aracer, I spent a few years rowing before doing any serious cyclng. But I don’t see many athletes in any fields trying to restrict their range of movement when developing power. Tennis, football (when trying to hit the ball hard), swimming, javelin throwing, sprinting, you name it, a good range of movement is pretty fundamental in just about everything. Obviously short cranks works well for many people, I’m puzzled why this is though.

One downside of shorter cranks is your seat will be higher – so not ideal on bikes with slack seat angles or if your dropper post doesn’t have as much drop as you’d like.

aracer, I spent a few years rowing before doing any serious cyclng. But I don’t see many athletes in any fields trying to restrict their range of movement when developing power. Tennis, football (when trying to hit the ball hard), swimming, javelin throwing, sprinting, you name it, a good range of movement is pretty fundamental in just about everything. Obviously short cranks works well for many people, I’m puzzled why this is though.

It isn’t about restricting range of movement, it’s about optimising range of movement:

[video]https://www.youtube.com/watch?v=6FEh7hDpGp0[/video]

thecaptain – Member
Obviously short cranks works well for many people, I’m puzzled why this is though.

reason 1 (of many): people are available in different sizes.

awhiles, if my aim was to get up a flight of stairs as quickly as possible, I wouldn’t take them one at a time (unless they were gigantic). Would you?

Look, I’m not arguing against people using short cranks if they feel it suits them. But I’m puzzled as to how this can be a good choice biomechanically.

people are available in different sizes.

Yeah, and cranks aren’t! At least, not to anything like the same degree.

awhiles, if my aim was to get up a flight of stairs as quickly as possible, I wouldn’t take them one at a time (unless they were gigantic). Would you?

No. But I don’t generally take them three at a time either – and I notice less long legged friends more often take one step at a time whilst I usually do two.

I used 170mm for years, then moved to 175mm for at least 12 years. Now on 165mm and I cannot tell the difference!

thecaptain – Member
…I wouldn’t…Would you?..

surely it depends how big the steps are?

But I’m puzzled as to how this can be a good choice biomechanically.

we’re a walking/running biped, we walk / run or more or less straight legs.

this:

not this:

(would form the basis of my literature review)

Good range != full range. Cycling with normal length cranks utilises a good range. Sure rowing uses a fuller range than most sports, but what would your coaches say if your bum was touching your feet at the catch, or you leaned right back and pulled the handle up to your chin? None of the other sports you mention uses anywhere near a full range of motion. I was curious whether you’d come up with gymnastics – compare the range of motion they demonstrate to that of any of those other sports.

Quite – I thought of running when the “full range” comment was first made. The range of motion when running isn’t markedly different to that used for cycling, even though it involves a lot more knee bend than walking. I’m thinking that rowing gets away with such a deep knee bend because little force is generated at the start of the drive.

I went down from 175 to 165 and my knee pain dissapeared completely when out on tje bike and i am 5’10 31 leg

One downside of shorter cranks is your seat will be higher – so not ideal on bikes with slack seat angles or if your dropper post doesn’t have as much drop as you’d like.

Yeah, but that really is one of the cases where the difference is too small to matter, my BB-saddle height is in the region of 660mm (depending on crank length 😉 ), changing crank length even 10mm is not going to cause a significant enough change in either height or setback that it wont work within the range of adjustment*, heck, 10mm is nearly the difference in thickness between my padded shorts and not padded shorts!

* unless you’re right on the borderdline of seatpost adjustment etc. in which case you’re more likely on the wrong size bike/length post

But I’m puzzled as to how this can be a good choice biomechanically.

There’s lots and lots of literature out there about how legs and knees work, and a lot of it specifically about how they work on bikes. To summarise, they only work efficiently and effectively within certain ranges, outside this range (but still within total range) they are significantly less effective at doing work, and with the added downside of putting undue loads on some of the joints.

Result -> optimum usable range =/= total range

175mm for me…

short little weak leggies need all the leverage I can get 😳

Height (and of course leg length) varies substantially between people. I’m exactly 20% taller than my wife, we are somewhat outlying but not all that extreme. Does it not seem reasonable that whatever the optimal value is in absolute terms, I should probably be using cranks roughly 20% longer than her? She actually rides with a more extreme knee bend (at crank top) than I do. And all frame sizes of any particular bike model are commonly sold with the same crank length. How can this make sense?

175mm for me…

short little weak leggies need all the leverage I can get

that’s why we have gears, and as a a short legged comrade you’re exactly the kind of person who might benefit from shorter cranks!

Does it not seem reasonable that whatever the optimal value is in absolute terms, I should probably be using cranks roughly 20% longer than her?

not necessarily no.

My Wife is 1 inch shorter than me, her legs are 3 inches longer*, she uses longer cranks than I do.

And all frame sizes of any particular bike model are commonly sold with the same crank length.

Not so, most big manufacturers, especially on road bikes, will spec shorter cranks on smaller bikes, and longer ones on bigger ones, the problem is that the range between short and long is not big enough (~10mm), and that this is also based on the false premise that big bike = longer legs, while it may hold true in general it doesn’t always.

How can this make sense?

It doesn’t that’s part of the argument, bikes are sold with a disappointingly small range of different crank sizes, and most retailers wouldn’t consider a crank swap when fitting a customer. Consequently we are left with a small range of sizes that ‘work’ for the most people, but not necessarily working optimally. You’d tweak a stem or saddle here and there to get the perfect fit, but apparently not a crank, despite it being the one place where we subject our body to repetitive motion under significant load when riding, yet it is often overlooked, partly because for most people it’s ‘good enough’ and partly because of the added complexity of swapping cranks to fit, and the lack of available options off the peg.

* her TT requirement is consequently shorter, and doubly so as she is less flexible at the hips/back than me, so I end up on long TT, low saddled, short cranked bikes, she ends up on short TT, higher stem, higher saddled, longer cranked bikes, despite there only being ~25mm difference in our hight.

I don’t see many athletes in any fields trying to restrict their range of movement when developing power. Tennis, football (when trying to hit the ball hard),

here’s some pretty dynamic football:

knees bent to something around 90degrees.
.
.
.
here’s some pretty powerful tennis:

knees bent to something around 90degrees.
.
.
.
here’s a fairly efficient cyclist:

knees bent to something around 90dgrees.

they all have the option of using more knee movement, but it seems approx 90degrees represents some kind of efficiency boundary, or something like that…

yunki – Member

175mm for me…

short little weak leggies need all the leverage I can get

gears for leverage, crank length for ‘fit’.

Love a good crank thread. I heard wiggo uses different crank legth for climb stages (long) to time trial (shorter)?
Agree with Ahwiles and other: “gears for leverage, crank length for ‘fit’.”
Don’t forget another property of crank length – saddle-arse clearance when descending a trail. I’ve moved from 170/175 to 180 on all my bikes, perception of benefits in order:
1) Lower saddle height / less drop to bars / more comfortable
2) Saddle clearance (ride a xc hardtail with 27.2 sp…but a dropper negates this)
3) Efficiency of stroke / range of motion as crank closer to 20% of leg length.
6’2″ with 35″ inside leg

perception of benefits in order:
1) Lower saddle height / less drop to bars / more comfortable
2) Saddle clearance (ride a xc hardtail with 27.2 sp…but a dropper negates this)

I’d take a poke at those 2 (or at least some of it)…

Yes your saddle is lower in relation to the BB. But the only reason it affected your bar drop is that you didn’t move the bars as well. The same change in bar drop/comfort could have been achieved by placing the bars higher in the first place.

What you’re saying here is that when you dropped your saddle height, you noticed you prefer having the bars higher* in relation to where your bum is sat, as the two are independently adjustable.

Saddle clearance, do you mean in relation to not smacking you undercrackers? cos that doesn’t make sense either, the saddle is in the same position relative to your (lower) pedal, so no clearance gained really when pedalling standing up, the only gain in clearance here is when the cranks are exactly level** and you’re not pedalling, so there is some truth in it, but as you say, mostly negated by dropper anyway if you have one.

* and **
case **, nullifies case * as when the cranks are level, your bar is at exactly the same height as when you had shorter cranks, so that comfort increase can only applly when seated.

Having said all that, it’s good that you’ve experimented, so many people don’t bother and could be missing out on increased comfort, performance, endurance or reduced injury. And even if they see no benefits at all they will get a better understanding of how they and their bike work and interact. At your height/leg length I wouldn’t be at all surprised if you might like even longer than 180mm if you could find them.

Interesting comment about shorter crank, means smaller perimeter, so even if your cadence needs to increase to maintain travelling speed, your actual foot speed probably stays the same. I can now get my head around how shorter cranks are not necessarily ‘harder’ work for mtb.

I just need to remember how hard that killer climb was, and do I really need a smaller ring up front, or a larger cog on at the rear, to make up for the change in crank.

I just need to remember how hard that killer climb was, and do I really need a smaller ring up front, or a larger cog on at the rear, to make up for the change in crank.

It’s entirely personal of course, but when I swapped from 170-165mm on the road I dropped 2 teeth off the front chainrings to compensate, felt about right and my average speed actually went up but I did notice a preference change to sitting and spinning more rather than standing to honk, so not all variables remained the same!

On MTB my SS (on 26er) ratio with 175mm was always 32/15 or 34/16. I did dabble with 170mm for MTB SS for a while and had to re-gear to 32/16 or 34/17. When I went back to using 175s for SS it also coincided with an increase in tyres size form 1.9/2.0 to 2.4 so I stuck with the easier gearing and it all kinda felt ‘right’

So if you’re talkign MTB, don’t forget to include tyres, cranks and gears in your calculations, but ultimately, fiddle until you’re happy and comfortable 🙂

170’s on everything here… Road, Full sus, hardtail, CX etc.

I’m 5ft10 with 33″ inside leg, but a history of knee pain. Moving to 170’s a decade ago solved that! I’ve ridden 172.5 on road bikes, but it’s just not as comfortable to ride for distance though doesn’t aggravate like 175’s do.

Both my current MTB’s have very low BB heights (which I like) so tempted to give 165’s a go off-road! But then it’d be a fair expense given is be replacing Carbon Raceface cranks on both bikes…

It’s entirely personal of course, but when I swapped from 170-165mm on the road I dropped 2 teeth off the front chainrings to compensate, felt about right and my average speed actually went up but I did notice a preference change to sitting and spinning more rather than standing to honk, so not all variables remained the same!

Nice to hear that somebody actually understands the theory behind it! We have a range of different gear ratios available to us for a reason. Most people just swap crank lengths, then moan that their gearing feels harder, without of course compensating by either riding in easier gears or dropping a couple of teeth off their chainrings!

Without knowing all the science behind crank length, I do find it odd that most bikes come with set cranks lengths according to bike frame size. Shouldn’t shops be supplying bikes with cranks that suit our own bodies?

I’m a bit weird, I have the legs of a “normal” ~5’5″ person and the torso of a normal ~6’2″, all meeting up to produce a 5’10” weirdo these days (5’11” in my prime).

So I often need a larger frame, which then usually gives me cranks that are at least 172mm…. Noooooooo! 😆

as a short legged comrade you’re exactly the kind of person who might benefit from shorter cranks!

I may well give it a try if that crankset you donated to my town bike ever wears out!!
My MTB is SS so I’m never sat down long enough for it to make a difference 🙂

n0b0dy0ftheg0at – Member
…I do find it odd that most bikes come with set cranks lengths according to bike frame size.

thing is, usually they don’t even do that.

Shouldn’t shops be supplying bikes with cranks that suit our own bodies?

at least providing the option to buy different cranks would be a bloody good start.

My MTB is SS so I’m never sat down long enough for it to make a difference

😉 Same here, 175s will give me knee pain on a geared bike within and hour, SS though I’m just fine with them.

thing is, usually they don’t even do that.

The big boys are actually not that bad at this, Spesh/Giant/Trek/Cannondale/Canyon all do, and I think some others as well, granted it;s only 10mm between extremes but at least it’s something!

at least providing the option to buy different cranks would be a bloody good start.

I think it’s a budgetary catch-22 situation, there aren’t many options off-the-peg, primarily because people don’t tailor their crank sizes, which in part is because there aren’t many options off-the-peg, because there’s not enough demand, because people don’t tailor their crank sizes…. repeat till implosion

…repeat till implosion…

agreed.

yes, specialized (and others) sometimes fit 165’s to their XS bikes, which possibly helps strengthen the myth that 165 is ‘extra small’, so people happy to experiment with an alternative to their 175’s only go as far as 170 (as they themselves aren’t extra small), then don’t notice much difference, and forget about it, so further weakening demand for alternatives…

Force X distance = work done.
Force is the grunt that can give.
Distance is the circumference of pedal spindle.
I went from 170mm. to 160mm cranks (road). I therefore have to pedal an extra 63mm now to achieve the same work done.

I am 5’4″ tall with an instep of 29″, and run a 48 36 26 triple with a 13 26 cassette. I climb better and feels a lot more spinny on the flats. Feels more Ferrari than Bolinder semi diesel!

Its just personal preference. This may be different on different bikes (different activities).

The crank arm is part of a SYSTEM of levers (gearing, wheel size, position of seat and handlebars, limb length etc), so cannot be separated alone as lots of people are doing in the comments above with their pseudo-science.

Various people have tried to calculate an optimum but failed. Even statistical evaluation of Pro riders choices revealed nothing.

The range is usually 160-180mm this was historically found by trlal and error and the best way to choose is those that feel most comfortable.

I’m 6ft 34″ inside leg and ride 175 as thats what i’m used to and so it is my preference. I don’t have extended time on any other length.

188cm tall 170mm cranks, I found going a bit shorter most noticeable on the turbo trainer.

wzzzz – Member

Its just personal preference … the best way to choose is those that feel most comfortable.

i think we’re closer to agreement than your post suggests 😉

… Various people have tried to calculate an optimum but failed.

a bit like saddle height then, lots of formulae around for calculating your saddle height, based on this, that, the other, but the best results are likely to come from personal preference. That doesn’t mean saddle height isn’t important, just that it’s difficult to predict.

which brings me to my central moan: there isn’t enough choice for people to try out a decent range, and say ‘A is too short, C is too long, i prefer B’

or, to drag out a goldilocks analogy, we’ve only been offered one bowl of porridge, and we’re poking around the edges looking for the hot/cold/sweet/salty bits that suit us best.

I ride 170 on my road and TT bike. Do you translate this directly to the mountain bike? I would assume so as the saddle height for optimal peddling I the same?

Maybe we all should have a set of these fitted for the first few rides of a new bike:

I ride 170 on my road and TT bike. Do you translate this directly to the mountain bike? I would assume so as the saddle height for optimal peddling I the same?

no the gearing is different. the bike is different, the activity is different, in fact all the levers are different.

Might make a good starting point though 😉 if you feel comfortable, carry on.

If you are chasing 10ths then experiment with longer and shorter arms.

175 gives me knee pain, 170 is marginal, but 165 gives no problem at all. I’ve chopped and changed between bikes for weeks at a time to prove this to myself before changing to 165 on everything (hardtail MTB, fat bike & road). I’m 5’6″.

Took some doing on the fat bike:

I am 6’2″ with 36″ inside legs but my lower legs (below the knee) are quite stubby and upper legs (above the knee) really long – what do you think that does to my ability to run long or short cranks?