Crank arm length: 172.5 vs 175. Is there an appreciable difference?

[Likes2ridefar]

My full suspension mtb has 165mm and my all road bike has 172.5mm. I’d prefer 165mm on both however I got a deal on the 172

I don’t notice huge change like some mention such as gearing impact, but I do find a slight increase in cadence average, which I like, and improved fit on my bikes vs longer cranks.

[IcySwan1]

I am 73, 5’ 11”, reasonably fit, and ride 5-6 days a week primarily on vintage Italian steel bikes. All are 172.5 except one is 175. I can’t tell the difference. As others have noted, I suspect I would notice if the change was 5mm or more but 2.5 is not really a factor in what bike I grab.

Mike

[dmitrik4]

It’s interesting that so many people are talking about their height rather than their inseam. I’m 6’ with a 30.5” inseam; am I using the same crank as a 6’ person with a 34” inseam?

I honestly don’t know; I’ve been using 172.5 cranks because that’s what was on my first real bike and they seem fine.

[litcrazy]

I'm 6'2" with longer legs and longer femurs and find 175-180 comfortable. I tend to go with a narrower q factor--I imagine this likely plays some roll too.

I find knee pain more associated with running out of gears on long days with lots of elevation. Avoiding grinding seems to determine my quad tightness and thus knee pain.

know this goes against current trends, but I am a bit uneasy about going shorter to limit my range of motion. Current PT in other areas often aims for an increased range of motion. Cycling already has such a circumscribed range of motion compared to running and hiking, that I'm disinclined to further reduce unless compelled to by pain or injury.

Does anyone discuss this in the lit?

[JMT3]

Started like most on 175mm. Me? 6’ 35” inseam. Went to 172.5mm over 20 years ago and loved it. Went to 165mm road and gravel and 170mm fat and mtb a few years ago. Feels great! I was a runner that went multisport and was a gear masher. Pushed a 55, 44 chainring set up with a seven speed 11-18 cassette and back then 70 RPM was a-lot. Now on 53, 39 165mm and my cadence averages mid 80 RPM and hit peaks of over 120 RPM on the road. Did hit 180 RPM a few years ago on a fixie and was bouncing all over the saddle.

[Nomadmax]

As someone who spins a little faster than most, I can tell a 2.5mm difference in crank length.

[Fat Cat]

2.5 mm equals +/-0.099". That is way less than 1/8" (0.125")

A difference that I openly admit is undetectable to me. But I almost exclusively use 175's since they're cheaper cos they're too long and nobody wants them.

[benb]

Originally Posted by Fat Cat

2.5 mm equals +/-0.099". That is way less than 1/8" (0.125")

A difference that I openly admit is undetectable to me. But I almost exclusively use 175's since they're cheaper cos they're too long and nobody wants them.

You always have to multiply the difference by 2.

The assumption is you are raising your saddle an equivalent amount (at least, some fitters seem to say some people need to raise it more) to the amount you shortened the crank.

Lots of them say change by 5mm at a minimum.

If you change by 5mm you've got your leg 1cm lower at the top of the pedal stroke. If that is enough to give your knee/hip the ability to control the pedal stroke all the way around and/or lets you handle 1cm more drop to the bars it can help things.

The other thing is if you raise the saddle you don't necessarily need quite as much setback. It's not as much as the difference in crank length, but if your leg is not bending as much your femur does not have as much of a vertical component. This means you don't have to be as far back to balance your torso, which means you can have the seat further forward, which opens your hips even more, makes it easier to handle a longer reach to the bars without weight on your hands, makes you more aerodynamic, etc..

This is seemingly an "expensive" thing to to try for a component swap, but it's dirt cheap compared to most aero equipment or aero bikes, it's totally UCI legal, and the gain you get from improving the torso angle you can hold and lowering the drag on your body is large.

If you are on cranks that don't stress your hip angle and don't cause you to have loss of control at the top of the pedal stroke this can all be meaningless.. but you want to actually know that's the case.

[Spoker]

6’5” used 180-175 but now on 172.5 for the better. Would like to try shorter. Not racing anymore.

[SlowPokePete]

I had Ultegra 172.5's on my Lynskey gravel bike, and when they were replaced through the recall, I realized I couldn't use my 46/30 Absolute Black chainrings on the new crankset.

So I ordered a set of GRX810's, but price-wise the best deal I could find were a set in 175.0, which I purchased.

I notice no difference at all while pedaling along on the bike, but one issue I have is it is far easier to incur a pedal strike when trying to pedal through a turn, which I am not really happy about.

SPP

[merckx]

What one feels is not necessarily congruent with the physics.

[benb]

Feel can be deceiving.

It took me a long time to accept my back giving out on hard efforts on the flats when I was trying to be aero but not uphill when I could sit up could be related to this.

It is super easy to be complacent about things just cause of the "perceived wisdom" of cycling.

I was completely bought into the dominant "taller people always need longer cranks on all bikes" to the point I've considered going and looking for 180s.

[Likes2ridefar]

Originally Posted by dmitrik4

It’s interesting that so many people are talking about their height rather than their inseam. I’m 6’ with a 30.5” inseam; am I using the same crank as a 6’ person with a 34” inseam?

I honestly don’t know; I’ve been using 172.5 cranks because that’s what was on my first real bike and they seem fine.

I’m 6’ and have an unusually high hip position resulting in a much longer saddle to pedal distance than anyone I’ve ever seen my height.

Everyone said I should rode 175 when I was measured or fit in the past, but I prefer 165 and output metrics support this.

[Fat Cat]

Originally Posted by benb

You always have to multiply the difference by 2.

The assumption is you are raising your saddle an equivalent amount (at least, some fitters seem to say some people need to raise it more) to the amount you shortened the crank.

Lots of them say change by 5mm at a minimum.

If you change by 5mm you've got your leg 1cm lower at the top of the pedal stroke. If that is enough to give your knee/hip the ability to control the pedal stroke all the way around and/or lets you handle 1cm more drop to the bars it can help things.

The other thing is if you raise the saddle you don't necessarily need quite as much setback. It's not as much as the difference in crank length, but if your leg is not bending as much your femur does not have as much of a vertical component. This means you don't have to be as far back to balance your torso, which means you can have the seat further forward, which opens your hips even more, makes it easier to handle a longer reach to the bars without weight on your hands, makes you more aerodynamic, etc..

This is seemingly an "expensive" thing to to try for a component swap, but it's dirt cheap compared to most aero equipment or aero bikes, it's totally UCI legal, and the gain you get from improving the torso angle you can hold and lowering the drag on your body is large.

If you are on cranks that don't stress your hip angle and don't cause you to have loss of control at the top of the pedal stroke this can all be meaningless.. but you want to actually know that's the case.

No. Diameter and radius are different measurements. Most of what you say makes no sense to me but then I'm old and stoopid

[mhespenheide]

^

A counter-example:

Let's say that you're happy on 170's, but you decide you want to try 175's. When the pedal is at the bottom of the pedal stroke, it's now 5mm lower. So you lower the saddle 5mm in order for the leg to feel the same at the bottom of the pedal stroke. That's dealing with the radius of the crank.

The top of the pedal stroke is 5mm taller relative to the bottom bracket (due to the increased radius), but you've also lowered the saddle. So the pedal, when it's at the top of the pedal stroke, it 10mm taller relative to the saddle.


Same effect when you're reversing the idea and going to shorter cranks.