[Closed] Crank arm length

not sure what you mean the crank arm length does not cause an compatibility issues. You probably would not notice the difference if you change either way by 5mm.

I did notice the change to 170 mm from 175 mm, but then again I am a princess.

If you have no reason to change then why even think about it! Unless you are particularly tall, small or prove to rock strikes then maybe just stick with what you're used to.

rough guide...

measure your inside leg.

if you've done that in inches, multiply by 25.4 to get millimeters.

divide your millimeter leg length by 5 = ideal crank length.

or, another rough guide based on your height:

6'2" = 175

6'0" = 170

5'10" = 165

neither is perfect, but they're both better than simply running the 175's that came with your bike.

Rode 170mm for years and years.

Then for some long, unremembered reason, born from a good long moment of monumental stupidity on my behalf. I decided I needed to ride 175mm cranks and indeed the online literature supported this change in kit spec.

Oh Lordy! What a total screw up! My most expensive fail, ev-vah!

Since my experience, I'd suggest sticking to what you know, I am thus, highly suspicious of generalized formulae for ascertaining one's most suitable crank arm length.

Firstly I'd echo what has already been said, if there's no problem with what you've used then...stick with it. I was having back and knee trouble so got geeky and read everything i could find on bike fit, including crank arm length. Found a proper study had been done on the 20% rule mentioned above and did change from 175 to 180s on my 29er and commuter (they both needed new cranks soon anyway). I have a 35" inside leg so 180 or 182.5 is 'better' than 175 and 180 seemed readily available.
A couple of months in and I can say the main things I notice is the benefit of a lower saddle (5mm!) on my hardtail - it helps my position (less bend-down to bars) and when it comes to rough trail/descending there's a bit more room to manoeuvre. A dropper post is the obvious solution if that's a concern though. I don't think pedal strikes have increased as the bb is same height and my commute doesn't have any fast bends (where you might want to lean and pedal).
There are many more important considerations, my knee seems to be mostly solved by switching off a road crankset to a wider stance (aka Q factor, tread) MTB crankset...the 180 can't be responsible as my knee never struggled on my 175 mtb cranks.

A hardy perennial along with clips vs flats.

Personally tried 180 which needed the saddle lowering for the bottom of the stroke. This caused a knee overbend at teh top of the stroke and a lot of pain....

Sheldon Brown suggests fit is important and any extra leverage theories are scotched by the simple process of selecting another gear...

[url= http://bikedynamics.co.uk/FitGuidecranks.htm ]This interesting article[/url] is an easy read and suggests shorter, which personally suits me now with either 165 or 170 on my bikes. Bonus for less pedal strikes too.

I guess my main motivation is pedal strikes.
Thanks for the tips I will read up and have a think.

Pedal strikes won't be worsened by adding 2.5mm!

Crank arm length, for me, is about bike fit, not clearing obstacles. Personally.

I have just experimented with 170mm cranks on the roadbike from my normal 172.5mm and it did not work for me. For the whole time I felt that I was not able to put the power down properly and my legs did not feel 'right' at all, including a lot of knee clicking in my right knee. Just gone back to 172.5mm and straight away my legs felt like they used to. Surprising how much difference it made, but is probably more noticeable on the road to the mtb. I run 175 on my mtb and don't notice the change switching to the road bike from them.

Oh, and I'm 5' 10" with about a 31" inside leg so personally do not agree with the post above indicating suggested lengths

I think you notice the difference, but I also think you need to give yourself time to get used to the new length. I used to hate shorter cranks because I always felt I couldn't get the power down. Now I have got used to them and I feel like longer cranks make me pedal slowly.

I think what you are used to has a lot to do with it.

It didn't feel any better for me after 800 miles, so pretty sure I gave it a chance 😉

When I switched to 175s is when I began to get issues with fit and comfort.
Had a spare 170 so swapped it out for the 175 I was using. Straight away, improved comfort, no loss in, err, "speed".
A relative term, in my case.
😉

If you are considering hope cranks, there is less... well crank to hit.

So a hope 175mm crank has less crank arm after the pedal thread than most.

Awhiles' formula for me suggests 162.5! Good luck even finding those.

I can tell the difference between the typical 175s on the MTB and 170 on the roadie. I like the shorter on road because it helps me spin better, on the MTB it helps me muscle up tricky stuff.

This thread has freaked me out by causing me to measure my inside leg - I thought it was 33" but using the book method it's 34.5" - I'm only 5'10.5"!

I have 175 on my hardtail and 170 on my full-sus and child-seat bikes. I thought I preferred 170 but my dodgy ankle seemed better after riding the hardtail last week and I don't know if it's pedalling bigger circles or the harder impacts when hitting the rough/jumping/dropping which are keeping it moving more freely...

Thinking about the biomechanics of it, surely all that matters is femur length?

I've just done the "ahwiles" calculation and it's come out as 193mm.
I think I'll stick with the 175's that are on all of my bikes..
I may be a bit of an anomaly though being 6ft 8. 😆

[quote=molgrips ]I can tell the difference between the typical 175s on the MTB and 170 on the roadie. I like the shorter on road because it helps me spin better, on the MTB it helps me muscle up tricky stuff.
I know that's the way it [i]should[/i] work but I've been trying some 165mm cranks on my 29er (down from 175mm) and I reckon climbing is actually easier. All my other bikes are 170s. Oh - 5'7" here.

Well, quite. The current 'range' of 165 to 175 is bizarrely narrow (considering the range of frame sizes on offer).

I wonder if part of the reason behind such limited crank lengths is that large changes in leverage mean you have to change gearing too - as cranks get shorter, torque goes down and cadence goes up which maintains power.