Crank length???

theBlob

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Sep 28, 2011
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#1
My bike came with a 170mm crank. I have quite long legs and am 188cm tall.

When I got a bike fitting the guy said I should have 172.5 or 175mm cranks.

My question is this if I go up to a 175mm I guess I have greater leg movement, so it will be harder to maintain a higher cadence, but each rotation will be easier.
So it should actually make my gearing a bit easier to spin

My question is this, will I notice the difference when I ride?

What benefits would there be in riding a larger crank? What benefits are there in keeping the shorter crank?

Thanks for your expertise and opinions in advance!:cool:
 

GSAstuto

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#2
Surprisingly crank arm length doesn't apply so rationally. In fact - many studies have shown that nothing is really gained (or lost) by most riders with longer legs using shorter cranks or vice versa. Within the range of about 150mm to 190mm all riders will develop roughly the same torque curve.

Now - what does change are other not so obvious factors:

1) On either end of the cadence spectrum (very low or very high) the individual may experience more optimization by using a longer or shorter crank.

2) Pedal Strike. Use longer cranks and you reduce the clearance you have in pedaling through a corner (or bank).

3) Aerodynamics - different length crank will position your body slightly differently on the bike while pedaling - this may affect your aero profile. Shorter cranks sit you a bit lower - and lower on the bike reduces drag - always.

A few years ago there was a big trend to increase crank length if you had proportionally longer legs. Now, there is a trend to use shorter cranks due to the aero factor and more riders working against metabolic cost vs performance as an important factor.

However - shortening the crank too much may cramp your body a bit - so other problems may arise.

At the end of the day, it all depends on you. What you prefer and / or provides the maximum performance given the ride style and events you participate in. Try some longer cranks for a week or so and see how you like them.

Personally I ride on either 170's or 172,5 , but I also have track cranks with 165. After a few km riding I honestly don't really think about the crank length. The only thing I do notice is with the slightly longer crank I feel a bit more power in the sub 70 rpm range - but then, my torque is dropping off so fast, it'd be better to be down a few gears and back into the 90rpm+ range where I don't really notice difference.

Out of the saddle I actually prefer the slightly shorter crank as I can get into higher cadence easier and it reduces my body vertical motion a bit.
 

andywood

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#3
A longer crank will give you more leverage. You canny change the laws of physics. I used to ride 180 mm DA cranks. Great for hill climbing for riders with long legs. After breaking those, I really wanted to buy the 180 mm compact Dura Ace crank set but it is hard to justify at twice the price of the Ultegra.

Longer crank arms might help you go faster up hills, but there are probably more effective ways to do it - like a lighter set of wheels.

Andy

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#4
My bike came with a 170mm crank. I have quite long legs and am 188cm tall.

When I got a bike fitting the guy said I should have 172.5 or 175mm cranks.

My question is this if I go up to a 175mm I guess I have greater leg movement, so it will be harder to maintain a higher cadence, but each rotation will be easier.
So it should actually make my gearing a bit easier to spin

My question is this, will I notice the difference when I ride?

What benefits would there be in riding a larger crank? What benefits are there in keeping the shorter crank?

Thanks for your expertise and opinions in advance!:cool:
My bad knee used to feel the difference between 170 & 172.5.
It shouldn't make all that much difference once you get used to a length.
You may have to also make a minor seat adjustment depending on how sensitive your leg joints and lower back is to a change...

Being tall (long legs) shouldn't really be a consideration although a lot of people will use that as their criteria to choose a longer crank for longer legs.

If you think about it, with a smaller crank you should be able to spin it faster... so you should over time experiment with different cranks until you get a feel for the length you feel best suits you.

Also remember if you never knew about different crank lengths you wouldn't even bother considering any of this. It can be a costly exercise to try them all over a long period of time and end up not feeling any difference.

I used to like a 175mm for my sprints... and since I've dropped down to 170, I find it a little harder to take off....

Listen to a few theories and go with the one that makes most sense to you... no matter what is said and done... in the end you will have to believe in it.
 

theBlob

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Sep 28, 2011
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#5
I like what I have, It certainly isn't giving me any trouble. But I am considering getting a power meter and compact crank, so I was considering if I do this should I be looking at getting a longer crank arm?

With my relative lack of experience and certaily lack of experience at trying different things to what I have right now, I wonder if I would even notice a small change like up to a 172.5mm?
 

kiwisimon

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#6
I used to ride a 172.5mm cranks but after switching to 175mm haven't noticed the change except when on my SS grinding up hills I appreciate the extra torque. All my bikes now run 175, from MTB to Track. Going on your height I would probably go 175mm. I'm 179cm and have no problems with them. Horses for courses but I can't tell the difference. Are you a spinner or a grinder. Longer cranks if you're the latter.
 

GSAstuto

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#7
Check out the Martin Study - while not necessarily definitive - it's quite interesting

http://www.plan2peak.com/files/32_article_JMartinCrankLengthPedalingTechnique.pdf

By the way - there are generally far more gains to be had by improving pedaling skills than changing crank arm lengths. I'd probably take the cash set aside for the new chainset and instead invest into a proper 'spin scan' and power analysis. You don't start tweaking an engine until you've at least dyno tested it. Benchmarks work the same in cycling. Until you have some baseline data it's very difficult to know exactly where or what you need to work on.
 

TOM

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#8
This topic has always intrigued me. I'm only 169.5 cm short but last year, I resolutely switched to "ultra"-short cranks, i.e. 165 and it "feels" like my climbing has improved in terms of lasting power :D. Biggest noticeable improvement however is on the descents...I can now attack those downhill corners with much more confidence than before (better gravity - better leaning :cool:).

Also, I would personally recommend shorter crankarms to riders who prefer to climb grinding up in heavy gear (like I do). I like the "reverse logic" approach, namely if one has powerful legs to start with, one doesn't need all that extra torque and one better makes that leg power last longer by forcing oneself to spin a bit more on those long hillclimbs". This approach seems to work for me but I'm not sure it is good for you...:D
 

GSAstuto

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#9
Personally I find this as well. My current chukokarb has 172.5 cranks - I've smacked the pedals a couple times, that plus the clip overlap is a bit unsettling when you're hard on either descents or climbs. With the 170's there's an ever-so-subtle increase in my cadence and actually a little easier to spin through the climbs rather than resort to pure mashing. Again - it's very slight - but I do notice it. Anthony, if you wanna swap and try, let me know. On my steel bike I always ride 172.5, but it has higher BB and bigger gearing. The carbon is set lower and has lower gearing.


This topic has always intrigued me. I'm only 169.5 cm short but last year, I resolutely switched to "ultra"-short cranks, i.e. 165 and it "feels" like my climbing has improved in terms of lasting power :D. Biggest noticeable improvement however is on the descents...I can now attack those downhill corners with much more confidence than before (better gravity - better leaning :cool:).

Also, I would personally recommend shorter crankarms to riders who prefer to climb grinding up in heavy gear (like I do). I like the "reverse logic" approach, namely if one has powerful legs to start with, one doesn't need all that extra torque and one better makes that leg power last longer by forcing oneself to spin a bit more on those long hillclimbs". This approach seems to work for me but I'm not sure it is good for you...:D
 
Oct 15, 2010
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#11
At most, we are talking about 5mm, right? More likely 2.5mm? Aero factor? I am sure pros and bike companies go on about it somewhere, but check it out on a ruler. There must be more variance in pedals and shoes than there are crank lengths. I'd say go with the standard length, or since your're taller, try the 175mm. I doubt it really matters.

For those with several bikes in their stables, some even riding MTBs now and then, do you really try to match all the goemetries and crank lengths? Probably better to train your body to accomodate some variance.
 
Jun 9, 2011
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#12
funnily enough the difference between a 170mm crankset and a 175mm crankset is 10mm.

i have 170, 175, and 180 cranks and for me there's a huge difference among them. the 170 is by far the easiest to spin and most comfortable in general. the 175 is ride-able but starts to get uncomfortable at higher cadences, especially when out of the saddle. The 180 is too big for me and very unpleasant to use.
 

Yamabushi

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#13
funnily enough the difference between a 170mm crankset and a 175mm crankset is 10mm.
To clarify this apparent "discrepancy", there is 5mm difference in the radius of the circle produced by rotating the cranks, and 10mm difference in diameter. The crank length measurement is equal to the radius. Additionally, someone mentioned earlier about how changing crank length affects seat height. For most people it's counter-intuitive, but when going to a longer crank, you need to lower the saddle. You have to do this in order to maintain the same reach to the pedals at the bottom of their arc. So with a longer crank you'll be sitting lower than with a shorter crank.
 

andywood

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#14
Found a big crack in the left arm and three small ones in the chain ring on my Ultegra 175mm compact crank set yesterday. I'm sure Shimano will come good on the warranty (they are only 5 months old) but kind of wish I'd paid for the Dura Ace (4 man Vs 2 man).

Currently on a butchered mix of DA crank arms and Sugino chain rings (50, 39 combination).

Always grateful for my mechanic Enosan...

Andy

www.jyonnobitime.com/time
 

joewein

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#15
Dura Ace components are lighter than Ultegra which, other things being equal, should make them break more easily. You primarily pay for the weight saving (which perhaps matters in a race), not for longer life of the components. Think formula 1 engine vs. road car.
 

andywood

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#16
Dura Ace components are lighter than Ultegra which, other things being equal, should make them break more easily. You primarily pay for the weight saving (which perhaps matters in a race), not for longer life of the components. Think formula 1 engine vs. road car.
I'd disagree with that. Having ridden a variety of components of Shimano 105, Ultegra and Dura Ace, I'd say you are primarily paying for durability or how long a part will last you. Sure the parts are lighter as you move up a grade but the biggest difference you will notice is in durability. Just my own impression, but a serious amateur racer will wear out 105 parts in 1 to 2 years, Ultegra in 3 to 4 and DA up to 10.

Price wise, DA tends to be twice the price of Ultegra. Crank set: 40,000 yen Vs. 20,000 yen. Pedals 20,000 yen Vs. 10,000yen etc.

Andy

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#17
IJust my own impression, but a serious amateur racer will wear out 105 parts in 1 to 2 years, Ultegra in 3 to 4 and DA up to 10.
Interesting. I'm not a racer so I can't offer a contrasting or confirming opinion. But it strikes me that there are not likely to be many Dura-Ace sets out there on racer's bikes that have worn out due to old age. If someone is sufficiently into racing that they are prepared to spend that much on components then they're going to want the next model two or three years down the road because it's 18 milligrams lighter. Or just because it's newer. That's if they haven't crashed and trashed it in the meantime. Good marketing.
 

FarEast

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#18
I have found in my years of riding that amatures ride more in a year than Pro's do - they also have very poor maintainance habits.

Shimano DuraAce parts are designed to be strong and light, it was part of the mission statement for the DA and XTR parts and they are pretty bomb proof as long as they are installed correctly and maintained.

I have had my 7900 since it came out and has not had any of its parts replace until two weeks ago where the back half of the rear derailiuer cage snapped in half.
 

joewein

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#19
A few people are reporting that the lighter titanium cogs of DA cassettes wear out more quickly than steel cogs on Ultegra cassettes. Also, DA front derailleurs made of aluminium can crack more easily than their steel Ultegra counterparts. Of course the same doesn't necessarily need to be true for all other parts of the group set, but to me it's two examples where weight and durability are conflicting goals.
 

FarEast

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#20
hhmmmm this is just conjucture Joe - without the actual returns or failure data from Shimano you can't really make such statements - remember it's the internet so you are obviously going to find people talking about failed parts - you'll never hear about people that have been happily putting in thousands of training km and racing kilometers in them without mishap.