Just need to replace a stripped crank. Can’t remember what std length is for avg height person? 5’9″, 31″ inseam. Is it 170 or 175

I’m a little shorter than that, and use 175 simply because it’s the ‘standard’. In reality, I don’t think crank length is that important. Go shorter if you’re getting lots of pedal strikes, otherwise 175 is fine.

Go short!

I used to be really sensitive to crank length (170mm is all I liked) – but I accidently bought a set of really cheap xtrs in 175 – now I can’t tell the difference between them!

Cool – thanks for the help!

All you should ever need to know and more here. http://sheldonbrown.com/cranks.html

it’s important to realise that there’s very little difference between 165, and 175.

it’s the same difference as found between a 56cm frame, and a 58cm frame – ie, 94%, it’s not much difference really, considering that 165-175 is considered the full range.

or, if you’re not a roadie: it’s less than half the difference between a medium and a large. We would laugh if an mtb frame builder suggested that their choice of ‘medium’ or ‘medium-and-a-bit’ covered a full range of sizes.

i’ve got the longest legs in the world; 36″, i’m happy with my 175’s – i definitely wouldn’t want longer cranks.

for your 31″ inseam, you’d get 150’s to keep the same proportion, if they were available, but for some crazy reason they’re not.

the reason that people don’t think it matters, is because the current ‘choice’ is an illusion: ‘different’ (but actually very similar) cranks are tried, no difference is felt, therefore crank length is unimportant.

I read a considered explanation reasoning that too short doesn’t really matter, but too long is bad.

That article recommended 170mm for me (I’m 5’9″ too) and I currently have 170mm on my commuter and 165mm (bought due to cheapness more than anything) on the mtb.

I prefer the shorter cranks for spinning on the mtb as my body movement is less and I find this helps with balance.

I used to run a 175mm crank on my older commuter and I think I preferred it for stand-up efforts in the big ring up shortish hills.

Of course it could all be in my head

here is the article I referred to (pretty good site for explaining various bike-fit issues)

I have 175 on my hardtail and 165 on my FS (Low bottom bracket) and I use them both a lot…. can hardly tell the difference and even that might be psychological

I’m 5’11” and could never tell the difference until I was on a fatbike, then it became *really* obvious. Spinning up I was bobbing around a lot with 175’s – switched to 170’s and it just sorted itself right out. Realised I was doing that on my other bikes too (but to a lesser extent) and since then they’ve all been 170, with the exception of my track bike which is 165.
I still could never tell the difference on a hardtail though, probably as I was in & out the saddle too often to really notice it, but on everything else 170 has been a lot better for my knees and the smoothness of my peddling over 175

jamcorse – Member

I have 175 on my hardtail and 165 on my FS (Low bottom bracket) and I use them both a lot…. can hardly tell the difference

that’s because there is very little difference.

ahwiles – Member

jamcorse – Member

I have 175 on my hardtail and 165 on my FS (Low bottom bracket) and I use them both a lot…. can hardly tell the difference

that’s because there is very little difference.

But sometimes a little difference can make a big change. 5mm at the crank-arm will make an overall change of 10mm in the overall vertical movement of the foot during one pedal rotation. When I make 10mm adjustments to my saddle height, I can certainly feel the difference in my legs while pedalling. I reckon that when I am approaching the ideal position, I can feel the difference with 5mm adjustments.

And that’s without even considering the fore-aft 10mm.

^this

In the past I’ve had 170 on one side and 175 on the other without even realising it after an emergency crank transplant. But I can tell the difference between 160 and 165.

In my case it’s pain in my arthritic knees with longer cranks, though… Currently running 165s on the good bike as that’s the shortest I could get quality parts. The nasty heavy 160s on the hack bike are more comfortable, though, and counter-intuitively seem to allow me to put more torque into the system. I have 155 cranks on my recumbent…

I’m 6ft tall, incidentally.

mickolas – Member

But sometimes a little difference can make a big change. 5mm at the crank-arm will make an overall change of 10mm in the overall vertical movement of the foot during one pedal rotation.

agreed.

but 5mm on a 170mm crank is only 3%

10mm on a 340mm turned circle is still only 3%

i am simply saying that 165-175 is not a wide enough range to reflect actual differences in height/leg length/bio mechanics.

if i wanted to reproduce the awful experience my wife must have with her 165’s, i’d need 210mm cranks – no wonder her knees hurt. My legs are 28% longer than hers, but my cranks are only 6% longer, this can’t be right.

I’m 5’10” and I used to run 175 on all my mtbs. Recently I tried 170s and the difference was immediate so now I run 170s on all of them.

ahwiles – Member

mickolas – Member

5mm on a 170mm crank is only 3%

10mm on a 340mm turned circle is still only 3%

Quite so.

For sake of argument, say 170mm is my ‘ideal’, and anything between 165 and 175mm is ‘tolerably good’, for me. If you will allow, I suggest that anything less than 150mm would be unacceptably short for me and anything over 190mm, unacceptably long.

That gives me a ‘tolerable’ range of 5mm either side of optimum and an an ‘acceptable’ range of 20mm either way. In this context, that 5mm change becomes a 25% movement away from optimum, within the range of possible acceptable sizes.

That is not to make any particular point, except that these figures can be argued in many ways and I don’t think it is useful to discuss %age changes in crankarm length.

I believe the metric to really be considered is the angle of the knee (and maybe hips too) at full extension, minimum extension and throughout the power stroke; for different crank lengths. Again changes in this measurement should be looked at, not as %ages of the whole, but as %age movements within a certain range.

It’s also important to remember that we are talking about a 4-bar linkage system (5-bar if you count ankle movement;6-bar if you count the ball of the foot) and relatively small changes of angle in a member can produce large differences in the distribution of forces within the system. That is why MTB manufacturers can produce bikes with very similar suspension systems that behave quite differently.

I ride bikes with 42cm and 44cm handlebars (C-C). This is only a difference of 5%, yet one feels right, the other is okay but not quite right.

In some ways it is fairly pointless to discuss any of this outside the context of a serious ergonomic study which would need to take into account a hefty amount of researched anthropometric data and results of efficiency tests/long-term joint-wear tests specifically and scientifically aimed at the action of cycling.

So maybe just ride what you like the feel of, eh? Personally, I spend far more hours commuting than I am comfortable with. This leaves me obsessed with clawing back any marginal efficency gains and I am sensitive to the cumulative effects of small changes.

i am simply saying that 165-175 is not a wide enough range to reflect actual differences in height/leg length/bio mechanics.

I completely agree with this. I am very glad to be a mediocre-sized person and to be able to take advantage of falling within this small range that is available to us.