Planemaker

My gravel and road bike both have 172.5 crank length while my hybrid has 175. I can honestly say that until I was ordering a new crank for my bike what the length crank any of my bikes had.

My LBS just ordered the cranks for the gravel bike and got 175 so, I'm just wondering what if any difference I will notice between a 172.5 and 175 crank.

FYI - if it helps I'm between 5'11' and 6'0" tall,

Metieval

the next time you walk up stairs/steps take one at a time, and then take 2 at a time.

some steps have smaller rise and some steps have a larger rise. Easiest way for me to describe crank arm length.

Usually it will come down to fit, fitness, and personal preference.

Going from a shorter crank arm to a longer crank arm, you may have to lower your seat a bit... it will change your leg extension length.

Marcus_Ti

Slightly different saddle height. Toe overlap in the front and heel-strike in the back might be slightly more of a thing.

That about does it.


AFAIK there isn't any conclusive research on what size of cranks is better for whom and when and why. Lots of builders and fitters have hunches and theories though.

Metieval

On bike fit. When you come up over the top, a longer crank is going to drive knee higher. Generally this doesn't matter on a MTB/ hybrid.
On a road bike/gravel bike, this will come into play depending on your saddle to bar drop, and how limber you are. The more flexible you are, the easier it will be for you to drive your knee into your chest. Most Gravel bikes have a pretty tall headtube, making this almost irrelevant.
Shorter crank arms allow for a higher saddle, pushing your butt up. Flattening your back. So now we are talking about aero gains though, more than crank power gains losses. On gravelbikes/endurance geometry, it's most likely irrelevant, unless you are slamming your stem and racing. Or in general how aggressive of a fit a rider likes to have. Along with that a riders flexibility comes into play.


The physics side of it, says that It's easier to put more force into a slower moving pedal. For a given cadence, the shorter the crank arms the slower the pedals are moving. However, The actual power difference would be minimal.

I am 5'8" On my road bikes, I prefer a shorter crank 170. My Supersix has 172.5 but at the Cost of Hollowgram crank arms, I am not changing them!! I have ridden hybrids with 175, my current hybrid is 172.5 just because that's the crank I had laying around. I've ridden MTB with 175 and 170. the 175 had a high BB and I rarely pedal struck. the 170 crank arm Mtb has a low BB and I hit the 170 cranks all the time.

bottom line, outside of bike fit influence, you'll most likely never see a performance difference with crank arm length.

dabac

Humans are tricky from the perspective that perceived change or ”feel” of the bike doesn’t have to be proportional to measured change.
And the feel of the bike can have considerable influence on mood and commitment to the ride.
Crank length, clips vs flatties among others are things that some riders consider hugely important, yet have shown in lab tests to have remarkabla little influence on output power.
I know one guy who was adamant that he absolutely had to have 172.5 cranks.... only to discover that his beater bike had one 170 mm crank and one 175 mm crank.
To his credit, he was then willing to reconsider his earlier statement.
For me, crank length have a significant impact on cadence.
On 175 mm cranks I pedal harder and slower on a higher gear, and often end up with sore knees.
170 mm cranks boost My cadence, ”never” cause knee issues, but, unfortunately also drop My average speed a little.

CliffordK

Each person is different.

I've tried some 180's and like them, but probably wouldn't go any longer than that. However, I ride bikes with 172.5's up to the 180's and hardly notice the difference.

In theory the longer cranks give you greater leverage. However, your pedaling circle also increases linearly with crank length. Thus, the longer the cranks, the slower the cadence, and no free energy

prathmann

Probably won't notice it at all. It's only a difference of a tenth of an inch. My bikes have crank lengths from 165mm (tandem) to 170 mm (folder and touring) to 175 mm (road) and I switch between them without really noticing any change. Yes, if I check average cadence then it's a little higher on the tandem than on the road bike - but both feel fine while I'm riding.

McBTC

Even shorter (e.g., 165s) cranks will better enable you to get into a more comfortable aerodynamic positon without your knees hitting your body, which is great for a road bike and windy return legs.

cobra_kai

As others have said, the biggest difference is in fit. Shorter cranks usually allow for a more aggressive aerodynamic fit without compromising comfort. Whether that is a concern on a gravel bike is questionable. There are multiple studies (too lazy to dig up links right now) that show there is no correlation between crank length and power production. Pedal strike could also be a concern if you are riding a low bottom bracket or ride through a lot of rough terrain.

I very much doubt you will be able to discern a difference between 172.5 and 175 unless you are very sensitive so such things because it is such a small difference. Probably if you didn't look at the actual lengths you would have never even known.

TimothyH

A shorter crank has the potential to open up the hip flexors and improve breathing and comfort.

With shorter cranks, the left leg of the guy in the image below won't come up so high toward his chest. His knee will be lower at the top of the pedal stroke.

Marcus_Ti

You're talking a fairly well insignificant amount though. The difference between a 175mm crank and a 172.5mm crank for reference... is about a bit over a nickel's thickness (1.95mm), and a bit thinner than a CR2032 battery (3.2mm).

And our friend in that photo with his flat back is going to probably have numbess problems...also that flat back exacerbates the breathing difficulties you point out. Which is to say, crank length isn't his problem nor is it really his solution.

McBTC

...and between 165s is 10mm, which means you must raise your seat by 10mm so... compared to 175's you're talking about a 20 mm change, which can spell the difference between spinning out using aerobars and, spending 6 months dealing with Greater Trochanteric Pain Syndrome from trying to spin long cranks in an aero position.

Marcus_Ti

Has any scientific study anywhere causally linked hip injury incidence with crank arm size? I have to ask because frame fitter/builder Lennard Zinn himself admits there's no scientific conclusive studies regarding what size crank a given person best makes advantage of.

I mentioned 2.5mm changes because that was the OP's question. As said a while ago, compensate in saddle height and you're good.

McBTC

I think there are reasonable grounds to support most any conclusion in this area, including personal experiences that call into question whether the ubiquitous 170-175 paradigm is ideal--e.g., this from Cycling Weekly:

McBTC

These observations are interesting to me in that they seem to take a more inclusive look at pedal stroke dynamics (from hip to toe) in relation to the desired activity (aerodynamics to brute power):

Metieval

sometimes personal experience with crank arms are like personal experience with 520 gram tires rolling just as fast as 215 gram tires! up hill, down hill, on a 16 lb bike on a 40lb bike makes no difference they all roll the same speed.

chas58

Did that answer your question?

My bikes range from 175 to 165, and I really can't tell much of a difference, unless I over think it.

I get a little less toe overlap with the 165
I get a little more torque with the 175.

On a mountain bike, cargo bike, or tandem, I want 175 as i need low RPM leverage (torque). Other than that I don't particularly care or notice

TimothyH

I don't know about injury but there are a few studies floating around the internet which claim better breathing with a more open hip angle.

Shorter cranks were a hot topic back in 2015. Several pro teams were reportedly switching some riders to shorter cranks.


-Tim-

ATPAH

Of course, if you're in the pro peleton, you're moving at 40 kph all day and are experiencing 4x the aerodynamic drag that the average gravel rider gets at 20 kph, so you want to squeeze-out every mm of advantage in achieving the optimal aerodynamic position on your bike. That's not really an issue if you're not racing. The only time that I fret about crank length is when I strike a pedal and suddenly find myself in a shrubbery. This doesn't really happen on my gravel bike, as pedal clearance is a constant and easier to manage, but I've been caught-out on my FS bike. I'll get shorter cranks for it when I've finished beating the current drivetrain to shytte. Like the other commenters, I wouldn't sweat it on the gravel bike.

TimothyH

Aerodynamics is your topic, not mine. I never said anything about aerodynamics. My words were "better breathing and more comfort."

Shorter cranks have less to do with aerodynamics and more to do with opening up the hip angle for better breathing and more comfort. I think better breathing and more comfort are universal and a few of the pro's claimed that they finished stages feeling fresher and less beat up with slightly shorter cranks.

If someone wants to use shorter cranks to get aero then that's available to them too, I suppose, but that isn't what I'm talking about.


-Tim-

ATPAH

Aerodynamics is not my interest, but you referenced the migration to use of shorter crank arms on some pro teams, and aerodynamics is always going to be central to their thinking. Shorter cranks allow a rider to raise his seat and assume a lower, more aerodynamic position more comfortably. Professional cycling is about performance and suffering. They generally don't tweak bike setups for comfort and are almost certainly going to shorter cranks because it allows them to get a more aerodynamic position. My point being, that we don't need to worry about that. We can simply assume a slightly more upright riding position and the breathing problem is solved.

Planemaker

Thanks for all of the input guys. Clearly, there are a lot of opinions about crank length.


At this point, I don't think there is much harm in going from 172.5 to 175. If at the end of the day they don't work out, I can always change them.

McBTC

Nope... no one is going to shorter because they want to raise their seat-- the seat is raised when you go to shorter cranks to maintain proper leg extension. That change, however, may then require other changes such as raising the bars, flipping the stem and moving the seat to maintain body position or to take advantage of a new positon like, adding clip-on aerobars.

TimothyH

To be clear, my posts in this thread have nothing to do with aerodynamics.

Shorter crank may open up the hip angle and allow for more comfort and better breathing. Comfort and breathing are my only point.

Many have bought into the idea that a longer crank arm provides more mechanical advantage (without understanding what that actually means) and have erred on the side of too long when choosing (or guessing) their proper crank arm length. I am certainly guilty here. This has resulted in riders with impinged hip angles and too much knee flexion. A slightly shorter crank arm can be a tremendous help for these riders, especially as they (I) age.

I consider aerodynamics as a secondary and tangential topic. Those discussing aerodynamics are doing so of their own volition. My point is hip angle, comfort and breathing.


-Tim-

McBTC

True, true... spend enough time looking at the research and learning what it takes to experiment with shorter cranks, it's clear why the industry goes with 'keep it simple, stupid,' when it comes offering shorter cranks on larger bikes: it's easier. Just that single simple consideration makes bike fitting much more difficult.