You really should get to a fit bike with a competent fitter to find this out. That is a huge purchase, would hate to not have it optimized. Likely you will do well on all sorts of length cranks, so you end up with the length that suits your position best, power usually doesn't really change much, it at all.

Take your exact position to a good fit bike with fitter and then play around. Shorten the cranks, raise you seat the same amount and forward a pinch, and then raise the bars about the same and forward. Once you get on 170's to 165's(what I think you will like best) you can then play around with your front end and probable Cda in different positions that are still at the same power outputs.

That is what I would do, but barring that I would probably just go with the 170's jack up the seat and bars and move forward a bit on the bike..That is my internet fit for you.. (-;

Thanks Monty. And I did also send a more detailed email to my fitter earlier today. Like you mention, he knows my exact position, flexibility, adjustment ranges of my bike, etc. better than anyone else at the moment, so I'll certainly defer to his advice. I was actually reluctant to start a thread but then figured why not, it'll be interesting to see the range of opinion (if there is much range).

Does he have a fit bike with adjustable cranks? If so, that is your next, best move...

Other than your saddle being too high...you look good.
I've always ridden 175's too...I tried the other route for a good two years.
Nothing but worse performance even with a big improvement in .cda
If I was you ....I wouldn't go down that road.
https://drive.google.com/file/d/1Km3YTOv29X1cT-ly4dMx8skl0M9qrDbR/view?usp=sharing

There are a few scientific papers on crank length and they essentially find that over a wide range it doesn't matter for power production. That leaves you free to choose crank length for other reasons, such as getting more aero. However, if you don't have a specific reason to change (such as you think it would make you more aero), I wouldn't change.

I have never ever not once (in 2000 fits with adjustable cranks) had a rider of any height prefer 175mm cranks in aerobars, WHEN THE OPTIONS WERE PROPERLY PRESENTED. That means on a fit bike, taken to the limit of their current cranks, under high load, before making a change.

You absolutely have zero downside and all upside in going down to 170 or shorter. And screw not changing it all at once. Pull the damn band aid off. Crank length range for triathletes is 140-170. You are tall but not super tall, so if you asked me to be a dictator I'd put you on 165s without a 2nd thought.

Totally disagree...sorry.
I am sure that you are wrong.
I understand that your view is of the average rider/triathlete...there are many exceptions.
A loss of power personally for me from 430w to 380w with a change from 175 to 165 cranks and with a much better .cda
and various position changes didn't come close to offsetting the power loss.

A loss of 430 to 380 over what kind of time interval? Hardly in the power range of a half or IM.

I have been playing with cranks length for the last four years. I started triathlon riding 170 cranks and then went to 165 and the experience for me was a how did you ever live with these type experience. That got me curious and I did a lot of research riding them for a while before trialing 150 cranks. These were too short and felt good at times but at times I just felt out of sync and struggled with producing power. I then went to 160 and had another how did I ever live without these moment and used them for the last few years. Several months ago they started to feel like I could go shorter so I went to 155 and you guessed I had another moment. Over this time period I think my muscle memory developed to be able to pedal shorter cranks and my position has definitely improved. My power numbers over that whole time have always been very similar but my cadence has dropped close to 15rpm average over that time and I am running quicker off the bike. Since going to the 155 my position is probably the most aggressive I have ever been and sustainable. I rode 220km on Sunday preparing for Ironman Cairns in just over four weeks and was still comfortable after 7.25 hours and felt good running off the bike. I can tell you then what my power numbers are like in the race but I would hope to average about 240w.

I personally would definitely try 170 with a minor position tweak and if that feels good give your body time to develop but keep going...

PS I'm just shy of 6' with long legs for my height

All the studies I've seen have been with riders in road positions. For me that bears out: there's no difference in a road position. In a TT position, it's a very different story. Going from a 172.5 crank to a 150 is worth ~15 watts for me. It doesn't erase the TT position power loss, but it definitely diminishes it.

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Agreed. I went from 175 to 170 on my road bike and prefer the change but tried 165 on my cross bike and it feels too short and unnecessary not having a closed off hip angle like a tri bike where going shorter felt immediately better.

Just to make sure I’m reading this correctly, you’re saying the change from 172.5 to 150 in the TT position resulted in a gain of about 15 watts? Or resulted in “-15 watts,” i.e. a loss of power?

Thanks for everyone’s input. I suspected there’d be a range.

I'm not a fitter, nor do I have any research articles to back it up but I'll share my bio-mechanics and exs phys knowledge from college.))

1) Provided all the knee levers and everything fits... a longer crank length would provide the same amount or in our case velocity at a lower power output. That just physics. Think a breaker bar. The long bar extends and you can move the same amount of torque and get your lug nuts off but it requires a huge movement.

2) Provided all the knee levers fit... A shorter crank length would spin around faster at a higher power output. Think more of a screw driver. Flick or the wrist instead of a giant half movement of the arm.

So in my opinion as an exercise physiologist I would look at your goals. Climbing, long distance riding, or anything that involves really stressing your legs and looking for easier gears you would, just for the sake of physics, likely be better off with a longer crank length. I would say most triathletes in most triathlons would benefit from the saved energy.

However, sprinters, short/prologue TT, short Aquabikes, or anything where flat out instant speed is more important than saved energy you would be better off, from a physics and biomechanics standpoint, with a shorter crank length.

To be true to ask a fitter everyone has different length bones and joint placement. To an extent anyhow. So the correct crank length for person A or person B might be different. But if a good fitter could fit you to either crank, without sacrificing position, refer to physics.

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I'd like to offer a testimonial for Dave's opinions - I used his private 'critique my fit' service and went from 172.5 to 165mm at his advice. That and a new Adamo saddle took some getting used to, but my power numbers in aero are now up, and I can now hold my trainer FTP in aero which I could previously only hold sitting up.

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For whatever is worth, I got my lg-SC used and it came with 172.5 and that was a bitch to say the least... switches to 165s and it was like having an epiphany right then and there!

I am looking to go down to 155s as soon as I can either buy find a used one!!!

Had 175 on my roadie (venge S-Works) and they were brutally painful! Switched to 170s and I was very happy but that damn BB wouldn’t stop creaking so I had to switch the BB and found FC 9000 crank so cheap I couldn’t say no to and it was 165s as well and I couldn’t be any happier with 165s on my roadie.

For me going lower in length worked wonders, iv seen other people who just seem unable to adapt to the differences i.e. having to increase cadence to maintain or gain speed (which is what I do) and most people rather muscle that shit up thru power and lower cadences which is something I used to do as well but find it better for me to up cadence and it is so easy to do so with shorter cranks than those 175 for me, That’s like a torture device for me to be honest but just adding my .2 and small story for the change

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I have to disagree with you. Your approach omits the fact that we have gears on our bikes.
Your description of levers is far more analogous to choosing a big gear on the flat and a small gear on a steep gradient. It's not the right way to approach this problem. Cycling is first and foremost about power. It makes no difference to your speed how you transmit the power from your legs to the rear wheel provided it gets there.

In a rotating system:
Power = Torque x Angular Velocity
Torque = Force x Moment arm

So,
Power = Force x Moment arm x Angular Velocity

Moment arm is the distance between the axis and the line of force. So it's the length of your lever if you apply the force at 90 degrees to it. For this subject this is the crank length. Angular velocity is normally measured in radians per second but we can use RPM so long as you keep unit in mind if you decide to do any calculations!

The product of those 3 variables, Force, Crank Length and RPM is what's important.
On a conventional bike, the crank length is fixed and you manage your power by how hard (force) and how quickly (rpm) you turn the pedals.
We control that with our legs and our gear selection to work at a manageable rate and with an appropriate balance of rpm and pedal force.

Longer levers are not needed for climbing because we can change gear to produce the same power with the same pedal force and rpm but a lower wheel speed. There is a difference in the distribution of pedal force through the stroke because you are forced to produce power continuously on a steep slope rather than letting inertia smooth your motion between the more powerful phases of the stroke, but I don't think we need to over complicate things by delving into that any further.

The question of crank length is not about getting free energy from longer levers (which is impossible) but rather about how our muscles work best (speed and length of contraction and force produced) and our position on the bike (to minimise impingement and allow a faster torso position while keeping hip comfortable).

It's interesting for me to observe how these conversations go. Some folks rely on studies and if that's the case you'll be motivated to go with shorter cranks. Some folks rely on basic logic and we come up with the long-lever-makes-it-easier-to-remove-lug-nuts perspective, and I get how that seems right at first blush (the inclusion of cogs - as mentioned - challenges that concept).

I fit a lot people. I've been fitting a lot people for many years. To see you in this position on 175 cranks makes me wince just a bit. I would recommend that you put a set of 165s on there. In your first ride you'll be thrilled (and probably a bit shocked) at how smoothly and quickly your leg comes over the top of the pedal stroke and finds an earlier start to the power phase of your pedal stroke. You'll be surprised at how high your cadence is, and, since your not riding a single speed, you'll drop a cog and have that same fluid feeling in a bigger gear. All I've spoken about in this paragraph is comfort.

For those who don't care about comfort and only focus on power - take the idea of spinning freely in a bigger gear and contemplate how that effects power.

Ian

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Ian Murray
http://www.TriathlonTrainingSeries.com
I like the pursuit of mastery
Twitter - @TriCoachIan

Thanks for your thoughts. Out of curiosity, what is it about my position and the 175 cranks that makes you wince? My guess would be something about hip angle, but I'm not a fitter and I feel like I've seen several really good riders with more "closed" hip angles who ride well (e.g. Kienle, but of course he's a whole different class of rider).

A few other bits of peripheral information, in case they're helpful:
I seem to be one of the relative few who doesn't experience a noticeable drop in power between sitting up or riding aero, perhaps because I train in the aero position a lot. I'm not a very fast runner but I seem to run ok off the bike: my last half iron (Sept) I ran a 1:31 and my last open half (Oct) I ran a 1:25. Seems like a typical delta.

Plenty of opinions on here, some from very smart people. I'll just add my own for the fun of it.

If we/they don't have all the information... ( and most likely you don't either) testing is the way to find out what is ideal for you. Crank length has an effect on the whole system...sometimes a very big one.

To answer the question of whether shorter or longer cranks are better...
Yes

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Northy,

Yes, you're right on with the assumption about the hip angle atop the pedal stroke. As your foot comes up around the backside and passes the 9 o'clock position and continues to climb the angle that is created between your torso (which is nearly parallel with your top tube due to what is perhaps a 120mm arm pad elevation AKA "drop") and your femur - that angle starts to get smaller and smaller. By the time you're at 11 o'clock I suspect there is some binding of soft tissue in that hip area (impingement). That gathering of tissue slows the legs circle as it comes over 12 o'clock and doesn't free itself up for more torque on the pedal 'til maybe 2 o'clock or even 2:30ish.

Going to a shorter crank - and I don't think you should hesitate or go in pieces, just commit to the 165 - would allow you to flow from 10 o'clock over the top and into earlier, quality torque at 1:15 maybe. I think it'll feel great.

Again, I'm still focusing on comfort.

But let's talk about power for a sec. Let's say you're ridding a 52x36 (mid compact) and an 11 speed 11x25. I don't know if that's it or not it doesn't really matter, this is just for sake of discussion. Let's say you're in your 52x17 at 92 rpm, and your making 200w. Here's the artists equation: Torque x Cadence = Power. I say "artists equation" because dedicated, left brain science/math folks will argue that it's not cadence it's foot-speed or something else even more accurate. Okay, great. But let's keep it to something we often measure and "know" - cadence. The thing we don't read on a daily basis is a measurement of torque - so let's use a simple 1 to 10 scale where 1 is feet floating so gently around the circle that we could have a tack under the ball of our foot and we'd barely feel the poke. And let's associate a torque of 10 with such freakishly heavy pressure that we're worried we'll break the pedal axle.

So there you are in your 52 x 17 at 92rpm with a torque of 5 and you're making 200w. With a 165 crank there would be such freedom across the top of the pedal stroke that your cadence would go up to 105 or 110 or 115 something you are capable of spinning but it doesn't feel optimal so you would drop to the 52x15 and settle into your favorite cadence of 92rpm and you'd be pressing down with your same "5" of torque. Now we have new formula: 5torque x 92rpm --- now equals 212w - because you're in a 52x15 now and not 52x17. This is just one way to look at it. My hope is that it will appease the completely sensible gut reaction to "If my lever's shorter I'll make less power". Perhaps if this were a track cyclists forum and we all rode fixies we cold go down that path but while you've got another cog down there the whole "don't shorten my lever" looses some of this value.

Ian

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Ian Murray
http://www.TriathlonTrainingSeries.com
I like the pursuit of mastery
Twitter - @TriCoachIan

Unfortunately my experiment with 150mm cranks was very neutral. They did feel better in a way because of the less restricted angle, but power gains never came even after a full year. Plus the aero was neutral too, even with lower bars (didn't move them). I kept normal cranks on my road bike which I ride a lot more, so maybe I never really adapted? It always felt a little funny with the short cranks, and I switched back.

Another thing to consider:

For a rider of your size, 175s are already pretty 'short' cranks ...

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I may as well report my own experience as well. I've never had access to 150s to try them, but I went down to 162.5 and found no effect on power at all, but definitely got more aero. My theory is that my knees stay more controlled but I'm really not sure what the exact cause was. Anyway I've stuck with them. My road cranks are 170s and I honestly can't really feel the difference.