Crank Lengths

[Jezston]

I have slightly odd proportions - I am 6 foot tall, but only have a 30-31" inside leg, so sizing a bike for me can be a little difficult! My current build is based on a 56cm frame which I believe is mostly correct for my leg length, and I'll be potentially requiring a longer stem than the relatively short quill stem that came with the frame (although this won't be decided until I sit on it).

What difference do different crank lengths make? Having relatively short legs for my height, should I be looking for slightly shorter cranks? Will my knees be happier if I do so?

 

[Gerry Attrick]

I have 31" inside leg too althiugh I am a couple if inches shorter in height. Most of my bikes have 170mm cranks, but one has 175mm and another has 165mm. To be honest, for everyday use I don't notice the difference but the bike with 165mm cranks is a great sprinter. The same bike runs tubs so the wheels are very light and the difference may relate to those rather than the cranks.

 

[zoxed]

Short cranks: if you have Googled you will have found lots of information, some conflicting (!!)
Personally I switched to 150mm cranks several months ago on my recumbents a few months ago, and I wish I had done it sooner !! I can spin much easier now and knee problems should be reduced as the max. knee angle will be less. Others rate 110mm cranks.
However a disadvantage for upright bike riders with a standard frame is that the seat height should be increased (to keep the leg nearly straight at the bottom) thus making it a bit harder to reach the ground.

 

[numbnuts]

I am 6 foot tall and have 32" leg I went for 175mm if that's any help

 

[dan_bo]

I am also longish with stumpy legs. I run a 55 cm square track bike with 165mm cranks and a 140 mm stem, A 54 cm crosser with 175 mm cranks and a 120mm stem and an 18" MTB with 175mm cranks.


I'm not sure it makes that much difference unless you're getting paid for it......

 

[HLaB]

One silly mistake I made a few years was with crank length. I'd got a new summer bike and had been riding it all summer (naturally enough ); it has 172.5mm cranks and is set up to a bike fit. I set up my winter bike to what I thought was the same spec; after a while my calf had the sensation it was becoming detached. To cut a long story short it has 175mm cranks and moving the saddle 2.5mm forward and lower made an almost instant difference.

 

[Jezston]

Thanks for your thoughts Gentlmen, I guess the conclusion is there aren't really any rules when it comes to crank length, and that isn't what might affect your knees?

I am a little paranoid about my knees, but I think this comes from previously riding in far too high a gear rather than high cadence, and on a bike that's too big for me.

 

[e-rider]

you'd surely be fine with either 170 or 172.5

I'm 6'2" and of normal proportions and I use 175mm but could happily use 172.5 I guess.

 

[rickangus]

There is a theory that crank length should be between 21 and 21.6% of leg length.

But leg length is not the samne as the size you use when buying trousers.

What I believe to be the most accurate way of establishing it is measure your height (without shoes) standing against a wall in cm. Then sit on floor, back to wall, and measure that height. Subtract that figure from the first and that is your true leg length.

Then take 21% of that to determine crank length.

If you're fortunate it will be near enough 165, 170, 172.5 or 175mm i.e. the easily available sizes.

But I know a number of people, self included, who fall outside these parameters who made the effort to obtain 'fitted' cranks and who are extremely pleased they did so. And for different reasons - to prevent knee pain, greater comfort and/or more power/efficiency.

 

[snailracer]

I have the opposite problem in that my legs are rather short.

To try and solve my chronic mild knee pain issues (old football injuries), I once fitted a set of 140mm kids cranks to one of my bikes (previously 165mm). It was very easy to spin ridiculously fast, which I got used to quite quickly. As far as my knee pain, I'm not sure it made much difference - the shorter cranks meant the force on the pedals loaded up very abruptly, more like walking than pedaling, making seat height and shoe sole thickness critical. It didn't make me faster or slower, although it felt different as I was spinning faster in lower gears, compared to 165mm.

 

[Jezston]

There is a theory that crank length should be between 21 and 21.6% of leg length.

But leg length is not the samne as the size you use when buying trousers.

What I believe to be the most accurate way of establishing it is measure your height (without shoes) standing against a wall in cm. Then sit on floor, back to wall, and measure that height. Subtract that figure from the first and that is your true leg length.

Then take 21% of that to determine crank length.

If you're fortunate it will be near enough 165, 170, 172.5 or 175mm i.e. the easily available sizes.

But I know a number of people, self included, who fall outside these parameters who made the effort to obtain 'fitted' cranks and who are extremely pleased they did so. And for different reasons - to prevent knee pain, greater comfort and/or more power/efficiency.

Hmm. Just did your measurement and got a leg length of 96cm, which is about 38 inches. At six foot with a 31 inch inside leg does that sound right? Just that 21% of 96 is 20.16, or 202mm, which seems pretty enormous!

 

[piedwagtail91]

i've three different bike sand all have a different length crank. it's hard to tell the difference ,especially after a couple of miles.
a few years ago a clubmate had his best ever racing season on a bike with different length cranks, 175, 172.5, he didn't find out til he stripped the bike at the end of the season.
two or three years ago i had no choice but to ride our clubs 150 mile ride with odd length cranks, when i noticed a crack i one the day before.
it didn't cause any problems.

 

[rickangus]

Jezston,

That does sound rather big/long! I'm 6' 5" and have 34 1/2" inseam for trousers. My measurement comes out at 39" so perhaps something isn't quite right with what you've done? I use 200mm cranks which most people are aghast at but actually they work very well for me.

A lot of people say they don't notice much, if any, difference when they try a 172.5 or 175mm crank compared to a standard 170mm - and they're probably right because there is very little difference. A 175mm crank is less than 3% longer than the standard 170mm

It seems that most folk are reluctant to go outside the 165 - 175mm envelope for which most crank and frame manufacturers are very grateful!

They like standardisation.