Crank arm length -How important is it?

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Daz555
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Re: Crank arm length -How important is it?

Postby Daz555 » Tue Aug 20, 2013 11:28 am

I rode my commuter for several months with one crank 5mm shorter than the other (long story - seized square taper). I could not feel the difference at all. After many months of riding without the crank bolt, the arm eventually worked itself loose. :mrgreen:
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MichaelW
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Re: Crank arm length -How important is it?

Postby MichaelW » Tue Aug 20, 2013 12:44 pm

A lot of misconceptions going on in this thread.
Saddle height is set from the 6:00 pedal position, not the BB centre. If some fit systems measure from BB, then they have to define the crank length and pedal stack. If you change from a 165 to175mm crank, you have to lower the saddle. You may also have to move it forward for the layback adjustment.
Pedal stack (from spindle to footbed) varies with different pedals, cleats, shoes. This makes no difference to the vertical deflection of the foot. 170mm cranks still go up and down 340mm and your hip still rotates through the same angle of movement.

What are the constants and what are the variables in pedalling. Under any one pedalling style (eg a sprint, a long cruise, a steep climb), the following should be true.
Power output is a constant. You cant increase your own power. If everyone could simply add power, there would be no limit to our speed. Nothing can increase your power once it leaves your foot.
Pedalling force is a constant. You cant just press harder.
Foot travel speed (FTS) is a useful constant. Travel is around the circumference and units are in distance, not angles.
One circumference is not just like every other circumference, they vary in distance. Imagine a 100mm crank and a 500mm crank. At constant foot travel speed, the 500mm crank would take longer to rotate and would require more work. The power output and pedalling force have not changed, neither has the bike speed.

Gear ratio is variable. (unless it is fixed) . As you alter cadence or crank length or anything else, you can change gear to find an optimum ratio.
Cadence here is a variable, you alter it to suit the crank length. Smaller cranks are pedalled faster because each rotation is less work. If you fix the crank length, you fix the cadence.


Any of the human factors can be improved over time by training but it is useful to consider them as fixed.

Slowbike
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Re: Crank arm length -How important is it?

Postby Slowbike » Tue Aug 20, 2013 13:01 pm

MichaelW wrote:Power output is a constant. You cant increase your own power. If everyone could simply add power, there would be no limit to our speed. Nothing can increase your power once it leaves your foot.
Pedalling force is a constant. You cant just press harder.
Imagine a 100mm crank and a 500mm crank. At constant foot travel speed, the 500mm crank would take longer to rotate and would require more work. The power output and pedalling force have not changed, neither has the bike speed.

Smaller cranks are pedalled faster because each rotation is less work.


How are you defining work?!

A longer lever requires less force to turn, but to achieve the same as a shorter lever it has to travel further ..

With everything else constant the watts required to move a cyclist along at a given speed doesn't change with a difference in crank length - as power in is pretty much power out (losses will remain constant) then it isn't harder to pedal a longer crank or easier to pedal a shorter one - with the exception of 1 factor - the engine.

The crank length needs to be matched to the engine - that is all.

Cav uses short cranks ... he's a short ars* - with short legs.
Wiggo uses 177.5mm cranks - he's tall ...

Apparently Chris Hoy uses 170mm cranks on a track bike ... but that's on the track ...

So, sample size of two (roadies) if you're short in the leg then go for a short crank, if you're long in the leg then go for a long crank!

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drlodge
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Re: Crank arm length -How important is it?

Postby drlodge » Tue Aug 20, 2013 13:10 pm

Slowbike wrote:So, sample size of two (roadies) if you're short in the leg then go for a short crank, if you're long in the leg then go for a long crank!


That about sums it up.

When I was being fitted up at Rourkes they mentioned 172.5mm is the new "standard" for Mr Average. At 5'9" I am Mr Average (in the height department anyway *cough*) so that's what I've got. Slightly prefer it over 170mm since I get a little more leverage and effectively a slightly lower bottom gear for steep hills.
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Daz555
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Re: Crank arm length -How important is it?

Postby Daz555 » Tue Aug 20, 2013 14:14 pm

drlodge wrote:
Slowbike wrote:So, sample size of two (roadies) if you're short in the leg then go for a short crank, if you're long in the leg then go for a long crank!


That about sums it up.

When I was being fitted up at Rourkes they mentioned 172.5mm is the new "standard" for Mr Average. At 5'9" I am Mr Average (in the height department anyway *cough*) so that's what I've got. Slightly prefer it over 170mm since I get a little more leverage and effectively a slightly lower bottom gear for steep hills.

Fair enough - but I suspect you "prefer it" because logic and new wisdom suggests it should be better - as opposed to actually being able to feel a 2.5mm change in crank length. If so, you are not alone - happens all the time with bike stuff. :mrgreen:
You only need two tools: WD40 and Duck Tape.

If it doesn't move and should, use the WD40.

If it shouldn't move and does, use the tape.

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drlodge
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Re: Crank arm length -How important is it?

Postby drlodge » Tue Aug 20, 2013 14:38 pm

Daz555 wrote:
drlodge wrote:
Slowbike wrote:So, sample size of two (roadies) if you're short in the leg then go for a short crank, if you're long in the leg then go for a long crank!


That about sums it up.

When I was being fitted up at Rourkes they mentioned 172.5mm is the new "standard" for Mr Average. At 5'9" I am Mr Average (in the height department anyway *cough*) so that's what I've got. Slightly prefer it over 170mm since I get a little more leverage and effectively a slightly lower bottom gear for steep hills.

Fair enough - but I suspect you "prefer it" because logic and new wisdom suggests it should be better - as opposed to actually being able to feel a 2.5mm change in crank length. If so, you are not alone - happens all the time with bike stuff. :mrgreen:


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bernithebiker
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Re: Crank arm length -How important is it?

Postby bernithebiker » Tue Aug 20, 2013 15:39 pm

Daz555 wrote:Fair enough - but I suspect you "prefer it" because logic and new wisdom suggests it should be better - as opposed to actually being able to feel a 2.5mm change in crank length. If so, you are not alone - happens all the time with bike stuff. :mrgreen:


If you think there's no difference in changing crank length by 2.5 or 5mm, then why do Shimano et al make 167.5, 170, 172.5, 175, 177.5, etc.?

Surely it would be easier for them to just give us all 172.5mm wouldn't it?
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MichaelW
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Re: Crank arm length -How important is it?

Postby MichaelW » Tue Aug 20, 2013 17:10 pm

Slowbike wrote:How are you defining work?!
A longer lever requires less force to turn, but to achieve the same as a shorter lever it has to travel further ..

With everything else constant the watts required to move a cyclist along at a given speed doesn't change with a difference in crank length - as power in is pretty much power out (losses will remain constant) then it isn't harder to pedal a longer crank or easier to pedal a shorter one - with the exception of 1 factor - the engine.

The crank length needs to be matched to the engine - that is all.


Work=power x time.
Time is the time taken for 1 rev which varies with the circumference of the rev so, bigger cranks take more work to turn (by a person of fixed power and strength/foot pressure)

Force is a constant in pedalling. A longer lever requires less force to move the same mass BUT a longer lever also allows the same force to move more mass. The work done over the greater distance of the longer lever. Instead of moving mass, cranks generate torque. More torque means that you can use a higher gear.

Agreed, if the rider produces 100 watts, then that is all they have, regardless of crank length. But at longer crank length, the rider will be outputting 100 watts for a longer duration doing more work per rev. It is for the gears to transmit that to the wheels, more work making the bike go further for each rev of the longer crank. The speed remains the same whatever crank length is used.

And I agree that the major reason for different crank lengths is different leg length. A secondary reason is different pedalling styles.
Last edited by MichaelW on Tue Aug 20, 2013 22:42 pm, edited 1 time in total.

samsbike
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Re: Crank arm length -How important is it?

Postby samsbike » Tue Aug 20, 2013 18:39 pm

I have 175mm on the mtb and 170 on the road. The latter surprised me as I thought a 56cm bike would come with 172.5.

However on my new build I fitted 175 and psychologically could not get comfortable, not sure what it was. I also have knee problems and a bike fit person said someone of my height should be on 172.5 if not 170, just I just changed back to 170s.

While I could not tell the difference I didnt want to risk any further injury to my knee. Its all a bit of a bummer because now I cant get my seat back just that little bit.

Armyofangels
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Re: Crank arm length -How important is it?

Postby Armyofangels » Tue Aug 20, 2013 18:40 pm

A 160 on the left and a 170 on the right works wonders for weirdos like me (and actually a lot of people who are riding unawares) who get a small hip roll and therefore ITB pain from the legs being slightly different lengths.

However, if you're in no pain over long distances your cranks are probably fine and it's an expensive potential fix if you haven't spent ages fiddling with seat height, spacers etc first!

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Re: Crank arm length -How important is it?

Postby philthy3 » Tue Aug 20, 2013 19:32 pm

mulletmaster wrote:I think all these people who think they can distinguish a 5 or even 2.5mm difference in pedalling radius should get blind tested on a turbo before spending a lot of money on a new chainset. I would be very surprised if the majority could tell more than 50% of the time.


Der, that's why you do the comparison on a turbo with a power reading. :roll:
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Daz555
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Re: Crank arm length -How important is it?

Postby Daz555 » Wed Aug 21, 2013 02:00 am

bernithebiker wrote:
Daz555 wrote:Fair enough - but I suspect you "prefer it" because logic and new wisdom suggests it should be better - as opposed to actually being able to feel a 2.5mm change in crank length. If so, you are not alone - happens all the time with bike stuff. :mrgreen:


If you think there's no difference in changing crank length by 2.5 or 5mm, then why do Shimano et al make 167.5, 170, 172.5, 175, 177.5, etc.?

Surely it would be easier for them to just give us all 172.5mm wouldn't it?

I did not say there was no difference. My comment was simply saying that I doubt any of us would be able to feel a 2.5mm difference in crank length. That's all.
You only need two tools: WD40 and Duck Tape.

If it doesn't move and should, use the WD40.

If it shouldn't move and does, use the tape.

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bernithebiker
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Re: Crank arm length -How important is it?

Postby bernithebiker » Wed Aug 21, 2013 10:07 am

Daz555 wrote:I did not say there was no difference. My comment was simply saying that I doubt any of us would be able to feel a 2.5mm difference in crank length. That's all.


My experience is this - 10 years riding on 172.5mm like most people.

Decided to try 175mm for the extra leverage - gave me all sorts of problems and aches - couldn't get on with it. Didn't feel right.

Back to 172.5 for a bit then decided to try 170's.

Feels good, can raise saddle a touch, can spin better and longer, no aches or pains anywhere. Riding performance up at least 10% year on year (compared over Strava segments), although there are other factors at play like more/better training, Q-rings, etc...

So yes, I can feel the difference and am sticking with 170's. (FI, I am 5'8 and 63kg.)
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Re: Crank arm length -How important is it?

Postby Slowbike » Wed Aug 21, 2013 10:12 am

bernithebiker wrote:So yes, I can feel the difference and am sticking with 170's. (FI, I am 5'8 and 63kg.)


Ah - a shortarse! ;)


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