Crank arm length -How important is it?

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TakeTurns
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Crank arm length -How important is it?

Postby TakeTurns » Thu Dec 06, 2012 18:05 pm

If one were to have crank arms a bit on the short side, how would this effect them? Cadence would be high, but would it effect performance and how?

Also, have you experimented with crank arm lengths and what's your experience?

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

Postby gloomyandy » Thu Dec 06, 2012 18:13 pm

I've often wondered about this. I have two bikes one my touring bike (a 29er) with 175 cranks and the other (my road bike) with 170. Both bikes are setup with pretty much the same riding position. I've never really noticed any issues with knees etc. between the two bikes. Hard to compare them in terms of performance as so much is different though.

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

Postby cougie » Thu Dec 06, 2012 18:23 pm

I've ridden crank lengths from 167.5 to 177.5mm. I really cant say I've noticed any difference. If you think - its only a small measurement compared to the combined length of your leg & foot.

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

Postby bernithebiker » Thu Dec 06, 2012 18:40 pm

This has always been a hotly discussed subject.

I've seen websites/people exhorting the benefits of short cranks (sub 170) and others doing the exact opposite and saying long (180+) cranks are best.

My view is that shorter cranks promote higher cadence, so if you're struggling to keep a high cadence, they may be a good idea.

Longer cranks give you more torque and may help with hills and powerful accelerations, but watch out for knee issues, and ground clearance in bends.

For my height, (5'8), I think 172.5 is pretty much spot on.
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SwainsL
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Re: Crank arm length -How important is it?

Postby SwainsL » Thu Dec 06, 2012 18:48 pm

My opinion is that if you're riding a crank arm length too short then you won't be maximising the potential of your legs. With longer legs comes longer leverage, therefore you'd be able to produce more torque without having to spin so much. If crank arm length is too long, then this could be effecting your stoke and possibly fatigue your legs sooner.

My inseam is longer proportion wise to my torso. I've used 172.5 and 180. I've been using a 180 ever since I had a bike fit. There's a noticeable difference between the 172.5 and 180. Especially on climbs. Although I haven't recorded myself, the 180 feels better when climbing.

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

Postby JGSI » Thu Dec 06, 2012 19:04 pm

Everytime I have to jump on a sh!tty spin bike, I have to contend with sh!tty 160 cranks... luckily 'they' designed a seat that goes up and down...
Pee all difference, except they'd look stupid on my road bike.

Lycra-Byka
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Re: Crank arm length -How important is it?

Postby Lycra-Byka » Thu Dec 06, 2012 19:14 pm

Am I right in thinking that different shoes ect would effectively alter crank length?
Ie different cleat systems, sole thickness, shims and pedals

????

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sub55
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Re: Crank arm length -How important is it?

Postby sub55 » Thu Dec 06, 2012 21:31 pm

Lycra-Byka wrote:Am I right in thinking that different shoes ect would effectively alter crank length?
Ie different cleat systems, sole thickness, shims and pedals

????


no you would not be
constantly reavalueating the situation and altering the perceived parameters accordingly

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sub55
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Re: Crank arm length -How important is it?

Postby sub55 » Thu Dec 06, 2012 21:34 pm

With reference to comments linking crank length with cadence. There maybe a colloration there , however foot speed pretty much remains constant .
constantly reavalueating the situation and altering the perceived parameters accordingly

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

Postby ChrisAOnABike » Thu Dec 06, 2012 23:00 pm

TakeTurns wrote:If one were to have crank arms a bit on the short side, how would this affect them? Cadence would be high

I don't get this. At a given speed, in a given gear, cadence is independent of crank length - it's still the same number of revs of the chain ring to get a given number of revs of the back wheel.

A shorter crank requires a larger force to exert the same torque. So why, if it's harder, would cadence go up with a shorter crank?
Is the gorilla tired yet?

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

Postby MountainMonster » Thu Dec 06, 2012 23:07 pm

ChrisAOnABike wrote:
TakeTurns wrote:If one were to have crank arms a bit on the short side, how would this affect them? Cadence would be high

I don't get this. At a given speed, in a given gear, cadence is independent of crank length - it's still the same number of revs of the chain ring to get a given number of revs of the back wheel.

A shorter crank requires a larger force to exert the same torque. So why, if it's harder, would cadence go up with a shorter crank?


Shorter distance to travel around?

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jonomc4
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Re: Crank arm length -How important is it?

Postby jonomc4 » Fri Dec 07, 2012 02:41 am

I am 5'10 - used 175 then used 172.5 - preferred the 172.5 as it helped my up my cadence to the 95 mark - maybe in my mind - dunno - but I will never go back to 175.

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

Postby JamesEs » Fri Dec 07, 2012 07:54 am

MountainMonster wrote:
ChrisAOnABike wrote:
TakeTurns wrote:If one were to have crank arms a bit on the short side, how would this affect them? Cadence would be high

I don't get this. At a given speed, in a given gear, cadence is independent of crank length - it's still the same number of revs of the chain ring to get a given number of revs of the back wheel.

A shorter crank requires a larger force to exert the same torque. So why, if it's harder, would cadence go up with a shorter crank?


Shorter distance to travel around?


But still the same number of revolutions (ie, the same cadence) for a given speed. Think of your current crank - the pedal end travels further than the middle, but they still complete a revolution in the same time.

A shorter crank may make it easier to maintain a given cadence, as you won't be moving your feet quite as fast, although how much difference 2.5mm or even 5mm will make is probably negligable. As with so many choices in cycling, I think it comes down to 'feel' - a particular crank length will just feel right for you.

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

Postby bernithebiker » Fri Dec 07, 2012 08:13 am

JamesEs wrote:
MountainMonster wrote:
ChrisAOnABike wrote:
TakeTurns wrote:If one were to have crank arms a bit on the short side, how would this affect them? Cadence would be high

I don't get this. At a given speed, in a given gear, cadence is independent of crank length - it's still the same number of revs of the chain ring to get a given number of revs of the back wheel.

A shorter crank requires a larger force to exert the same torque. So why, if it's harder, would cadence go up with a shorter crank?


Shorter distance to travel around?


But still the same number of revolutions (ie, the same cadence) for a given speed. Think of your current crank - the pedal end travels further than the middle, but they still complete a revolution in the same time.

A shorter crank may make it easier to maintain a given cadence, as you won't be moving your feet quite as fast, although how much difference 2.5mm or even 5mm will make is probably negligable. As with so many choices in cycling, I think it comes down to 'feel' - a particular crank length will just feel right for you.


True.

Imagine you're doing 30km/h in say 50/16 gear and your cadence is 80 and your crank arm is 175mm.

If you make your crank arm 170mm then everything stays the same; cadence is still 80.

BUT, the difference is that the 80 on the 170's MIGHT feel easier to spin as it's a smaller circle.

If you go up to a racey 100 or 110 cadence, you MIGHT struggle to maintain that on the 175's, but find it easier on the 170's.

Someone mentioned earlier that foot speed is constant. But I don't think it is. For a foot on a 175mm crank to complete 1 rev in the same time as a foot on a 170mm, it has to travel faster as it is describing a larger circle.

Again, it boils down to how you ride, your flexibility, your position on the bike, etc.

O-Level Physics kind of suggests go for the longest lever you can get for best torque, but watch out, because a) this could cause knee problems to creep up on you, and b) you might find your average cadence reduces which would drop power (Power = Torque x rotational speed).

Hope that's all correct, I haven't had my coffee yet.....
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Herbsman
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Re: Crank arm length -How important is it?

Postby Herbsman » Fri Dec 07, 2012 09:33 am

Longer cranks mean you have to bring your foot up higher at the top of the pedal stroke, and put it down further at the bottom of the stroke. This means the range of motion required is greater. Probably not a problem for most people but those with flexibility issues may benefit from shorter cranks.

Have a read of this CRANK LENGTH – Which one? by Steve Hogg

d. Flexibility: If a rider is tight in the hamstrings they cannot effectively have as high a seat height as would be possible with more flexible hamstrings. The lower the seat height for a given leg length, the higher the knee will rise for a given crank length.

e. Potential for an increase in knee problems: Longer cranks bring the potential for greater loads on the knee. If the rider has problems with foot/ankle or hip/lower back that set the scene for knee problems (in the sense of the knee being a single plane joint positioned between two multi plane joints the movement of which can dictate the plane of movement at the knee) then longer cranks can sometimes be enough to push that rider over the edge into injury through increasing shear forces on the knee. Note: Shear force is when the upper leg tries to push forward over the top of the tibia and fibula. Too much shear force = pain or injury. A longer crank for a given rider increases shear force disproportionately for the following reason. For sake of argument, assume a change in crank length of 10mm. To maintain the same effective seat height (as measured from centre of pedal hole in crank to top of seat), the seat post will need to be dropped 10mm further into the frame. That will mean however, that the knee will rise 20mm higher at the top of the pedal stroke. 10 mm because of the 10mm longer crank length plus 10mm because of the seat post being dropped in to the frame.

f. A longer lever doesn’t necessarily mean more power: A longer crank length (lever length) increases the multiplication of the muscular force that the rider applies to the pedal than a shorter crank length. However the rider doesn’t get something for nothing other than spending money on cranks. As you read in e. above, the knee will have an increased bend at the top of the pedal stroke with a longer crank length. This in turn means that the longer crank arm must progress further past TDC (top dead centre / 12 o’clock in the pedal stroke) than a shorter crank arm before the rider can apply the same mechanical advantage to it. The best example here I have is a personal one and I don’t pretend an argument for the entire cycling population can be extrapolated from one person’s experience, but it is worthy of mention. During one of my 3 month ‘experiments’ with crank length., I moved from 172.5mm to 185mm as 185mm is very, very close to my inseam x 0.216. to see what happened. At the time my racing was local crits and a monthly time trial on a course which isn’t mountainous but is far from flat. That increase in crank length is 7%. The leverage felt enormous and my PB in TT’s improved as did my hill climbing. All of which was great. The story was quite different in fast bunch rides and crit racing though. Never mind ground clearance issues, I worked around those. The problem was high speed. I found at light load on a trainer I could pedal the 185′s as fast as I could ever pedal the 172.5′s when playing games with maximum cadence (but was more untidy in the process) but pedaling at high load was a different story. My sprint top speed in local crits dropped by 7 km/h and I struggled to ride much above 50 km/h on the flat. As any crit rider knows, there are periods where the rider must be able to sustain really high speed (usually desperately trying to hold on to a wheel). and I really struggled with this. Why?

Because I could not apply the extra leverage through as many degrees of crank arc as I could with a shorter crank. Which meant my pedal stroke tended towards ON / OFF / ON / OFF if you follow me. Less smoothness meant less ability to pedal fast under high load and riding a bigger gear didn’t work as this decreased ability to accelerate and I was constantly being gapped when the pressure was on. Torque analysis confirmed this. I went from being a reasonably smooth pedaller at high load with shorter cranks to being a less smooth pedaller with longer cranks. In the type of racing where there are plentiful changes in speed, this was a disadvantage when the pressure was really on. Your experience, if you choose to experiment, may well be different to mine, particularly if you are REALLY long legged or your type of riding differs from mine.

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Monty Dog
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Re: Crank arm length -How important is it?

Postby Monty Dog » Fri Dec 07, 2012 11:37 am

I have both 170s and 165s on my bikes - I know I can manage a higher cadence on 165s that I can with 170s which can be as much as 20rpm. Likewise, I find the shorter crank easier for seated climbing as it feels I can keep my power output more constant as well as easier pick-up if I need to accelerate. Horses for courses really - some people like to spin and others prefer to crank it out on a bigger gear. Chris Hoy manages to ride at 80kph using 165mm cranks and a 108" gear!
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Stedman
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Re: Crank arm length -How important is it?

Postby Stedman » Fri Dec 07, 2012 12:53 pm

I am 5’ 6” tall and I ride 165 cranks on my road bike. I find it gives a much smoother cycling style and for longer rides it keeps my legs fresher and I frequently cycle up to 200 miles in a day.

For those using the long crank power hypothesis, Mark Cavendish only uses 170 mm and he is 5’ 9” tall!

Arguably too many riders (especially shorter ones) tend to have cranks which are far too long and if you have a tendency to creep forward on your saddle that is also an indication of this problem.

The biggest problem I find is trying to get hold of 165 cranks in the first place!

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

Postby snickwell » Fri Dec 07, 2012 15:18 pm

Genuinely think its psychological thing (unless you're moving around the extremes obviously). I've tried 172.5s and 175s and I can't say it made a difference to me. I just pedal as hard as I can for as long as I can...

ps. this is only an opinion, based on my real world findings. Please do not berate me.

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

Postby Herbsman » Fri Dec 07, 2012 15:45 pm

snickwell wrote:Genuinely think its psychological thing (unless you're moving around the extremes obviously). I've tried 172.5s and 175s and I can't say it made a difference to me. I just pedal as hard as I can for as long as I can...

ps. this is only an opinion, based on my real world findings. Please do not berate me.

You're wrong, this is total rubbish, also you are an idiot.


















:wink:

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

Postby Bozman » Fri Dec 07, 2012 15:57 pm

I've got different crank lengths on different bikes, this has happened on several occasions over the years and there's no difference.......apart from the 2.5mm.


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