Crank Length !

EasyDoesIt said:

Ok stop sniggering and well i know i'm gonna get some smart ass answers here but could someone please tell me what difference the crank length makes. I'm currently on 170's and i sometimes get aches in the back of my knees so i've ordered 172's for my new bike will these help or what ?

Aches and pains come from many different sources; seat position, seat height, cleat position, stem position, etc...

Not knowing your body specs there is no way to determine what is right; the 170s or the new 172.5s. Since you already ordered them (why are you asking for input now?), might as well install the cranks and see how they feel.

EasyDoesIt said:

Ok stop sniggering and well i know i'm gonna get some smart ass answers here but could someone please tell me what difference the crank length makes. I'm currently on 170's and i sometimes get aches in the back of my knees so i've ordered 172's for my new bike will these help or what ?

Last year I bought a new crankset with arms that were 2.5mm longer than what I had been using. I can't tell any difference at all. The research that's been done on crank length hasn't been real consistant. I kind of suspect that you'll get better results fiddling with your saddle height than you will by using longer crankarms.

no. I'd be very surprised if it made any difference.

In fact, if you went longer than a 2.5mm increase, I'd guess the problem could worsen because the thighs go through a larger range of motion (with the same extension at the bottom of the stroke, once the saddle has been adjusted for the different length), so the hamstrings are under slightly more stress, especially under heavy load.

I'm no Dr Butt, generally, posterior discomfort is caused by a too high saddle and/or tight hamstrings/lower back/hip flexors

http://www.cptips.com/knee.htm

total bs. Crank length can make all the difference in the world! On average, I could ride 3mph faster with my 172.5 than my old 175. If you think about it, the circle actually gets smaller by 5mm, which then means you can raise your seat by 2.5mm since when your foot is at the bottom stroke, it is now 2.5mm higher up.

These things have to be considered when getting new crank lengths. Raising or lowering the saddle (depending if you are getting longer or shorter cranks), makes all the difference. Since a smaller circle lets you spin more efficiently, and lets you use your calf muscles more.

- to reply to your original post, the 172.5 should be fine. Especially if it is for a road bike. But for mountain bikes I generally lean more to 170mm just because they are smaller and let you have more control (also a lower saddle height).

Thanks for the replies

My saddle height is grand and my stem length is grand too, tinkered around with the saddle a few months ago and found where it is now is perfect. My height is 173cm and inside leg is 84 cm. I use Look pedals with floating cleats so they shouldn't be a problem also the stem i got with the bike was too long and i've since shortened it but the ache behind my knees is still there. It only seems to ache though if i'm off the bike for over a week and go up hard hills, just curious if the new cranks will help.

free_rideman said:

If you think about it, the circle actually gets smaller by 5mm, which then means you can raise your seat by 2.5mm since when your foot is at the bottom stroke, it is now 2.5mm higher up...

....which = a relatively 5mm higher position through the top of the stroke. So, just when we apply power, at about 1 o'clock on the pedal circle (no one can convince me they start pushing at 12 o'clock 🙂), the saddle is 5mm higher, relative to the knee, but it obviously levels out as it approachs 6 o'clock.

I agree about the saddle height/position thing, and I can't BELIEEEEEEVE it's underemphazied in all the crank length discussions and on the websites. Most people talk about extra leverage with long cranks, but ignore that their position is screwed up. I figure most people who don't feel as though their position has been significantly altered with different length cranks had a crappy position in the first place!! 🙂

I recently went from 180 back to 175 (I'm 6ft, with long legs), and I suspect I could be a little faster due to the relatively higher saddle position, but if height is good, do I keep going to 170 or 165? I don't really think so. There's gotta be a happy medium, and THIS IS WHAT'S BEEN DRIVING ME FKN CRAZY FOR THE LAST TWO YEARS!!! 😄

EasyDoesIt said:

My saddle height is grand and my stem length is grand too, tinkered around with the saddle a few months ago and found where it is now is perfect.

And yet you have symptoms that are classic of a seat height being wrong... If the pain hasn't gone away, there is a problem, and it is not likely to be that the cranks are 2.5 mm too short.

But, since I am always seeking to learn, what makes you say that your seat position is perfect?

For now, I will simply assume that it is perfect. Since you are still having the pain, I would suggest (based only on my personal experience) that you should select your gears differently (unless you are already spinning at 90+ rpm). My suggestion is that when you are riding in a gear that feels comfortable to you, drop one more gear. You will feel like you are pedalling just a little too fast without enough resistance to your pedals. Do this continually for about 2 weeks and, if you are a masher like I was, the problem might go away like mine did. If you are a masher you will actually slow down a little until you get used to the higher cadence, but your legs will last longer.

Good luck. And based on what others have mentioned... that crank change will make you need to change your perfect setup anyway, so a small tweak now could save you the headache of switching cranks... and hurt nothing.

First, calf muscle recruitment has very little to do with power output or efficiency in cycling.

Second, as others have mentioned, you can't look at crank length in isolation, given what you've said. Before making any crank length changes, a person oughta be sure that their bike fit is right. Then crank length changes can be considered.

Some people are sensitive to crank length changes, and some aren't. Longer cranks do provide more leverage and can reduce cadence a tad, but cadence changes are not writ in stone. I went from 172.5 to 175 'cuz my spin felt "confined" or restricted. I just felt like my legs wanted to spin a bigger circle. The change fixed that, and as a bonus, my cadence went up. How your body responds to the crank length change will depend on your physiology.

As a few have mentioned, though, the pains that you feel are likely from fit issues, not crank length issues.

A smaller crank is more suited to spinning faster where a long crank is more suited to slower spinning.

Some cyclists get knee problems from having the seat too high.

Remember to lower the seat when you put longer cranks on.

mikesbytes said:

A smaller crank is more suited to spinning faster where a long crank is more suited to slower spinning.

Some cyclists get knee problems from having the seat too high.

Remember to lower the seat when you put longer cranks on.

All good advice!

However, if you subscribe to the KOPS system, then the seat must still be adjusted at the 90 degree crank point regardless of the crank length.

Why not try the sadddle forward a little first, it will bring up the cadence.

Completely unrelated thing. But George, your'e not serious about the comment you got down the bottom there in red, about distributrors being annoyed with your recommendations, or are you?

gclark8 said:

All good advice!

However, if you subscribe to the KOPS system, then the seat must still be adjusted at the 90 degree crank point regardless of the crank length.

Why not try the sadddle forward a little first, it will bring up the cadence.

mikesbytes said:

A smaller crank is more suited to spinning faster where a long crank is more suited to slower spinning.

Some cyclists get knee problems from having the seat too high.

Remember to lower the seat when you put longer cranks on.

This is good advice. Pains behind the knees often means the seat is too high and/or too far back. If the position remains the same, longer cranks could likely make the knee pain worse.

free_rideman said:

total bs. Crank length can make all the difference in the world! On average, I could ride 3mph faster with my 172.5 than my old 175. If you think about it, the circle actually gets smaller by 5mm, which then means you can raise your seat by 2.5mm since when your foot is at the bottom stroke, it is now 2.5mm higher up.

These things have to be considered when getting new crank lengths. Raising or lowering the saddle (depending if you are getting longer or shorter cranks), makes all the difference. Since a smaller circle lets you spin more efficiently, and lets you use your calf muscles more.

- to reply to your original post, the 172.5 should be fine. Especially if it is for a road bike. But for mountain bikes I generally lean more to 170mm just because they are smaller and let you have more control (also a lower saddle height).

1. How did you assess your increase in speed?
2. How can you advise someone on crank length when they have told you nothing about their body dimensions?

There are plenty of sites that will give objective suggestions on the effect of different crank length and crank lengths for people of different heights (effectively leg length).

gclark8 said:

However, if you subscribe to the KOPS system, then the seat must still be adjusted at the 90 degree crank point regardless of the crank length.

How does the KOPS system work?

mikesbytes said:

How does the KOPS system work?

KOPS= Knee Over Pedal Spindle. It is what it says, the belief that the bony protruberance beneath your kneecap (i.e. on the proximal tibia) should be located on a line that is perpendicular to the ground, passing thru the pedal spindle (when the rt. pedal is in the 3 o'clock position or the left pedal is in the 9 o'clock position). Some people take view it as a fact. Personally, I think it's just a nice ballpark in which to start fine tuning a fit. Always keep in mind that fit is highly personal, and while some of the old nuggets (KOPS, for example) might work for some or most people, they do not work for everyone.

alienator said:

KOPS= Knee Over Pedal Spindle. It is what it says, the belief that the bony protruberance beneath your kneecap (i.e. on the proximal tibia) should be located on a line that is perpendicular to the ground, passing thru the pedal spindle (when the rt. pedal is in the 3 o'clock position or the left pedal is in the 9 o'clock position). Some people take view it as a fact. Personally, I think it's just a nice ballpark in which to start fine tuning a fit. Always keep in mind that fit is highly personal, and while some of the old nuggets (KOPS, for example) might work for some or most people, they do not work for everyone.

So KOPS would say that if you put 2.5mm longer cranks, then (assuming the seat was perfect before) you need to move the seat forward 2.5mm.

mikesbytes said:

So KOPS would say that if you put 2.5mm longer cranks, then (assuming the seat was perfect before) you need to move the seat forward 2.5mm.

Not necessarily. If you lower your seat to suit the longer cranks your seat will have some forward movement because of the angle of the seat tube.

Assuming you're happy with your seat height setting you will have to drop a plumbbob from your knee joint again to get it exact but it will be less than 2.5 mm.

As someone stated earlier this is only a starting point because this method varies depending on whether you have your shoe level or your pedal level when you line up your knee with the pedal. It also varies depending on where your bum is on the seat when you line it all up.

Climbers tend to move the seat back while time triallers (triathletes especially) tend to have their seat well forward of knee over pedal axle.

dgregory57 said:

And yet you have symptoms that are classic of a seat height being wrong... If the pain hasn't gone away, there is a problem, and it is not likely to be that the cranks are 2.5 mm too short.

But, since I am always seeking to learn, what makes you say that your seat position is perfect?

For now, I will simply assume that it is perfect. Since you are still having the pain, I would suggest (based only on my personal experience) that you should select your gears differently (unless you are already spinning at 90+ rpm). My suggestion is that when you are riding in a gear that feels comfortable to you, drop one more gear. You will feel like you are pedalling just a little too fast without enough resistance to your pedals. Do this continually for about 2 weeks and, if you are a masher like I was, the problem might go away like mine did. If you are a masher you will actually slow down a little until you get used to the higher cadence, but your legs will last longer.

Good luck. And based on what others have mentioned... that crank change will make you need to change your perfect setup anyway, so a small tweak now could save you the headache of switching cranks... and hurt nothing.

Because when i lowered the saddle the pain was worse and when i highered the saddle back up the pain wasn't nearly as bad. Was out last sunday and had no pain whatsoever.

mitosis said:

1. How did you assess your increase in speed?
2. How can you advise someone on crank length when they have told you nothing about their body dimensions?

There are plenty of sites that will give objective suggestions on the effect of different crank length and crank lengths for people of different heights (effectively leg length).

The speed assessment is a very rough one. Nothing scientific, but I did feel like my high cadence was a lot easier to maintain, thus I didn't get tired as quickly. So then when I rode on this strip of road that is flat, and quite long, I noticed something. There I would see that I could keep a faster speed (~3mph) and not get as tired as I would before when I had longer cranks, and a slower cadence.

To explain your other question, I was just stating the fact that crank lengths are important. Too many people today don't stress the fact. I guess I should have clarified it more. But by all means, if your fit on a bike sucks in the begining, than changing crank lengths in the middle of it will actually get you more confused. So having a good fit on a bike is definitely more important. (unless you are really small, and your bike came with 185mm cranks, then you have a fit problem) 😉