79pmooney

My take on this? Each person will find that a crank length works best them. And all the measurements, etc, in the world cannot do more than narrow that down some. 165 and 175 are very far apart. Like an alien coming to this galaxy to live and only looking at Mercury and Neptune. Very, very hot and very, very cold. Not the planets that could have water - as steam - Venus, in all three phases - Earth or as ice - Mars.

I started as most of us did on 170s and 171s. Never thought about it. Bought a race bike with 175s. First ride it was very obvious that bike was "right". That feeling never left as I went on to achievements that summer I did not think possible for this body. In the 45 years since, going to smaller cranks has never been a gain for me. I don't spin faster. Just suffer higher forces on my knee from the reduced leverage.

I am not saying my experience carries over to anyone else, other than we each probably have a "best" crank length for our bodies. I'd first look at my body, especially leg length. 170/171 is probably a reasonable first guess for most people around 5'10 with average leg length. (I was 6'1/2" with very long legs before I started shrinking. And all that shrinking is above my hips.) That person could try 5mm bigger or smaller (say if he chanced on that crankset at a good price), ride it several months and see if that was better or worse than the 170s. If worse, maybe try the other direction. He/she might well find that midway between one of those extremes is "right". But 2.5mm from "right" is still plenty close. (I can ride 172.5s. I can tell they are a little short but it's not an issue. 177.5s might be sweet but 175s are so right for me that I have to ask if my life is improved banging my head against the "I gotta find those uncommon 177.5s" wall for the rest of my life. And with chondromalacia patellae, my knees REALLY don't like crank length changes so it is stay 175 or change all 6 bikes.)

Edit: Changing seat height for crank changes - don't change it much if you have it right currently. The BB is is the center of action if you will. It is not changing. At most, go down 1/2 the distance of the crank length change. Biggest change for your body will be how high you knees come.

Don't sweat the center of gravity change. It won't make a difference that matters. Now pedal clearance on corners if you ride fix gear or criteriums, now that's another matter entirely but it is better addressed with bottom bracket height (and skinny, high bottom pedals) for most road use.

Steel Charlie

Fun Fact
Quickie calc will show that 175mm are less than 1/4" longer than 170mm and less than 1/8" longer than 172.5mm. I'm just a more or less normally proportioned human and I am unable to detect the difference when on the bike. I shift gears.
That's why there are so many different gear ratios available from cogs and chain rings.

DiabloScott

I give this comment a thumbs-up for the most abstract analogy I've ever read on BF.
I would've expected a Goldilocks comparison, but this was stellar.

CrimsonEclipse

Was hesitant to respond to a zombie thread, but.... since it's still alive:

Crank length is roughly proportional to your inseam.

I am short, thus, 175mm crank arm exceeds my ability to operate it without tilting my hips and increase movement against the saddle increasing saddle sores even with padding, fancy saddles and booty creme.

With an inseam around 28-29", 160-165mm (MTB and fat bike) fits me fine and nearly eliminated saddle sores.
I use 150mm crankarms on a different bike for certain recovery days on paved trails.

If you reduce the crank arm by 10mm, you will need to raise your saddle height by roughly the same distance.

Changing from a longer crankarm to a shorter is like going from a diesel engine to a gas engine.
Less torque and higher RPM of the motor (cadence) and usually easier on the knees.
You'll likely end up shifting more and mashing less.

Once you go too short you can feel the effort more in the quads.

It takes about 5-10 rides to get used to the new cranks and saddle height.

RB1-luvr

I ride 170s and 172.5s, and have one bike (Centurion Accordo) with 165s. It feels a little odd every time I ride it. I'm hoping to find an appropriate 170 crank for it.

tungsten

I seen on the u tubes a guy who bike fits/coaches MAMILs who said they all started posting personal bests when he got them on shorter cranks.
So I drank the kool-aid and went from 175 to 170 on my mtn bike (was smashing rocks 'n roots anyways so there's that).
And went from 172.5 carbon (sob!) to 170 alloy (should be even shorter as per dude above, apparently) on the road bike and from 175 to 170 on my grocery getter so I at least subjecting my body to spinning same size circles.
Cool story, eh?

oldbobcat

Having long legs, I switched from 170s to 175s decades ago. My sprint lost some snap but I felt more comfortable with the longer stroke and my climbing improved. I've been reading about riders claiming all sorts of benefits from shorter cranks, like improved aerodynamics, opening the hip angle, and reduced knee problems, and I have no reason to doubt them. I even read that Lance Armstrong gave up his 175s when he stopped riding Treks in sportives. They also acknowledge they have to turn them at a higher cadence to realize those benefits. So if you want to be on the bleeding edge and you're ready to spin, baby, spin, by all means try shorter cranks. But I see no reason for anyone to go longer than 172.5 unless they have really long legs or they think they're Greg LeMond, Bernard Hinault, or Jacques Anqutil.

Bike Gremlin

My 2c long-winded drivel about the pros and cons of longer/shorter cranks:
Optimal bicycle cranks(et) length

TL/DR:
It boils down to personal preference.
For those who prefer to spin faster, shorter is better - as I often like to joke:


Shorter IS better

For those who prefer to use more torque/force, or ride technical mountain trails, longer cranks usually work better.

Height (leg length) also plays a part of course, and if you go to extremes relative to your leg length, it starts getting worse (there is a point past which the cranks are either too long or too short, regardless of your pedalling style and type of riding you do).

Relja

PeteHski

The higher cadence from shorter cranks happens automatically because of the shorter distance your feet travel per revolution.

One crank length study I read mentions that cardio loading was proportional to foot velocity rather than cadence.

But you also have to lower gearing to compensate for the shorter lever and naturally higher cadence. This is usually only an issue if you are already grinding away at your limit in your lowest gear.

GamblerGORD53

I'm 5' 8". My 3 bikes have 3 sizes. I do 100+ milers on all of them. The 3 speed has the short ones.
I did both my tours on a Rohloff 120 lb bike with 180 cranks. ZERO knee pains. I needed a thinner arm crank, so it's now 175. Stupid thing didn't have a proper 1/8" ring either. The 180s feel like a straight 8 Packard. LOL.
The way I see it is less revs = less leg wear and easier high speed downhill.
My hybrid 3x8 had 165s, fricking useless.

tungsten

Ha ha ha I am! But the corollary is that's there's a smaller torque "dead spot" in the stroke. So I have been informed.

PeteHski

I find it pretty much academic. Unless you are going from extremely long to extremely short cranks, the difference in gearing is pretty insignificant.

If in doubt then shorter cranks generally have more pros than cons.

Garthr

To throw a monkey wrench into using short-er cranks is where you place your feet on the pedals. Moving your feet forward, more towards middle of the pedal rather than BOS. Doing this I effectively use 150mm cranks and I have a 36.5" floor to crotch legs and sz 14 feet. I have the ability to sit and spin or grind as I wish. It's a very different feel, I'd say I'm using my core/trunk muscles more effectively than I ever did otherwise in my life. I also found I needed to gradually move my saddle all the way forward on a Cobb San Remo, from being all the way back. This is what works for me though, as if things aren't working out I have no qualms trying different ways to do things. It's fun, it's limitless, it's LIFE !

Trakhak

Leg wear = muscle work = developing strength and endurance?
Less revs at the same speed = higher load = joint wear?
Pedaling downhill = sucker?

grumpus

Or just shift into a lower gear. :-)

smd4

He said he was in the “lowest” gear. Nothing lower than lowest.

grumpus

We're discussing choice of crank length - you won't change that on the fly either. Well, you could - people have made variable cranks, useful for testing or bike fitting.

kommisar

Advantages of shorter crank include less chance of pedal strike, knee doesn't rise as high at the top of the stroke requiring a smaller range of motion.
Disadvantages of shorter crank include less leverage so your gears will seem bigger, will require raising seat and the associated changes in center of mass and seat to bar drop.

icemilkcoffee 

One thing not mentioned is , with shorter cranks, you can actually get lower on the drop bars, because your knees don't come up as much. Short cranks => more aero.

grumpus

Does it, though? The main part of the power stroke is centred on roughly horizontal cranks, that pedal height varies very little with crank length, why does saddle to pedal distance need to be different?

kommisar

Seat height is measured from the bottom of the pedal stroke, not the middle so yes the seat will be raised approximately by the difference between the long crank length and the short crank length. This has the advantage that knee height at the top of the stroke is lower relative to the hip by 2x the difference in crank length.

grumpus

Just because saddle height is usually measured from the furthest position of the pedal doesn't mean you have to raise the saddle if you fit a shorter crank. If you leave it your knees will be bending a little more at the bottom of the stroke and a little less at the top. Is there some biomechanical principle that states you must maintain maximum extension, or are you just assuming that?

PeteHski

I agree and some pro fitters suggest initially leaving your saddle height alone when fitting shorter cranks. So, as you say, the reduced leg motion is then split equally between the top and bottom of the stroke. From there you can fine tune saddle height to personal preference. Some might prefer to raise it to restore their previous leg extension and others might actually prefer the reduced extension. There is no right or wrong here.

FWIW many people measure their saddle height to the BB centre rather than the pedal.

1970bikes

Back in the day, we called 165 cranks velodrome cranks.

Are you going to ride on a velodrome? if not, stick with the longer crank.