater stress on my knees) I put the longer cranks on the "fast" bike and the shorter on the "distance" bike.
Having said all that, I didn't set out to buy different length cranks (the 172.5mm set were just a good price) and wouldn't go out of my way to do so in the future: the affect is marginal at best.
170mm cranks will suit 90% of people 90% of the time [N.B. these figures are plucked out of the air and have no basis in fact].
fossyant
Ride It Like You Stole It!
- Location
- South Manchester
Most folk would be hard pushed to tell any difference. Ideally, the longer the legs you have the longer the cranks.
Theory is you'll get more leverage out of longer cranks, but less ability to spin. Whether anyone really notices this is dabateable.
I have always stuck to 170. When on the track, it's stipulated that you need to use 165's, and I can't tell the difference
Theory is you'll get more leverage out of longer cranks, but less ability to spin. Whether anyone really notices this is dabateable.
I have always stuck to 170. When on the track, it's stipulated that you need to use 165's, and I can't tell the difference
YahudaMoon
Über Member
It's my understanding 165 is the only size allowed, along with many other specifications for track bikes?
fossyant
Ride It Like You Stole It!
- Location
- South Manchester
It is, and certain BB heights !
YahudaMoon
Über Member
Correct :-)
slowmotion
Quite dreadful
- Location
- lost somewhere
Rich, Christopher wants a word in your ear....See 4:25.
EDIT: posted wrong clip!
EDIT: Here you go..
[media]
]View: http://www.youtube.com/watch?v=FAl_5e7bIHk[/media]
EDIT: posted wrong clip!
EDIT: Here you go..
[media]
]View: http://www.youtube.com/watch?v=FAl_5e7bIHk[/media]
GrasB
Veteran
- Location
- Nr Cambridge
Naturally a rider in any kind of shape will tend to keep their foot speed constant, that is to say if you nominally ride at 1.8m/s foot speed you'll simply change your cadence depending on what crank length you use. The theory goes is that the power production will be the same across a wide range of cranks. However your bike fit can make getting the crank size right suddenly much more efficient at a certain threshold. The only thing you can do is try, which is an expensive road to go down.
There's 2 schools of thought with regard to crank length;
1. Grinders should go for longer cranks to maximize torque, spinners ride on shorter cranks to maximize rpm
2. Spinners should go for the longer cranks to maximize torque in their low rpm registers. Grinders should use longer cranks to help push their rpm up the scale.
The current fashion in the UK seems to be option 1 but there are lots of people out there who think option 2 is the right answer.
There's 2 schools of thought with regard to crank length;
1. Grinders should go for longer cranks to maximize torque, spinners ride on shorter cranks to maximize rpm
2. Spinners should go for the longer cranks to maximize torque in their low rpm registers. Grinders should use longer cranks to help push their rpm up the scale.
The current fashion in the UK seems to be option 1 but there are lots of people out there who think option 2 is the right answer.
Oooh, I hadn't given the hip any thought TBH. Would 2.5mm make any real difference though considering the number of revolutions? Spinning more, in a lower gear, feels more comfortable in practise though so maybe the shorter ones would help in that respect.1823195 said:
sidevalve
Über Member
Surely the length of crank does'nt alter the speed of rotation, one turn of crank is the same at the rear wheel whatever the crank length, only the leverage changes [slightly]. Would it not be simpler to use a diferent chainring size to achieve the desired result [for all normal uses anyway] ?
GrasB
Veteran
- Location
- Nr Cambridge
^^ yeah that
165mm = 1036mm foot circle = 1.55m/s @ 90rpm
175mm = 1100mm foot circle = 1.65m/s @ 90rpm
So for the same rpm your foot on a 175mm crank is moving as thought you're a gear lower compared to 165mm cranks.
The problem is that though you increase the rpm you don't actually reduce the joint load as the all important foot speed is the same. It does however reduce joint movement which may or may not be advantageous.
165mm = 1036mm foot circle = 1.55m/s @ 90rpm
175mm = 1100mm foot circle = 1.65m/s @ 90rpm
So for the same rpm your foot on a 175mm crank is moving as thought you're a gear lower compared to 165mm cranks.
The problem is that though you increase the rpm you don't actually reduce the joint load as the all important foot speed is the same. It does however reduce joint movement which may or may not be advantageous.
