Being a mechanical engineer and a keen biker I was surprised when someone recently wanted to buy a set of crank arms off me but then lost interest when he found out they were 170mm not the 175mm that he wanted – WHY ?
Maybe half an inch might make a bit of difference but surely he'd not notice the difference that 5mm would make ?

some people think it's absolutely vital, though from a mechanical point of view it just makes a small difference to the mechanical advantage and the tests I read about said different lengths make little difference to the power transferred – however, roughly the right length will be most comfortable. I rode for months with a 170 one side and a 175 the other and couldn't feel anything different

Once you're used to it you're used to it – like a bent pedal axle, after a while you just don't notice it until you get on someone else's bike.

Having said that, I won't now buy any cranks longer than 165mm as I've got short 29" legs and I'd and I'd prefer to have the ground clearance.

Good point, I'm after new slx double ring groupset and can get a good price from somewhere which only has 170's – it has put me off, but perhaps I'd not really notice…….

I replaced the crank on my Stumpjumper FSR 01 with a 170mm one (from 175) as my pedals used to strike the ground quite often.

This small change, pretty much eliminated pedal strikes.

Did you notice any difference in the actual pedaling action ?

I used to get pedal strikes all the time, and changing the cranks made ****-all difference. I fixed it by fitting longer travel forks 🙂

as above I've had one 170mm and one 175mm on the same bike and dint feel anything odd.

Now I think of it, I put a spare crank on the comuter last week after a pedal-seizing incedent. Didn't even check the size so may be doing it again for all I know.

I'm sure you coul tell 165mm and 180mm appart though.

Not hugely sure what difference it make really – I suspect that it has more relevance to road bike the MTBs – I’m finding the road bike really do very fine adjustment to make them work/comfortable

I run 175mm on one bike and 170mm on the other, doesn't feel any different at all.

@hilldodger – couldn't tell the difference whatsoever….(apart from not whacking my pedals on things….)

Thanks, thought as much really…..

My knees noticed a change from 170 to 165 but only on road sections not off road where your shifting around a lot more.

I'm amazed no one has come on to say how critically vital the exact length is yet – many claim to be able to tell blindfold…

I use 175mm on the bouncer & did have 170mm on the road bike which I didn't like. It really felt cramped, like I wasn't able to stretch my legs although the seat height relative to the pedal at bottom dead centre was the same.
It may only be a difference of 5mm but as the pedal revolves the crank arm lenght of 170mm makes a circumference of 534.07mm as opposed to a 175mm lenght which has a 549.77mm perimeter. So as you can see the 175mm crank arm has a longer arc to travel, creating the feeling of being roomier.

<thatsmythoughtonitandnowexpecttobeflamedforinncorrectcalculations>

I too had 175 on my mtb and 170 on my road bike, and did notice a difference for about the first 3 minutes, then forgot about it!!!

Jay

The cranks on my Genesis Flyer are something stupid like 167.5, all my other bikes are 175. I notice the difference all the time (and don't like it) but maybe I'm just sensitive 😉 because I can also notice if my saddle is 5mm too high or low.

It's much more noticeable on road and (especially) track bikes.
All my bikes have 175mm apart from my roadie SS which has 170's and the difference is noticeable but, switching between bikes as I do, it's easy getting used to it.
Riding track, all the hire bikes have 165mm – makes it easier to spin at the slight expense of getting the power down initially.

crazy-legs you beat me to it, track bikes. The difference can be felt.
I went to 170 on road and cross from 172 and felt the difference (170 is the right size for me) 170 real or imagined seems to give me a bit more acceleration.
I can't say I can notice it so much on the MTB.

Same here Dibbs, 5mm out and I can throw my toys out of the pram.

I think we have established from previous posts that those with shorter legs who tend to ride with a higher cadence find shorter cranks beneficial. I know I do – less strain on the knees and a smoother pedalling action.

For the record, I once bought a new Orange from Neil Waltons' in Didsbury and felt very awkward and uncomfortable on the bike until I discovered that one crank arm was 175mm and one was 170mm.

Just contributing 'I can feel no difference' to a thread like this is pointless and unhelpful unless you state your leg length, pedalling style etc.

Sorry to bang on about this, but using shorter cranks has really contributed to my enjoyment of cycling over the last few years and I'd hate someone to be denied that because 'someone on the internet' told them it didn't matter.

Try a few different crank lengths and make your own mind up, but at least, give it a go.

I have 110s on my unicycle. Now that is bloody confusing when you get on after you've been riding 175mm bike cranks.

Well i have very short legs & i ride standard 175mm XT cranks – doesn't cause me any problems whatsoever. I've used 170mm in the past & not noticed owt different.

Fair enough MD!

Didn't seem to slow you down much – how's the Boardman going BTW?

Only been out on it once – full of a cold & now coughing like terminal consumptive!
Hope to give it a whirl next week if i can breathe… 😕

It's the crank length that's brought that on! 😉
The strain on your knees has spread upwards.

Y'know, that was one thing i never checked when i bought the bike – hang on…

Hmm, seems i was wrong!

Cranks on the Marin are 170mm XT's & the cranks on the Boardman are 175mm Truvativ's.

I'm obviously going to suffer horribly, my knees will explode and my spine will collapse if i ride the Boardman!

Just to reinforce a point that Rusty Spanner has made, if you suffer from knee pain at all, try running shorter cranks. This helped me out loads years ago when I used to suffer really badly from knee pain whilst riding.

5ft 11 here, 33" inside leg, which probably means I should be firmly on 175's all the way, but 3 out of my 4 bikes have 170mm cranks. The other only has 175's cos I'm skint and can't afford to change them (and it has less gears than the others, so I have to get out of the saddle a bit more too).

Shorter cranks are also great for applications where pedal clearance is an issue, ie. DH and freeride bikes.

And as has been said, on track bikes you'll often see very short cranks as a high cadence is preferable to being able to put the power down initially. And it's this that I notice a big difference between 175's and 170's on my MTB's. I find maintaining a decent cadence comes pretty easy with 170's, but on 175's it feels less natural.

Could I swear blind the difference between 170's and 175's just by jumping on a bike and taking it up the street? Not sure I could to be honest, but give me 30 mins or more in the saddle over some terrain I know, and I'm confident I could tell the difference. Sure, it's a small difference, but it is there.

One of My cycling GoDs/GuRus – Sheldon says
"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." found at
http://www.sheldonbrown.com/cranks.html

but I still have different crank lengths on different bikes for different things 165 for DH 170 FS trail, 175 HT xc, and 180 for SS, I think I can tell the bigger differences both in leverage and pedal strikes. 😕

"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."

Yep, but we have no choice about the staircases we use, and we don't use them for hours on end, for pleasure.

When climbing up the large zig – zag steps from Llanberis to Dinorwic I have major problems with leg extention/flexation, but the longer legged aren't troubled at all!

It's all about proportion and simple mechanics isn't it?
Don't get me started on proportional fork travel BTW, we'll be here all day 😀

I notice the difference on the race BMX. I have 180's on that.

However, I have never, ever noticed the difference on the MTB. It has 175's as that's what was available…

I think people really obsess too much about crank length.

Yes and no.

It's one of those things that some people really do notice the difference with, and some people don't. Shouldn't knock someone for wanting to run different crank arms for a reason, or likewise someone who says they can't notice any difference.

Plenty of people couldn't tell the difference between 2 identical frames, but say one made of normal double butted 4130 cromoly (or Reynolds 520 as it's often badged) and the other made out of 853. But then again, some people can…

Some people obsess about gear ratios (especially singlespeeders), others just ride what they've got… Who's right?

Full Sus Vs Hardtail anyone? 😉

Sorry, I'm getting carried away… The point I'm trying to make though is people are different, and different people notice different things. So if you're happy running whatever length cranks are on your bike, great, if you obsess over what crank length for what bike, that's also fine…

I've 3 bikes and I couldn't tell you what crank lengths are on any of them. I'm comfortable riding all 3.

i always to get 170's although tbh i doubt it makes a difference.

My HT has 170mm cranks on it, the FS has 175mm cranks on it, my road bike has 172.5mm cranks on it and my commuter has 175mm cranks on it.

Can't tell the difference in any of them.

34" inside leg and I ride with a high cadence.
165mm on the FS and 175m on the HT.
I can't tell the difference.

Just contributing 'I can feel no difference' to a thread like this is pointless and unhelpful unless you state your leg length, pedalling style etc.

I disagree, it doesn't matter how long your legs are if the cranks are different, you either can or can't feel the difference. Also I wonder if "pedalling style" isn't a mental perception rather than a physical thing as the path of your feet is necessarily a circle defined by the crank, however you care to describe it

Sorry to bang on about this, but using shorter cranks has really contributed to my enjoyment of cycling over the last few years and I'd hate someone to be denied that because 'someone on the internet' told them it didn't matter.

There seems to be consensus that long term, there will be an ideal length for each rider, but that doesn't mean you'll instantly be able to tell what it is.

Its probably similar to compression and rebound on my forks – I can't tell the difference when they're adjusted so I just set them bang in the middle and ride 🙂