Crank lengths

[Ben Wight]

I haven't seen this being discussed much, and haven't been able to find much info at all from
web searches.

I currently am using a 48 tooth chain ring with 175mm cranks, and have been on the search for at
least 180mm crank and 52 tooth chain ring combo for my MTB. Local bike shops look at me strange when
i ask them about 180 or above cranks, and all tell me that 175 is the biggest normal size.

I am relatively tall, about 6'1", and think I could easily push a 180 or higher crank, with the only
problem I've heard about being knee soreness and cadence drop.

I dont think I will have knee problems, as I have been using the 175mm crank for about 13 years,
(since I was 10), and never had any knee probs then, so using the logic that I am much taller now, a
bigger crank should be no probs.

I also feel that the extra torque available from the bigger crank will let me spin a bigger chain
wheel with the same effort.

1. So my questions are, does anyone see any flaws in my pretty simple theory.
2. Why isn't it more common to see larger cranks for taller riders? Obviously, if you have a
cadence drop, but you are putting in more torque, then if you gear the bike correctly, it should
be faster.

Ben Wight

 

[Mike]

Ben Wight wrote:

> I am relatively tall, about 6'1", and think I could easily push a 180 or higher crank,

Sure you could, but is there anything to gain??

> I also feel that the extra torque available from the bigger crank will let me spin a bigger chain
> wheel with the same effort.

Again, yes, but at a lower speed. No real gain.

> 2. Why isn't it more common to see larger cranks for taller riders?

Because it makes little difference. A _very_ tall rider might gain, but you are not that far
from the mean.

> Obviously, if you have a cadence drop, but you are putting in more torque, then if you gear the
> bike correctly, it should be faster.

I don't see that at all. Yes, a longer crank will mean slightly lower cadence, and higher gear.
Well, with only a 1.5% difference it really means you shift up slightly sooner, and down slightly
later. More significantly, it means your muscles move through a greater range, but unless you are 7'
tall, that doesnt necessarily help. Its the speed of muscle contraction that is important, and you
control that by selecting an appropriate gear.

HTH

--
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[Ben Wight]

Okay, someone to discuss this with. I am vaugely familiar with the theory of changing gears to keep
the cadence constant, I dont exactly know why this is though. Is their an optimum speed to max
efficiency?

My theory is if you push a bigger crank, you will generate more torque, then you gear it taller, to
keep the same cadence. Yes you will be moving in a bigger swept circle, but I would guess for the
average rider, most of that movement is not caused by muscle contraction, but simply the momentum of
the moving masses. You actually use your quads, calves and glutes for less that half a rev right????

What I am trying to say is that I dont think a bigger swept range will take much toll at all with
respect to muscle fatigue. The extra torque available from the power stroke should be able to
generate more speed at the wheel.

What about the speed of the muscle contraction business, how does that work in summary?

Ben Wight

Mike <[email protected]> wrote in message
news:[email protected]...
> Ben Wight wrote:
>
> > I am relatively tall, about 6'1", and think I could easily push a 180 or higher crank,
>
> Sure you could, but is there anything to gain??
>
> > I also feel that the extra torque available from the bigger crank will
let
> > me spin a bigger chain wheel with the same effort.
>
> Again, yes, but at a lower speed. No real gain.
>
> > 2. Why isn't it more common to see larger cranks for taller riders?
>
> Because it makes little difference. A _very_ tall rider might gain, but you are not that far from
> the mean.
>
> > Obviously, if you have a cadence drop, but you are putting in more
torque,
> > then if you gear the bike correctly, it should be faster.
>
> I don't see that at all. Yes, a longer crank will mean slightly lower cadence, and higher gear.
> Well, with only a 1.5% difference it really means you shift up slightly sooner, and down slightly
> later. More significantly, it means your muscles move through a greater range, but unless you are
> 7' tall, that doesnt necessarily help. Its the speed of muscle contraction that is important, and
> you control that by selecting an appropriate gear.
>
> HTH
>
> --
> make nospam into oz to reply.

 

[Mike]

Ben Wight wrote:
> Okay, someone to discuss this with. I am vaugely familiar with the theory of changing gears to
> keep the cadence constant, I dont exactly know why this is though. Is their an optimum speed to
> max efficiency?

Think about what you mean by "efficiency". We dont care about fuel efficiency like you would with a
petrol engine or battery. Its more a matter of maximum sustainable, or comfortable, power.
Drivetrain losses are utterly insignificant - its your legs that matter. I dont want to get too
technical, but know this one equation: P=Fv, ie power = force x velocity. So pedalling faster would
mean more power, except that you cannot keep up the same force as with a slower cadence.

> My theory is if you push a bigger crank, you will generate more torque, then you gear it taller,
> to keep the same cadence. Yes you will be moving in a bigger swept circle,

Yes, more torque at the same force. But with a bigger circle, it will take your legs longer to go
around. Unless you push harder. It sounds like you are wanting something-for-nothing here. There is
no free lunch. If it were that easy, wouldnt everyone do it?

> but I would guess for the average rider, most of that movement is not caused by muscle
> contraction, but simply the momentum of the moving masses.

That doesnt really make much sense. If you are keen, dig out your old school physics books and look
up mechanics, Newtons laws, etc.

> You actually use your quads, calves and glutes for less that half a rev right????

The muscles are still busy between pushes. Lots of chemistry happening. Sorry to be so negative. I
wish you were right. No free energy though.

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[Ben Wight]

Mike <[email protected]> wrote in message
news:[email protected]...
> Ben Wight wrote:
> > Okay, someone to discuss this with. I am vaugely familiar with the
theory
> > of changing gears to keep the cadence constant, I dont exactly know why
this
> > is though. Is their an optimum speed to max efficiency?
>
> Think about what you mean by "efficiency". We dont care about fuel efficiency like you would with
> a petrol engine or battery. Its more a matter of maximum sustainable, or comfortable, power.
> Drivetrain losses are utterly insignificant - its your legs that
matter.
> I dont want to get too technical, but know this one equation: P=Fv, ie power = force x
> velocity. So pedalling faster would mean more power, except that you cannot keep up the same
> force as with a slower cadence.

Ahh, so this is what I was getting at. There is an optimum speed where the greatest force can be
delivered comfortably. This is what I meant by max efficiency, just like peak torque of a motor.

Yes that equation is right, Watt=Nm/sec, I understand that simply by installing a larger crank, you
cannot suddenly magically output more power. I was coming more from experience with riding a 170mm
crank equipped bike, compred to my 175mm equipped bike. Everytime I ride the bike with smaller
crank, I feel like I am pedalling hader to get the same result. There must be an optimum crank
length for the length of the riders leg. I feel that if I go to a 180 or bigger, there will be more
potential unleasehed.

> > My theory is if you push a bigger crank, you will generate more torque,
then
> > you gear it taller, to keep the same cadence. Yes you will be moving in
a
> > bigger swept circle,
>
> Yes, more torque at the same force. But with a bigger circle, it will take your legs longer to go
> around. Unless you push harder.

What about a 50mm crank, surely you cant just spin it faster with the same force to generate the
same power though. I know the calc works, but I wonder if you actually tried it in practice what
would happen.

> It sounds like you are wanting something-for-nothing here. There is no free lunch. If it were
> that easy, wouldnt everyone do it?

Well, why limit the average crank length to 170mm? If a 5" rider can use a 170mm crank optimumly,
then it just seems logical that a 6" rider can use a longer crank more effectively, but surely 175
isn't the limit, maybe its more of a convention.

> > but I would guess for the average rider, most of that movement is not caused by muscle
> > contraction, but simply the momentum of
the
> > moving masses.
>
> That doesnt really make much sense. If you are keen, dig out your old school physics books and
> look up mechanics, Newtons laws, etc.

What I am getting at is followed up in the next line below. The average rider is not really exerting
themselves on the upstroke, or at BDC or TDC, but mostly on the down stroke after TDC. This is what
I meant by using your muscles for only half a rev (for each leg of course).

> > You actually use your quads, calves and glutes for less that half a rev right????
>
> The muscles are still busy between pushes. Lots of chemistry happening. Sorry to be so negative. I
> wish you were right. No free energy though.

I have just had a brainstorm which has set my original thoughts out more clearly. When climbing and
out of the saddle, you a using your body weight to be the maojor contributing force to the resultant
force down on the pedal, to generate a torue. With keeping a constant cadence not being as big of an
issue when slowed down to the point where you are up out of the saddle to climb, a bigger crank will
give you the same cadence but with a taller gear right?????

Meaning, a faster climb, surely????

I will be getting a 180 crank to determine any difference, and I will be watching my results from
the speedo with interest.

 

[John Retchford]

"Ben Wight" <[email protected]> wrote in message news:[email protected]...
>
> I have just had a brainstorm which has set my original thoughts out more clearly. When climbing
> and out of the saddle, you a using your body
weight
> to be the maojor contributing force to the resultant force down on the pedal, to generate a torue.
> With keeping a constant cadence not being as big of an issue when slowed down to the point where
> you are up out of the saddle to climb, a bigger crank will give you the same cadence but with a
> taller gear right?????
>
> Meaning, a faster climb, surely????
>
> I will be getting a 180 crank to determine any difference, and I will be watching my results from
> the speedo with interest.
>
Ben, I hope the storm in your brain has now abated. Your reasoning has it that the heaviest person
with the longest cranks will be the best climber. Weren't we all silly to think that power/weight
ratio and oxygen uptake were the important factors? Do you think that changing crank length by less
than 3% will make a measurable difference? Nobody that I am aware of has produced compelling
evidence of efficiency gains from small changes in crank length.

John Retchford

 

[John Retchford]

"John Retchford" <[email protected]> wrote in message
news:[email protected]...
>
PS Have a look at : http://sheldonbrown.com/cranks.html

John Retchford

 

[Super Mario]

I used to use 170mm cranks. I am 5 foot 8 inches and weigh 71kgs. Using 170mm cranks and a 52-111 drivetrain used to be able to time trial at 43kms comfortably.

I started using 180mm cranks about 3 months ago with the same 52-11 drive train and can time trial at 48kms an hour comfortably.

Coming from an engineering background with a degree in mechanical engineering it is easy to work out.

You have a bigger lever (which is the crank), the bigger the lever, the less work that is needed to move the object, (which is the bottom brcket)

Your legs go around a much biggher circle, but if you maintain the same cadence, you will generate a faster speed.

The only injury that you have to watch for, is making sure your inside pedal is up going around a corner!

 

[Peter Signorini]

"Super Mario" <[email protected]> wrote in message
news:[email protected]...
> I used to use 170mm cranks. I am 5 foot 8 inches and weigh 71kgs. Using 170mm cranks and a 52-111
> drivetrain used to be able to time trial at 43kms comfortably.
>
> I started using 180mm cranks about 3 months ago with the same 52-11 drive train and can time trial
> at 48kms an hour comfortably.
>
> Coming from an engineering background with a degree in mechanical engineering it is easy to
> work out.
>
> You have a bigger lever (which is the crank), the bigger the lever, the less work that is needed
> to move the object, (which is the bottom brcket)

Coming from an engineering background perhaps you can explain the concept of "work", especially as
it relates to travelling 5 kmh faster
>
> Your legs go around a much biggher circle, but if you maintain the same cadence, you will generate
> a faster speed.

Doesn't this translate as more "work"?

Cheers Peter

 

[Mike]

Super Mario wrote:
> I used to use 170mm cranks. I am 5 foot 8 inches and weigh 71kgs. Using 170mm cranks and a 52-111
> drivetrain used to be able to time trial at 43kms comfortably.
>
> I started using 180mm cranks about 3 months ago with the same 52-11 drive train and can time trial
> at 48kms an hour comfortably.

Thats 12% speed gain, around 30% power gain, from a small geometry change? Whats a polite term for
"********!!" ? I calculate that with 210mm cranks, you will set a new world record.

> Coming from an engineering background ... You have a bigger lever (which is the crank), the bigger
> the lever, the less work that is needed to move the object, (which is the bottom brcket)

Well, you just failed mech.eng 101 . A longer lever allows less force, but over a greater distance.
"work", ie energy (joules), is the same.

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[Mike]

Ben Wight wrote:

> Ahh, so this is what I was getting at. There is an optimum speed where the greatest force can be
> delivered comfortably.

I guess so. That speed is ZERO. Force is limited by your weight. The biggest, most
overweight guy wins.

> compred to my 175mm equipped bike. Everytime I ride the bike with smaller crank, I feel like I am
> pedalling hader to get the same result. There must

Changing gear will have a much greater effect.

> What about a 50mm crank, surely you cant just spin it faster with the same force to generate the
> same power though.

At that rate, losses in your leg from reciprocating motion would be too great. You would find that
you were doing all the work with calves, not the big thigh/bum muscles.

Somewhere around 165-175 works fine for most people. You are tall-ish, so stick with 175. +/- 5%
wont make a meaningful difference.

> I have just had a brainstorm which has set my original thoughts out more clearly. When climbing
> and out of the saddle, you a using your body weight to be the maojor contributing force to the
> resultant force down on the

No. You are still confusing force with power. ALL force you apply to the pedals must be
counterbalanced. Your weight can do this, but it doesnt do work, unless you are going downhill. If
you rode a bike on the moon, you would want to be strapped in to go faster. Climbing out of the
saddle allows you to put more weight on the pedals, but doesnt make you more powerful. In fact, you
actually loose power, because the muscles are less efficient at the lower speed. So why do it? You
should only climb out of the saddle to grind up a hill when you have run out of gears. Its a poor
second choice to shifting down.

> pedal, to generate a torue. With keeping a constant cadence not being as big of an issue when
> slowed down to the point where you are up out of the saddle to climb,

Pardon? You are up out of the saddle BECAUSE your cadence is way too slow, because you dont have
low-enough gears. If you are on a racer with 40:23 bottom gear, it happens a lot. If you are on a
22:34 mountain bike, you may never need to stand. And you'll climb faster than on the racer - with
compensating weights, of course.

> bigger crank will give you the same cadence but with a taller gear right?????

Yes, but its much better to just shift down a gear. Or use smaller wheels.

> I will be getting a 180 crank to determine any difference, and I will be watching my results from
> the speedo with interest.

I would be very interested to measure the difference, but would need you in a lab on a stationary
bike with V(O2) and power meters.

The hill-climb gain might be measurable with a stopwatch, assuming you dont have a low enough gear
to keep up 90rpm cadence. Will your new crankset have triple chainrings?

Let us know if it makes a difference to comfort. I once noticed I couldn't bend as low with a big
crank, because the thigh was pushing into my gut on the upstroke. Time for a diet

Thanks for the post. Its good food for thought. I wish I had the equipment to test it empirically.

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[Peter Signorini]

"Mike" <[email protected]> wrote in message news:3e9cd806$0$1031
> Climbing out of the saddle allows you to put more weight on the pedals, but doesnt make you
> more powerful. In fact, you actually loose power, because the muscles are less efficient at the
> lower speed. So why do it? You should only climb out of the saddle to grind up a hill when you
> have run out of gears. Its a poor second choice to shifting down.

Tell this one to Lance Armstrong and the other TdF riders as they climb the Pyrenees cols. A
standard tactic in climbs is to chnge *up* a gear and stand to accelerate up a hill.

Cheers Peter

 

[Mike]

John Retchford wrote:
> "John Retchford" <[email protected]> wrote in message
> news:[email protected]...
>
> PS Have a look at : http://sheldonbrown.com/cranks.html

I second that. then read the whole site. Sheldon's explanations are clear and accurate. (He does say
when something is controversial.)

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[Jose Rizal]

Super Mario:

> I used to use 170mm cranks. I am 5 foot 8 inches and weigh 71kgs. Using 170mm cranks and a 52-111
> drivetrain used to be able to time trial at 43kms comfortably.
>
> I started using 180mm cranks about 3 months ago with the same 52-11 drive train and can time trial
> at 48kms an hour comfortably.
>
> Coming from an engineering background with a degree in mechanical engineering it is easy to
> work out.

Not so for some, as you demonstrate.

> You have a bigger lever (which is the crank), the bigger the lever, the less work that is needed
> to move the object, (which is the bottom brcket)

No, the longer the crank, the less *force* is needed to rotate it at the same speed since the moment
arm is longer. That's why it seems physically easier. The *work* required to rotate the crank is the
same at the same speed. Work and force are two different physical concepts, the former being the
product of force and the distance it acts on.

Neglecting gravity effects, and in simple terms which may jolt back your memories of first year
mechanical engineering, to maintain the same speed of rotation for cranks of different lengths,
torque must be the same: that is,

Fo * ro = F1 * r1 (1)

where Fo and ro are the force and crank length for a short crank, and F1 and r1 are those for a
longer crank.

For Fo acting on a crank ro and rotating it a full turn, the work done is

Wo = Fo * circumference = Fo * 2 * Pi * ro (2).

For a crank r1, W1 = F1 * 2 * Pi * r1 (3).

From (1), F1 = (Fo * ro)/r1 so

W1 = (Fo * ro)/r1 * 2 * Pi * r1 = Fo * 2 * Pi * ro. That is,

Wo = W1.

Work is the same for both short and long cranks rotating at the same speed. The force F1, although
less than Fo and seems "easier" to apply in terms of physical effort, has to act over a longer
distance per crank revolution, while Fo, which seems "harder" to apply, has to act over a shorter
distance per revolution.

Nothing's ever free in physics.

> Your legs go around a much biggher circle, but if you maintain the same cadence, you will generate
> a faster speed.

You're now confusing longer cranks with higher gearing. Using the same gear ratio, if you maintain
the same cadence, your speed will not increase if you use longer cranks. If you want to go faster,
you need to rotate the cranks faster or shift into higher gear.

 

[Ben Wight]

> The hill-climb gain might be measurable with a stopwatch, assuming you dont have a low enough gear
> to keep up 90rpm cadence. Will your new crankset have triple chainrings?
>
> Let us know if it makes a difference to comfort. I once noticed I couldn't bend as low with a big
> crank, because the thigh was pushing into my gut on the upstroke. Time for a diet

At the end of the day, the longer crank for me may just be a mental boost.

So in summary, I understand the concept of power and work. At the end of the day, to go faster, you
must generate more power and do more work, but is it possible that a longer crank will allow one's
body to output more power easily, when taking the comfort factor into account?

I haven't checked out any referred links yet, so I still maybe talking out my ass!

 

[Super Mario]

I mightbe generating more work, but the speed generated is more for the same amount I put in genious!

All the formulas you put out are ****! It's the reality of the actua; experiment!

What I wrote is true. And I have made 190mm cranks in my workshop! Which I will use to race this weekend!

there is no rule in stateing how long crank arms have to be!

 

[Jose Rizal]

Super Mario:

> I mightbe generating more work, but the speed generated is more for the same amount I put in
> genious!

It's "genius", not "genious". And no, any engineering student can tell you that you're wrong, have
no concept whatsoever of any physical laws and mechanics of solids; in fact, no understanding of
engineering at all. "Engineering background" indeed... If you're going to make up things, at least
read up a bit and make it read like you know what you're talking about.

> All the formulas you put out are ****!

Sorry, you can't dismiss the laws of physics just because you can't understand it.

> It's the reality of the actua; experiment!

Ah, on your make-believe world where the laws of physics do not apply? Of course, I see it now...
chocolate streets and chocolate dogs, and a chocolate sale in a chocolate shop!

> What I wrote is true.

In the land of chocolates....

> And I have made 190mm cranks in my workshop! Which I will use to race this weekend!

And I have turned lead into gold! Which I will sell this weekend! I'll be rich, rich I tells ya....

> there is no rule in stateing how long crank arms have to be!

It's "stating", not "stateing", and it's "cranks", not "crank arms" (in every sense of the word).

Your poor literacy skills and lack of any clue of the world around you highlight the comical claim
you made of having a mechanical engineering degree and an engineering background. Go back to your
room, little boy.

 

[Peter Signorini]

"Super Mario" <[email protected]> wrote in message
news:[email protected]...
> I mightbe generating more work, but the speed generated is more for the same amount I put in
> genious!

You certainly are ingenious if you can generate more speed for the same work, simply by using a
longer crank.
>
> All the formulas you put out are ****! It's the reality of the actua; experiment!

Yes, the experiment is crucial to prove the formula. So tell us about the experiment you used to
prove that you could override the laws of physics re. work.

> What I wrote is true.

Ahh, the truth! Is that like the truth of the immaculate conception?

> And I have made 190mm cranks in my workshop! Which I will use to race this weekend!
>
> there is no rule in stateing how long crank arms have to be!

Certainly not. Show us how your 1000mm cranks can help you achieve the speed of sound for the
same work.

Cheers Peter

 

[John Staines]

Errr I don't think picking on his literary skills was warrented or added to the discussion.

It just made you look abit silly and narky.

all IMHO of course. )

Kind regards

Jose Rizal wrote:
>
> Super Mario:
>
> > I mightbe generating more work, but the speed generated is more for the same amount I put in
> > genious!
>
> It's "genius", not "genious". And no, any engineering student can tell you that you're wrong, have
> no concept whatsoever of any physical laws and mechanics of solids; in fact, no understanding of
> engineering at all. "Engineering background" indeed... If you're going to make up things, at least
> read up a bit and make it read like you know what you're talking about.
>
> > All the formulas you put out are ****!
>
> Sorry, you can't dismiss the laws of physics just because you can't understand it.
>
> > It's the reality of the actua; experiment!
>
> Ah, on your make-believe world where the laws of physics do not apply? Of course, I see it now...
> chocolate streets and chocolate dogs, and a chocolate sale in a chocolate shop!
>
> > What I wrote is true.
>
> In the land of chocolates....
>
> > And I have made 190mm cranks in my workshop! Which I will use to race this weekend!
>
> And I have turned lead into gold! Which I will sell this weekend! I'll be rich, rich I
> tells ya....
>
> > there is no rule in stateing how long crank arms have to be!
>
> It's "stating", not "stateing", and it's "cranks", not "crank arms" (in every sense of the word).
>
> Your poor literacy skills and lack of any clue of the world around you highlight the comical claim
> you made of having a mechanical engineering degree and an engineering background. Go back to your
> room, little boy.

 

[Jose Rizal]

John Staines:

> Errr I don't think picking on his literary skills was warrented or added to the discussion.

It's "literacy", not "literary". It's "warranted", not "warrented".

> It just made you look abit silly and narky.

It's "a bit", not "abit".

Sorry, didn't know you two were related.

> all IMHO of course. )
>
> Kind regards
>
>
>
> Jose Rizal wrote:
> >
> > Super Mario:
> >
> > > I mightbe generating more work, but the speed generated is more for the same amount I put in
> > > genious!
> >
> > It's "genius", not "genious". And no, any engineering student can tell you that you're wrong,
> > have no concept whatsoever of any physical laws and mechanics of solids; in fact, no
> > understanding of engineering at all. "Engineering background" indeed... If you're going to make
> > up things, at least read up a bit and make it read like you know what you're talking about.
> >
> > > All the formulas you put out are ****!
> >
> > Sorry, you can't dismiss the laws of physics just because you can't understand it.
> >
> > > It's the reality of the actua; experiment!
> >
> > Ah, on your make-believe world where the laws of physics do not apply? Of course, I see it
> > now... chocolate streets and chocolate dogs, and a chocolate sale in a chocolate shop!
> >
> > > What I wrote is true.
> >
> > In the land of chocolates....
> >
> > > And I have made 190mm cranks in my workshop! Which I will use to race this weekend!
> >
> > And I have turned lead into gold! Which I will sell this weekend! I'll be rich, rich I tells
> > ya....
> >
> > > there is no rule in stateing how long crank arms have to be!
> >
> > It's "stating", not "stateing", and it's "cranks", not "crank arms" (in every sense of the
> > word).
> >
> > Your poor literacy skills and lack of any clue of the world around you highlight the comical
> > claim you made of having a mechanical engineering degree and an engineering background. Go back
> > to your room, little boy.