What guidance are you looking for?

My advice would probably be to listen to your fitter since we know nothing about you and he/she does.

I can only attest to my fit. I'm 5' 11" with a 32" inseam. I switched from 175mm to 150mm cranks. Once the seat height was reset for the shorter cranks (5cm up)... I could barely tell a difference. After one 3 hour ride... It felt like I'd always ridden them.

There was no change in my sustainable power (30+ minutes) in aero or upright following my fit with the shorter cranks.

So, I agree with dfw_tri... Trust your fitter.

I started racing tri's and IM on 170 and trialled down in 5mm steps to 150 which I found too short but 155 seemed to be the limit before my ability to generate sustainable power dropped away. Over that time my cadence dropped from about 99 average over an IM to 83 now on 155. I used to have a high cadence from a cycling background.

P.S. I'm 5' 11 1/2" with long legs for my height...

I’m 5’8” and recently switched from 165 to 145 cranks. Much more comfy and can’t tell any power differences.

I switched from 172.5mm to 160mm cranks a few months ago. The first hard ride did feel noticeably different and I was a bit worried I'd gone too short, but by just the second ride (with no tweaks) I felt perfectly normal again and the only obvious difference following the change was a smoother top of stroke and happier knees.

  
Shorter crank :
less angle in the knee and at the hips
BUT
for same power and same RPM, more force need to be produced to the pedal (power = force at pedal x pedal speed), as reducing crank length, for a given RPM, reduce pedal speed

So it is a balance between knee/hips angle (confort and aero) and force/RPM (what is the force where pink muscle fiber get involved, what RPM you can sustain).

RPM not being easy either, the slower being the most efficient energetically, but again involving pink muscle fiber with will lead to some potentials issues, and higher RPM losing efficiency and possibly generating too much friction with saddle.

Sustainable knee angle, hip angle, aero and breathing implications, RPM capabilities, force capabilities, impact on the Run capabilities... all this being highly individual from your skeleton, muscles, training, preferences, history, ...

... nobody can choose for you. If you try something different, take time to adapt, and be aware there is multiple possibles impact, most cannot be predictable.

Change is an opportunity and also a risk... like... ahem... did you get married several times ?

So, who makes shorter cranks? I’m having a hard time finding anything less than 170.

Rotor and Cobb

Shimano, Cobb just to name a few.

Okay, I think I may just split the difference and look for a 160mm crankset to throw on my rig. I know that 10mm will make a significant difference for me from a breathing and knee comfort standpoint.

Everyones comments here have made my decision that much easier.

Thanks!


EDIT: Am i going to have any compatibility issues going to 160mm when I use shimano? I dont see a 160mm option with shimano.

I started on 172.5 switched to 165 for almost 2 years and I am now 155 on my TT and 165 on my roadie!

You will definitely spin more on the 155 so if that's what you like then go for it, honestly after several TR workouts I can't exactly tell the difference between either crank other than the increased cadence.

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Speed kills unless you have speed skills!!!

I know this is an oft mentioned effect that's attributed to shortened cranks, however I haven't seen it.
My cadence tends to be rather low. I haven't noticed any real tendency to increase it since significantly reducing crank length from 172.5 to 160.

I agree, both with shorter cranks (for most) and trusting one's fitter. I'm a little surprised at your saddle change. It looks like you shortened your cranks by 2.5cm, but raised your saddle by 5cm. What am I missing?

The difference in force is minimal. For example, at 300w, 80rpm, you have to exert 20N more force for 155mm cranks than you do 170mm. As studies have shown, there are few, if any, advantages to having longer cranks for time trialing.

Basically all I'm trying to say is you're making this out to be a big change with potential negative side effects, whereas it's more likely the OP will switch cranks and notice an immediate positive one in aero.

That my seat might have been too low to start with. :-)

When I built my tri-bike this summer, I built it up with 150mm cranks (my road bike is 175mm). I chose the 150mm cranks based on FindingFreestyle's thread "maybe your seat is too high because your cranks are too long":



I set my initial seat height on the TT bike at my road bike seat height (730mm). I might have raised it a little bit, I don't recall. I road the TT bike that way for a week until I could get an appointment with a fitter. During that first week, my comfortable cadence was elevated by about 12rpm. I also felt like I was pedaling in little tiny circles like I was on a kid's bike. I did my usual 2x20s, power was ok...but, cadence was up (upper-90s vice mid-80s).

The following Saturday I had my fit appointment, where my seat was raised the 5cm (to 779mm), amongst the other usual fit changes. As soon as my saddle was raised my pedaling sensations immediately returned to normal. It was one of those Ah-Ha! moments. The following week, I did the 2x20s and power was again normal, and cadence was back into my typical range (mid-80s). The week after that I did the 2x20s again, 1st on the horns 2nd in full-aero. And again power and cadence were normal between the two positions. The week after that I did the full 2x20 in aero with the same results as previous.

That's true....on a fixie, and the corollary to a fixie the minimum gear on a muti-speed bike. Otherwise, cadence is self-selected to a comfortable combination of foot-speed and pedal-force. So, any effective gear-inch change resulting from shorter cranks is offset by a slight change in shifting points by the rider.

"minimal" mean nothing. There is an impact, it can be calculated, any comment on its possible consequences on someone is... a comment, an uneducated guess, not a fact, not a proof.

Wich studies ? Didn't noticed that TT serious peoples goes with 150mm crank for TT or Hour Record.
Dumoulin goes 150mm ?
Wiggins or Dowsett went 150mm for Hour Record ?
How stupid they are...

What I say is : there is some impacts, some can be positive, some can be negatives, and it is hard to tell, because this is very personal.

Maybe it works for you, fine. Does not mean it will work for someone else. An exemple is never a proof. A cohort result can be a proof, in a limited context. Many PhD thesis have been made on crank length and RPM. And the results are... well, not so easy to tell.

Not trying to push in one direction (short, short, short....or long, long, long, ...), just trying to give a view of the possible impacts.

Wandering what kind of "minimal" thing pushed Frodeno out of Kona podium the 2 last years.

As a very short (5') rider, shortening my cranks from 170 to 165mm was really helpful. It allowed some space to open up, get more aero and honestly fit my body better.

Thanks for your reply. That was my guess, but without evidence, I didn't want to make the accusation. I've known FindinFreestyle for quite a while. Can't go wrong listening to him.

The new 105 cranks (FC-R7000) are now available in 160mm...

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"I'm thinking of a number between 1 and 10, and I don't know why!"

Sorry, dont get it

What is true on a fixie ?

The coupling between power, pedal RPM, and torque...or foot-speed and pedal-force, if you prefer. As long as we are riding multi-gear bikes, we can change the speed/force or RPM/torque relationships by shifting gears to whatever our body finds most comfortable.

Going to shorter cranks doesn't REQUIRE me to sit and grind at a higher pedal-force/torque (thus recruiting different muscle fiber types), because I can down shift.

If your bike has a BSA or BB30 BB, then there are loads of BMX or other kids cranks that come in short lengths.

There is also a guy who will shorten cranks for a nominal fee. I had him do some Ultegra Octalinks a few years back and he did great work. Think it was about $80 for the pair. http://bikesmithdesign.com/...t_Cranks/hollow.html

I think you just saved me $100.

Yes, that is why I wrote :

"For the same power, AND SAME RPM, more force needed..."

I know that some people will avoid the additional force needed by increasing RPM (also true on fixie :-), that is why I also discussed RPM right after...

This is true but its not the whole story.
First, if rpm is the same, the pedal will move slower on a short crank. This will result in muscle contracting slower which increases the muscle's ability to produce force due to force-velocity characteristics. See the figure I linked to in the Rohan Denis thread.
Second, if you raise your saddle so as to maintain maximum extension, the leg will be more extended throughout the rest of the cycle. A more extended leg is stronger. You probably know you can do more weight with a partial squat than a deep squat. In my field this is called biomechanical gear ratio.
For those two reasons your muscles are capable of greater force production. And, as you mentioned, you can always change gear too.
But all these are in the nuance. McDaniel and colleagues showed clearly that crank lengths between 145-195mm have no effect on metabolic cost / efficiency.
Cheers,
Jim

Interesting, thanks

Not just "some people". I'll stop short of saying "all people", but I would certainly argue "most people"....and probably everyone here. We shift when our legs tell us "that's too hard, we can't keep this up for very long". Case in point, here is some data from me. Two rides, one on the road bike with 175mm, and one on my TT with 150mm. Both are 20min 90% IF efforts:

road (175mm): 5 September 2019, AvgPwr = 214 watts, AvgCad = 91 rpm, Speed = 21.1 mph, position=On-Hoods
TT (150mm): 9 October 2019, AvgPwr = 217 watts, AvgCad = 92 rpm, Speed = 24.7 mph, position=Full-Aero

I have no idea what gears I was in each time. I don't ride to a set cadence---Often I don't even have it displayed on my head unit. I simply select the gear that feels good at the time, for an effort level that my brain tells me I can hold for the remaining duration. I don't even ride to a set power---I ride to an effort level, and expect power to be in a certain range.

At 25mm difference, my TT cranks are 14% shorter than my road bike. That's a pretty extreme change by most peoples' standards. Yet, my power and cadence differ by 1/10th of that (~1.5%).

Yes, that's just one example---which does not a proof make. But, its a pretty extreme example of changes in bike, position, and crank length....which are indistinguishable when looking at power output and RPM.

I made a decision,

I am going with a 50/34 160mm length crank and an 11-23 cassette on the rear.

I am super stoked to run this starting in December.

This both makes sense objectively and tallies with my subjective experience when I switched from 172.5 to 160 cranks.
I haven't changed cadence significantly and have maintained the same power. Part of the reason I changed was that the load on my knees sometimes felt uncomfortable at the top of the stroke - possibly die to a bit of instability in my knees. Since dropping the top of stroke position by 25mm, the angle of my knee at that point in the stroke is straighter and thus it's both more stable and able to produce higher force. This increased ability to produce force seems to easily compensate for the reduction in crank length and I can produce as much or more torque compared to before, while seated. My knees are feeling much more comfortable for the same power output and my stroke is smoother.

Cool, thanks for sharing.
The issue of tight hip angle at the top of the pedal stroke was a major part of my recent presentation at ISCO. Everyone knows you should set maximum leg extension. Why not set minimum leg extension (by chancing crank length) too?