Crank length

[The utility cyclist]

one of my chums used 170mm cranks for years, tried 175mm and found it made his knees hurt, then changed to 172.5mm and has been happy ever since.

To my annoyance many years ago I found I'd become habituated to 6-5/8" cranks, which you can't buy these days. 170mm is just a few mm longer but enough to be felt.

cheers

You can notice the difference in 1.725mm, how?

How? who knows the mechanism but i can feel different cranks, tyres and bar widths. Certainly if you regularly use one size, and variance from that can be felt by pretty much anyone, i'm guessing a lot of people put it down to just being different rather than identifying what is different.

i have bikes with 170, 172.5 and 175mm cranks and when swapping machines, i can tell the difference but soon become accustomed. i have quite long levers and find i'm most comfortable with the 172.5 size which is my default, the 170 are an historic aberration and the 175's give me some extra leverage on the poor climbing small wheeler.

Personally I don't believe anyone can notice 1% difference, it's in the head.

[Brucey]

believe what you like; for years I thought my 6-5/8" cranks were 170mm and it was only after I persistently felt pedalling was somehow different on the bikes with the 6-5/8" cranks that I bother to measure them accurately and the penny finally dropped. FWIW I'd describe a 2.5mm difference in crank length as 'very noticeable indeed' and a difference of 5mm as 'massive'. If you are racing you will certainly notice if you go 1% faster or slower....

Some folk say they don't feel any difference in cranks of different lengths but that still might make them go faster or slower whether they feel it or not. I've even known folk ride for years with cranks that are different lengths each side and not notice....

Fortunately we are not all the same....

cheers

[Debs]

When i started time trialing in the 1980s i used bikes with what was standard 170 mm cranks. I never thought anything otherwise about cranks in those days.
In the 90s i had a few seasons of road racing, and as a reasonable climber i swopped over to 172.5 cranks due to the theory being the longer length crank gives better leverage when climbing up hills; longer crank produces more torque at lower rpm while climbing. I was strong and fit in those days and got on fine with the 172.5 crank length which i also carried on using after retiring from racing.

It wasn't until a few years ago getting into my late 50s that knee trouble became an issue, it took some time but i noticed it was worse on my bike with the 172.5mm crank length, my other bike which incidentally had 170mm crank length was obviously far kinder to my knees, and also [ and what became a revelation ] was also easier for me to commit to a very efficient cadence on the flat which for me is around 95rpm. The 170mm size simply feels comfortable and ergonomic for my physic.

Being young and strong it's probable one can adapt and cope fine with varying crank lengths even if they're not the best size, but with age comes a degree of fragility and a need to get the bike set-up more appropriate.

[KM2]

Isn't crank length going to be a function of thigh length and not total leg length? Within the parameters of normality!!!

[foxyrider]

Isn't crank length going to be a function of thigh length and not total leg length? Within the parameters of normality!!!

But what is normal?
For my height, i have long legs, others will have longer but they are generally taller than me - i also have 'Gibbon' arms so the high saddle, low bar position i favour is right for me but not appropriate for someone who sits elsewhere on the limb/torso length equation.

Fwiw, i used to have 180mm cranks on the back of the racing tandem which felt a bit odd at times but allowed me to get on top of the 62x11 gearing!

[foxyrider]

Being young and strong it's probable one can adapt and cope fine with varying crank lengths even if they're not the best size, but with age comes a degree of fragility and a need to get the bike set-up more appropriate.

oh to be young again! I'm not fragile, i just don't bend as much as i did!

[Marcus Aurelius]

believe what you like; for years I thought my 6-5/8" cranks were 170mm and it was only after I persistently felt pedalling was somehow different on the bikes with the 6-5/8" cranks that I bother to measure them accurately and the penny finally dropped. FWIW I'd describe a 2.5mm difference in crank length as 'very noticeable indeed' and a difference of 5mm as 'massive'. If you are racing you will certainly notice if you go 1% faster or slower....

Some folk say they don't feel any difference in cranks of different lengths but that still might make them go faster or slower whether they feel it or not. I've even known folk ride for years with cranks that are different lengths each side and not notice....

Fortunately we are not all the same....

cheers

You’re correct. The difference in pedal circle per unit change in crank length, is noticeable. The feel of a 170mm crank, over a 175mm crank ( for example ) is night and day.

[Debs]

Back in the early 90s there was a fashion trend [ flavour of the month? ] for long cranks pushing high gears for time trials.
I was given an opportunity to try 175 cranks with 54 tooth outer ring for an up and coming 100m TT.
The race was not on home ground for me, but was a fast course and a good day, and i did achieve a PB, just.
However i really hated those long cranks, and really hated the high 54 tooth ring, couldn't wait to get that off the bike.

In hindsight it was silly of me to experiment with a 100m event.
I've always believed if i'd had a 170mm crank with my usual 51 tooth outer ring for the day, my PB would have been another 10 minutes up at least.
We live and learn from experience

[Marcus Aurelius]

Back in the early 90s there was a fashion trend [ flavour of the month? ] for long cranks pushing high gears for time trials.
I was given an opportunity to try 175 cranks with 54 tooth outer ring for an up and coming 100m TT.
The race was not on home ground for me, but was a fast course and a good day, and i did achieve a PB, just.
However i really hated those long cranks, and really hated the high 54 tooth ring, couldn't wait to get that off the bike.

In hindsight it was silly of me to experiment with a 100m event.
I've always believed if i'd had a 170mm crank with my usual 51 tooth outer ring for the day, my PB would have been another 10 minutes up at least.
We live and learn from experience

Yep, we now know, what we previously didn’t about efficient ( lower power ) efforts versus ( overpowered inefficient ) efforts. Short sprints / TTs, it’s no great shakes, longer exertions show the differences.

[Rod Goodfellow]

Back in the 1970s I won a couple of Mersey RC 24hr TTs using 175 cranks in the daylight hours and 170 cranks on my night bike.I noticed the change for about a mile after restarting after each change but not thereafter. I was used to 175 for tts but 170s for training and utility riding. Currently I do nearly all my riding on 175s but occasionally ride a 1960s fixed bike with 165s ,again only notice the difference for the first few minutes. So for me crank length within limits does not make any appreciable difference.

[Marcus Aurelius]

Back in the 1970s I won a couple of Mersey RC 24hr TTs using 175 cranks in the daylight hours and 170 cranks on my night bike.I noticed the change for about a mile after restarting after each change but not thereafter. I was used to 175 for tts but 170s for training and utility riding. Currently I do nearly all my riding on 175s but occasionally ride a 1960s fixed bike with 165s ,again only notice the difference for the first few minutes. So for me crank length within limits does not make any appreciable difference.

The difference you ‘feel’ initially, does continue, you ‘acclimatise’ to it, which is a bang up guarantee that it’s working.

[thatsnotmyname]

Interesting article on crank length.

https://cyclingtips.com/2017/09/crank-length-forget-leverage-power-fit/

I will discuss this in more detail below, but for those hoping for a quick answer, here it is: there is no evidence that crank length has an effect on a road cyclist’s power or speed.

There is too much to summarize, but it is a comprehensive review of the literature.

Another interesting summary of crank length studies and their effect (or non-effect) on performance..

TL:DR = it's all about fit - next to no impact on performance.

http://myworldfromabicycle.blogspot.com ... -just.html

[mercalia]

In physics terms, the crank is just a lever than amplifies the input force. Think trying to undo a bolt with a regular spanner, vs an 18-inch long breaker bar. The longer the spanner, the easier the job is (potential bolt-shearing shenanigans aside).

That said, the noticeable difference between one crank length and the next, would likely be negligible as most are in the same ballpark.

there is maybe as little as a 3% "advantage" in using 175mm over the standard 170mm cranks? Not a great deal? Might just be useful in fine tuning riding position ( saddle height v handle bars on a frame that is really too small) or if you have thick soled shoes?

[fastpedaller]

An interest subject, but so many variables and approaches to analysing it. As an example, if measuring power output at the cranks it isn't surprising that there is very little difference (by changing crank length), as the rider will be putting the same effort out (but of course the range of limb movement will be changed, depending on crank length. It's not as simple as "a longer lever is better", because as others have said the variable gearing of a bike is a bigger factor. One thing which the studies do not appear to take into account is how an individual adapts (or not) over time to a crank length change. Just taking a rider from (say) 170mm cranks and getting them to ride with 160mm cranks whilst taking measurements may not be conclusive, as the rider may present with different figures after riding the 160mm cranks for an extended period. I appreciate this would be difficult to ascertain. I used to ride 150mm cranks with fixed wheel for time trialling many years ago, and when changing to another bike with 170mm cranks it felt rather odd. once 'accustomed' it also felt odd to go to 150mm. At present (although not racing any more) I'm happy with 160mm.

[PT1029]

A friend in our local CUK group has 170/172.5/175 mm cranks on his 3 bikes, says he doesn't notice any difference.
I am used to 170 mm cranks. When building up a mountain bike (which later morphed into a camping/poor tarmac tourer) in the 1990's I was told mountain biking, best use 175 mm cranks (a new thing then).
While I could use 175 mm cranks without issues, I just couldn't get on with them, as the saddle simply felt too low*, so I replaced them with 170 mm cranks, and found them much better (for me at least).

* The sums being, longer (by 5 mm) crank means putting the saddle down 5 mm to get the saddle correct relative to the bottom pedal position.
Thus on the top stroke saddle feels 5 mm too low (it's just been moved down 5 mm) PLUS a further 5 mm because the longer crank comes up 5 mm higher, so in all it feels 10 mm to low when feet are at top dead center. It just felt so wrong.

Changing the rear cranks on the tandem from 170 mm to 160 mm (same as her solo) means I can get away with a higher cadence on the tandem. With 170 mm cranks on the back of the tandem, her upper limit for cadence was some where near my lower cadence.