170mm vs 172.5mm crank length

[Froggie]

Does it make a big difference - I guess if you are doing high mileage then it becomes more critical, but for someone doing short rides <50miles will it be noticeable?

[woollypigs]

I can feel the difference between 165, 170 and 175, never tried a shorter gap.

Tried it on my fixed.
165 - plenty of spin but no power on the hills
170 - good spin and got more power on the hills (this combo I like)
175 - less spin and lots of power on the hills

[Biggsy]

Not a big difference, certainly, but I noticed the difference between 172.5 and 175mm even on short rides in terms of one feeling more ergonomically suitable for me than the other.  I went to the trouble of buying a 172.5mm chainset to experiment with, then selling it on after deciding to go back to 175mm.

(Don't forget, by the way, that changing cranks to longer ones requires the saddle to be lowered, and vice versa).

Cue posts from those who accidentally fitted a left 160mm crank and a right 180mm crank and never noticed until ten years had passed.  There's always one!  

[MSeries]

It is noticeable but for me not a problem at all. I have bikes with 165, 170 and 172.5, might have 175 too, I can't remember. I am a high mileage rider

[Greenbank]

I've a luddite. I'm rubbish at determining ride quality, differences in tyres, handling or anything like that.

I have bikes with 165mm (fixed), 172.5mm (Audax and Wilier), 175mm (Colnago) and 170mm (hackbike) and can't notice any difference between any of them (as far as pedaling and cranks are concerned).

I've done rides over 200km on most of those bikes too, and do over 5000 miles a year.

[SpaceBadger]

Michael Hutchinson in this week's CW says that crank length doesn't make a chuffing bit of difference in his opinion, or words to that effect.

Never tried anything other than 175mm personally. Except for 152mm on a child's bike on Sunday. That was just wrong (on both counts!).  

[aidan.f]

If you are 4'8" fully upright it will certainly make a difference for the (generally)
women I see on off the peg MTB's with knees up to their elbows on 175's

[Deano]

I notice the difference between 165 and 170 on fixed, but only when I'm spinning at 150 rpm+

Otherwise, I'm completely insensitive to such slight differences. 

[Wibble]

Given the very slight differences between your two crank lengths I'd suggest replacing the left hand crank so you can directly compare the two.  Seems a better way of doing things than splashing out £££ on a new chainset right away.

Most of my bikes had 175mm cranks, but the fixie started its life with one side 170 & t'other a 160 crank.  Very noticeable difference!  Lots of extra power on the longer crank.  They're now at 165 and 160 - almost no difference, but it's still there.

The fixie will eventually be moving up to 170mm cranks as 165 feels a little too small (prolly my big feet).

I suspect that getting crank length 'just so' really only matters to those putting lots of power down.  When fit I was cycling between 25 and 30mph (but short rides!  I average a trundly 12mph on centuries) and 'needed' the bikes various wossnames adjusted to the mm or would notice the imperfect fit holding me back.  Now injury has left me both unfit and slow the fit of the bike seems to matter less.

[Zoidburg]

2.5mm is not going to that noticable to be honest, you can loose or gain that much just through using different combinations of shoe and SPD pedals, most people will have that much difference at least in leg length as well.

Like the others said 5mm graduations in length start to get noticable but 172.5mm often strikes me as the result of differing manufacturers interpreting metric/imperial measurements differently so you get 170mm stamped on one manufacturers chainset and 172.5 marked on another.

[blackburnrod]

During 2 24hr events I have used 175 in daylight and 170 at night.I only noticed the difference for the first mile or two after each change.

[LittleWheelsandBig]

Not so Zoidburg.  I've seen several older 171 mm cranks (measured and labeled as such), probably as a result of direct conversion from Imperial castings.

Personally, I can't really feel 2.5 mm difference in crank length but 5 mm is obvious.  This was a big reason I selected 172.5 mm cranks for the road and 170 mm for the track.  I could swap between them without noticeable effect (and to 175 mm off-road).  Any wider difference and some cranks felt 'weird'.