Any reason to not use super short cranks?

I’m a 181cm male with an unusually short inseam (77cm). I also have very tight hips and pre-existing knee injuries, I’m addressing these issues with regular mobility and strength training but at this point it’s just holding things steady.

After addressing some other issues, my bike fitter suggested that I run the shortest cranks I can find, in my case that’s a 155mm Rotor crankset. His reasoning is that the shorter cranks will give me a wider hip angle which will help with some pain I’ve been having. I also put out more power in a TT position so having shorter cranks apparently will help with comfort at a more aero position.

That being said, I’ve been researching reasons NOT to go with shorter cranks. Below is a list of a few reasons I came across as well as my responses:

  1. Less torque: I can accept that my top end sprint power won’t be as high, but ultimately if I’m running 2x11 isn’t torque a non-issue? Meaning, can’t I just change gears to get a higher torque?
  2. Less aero: I can’t find any solid numbers on this, but if I feel more comfortable in an aero position I’m sure that will negate any aero savings I’d get from longer cranks.
  3. Limited supply: So far the only manufacturers making cranks at 150/155mm are Rotor and Cobb. I’ve heard bad things about Cobb’s shifting which is why I went with Rotor.
  4. Makes you look like the roadrunner in Looney Tunes: I’m fine with that.

Does anyone have any thoughts on the negative aspects of going with shorter cranks?

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You are correct…if you can notice the difference in torque, simply change gears. IOW, don’t worry about torque.

Not following you here…on a TT bike you should be more aero with shorter cranks because you can rotate forward more while maintaining a more open hip angle.

Something to consider, but if you can find what you want now, I wouldn’t worry too much. If anything, availbility will likely increase over the short-term. More people are using short cranks now.

Again, just shift gears to a larger gear so you can maintain your “natural” / preferred cadence.

I’m not a fan of using shorter cranks for the sake of just using shorter cranks, but it sounds like you have specific fit issues which they will help address. I’ll defer to your fitter’s advice re: his length recommendation…seems a bit extreme to me, but he has seen and fit you on the bike. If you like / trust what he ahs told you so far, go for it.

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Going from 172.5 to 165 I have definitely noticed the torque difference. But yeah you either get used to it / get stronger etc or change gearing to adapt. That said I’m never going back to longer cranks because in my case, much like yours, there were fit issues with longer cranks for my build. Opening hip angle and having more room so that my thighs don’t hit my chest in an aero position were the main ones.

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One of the issues I had noticed using shorter cranks was handling on my road bike. I’d raise my saddle to keep my leg in the same position at BDC and this would raise my center of gravity. This took a bit to get used to (and in a TT position isn’t something I’d overly care about).

Limited supply is annoying - that’s the main reason I went back to (slightly) longer cranks.

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Adding to the required saddle height change, should be consideration of the front end height too.

If you aim to keep the same saddle to bar drops (or extensions for Tri/TT) you will need to raise the front end a similar amount as the saddle height increase.

This gets overlooked too often and people looking to open a hip angle accidentally end up restricting it by keeping the front end at the lower (original) height.

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I’m slightly taller than you with a longer inseam and went down to 165mm from 175mm.

I raised the saddle 10mm and dropped the front end 10mm for 20mm total with no drop in power. Almost like free aero.

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Listen to your fitter. I used the Cobb cranks before and I can confirm the shifting is atrocious. I’m on Shimano 105 160mm now and I’m happy with them.

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My bike fitter suggested I should lower my saddle with shorter cranks to keep the top of my stroke the same height. That would make you more aero.

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Find a new bike fitter…seriously.

If you change to shorter cranks, you move the saddle up and forward.

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Then you would end up with a more closed leg extension angle. That’s potentially fine if you guys decided you were previously over-extended prior to the crank length change.

But that is an odd conclusion, IMHO, if the leg extension was “good” prior to the change.

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I had heard on a YouTube channel that Specialized had run aerodynamics on short vs long cranks in the win tunnel. Something about how shorter cranks having a higher cadence than longer cranks led to shorter cranks creating more drag.

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And less powerful. Typically your relationship with bottom dead center is kept constant and top dead center is what it is as a result. Optimal power tracks fairly well with saddle height right up to the point that you either get injured and/or made fun of on slowtwitch for having a saddle that’s too high. Mostly thinking of Phil Burt’s book which has a plot of what I’m trying to describe.

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If you just switch to shorter cranks and bump your saddle up and do nothing else it increases your drag because you’re now a centimeter taller and your frontal area is a bit larger. That said, if it allows you to tuck lower for the same (or more open) hip angle then it’d be a net aero gain (or neutral).

No idea re: cadence, although you can pedal slowly on short cranks if you want to. Crank length is just another part of gearing.

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Forget about torque, the most it matters is power. Diesel engines usually have the double of torque than a petrol engine. A 200hp R bike or a F1 engine has a ridicously small torque, but has the proper gearing to transmit that toque multiplied in the wheel/s at certain rpm is what really matters

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I have no idea of the impact of cadence on aero (never really looked into it) but the issue is easily remedied…shift up a cog or two and resume your “normal” / preferred cadence.

You would likely do this automatically w/o thinking about it because we tend to settle into our preferred cadences when we ride using our gears.

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Slightly off topic: I’ve only ever used 175 or 172.5, like ever. I’m about to run a set of 170 on my mountain bike due to pedal strikes with a particular bike. I’m wondering if I’ll even notice the torque “issue”

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Totally worth trying. My trusted fitter helped me deal with some similar issues and we went down to 165mm cranks. It was weird for the first 100 miles then it’s less weird. Then it’s normal.

You will need to “re-find” your cockpit dimensions . . . and it’s not as simple as 170mm - 165mm = +5mm to saddle and stem height. Seat tube angle and head tube angle will affect things and those things will, in turn, affect saddle setback and saddle to bar reach.

We’re not cars + trucks. So while torque might be affected, it’s a minor part of a larger equation . . . unless you make your living riding a bike or are always at the pointy end of the pointy end.

Finally, I just did a bit of math to figure some things out.
With 170mm cranks @ 90rpm your foot is moving at .060mph
With 165mm cranks @ 90rpm your foot is moving at .058mph

To pedal 165mm cranks @ .060mph you will need to hit 93rpm.

I’m not really sure why this matters though because I’m assuming that you’d be using the same chainring/cog combo. So at 90rpm you’re still going the same speed. You just might be slightly more efficient because you’re legs are moving .002mph slower.

What do I know? I tend to calculate shit like then and then tell people that they’re overthinking.

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I switched to 165s and wouldn’t go back, can maintain a higher cadence now and its opened up my hip angle so if you’ve got a bit of a belly you can ride in the drops without hitting it :laughing:

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I’ve had 7 knee surgeries and had issues with the 172.5mm cranks (knee flexion). I switched to 165mm cranks on all bikes after one of my bike fits and recommended by my fitter. Magical. I probably would have gone shorter, if I found them available. Didn’t know Rotor offered 155mm.

I think it’s been said, but shorter cranks mean you raise your saddle up and thus a bigger frontal area (assuming cockpit is raised too).

Thank you. I have been meaning to do this for a few years. Very interesting.

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Just to share my math in case I did something wrong.

RPM * pi * diameter = linear speed

so

90(pi)(340) = 96,132mm
96,132 = 0.0597 miles.

and here’s where I discovered my mistake. That’s .06 miles per minute not MPH.

So
.06(60) = 3.6 MPH

90(pi)(330) = 93,305
93,305 = .058 MPM
.058(60) = 3.48 MPH

I think everything else works out the same.

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