Author Topic: Shorter cranks?  (Read 2229 times)

quixoticgeek

  • Mostly Harmless
Shorter cranks?
« on: 22 November, 2018, 10:37:45 am »
Currently I run 170mm cranks on both my bikes[1], I'm idly wondering what effect 165mm cranks would have for me when riding, primarily I'm thinking it would give me less issues with thighs/knees banging the beer belly when on the aero bars, and wondering if it would give me less strain on my knees after 20 hours on the bike. Is this realistic analysis? What other effects would having shorter cranks have?

J

[1] 1 is a Brompton tho...
--
Beer, bikes, and backpacking
http://b.42q.eu/

Re: Shorter cranks?
« Reply #1 on: 22 November, 2018, 11:25:03 am »
How tall are you?  There are various formulae around that recommend crank length based on leg length, height, etc.

Most people use the cranks that came on their bike and, in many, if not most, cases, such a measurement would suggest they are using cranks that are too long.

I switched to 165mm a few years ago, mainly so that I could be more comfortable in a TT position with a less-sharp hip angle.   I think it's a good idea for endurance riding, for the reasons you cite.  As well as knee benefit, opening up the hip angle helps your back.

One disadvantage is that it makes you less aero.  Seat position is dictated by the low point of the cranks, and shorter cranks make it higher, so everything has to move upwards.

Also, as you don't have as much leverage with a shorter lever, you might want lower gears.   

I still have 170mm on a couple of old bikes and they are perfectly rideable, but I notice the difference straight away.

quixoticgeek

  • Mostly Harmless
Re: Shorter cranks?
« Reply #2 on: 22 November, 2018, 11:31:01 am »
How tall are you?  There are various formulae around that recommend crank length based on leg length, height, etc.

1.7m tall.

Quote

I switched to 165mm a few years ago, mainly so that I could be more comfortable in a TT position with a less-sharp hip angle.   I think it's a good idea for endurance riding, for the reasons you cite.  As well as knee benefit, opening up the hip angle helps your back.

Interesting. Back pain reduction would always be welcome, esp after 20 hours on the bike...

Quote
One disadvantage is that it makes you less aero.  Seat position is dictated by the low point of the cranks, and shorter cranks make it higher, so everything has to move upwards.

Also, as you don't have as much leverage with a shorter lever, you might want lower gears.

Less aero at this level is probably not going to be noticed, I'm not riding an aero bike, I have luggage attached, and I could probably counter any difference in aero by doing my jersey zip up or something...

Low gears however is going to be an interesting one. Need to see if I can squeeze the 14-40 rear cassette into the RX-805 RD rear mech...

J
--
Beer, bikes, and backpacking
http://b.42q.eu/

Kim

  • Timelord
Re: Shorter cranks?
« Reply #3 on: 22 November, 2018, 01:10:46 pm »
I'm almost as tall as you (though with short legs for my height), and find 170mm a smidge 'too long' (resulting in a flapping about sensation almost but not quite entirely unlike having the saddle too high, that you'll recognise if you've ridden bikes with a variety of crank lengths, and consider normal if you've spent your entire adult life riding bikes with cranks that are slightly too long).  I like 155mm cranks on recumbents, but with appropriate gearing[1] anything up to 165mm is fine on an upright (indeed, a longer lever seems preferable for technical off-road riding, even if you do get more pedal strike).  I have a relatively upright riding position anyway, so thighs vs belly has never been an issue.

I didn't find it made a direct difference to my knee problems (which to a first order approximation came down to excessive force at the wrong foot angle), other than by curing me of my pushing-back-with-my-shoulders habit by making it much more comfortable to spin lower gears on a 'bent.  Uprights were never an issue, once I'd learned to start off with the other foot, and habitually use the gears.

Cranks shorter than 165mm are an exciting new world of unobtanium bike components...


[1] If you change the crank length significantly without altering the gearing to preserve the gain ratio at the bottom end, it's liable to be an own-goal knees wise.

Re: Shorter cranks?
« Reply #4 on: 22 November, 2018, 01:54:18 pm »
I'm about 5' 8" (just) and both times I've been fitted for a bike have been told 170mm cranks are the right length for me, so that's what I've been using. I don't have a problem riding my wife's bike, which has 165mm cranks, though it does feel slightly odd, not really tried going the other way up to 175mm except for very short rides, it was okay, though did feel "different".
Old enough to know better, but young enough to do it anyway

Kim

  • Timelord
Re: Shorter cranks?
« Reply #5 on: 22 November, 2018, 02:07:48 pm »
It's a bit like saddle height, in that you can tolerate a suboptimally short crank much more than you can tolerate an excessively long one.

fboab

  • Getting fatter every day
Re: Shorter cranks?
« Reply #6 on: 22 November, 2018, 02:46:16 pm »
It's 5mm. I think I've got socks that make that much difference- I certainly have soles that do.

(I know, circumference difference is 32mm, but, really. I definitely have at least that 5mm difference in seat height on my bikes, depending what I might be doing with the bike)

I think it's a little bit like people recommending 650 wheeled bikes for shortarses. Doing a few stretches probably makes more difference unless you're really at the pointy end of athleticism- Frank's closer to that than I am.

I'm 160cm- I 'should' be riding 160mm cranks and most of mine are 170. Though to be honest, I've never checked, they could easy be 175
TSS is not Total Sex Score, Chris!

Re: Shorter cranks?
« Reply #7 on: 22 November, 2018, 02:48:36 pm »
I'm 171cm, and find 170mm cranks a good fit (and wasn't recommended a different length when I went for a bike fit).

Re: Shorter cranks?
« Reply #8 on: 22 November, 2018, 03:36:36 pm »
Leg length (or pubic bone height) strikes me as a more relevant determining factor than overall height.  FWIW, I'm 175 cm tall, leg length/pubic bone height of 87 cm, and I switched from 170mm cranks to 172.5mm about 20 years ago. My hill climbing ability improved a tiny bit, and I haven't noticed any ill effects.

Re: Shorter cranks?
« Reply #9 on: 22 November, 2018, 03:48:26 pm »
I'm 183 cm, and I've got bikes with 165mm (fixie), 172.5mm (road) and 175mm (MTB). They are all sufficiently different (including different shoes for MTB, different saddles on all 3) that the crank lengths are lost in the noise. Changing the crank length setting on the pedal calibration is annoying when switching them between bikes though!
On the TT front, shorter means that the saddle has to go up -> less aero, but that your back angle might come down a little (reduced max hip angle and/or better belly clearance) -> more aero. How it actually works out is going to be rider specific, but in this case it sounds like it would be advantageous for you (at the moment).
Shorter than 165 is possible but rare, and difficult to experiment with - if you are convinced that shorter would be better there are some people who will re-drill and tap solid alloy cranks.

Re: Shorter cranks?
« Reply #10 on: 22 November, 2018, 03:51:39 pm »
It's 5mm. I think I've got socks that make that much difference- I certainly have soles that do.

You're meant put your seat up by 5 mm so that your full extension is the same, and if you do that your knee compression at the top of the stroke reduces by a total of 10 mm, which starts to seem like it might be a significant difference.

But as a fellow shortarse, I'd be much more inclined to try shorter cranks if they were *much* shorter and much more available.

Karla

  • car(e) free
    • Lost Byway - around the world by bike
Re: Shorter cranks?
« Reply #11 on: 22 November, 2018, 04:27:11 pm »
I'm 181 cm and am marginally more on the gangly legged side (don't ask me for thigh bone measurements).  I run 165 on the TT bike, because I wanted to get my saddle lower and (contra Frank's advice) would prefer to lose 5 mm off the bottom of my pedal stroke if it means losing the dead spot at the top.  None of my other bikes are more than 170, but that's due the sake of consistency with the TT bike. 

Given the differences in people's heights, crank lengths are basically all the same proportionately.  Try the shorter cranks if you like but don't worry too much about it.

Re: Shorter cranks?
« Reply #12 on: 22 November, 2018, 04:29:19 pm »
Currently I run 170mm cranks on both my bikes[1], I'm idly wondering what effect 165mm cranks would have for me when riding, primarily I'm thinking it would give me less issues with thighs/knees banging the beer belly when on the aero bars, and wondering if it would give me less strain on my knees after 20 hours on the bike. Is this realistic analysis? What other effects would having shorter cranks have?

J

[1] 1 is a Brompton tho...

Hi H. I run 140mm Thorn cranks on both of my bikes and have done since 2007 when I had a total left knee replacement.
I was the only way that I could achieve the full circulatory action of the pedals.
The shorter length cranks gave me an equivalent of two teeth high on my chain-rings.
It also gives a spin rate.
I am using Stronglight Triples on both bikes with Campagnolo  10sd on the rears.
I stand at 4'10" tall with a 23" inside leg and ride a 51cm Bianchi Via Nirone 7 Alu Carbon Flat-bar Hybrid and a 50cm Steve Goff on dropped bars.
One of the not so obvious advantages is that pedals are not at risk on the kerb edges unless you are riding the roads with 'Flood Prevention' kerbs.
Your ears are your rear-end defenders,keep them free of clutter and possibly live longer.

Re: Shorter cranks?
« Reply #13 on: 22 November, 2018, 06:06:42 pm »
I like 165s better than 170s.  I find 175s a bit annoying, although I can ride all of these and do.  A lot depends if you prefer spinning to mashing.  165s are best for fixies because of ground clearance and the ability to keep up with the pedals on descents.
And Darkness and Decay and the Coronavirus held illimitable dominion over all.

Re: Shorter cranks?
« Reply #14 on: 22 November, 2018, 06:49:56 pm »
It's all bullshit. I've got bikes with 165, 170, 172.5 and 175. It makes no difference to me. I do generally go for 172.5 when building up a new bike, but that's only because I have to make a choice when ordering. Unless you're exceptionally tall or short, I doubt anyone would ever really notice*

*OK, if you rode 175 exclusively for months every day, 165 would probably feel weird to start with, but you'd soon get used to it....
Those wonderful norks are never far from my thoughts, oh yeah!

Re: Shorter cranks?
« Reply #15 on: 22 November, 2018, 08:08:13 pm »
When I was young I ran 175mm cranks (only had one bike so they did for fixed, gears, tt, touring and whatever else) - until the bike got stolen. They were great and, surprise, surprise, I span the pedals really fast on fixed. Afterwards due to being impoverished and riding whatever was cheap I used 170mm and always hankered after the sensations of 175. Eventually came the mtb, hacked into tourer, with 175mm cranks. Really nice! Until one day going to fetch the bread I had trouble and enormous pain getting over the top of the pedal stroke (this was about five years ago). I could no longer bend the right knee beyond 90°, my arthritis had caught up with me. (I had already had one op to eak out the life of the knee joint and was pushing things as far as I could). Fortunately I had a set of 165mm cranks and they kept me riding. Even after the knee replacement earlier this year I am still on 165mm. Moral of story if you haven't got a problem you might not notice the difference but when you need them don't believe a word anyone says, just get comfortable. Pushing the saddle up can have consequences for your back unless you have the possibility of raising the bars to compensate (which I don't really). It's not all benefit!

Re: Shorter cranks?
« Reply #16 on: 22 November, 2018, 08:44:49 pm »
IIRC attempts to correlate leg length with

a) folk's preferences for crank length and
b) biomechanical performance varying with leg length

have each produced graphs which look like a 'vague splodge' of data points rather than a 'line'.  This  means that there is a weak correlation at best, and/or that there are other factors which are more important.  I suspect the latter since responses on this topic always vary from 'it doesn't matter' to 'I changed by a small amount and it made a difference'.

Regarding 'making a difference'; it is possible for changes to make a difference that is not really noticeable (either immediately or at all) to some riders, but the differences are often shown by careful measurements.

These days I use 170mm cranks on the road and 175mm cranks on my MTBs. I have MTBs that are set up for the road and on these bikes I very much prefer to use 170mm cranks too.  I suspect that I spin on my road bikes but I am OK to 'mash' slightly on my MTBs. I definitely don't pedal in the same way on each sort of bike and it isn't just the riding position being different; (I would be unhappy riding 175mm cranks on a bike with dropped bars for sure).

Years ago I (unknowingly, I thought they were 170mm for ages) had 6-5/8" cranks on my training bike and I couldn't for the life of me work out why I pedalled so badly on my race bike after a winter spent almost exclusively on the training bike, where I pedalled 'very nice circles'.  Eventually I measured carefully and found a reason why my pedalling was so much more fluid on the training bike; 6-5/8" is 168.275mm which means that my circles were just over 3.4mm smaller on the training bike than on the race bike. 

As others have said, a good principle is to 'get comfy'.  By all means try different cranks and see how you get on.

cheers

Ben T

Re: Shorter cranks?
« Reply #17 on: 22 November, 2018, 09:25:05 pm »
I've been wondering the same thing - I've currently got 175mm cranks but I think 170mm would be too short so I'm thinking of how I can get some 172.5mm ones  ::-) ::-) ::-)

Re: Shorter cranks?
« Reply #18 on: 23 November, 2018, 09:14:45 am »

Also, as you don't have as much leverage with a shorter lever, you might want lower gears.   


IME it's more complicated than this, as you need to take the biomechanics of the leg into account. I switched from 175 to 155 on my MTB to combat knee problems, and found that, if anything, I could push one gear *higher* with the short cranks. I hypothesise that this is because the power part of the pedal stroke is done with the leg significantly straighter. Pedalling style might have a big effect though - I imagine someone who 'pedals circles' more than I would notice that effect less.

FWIW, I eventually decided that 160mm is about optimum for me ( I'm 1.83m tall), but I've ended up using 165mm for the simple reason that it's easier to find quality cranks in that size.
Life is too important to be taken seriously.

Re: Shorter cranks?
« Reply #19 on: 23 November, 2018, 09:47:43 am »
Quote
It's all bullshit. I've got bikes with 165, 170, 172.5 and 175. It makes no difference to me

Good for you, count yourself among the lucky group of people with a wide tolerance for crank length*, not everyone is so lucky.

Changing crank length (going shorter, I'm 1.74m tall with short legs) for me sorted out** a persistent knee issue that would have me uncomfortable after 30miles, and in agony after 60, and struggling to walk the next day after 100 to someone who can happily ride 200+miles in comfort.

It may be only 5mm, but it's 10mm on the circle (or more if you change cranks by more than 5mm) and can affect hip and knee extension angles significantly and I can tell you the 'wrong' cranks are fitted within a few revs, and can probably tell you the length in a blind test with a fair degree of accuracy after a few minutes. I'm a lot less sensitive to it on fixed/SS or offroad though and can run longer on those bikes as I spend a lot more time stood up so the effect on hip and knee angles can be somewhat mitigated when standing.

If you're one of the lucky ones then that's great ignore crank length and crack on, if you're unlucky it pays to get it right.

* FWIW I'm the same with bar width, 2cm 'wrong' can give me pain, but not with TT or stem length, you can vary that 3-4cm and I'll likely be fine, other people are very TT/reach sensitive.

** was definitely the crank length that did it as it was a common issue across a number of bikes with differing setups in terms of reach and saddle-bar drop, and one by one I swapped them over to shorter cranks, riding one of the ones with the wrong cranks would bring the pain back until the cranks swapped and then pain gone.

Re: Shorter cranks?
« Reply #20 on: 24 November, 2018, 10:56:15 am »
I'm sorry if I caused offence. That wasn't my intention  :-[
Those wonderful norks are never far from my thoughts, oh yeah!

Re: Shorter cranks?
« Reply #21 on: 24 November, 2018, 07:53:16 pm »
None taken, it was just the bullshit bit that made me bite as I probably could have solved my knee problems about a year sooner if I hadn’t got similar comments when I asked around. No hard feelings :-)

Re: Shorter cranks?
« Reply #22 on: 24 November, 2018, 08:18:45 pm »
For taller people, say over 1.85m, all cranks are short anyway.

A short person using 165 cranks will be pedalling a bigger circle relative to their leg length than a taller person using 180 cranks.

Re: Shorter cranks?
« Reply #23 on: 14 January, 2022, 11:52:22 pm »
I was finding  my croix de fer really uncomfortable on my knees.  I have used 175's for decades on all my bikes, but the pain in my knees called for some drastic action.
Out went a 36/26 175 mtb chainset
In came a 40/33 170 Spa chainset

Out went the wide mtb b/b with spacers to fir the CdF
In came a 110mm sq taper b/b for a nice chainline and a lot lower Q

Changed saddle to my preferred selle italia from some performance Trek thing

Dropped bars by quite a bit (~40mm)
 
Changed shoes - (realised I only wore my touring/training shoe type spd shoe on this bike and cleats were maybe too far back)

Results so far are very promising, the shorter cranks and lower Q really feel much kinder to my knees.
I will try a 175 bike again soon, my Domane always felt very comfortable and easy and will see if it feels a backward step to persist on 175's.   The way the 170's feel on my CdF  I think I might go 170's or maybe 172.5's for all my bikes.

Re: Shorter cranks?
« Reply #24 on: Yesterday at 12:42:50 am »
I dropped down to a 170 crank for my Rando/endurance bike build. The gearing is a good bit lower than the other rigs too and I find it very comfortable humming away with the smaller cranks. The BB is also lower so frankly a longer crank would be hazardous with pedal strike what with big flat pedals and heading off the tarmac..
My partner is about your height and has 165's on her Rando. Low gears like you too. Loves it.
As others have mentioned,  Ive found age related knee damage is more niggly with larger cranks. Not sure of the mechanics regards to that.
I do still road race with longer cranks, but accept there will be a price to be paid. Endurance stuff though, when power output is dialled way back Im loving the shorter cranks.
often lost.