Getting Cranky

Page may contain affiliate links. Please see terms for details.

lejogger

Guru
Location
Wirral
In short, when looking to replace my current chainset, is the crank arm length going to make a blind bit of difference? Both my bikes came with 175mm but before I took any sort of plunge into a replacement I thought I better run it past you chaps. I'm 6ft, the bikes are a size L, so a crank arm at the longer end of the scale seems to make sense.

SRAM have the ability to offer me 170mm, 172.5mm, or 175mm.
Now why on earth do they bother? 5mm difference???? Is that going to make the slightest bit of difference????! I mean I know you can change your stem length in 10mm increments or move your seat down, up, forward or back by fractions, but surely there is a tolerance for most people with a modicum of flexibility? So does a 5mm crank range serve any purpose?

I'm not saying that it doesn't, I'm just asking the question of more knowledgeable people :thumbsup:

It's not even as simple as getting a bike fit, because from a bit of googling there are so many schools of thought on what is 'optimum' crank arm length that I'm not even sure that there is an optimum, so one bike fitter may tell me something completely different to another.

Reading between the lines, in 'the old days' it was thought that longer gave more leverage and power, but recently there's been a shift towards a more compact pedal circle providing a more balanced stroke?

To be frank, I'm confused. I've had knee niggles for a few years, but related to football, not cycling. I occasionally get slight discomfort but I presume I'd get that regardless of the cranks - it's just my dodgy knee, so the cycnic in me thinks I should just stick with 175. Both my bikes will therefore be the same, and the status quo will be restored. But what if it WOULD be more comfortable with a shorter crank? But then 2.5mm? What will that do??! Argh!

I guess what I'm looking for is a concrete reason to change i.e. at Xft tall with a XXinch inseam you WILL need a XXXmm crank arm length because...

a) it will be kinder on your joints
b) it will make me more efficient
c) it will increase my prowess with the ladies etc etc.

Help :sad:
 

fossyant

Ride It Like You Stole It!
Location
South Manchester
Stick with what you have. You are a taller fella and 175 is ok.
 
OP
OP
lejogger

lejogger

Guru
Location
Wirral
Stick with what you have. You are a taller fella and 175 is ok.
Cheers Fossy.

I guess my issue is that if it's as fluffy as simply being 'you're tall so that'll be fine' then why have the option of such tiny incremental adjustments?

If there are only really 3 mainstream options, and they are all so close to each other, why bother having the options at all unless there is some clear science behind it?
 

fossyant

Ride It Like You Stole It!
Location
South Manchester
Its more what you are used to. I am 5'9 short arse and run 170 on all my bikes. Ive never noticed the 165 s on the track feeling any different

Smaller cranks allow you to spin more, larger more leverage.
 

snailracer

Über Member
I have very short, chunky legs and bad knees (from football) and I definitely noticed an improvement in both comfort and efficiency going from 165 to 152mm cranks. I reckon, in my case, that reducing the angle of my knee at the top of the stroke is more efficient, because the tissues around the knee are less compressed, however this might be less of a factor for thin-legged riders. Leverage is reduced, but I can spin faster which compensates.

Note that the difference in your stroke is double the difference in crank length, so going from 175 to 170mm means your knee rises 10mm less over one whole pedal rev.

One "authority" recommends a crank length of 21% of your leg length, however there is a bit of practical difficulty in accurately measuring leg length (it's not the same as your inside leg measurement, usually).

A short crank also makes the thickness of shoes more noticeable, as the downward push is concentrated in a shorter pedal arc. If you tend to wear various different shoes when riding, it is something to consider.
 
D

Deleted member 1258

Guest
Its more what you are used to. I am 5'9 short arse and run 170 on all my bikes. Ive never noticed the 165 s on the track feeling any different

Smaller cranks allow you to spin more, larger more leverage.
If your a short arse at 5'9 whats that make me at 5'6? ^_^ Like you the165's on my fixed and the 172.5's on my geared bike feel the same, though its easier to spin up the fixed on the shorter crank.
 

Mr Haematocrit

msg me on kik for android
I run short cranks with a narrow Q factor which addressed knee issues I had prior to cycling (from running) so I'm a big supporter of getting the correct length cranks.I was advised to change my crank length during a bike fit
 
Heh. I had a bike with two cranks of different lengths, you can feel the difference if you do that.

175mm should be fine. As for the knees, choosing a different gear will make more difference than crank length.
 
FWIW I'm 6' 5" and run stock 175mm cranks on my bikes. I've ridden track as well, I even bought a track bike at one point, and to comply with regs they have to run 165mm cranks which I did, and I never felt any discernible difference :thumbsup:
 

Shut Up Legs

Down Under Member
Slightly OT: I reckon Frodo and Sam should have ridden bikes to Mordor: they would have got there a helluva lot faster. I'm thinking some variety of MTB, mainly for those tricky mountain bits. Just think how much fun the downhill ride off Mt Doom dodging lava flows could have been! :hyper: Of course, to remain (slightly) on-topic: they would have needed very, very short cranks.
 
Top Bottom