Crank Arm Length
07-15-08, 07:21 AM
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Crank Arm Length
This may seem like a dumb question, but I have two bikes, both are Campy equipped, one has Campy Alloy 170mm crank arms, the other has FSA SLK Carbon 175mm crank arms. I seem to be faster and more comfortable on the bike with the 175mm crank arms. I have been thinking about changing the 175mm to Campy carbon crank arms to make the entire drivetrain Campy and hopefully improve shifting on the front rings. My mechanic said I could do 170mm if I wanted to. My question is what is the advantage/disadvantage of the 170mm over the 175mm crank arms?
Any thoughts/comments/jeers are appreciated.
Thanks!
Any thoughts/comments/jeers are appreciated.
Thanks!
07-15-08, 07:34 AM
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there are formulas out there that calculate the "ideal" crankarm length based on your inseam. If 175s feel better and you have an inseam of 33+ inches, I say it's ok to use 175s.....but I have 2 bikes, one with 170 and one with 175s and damned if I can tell the difference!
07-15-08, 08:34 AM
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From sheldonbrown.com. I win!
So when I was looking at crank length to determine whether it would help with some knee pain, I determined that unless range of motion is a problem, it makes no difference if a crank is 170 or 175. Crank length only matters if your legs are really long or really short compared to the general population. So if your bike is over 60cm maybe you would want to be sure your crank is 175mm. If your bike frame is under 50cm, maybe you want to go with 170, or even 165mm. Or get a really short crank if you have range of motion issues, like mild arthritis.
But in the end, it doesn't matter. Why? Well, if you were on a single-speed bike, the larger crank would give you more leverage and thus more power. But since you have gears, that rationale is out the window. So just get whatever is comfortable/cheap.
Crank Length
Different cyclists have different leg lengths. It seems obvious that crank length should be proportional, so long legged cyclists should have long cranks, short-legged cyclists should have short cranks....and yet, 99.9% of adult bicycles have crank lengths between 165 and 175 mm. Have the bicycle manufacturers joined in a great conspiracy to force everybody to ride the same length cranks, regardless of their needs?
This is a common misunderstanding. The "leverage" of a bicycle drive train, also known as "gain ratio" depends on the crank length, wheel diameter and the sizes of both sprockets.
Yes, if you go to longer cranks without changing any of the other variables, you will have more "leverage", which is another way of saying you'll have a lower effective gear...but on a multi-speed bike, you can change gears at will!
Ay, there's the rub! Assuming you adjust your gearing appropriately, crank length has no effect on leverage, it just has to do with the range of motion of the knee and hip joints.
Too long cranks cause excessive knee flex, and can cause pain/injury if it causes your knee to flex more than it is used to.
I learned this the hard way when I bought a used mountain bike that came with 180 mm cranks. I found that it made my knees hurt every time I rode it.
On the other hand, there doesn't seem to be any deleterious effect from shorter cranks.
I've been experimenting with this a bit myself lately. For my fixed gear, I commonly ride 165 mm cranks with a 42/15 ratio on 700c or 27 inch wheels, when I'm riding fixed. This gives a gain ratio of 5.8.
My latest experiment is taking place on plastic Trek frame I picked up in a barter deal. I had a pair of TA 150 cranks that used to be on my kids' Cinelli BMX bike, so I've put these on the Trek. I'm running a 45/17, which gives a gain ratio of 5.9, just a bit higher.
When I first get on the bike after riding with longer cranks, it feels a bit funny at first, but within a very short distance it's just fine. I go just as fast, climb just as well. For a given speed, my pedal rpm is higher (though my pedal speed is the same) but the short cranks make it easy to spin much faster than I normally would.
After riding this bike for a few miles, when I get back on "normal" cranks, they feel a bit weird and long at first, then I get used to them after riding a couple of minutes.
I think people really obsess too much about crank length. After all, we all use the same staircases, whether we have long or short legs. Short legged people acclimate their knees to a greater angle of flex to climb stairways, and can also handle proportionally longer cranks than taller people normally use.
Different cyclists have different leg lengths. It seems obvious that crank length should be proportional, so long legged cyclists should have long cranks, short-legged cyclists should have short cranks....and yet, 99.9% of adult bicycles have crank lengths between 165 and 175 mm. Have the bicycle manufacturers joined in a great conspiracy to force everybody to ride the same length cranks, regardless of their needs?
This is a common misunderstanding. The "leverage" of a bicycle drive train, also known as "gain ratio" depends on the crank length, wheel diameter and the sizes of both sprockets.
Yes, if you go to longer cranks without changing any of the other variables, you will have more "leverage", which is another way of saying you'll have a lower effective gear...but on a multi-speed bike, you can change gears at will!
Ay, there's the rub! Assuming you adjust your gearing appropriately, crank length has no effect on leverage, it just has to do with the range of motion of the knee and hip joints.
Too long cranks cause excessive knee flex, and can cause pain/injury if it causes your knee to flex more than it is used to.
I learned this the hard way when I bought a used mountain bike that came with 180 mm cranks. I found that it made my knees hurt every time I rode it.
On the other hand, there doesn't seem to be any deleterious effect from shorter cranks.
I've been experimenting with this a bit myself lately. For my fixed gear, I commonly ride 165 mm cranks with a 42/15 ratio on 700c or 27 inch wheels, when I'm riding fixed. This gives a gain ratio of 5.8.
My latest experiment is taking place on plastic Trek frame I picked up in a barter deal. I had a pair of TA 150 cranks that used to be on my kids' Cinelli BMX bike, so I've put these on the Trek. I'm running a 45/17, which gives a gain ratio of 5.9, just a bit higher.
When I first get on the bike after riding with longer cranks, it feels a bit funny at first, but within a very short distance it's just fine. I go just as fast, climb just as well. For a given speed, my pedal rpm is higher (though my pedal speed is the same) but the short cranks make it easy to spin much faster than I normally would.
After riding this bike for a few miles, when I get back on "normal" cranks, they feel a bit weird and long at first, then I get used to them after riding a couple of minutes.
I think people really obsess too much about crank length. After all, we all use the same staircases, whether we have long or short legs. Short legged people acclimate their knees to a greater angle of flex to climb stairways, and can also handle proportionally longer cranks than taller people normally use.
But in the end, it doesn't matter. Why? Well, if you were on a single-speed bike, the larger crank would give you more leverage and thus more power. But since you have gears, that rationale is out the window. So just get whatever is comfortable/cheap.
Last edited by bryroth; 07-15-08 at 08:43 AM.
07-15-08, 08:57 AM
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Really? I find that my knees never have a problem on the bike that has 175mm cranks, while I simply cannot get the bike with 170mm cranks setup so that my knees don't start hurting after 10-15 miles...
07-15-08, 11:05 AM
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I have a 31” ish inseam and I started riding a bike with 172.5 cranks. After a few thousand miles I wanted to upgrade my cranks and I got a good deal on some with 170 arms. I figured I'm not going to feel the difference of 2.5mm. Boy was I wrong. To me, it was a big difference. It felt like I had to spin about 10-15 rpm faster to get the same feel. Especially going up hills. .
Sorry, I know this isn't a technical answer, I'd say just go with what you feel is more comfortable for you.
Sorry, I know this isn't a technical answer, I'd say just go with what you feel is more comfortable for you.
Last edited by Medpilot; 07-15-08 at 11:06 AM. Reason: spelling
07-15-08, 11:44 AM
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07-15-08, 11:59 AM
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I am also one who notices.
I've tried 165, 170 and 175. 165 feels like I'm spinning like a top. 175 feels great on a MTB trying to lever over rocks and roots, but is sluggish and awkward on a roadbike. 170 is my sweet spot, not a mm bigger, where I get a good balance of both.
I actually prefer 165 for long, flat rides without stops, but since I live in hill country and spend lots of time powering up I appreciate the extra 5mm for that.
Oh yeah, I'm 5'10" with a 33.5" cycling inseam.
I've tried 165, 170 and 175. 165 feels like I'm spinning like a top. 175 feels great on a MTB trying to lever over rocks and roots, but is sluggish and awkward on a roadbike. 170 is my sweet spot, not a mm bigger, where I get a good balance of both.
I actually prefer 165 for long, flat rides without stops, but since I live in hill country and spend lots of time powering up I appreciate the extra 5mm for that.
Oh yeah, I'm 5'10" with a 33.5" cycling inseam.
07-15-08, 12:14 PM
#9
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Unless you are totally focussed on high rpm efforts (120+ rpm), I think 175s will help. I have a 29" inseam and have ridden 170s and 175s.
https://sprinterdellacasa.blogspot.co...m-lengths.html
https://sprinterdellacasa.blogspot.co...ng-cranks.html
Longer cranks, even at your/my size, will help your low/mid range power (under 90 rpm). In the 90-110 rpm range I think there isn't a big difference.
This year I went back to 170s and I'm not convinced it was a good idea. I have more pedal speed (i.e. cadence) but my top speed (mph) is lower.
cdr
https://sprinterdellacasa.blogspot.co...m-lengths.html
https://sprinterdellacasa.blogspot.co...ng-cranks.html
Longer cranks, even at your/my size, will help your low/mid range power (under 90 rpm). In the 90-110 rpm range I think there isn't a big difference.
This year I went back to 170s and I'm not convinced it was a good idea. I have more pedal speed (i.e. cadence) but my top speed (mph) is lower.
cdr
07-15-08, 09:44 PM
#10
Aluminium Crusader :-)
It's not only about "power vs rpm", it's also about position and torque. If someone uses way long cranks, they're too hard to 'get on top of' and push down if their knee is in their chin at the top of the pedal stoke. Long cranks have more leverage, but the rider is in a lower/waeker position, and shorter cranks have less leverage, but the rider is in a higher, more powerful position.
Here's a good bit on torque
https://www.arniebakercycling.com/pub...m%20Length.pdf
Here's a good bit on torque
https://www.arniebakercycling.com/pub...m%20Length.pdf
