Hey Folks, I just started cycling and triathlons about a year ago. My previous bike had 175mm cranks. I just bought a brand new bike and it has 170mm cranks. I hear that the shorter cranks benefit cadence and leg fatigue, so many people are switching to the shorter crank arms. I am trying to decide if I should get a longer set of crank arms and I will tell you a lil bit about myself for you to understand my question more. Ex collegiate Football player and Sprinter. I am 5’10’’ with average leg length. I believe my background helps me maintain a high cadence fairly easy and my background developed my legs to prefer a full range of motion. My first IM is at the end of June. So…If you were my Coach and had my best interest in mind…what crank arm lengths would you suggest to me and why?
a shorter crank arm length will help prevent your knees coming into your chest in the aero position, which is usually the limiting factor.
170 to 175 doesnt sound like much but you need to add the 5mm at the bottom of the stroke with the 5mm at the top off the stroke.
If you really want to analyze, you need a power meter, anyone can spin like a gerbil on a wheel and put out little power, different story with higher power. Technically higher cadence, through lower force benefits guys with slow twitch fibers, it’s not a question of speed, you don’t need fast twitch fibers to pedal 95 rpm. Guys with bigger, fast twitch fibers can put out more power at low cadences. Also, if your rpms stay constant, a shorter crank takes more force to turn, so keep that in mind. If I were you, at such an early stage, just stick with what you’ve got, honestly the only reason to switch crank arm lengths is due to positional considerations.
My wife is 5’10" and was state tt champ and undefeated at duathlon with 170 cranks.
IT doesn’t “benefit cadence” or “help leg fatigue”. It can, in some cases, open up your hip angle in aggressive aero positions. If you are competitive and setting yourself up in an aggressive aero position, you might make more power with shorter cranks than you would with long cranks.
The worst case would be that it doesn’t matter. Unless they FEEL terrible to you, I wouldn’t bother changing. I would probably suggest 170mm cranks to you if I was your coach.
Hey Folks, I just started cycling and triathlons about a year ago. My previous bike had 175mm cranks. I just bought a brand new bike and it has 170mm cranks. I hear that the shorter cranks benefit cadence and leg fatigue, so many people are switching to the shorter crank arms. I am trying to decide if I should get a longer set of crank arms and I will tell you a lil bit about myself for you to understand my question more. Ex collegiate Football player and Sprinter. I am 5’10’’ with average leg length. I believe my background helps me maintain a high cadence fairly easy and my background developed my legs to prefer a full range of motion. My first IM is at the end of June. So…If you were my Coach and had my best interest in mind…what crank arm lengths would you suggest to me and why?
Hi Cojo,
We worked this out to our Pro Team’s satisfaction a few years ago. I wrote about that thinking here:
http://cervelo.com/en/engineering/ask-the-engineers/crank-length.html
Cheers,
Damon
Thank you all for your responses. I am definately gaining valuable insight. My dilemma started with the new bike and shorter cranks. I noticed that my legs seemed to be stressed more. I figured it must be the difference in crank arm length. All other variables were improvements as I went from a Schwinn(175mm cranks) to a new Shiv(170mm cranks). I realize that my body can and will adapt to the shorter cranks but, I just want to make sure that I am as efficient as possible in and with my pedal stroke. So heres another question to help my understanding of Power Measurement.
Consider 2 cranks, 1 short and 1 long. All VARIABLES are EXACTLY the same with the 2 cranks except for the arm length. Will the longer crank arm accumulate a larger amount of power since it is being ‘powered’ over a longer distance through its 360 degree rotation? I believe it would…which to my understanding…means you need to generate more power with a shorter crank to keep up. Please set my mind straight. 
Perfect article Damon! Thank you. Your post showed up while I was writing my last one and I didnt see it until I had already posted. Thanks to all. I believe I will maintain my current set up and evolve into it. The opening of the hips makes sooooo much sense and my battered ribs will appreciate the shorter crank arms. So will my wallet. Thanks Guys!!
Hi Damon - in your article you specifically mention using different size cranks arms between TT and road…do you consider that a rule of thumb or moreso in ‘if you want to experiment, start -5mm from your road crank arm length’. Just curious…I have currently settled on 170mm arms and have ridden that length for both TT and road and had positive experience with both.
Oh, and as to the original topic…I believe it’s something you have to play with. I was using 172.5’s which were fine, but I was curious…so I tried 165’s, 175’s, and finally 170’s…the last of which just felt the best IN AERO…no science involved.
Hi JA,
Yep, I consider it a rule of thumb: 5mm longer on the road bike than the TT bike.
This is based on the typical differences in pros’ positions on their road and TT bikes. The idea is to respect the limits of the hip’s range of motion, which depend on seat angle and torso angle as well as crank length. Most experienced amateurs probably have a similar difference in hip angle, so I would assume this rule of thumb could probably be applied to most readers of Slowtwitch.
But because crank length has so little effect on power, it’s not a firm rule to me.
As an illustration of an exception to this rule, my personal bikes all have 170mm cranks, both road and TT, mainly because I already own those cranks. I’m comfortable with that. If I were buying new I’d put 165s on my TT bike.
Cheers,
Hi Cojo,
Glad to help! Enjoy your riding.
Another thing I’d consider if I was your coach: what kind of terrain you’re riding on. On totally flat or mostly flat courses, you could get away with most anything, with most factors favoring set ups that reduced your total aero drag.
But if there is a lot of significant climbing, some average height and taller riders report more difficulty with short cranks.
Nope, no power is lost. If your legs make 200 watts, 200 watts will hit the crank and 196 watts will hit the rear wheel (a few being lost to drivetrain friction!)
Crank length has no impact on this. It would only affect eyour cadence, and/or what gear you choose to use at a given speed/power.
Consider 2 cranks, 1 short and 1 long. All VARIABLES are EXACTLY the same with the 2 cranks except for the arm length. Will the longer crank arm accumulate a larger amount of power since it is being ‘powered’ over a longer distance through its 360 degree rotation? I believe it would…which to my understanding…means you need to generate more power with a shorter crank to keep up. Please set my mind straight.
Changing crank length is essentially changing gear ratio. If you push 6 inches around the pedal stroke on a longer crank you are physically advancing the wheel a smaller distance than with a shorter crank. I believe this is why people long thought that “leverage” was an advantage. It is easier to push the same gear, but you are also going slower. Sheldon Brown’s site has the idea of “gear ratios” instead of using gear inches so that gearing choices can account for this.
I would pick what fits best and feels best. For some reason unknown to me I prefer the “feel” of longer cranks, but nothing longer than 172.5 works for my fit, so I run 172.5’s.
begin n=1 comparison
I have always ridden 172.5s on all my bikes from MTB’s/Tandems/singles/TT. that’s roughly 20 different bikes all with the same size cranks.
i bought one particular road bike along the way (still have it) and i immediately set it up to my known coordinates that i have loved for so long. (based on the center of the crank for seat height)
i hopped on it on the trainer and ‘tweaked’ the seat height a smidgen for feel as i do often because i’m sensitive to about 3mm difference in seat height i can FEEL.
i set out on a ride and things felt REALLY weird. i chalked it up to the handlebars being 40’s instead of 42’s that i’m riding on my other bikes.
after an hour on the bike, in mid season form, i could barely get up and over these little hills/bumps and i was extremely winded. I was riding with a buddy that i normally have no issue haveing a heated ride with, only now i could barely hold his wheel. I wanted to throw the bike as i was so angry with it. (this is consistent still with this bike)
i get home and finally figured it out. THIS bike had 165’s on it. i have to say this was as close to a blindfolded comparison i could have ever done with crank arm length. truly never noticed it or gave it much thought until i figured it out.
moral of MY story, stick to what you’ve trained on, or at least be aware of the change. in MY case it felt like (and still does) i had to push REALLY hard on the pedals. i would have never thought 7.5mm of crank length would make that much of a difference. but for ME it seems to be huge. No i don’t have a PM to test the output, but i can tell you the RPE is very different.
i measured the seat height based on the pedal in the lower position and i had instinctively adjusted it to exactly the same as my other bikes +/- 2mm so i think i have the feel for what i prefer for leg extension.
/end N=1 comparison.
Tim
I’ve had nearly the same experience going from 170 to 165 but I knew it. I went back to my 170’s and put the 165’s on the GF’s bike.
She apparently doesn’t mind them at all.
jaretj
Tim, how tall are you?
I ride 172.5 normally and 170 feels immediately odd, even that tiny difference.
But yeah if you have trained your nervous system on one length for a long time, switching is going to be weird for a while.
The OP could always split the difference! 172.5!
I’ve had nearly the same experience going from 170 to 165 but I knew it. I went back to my 170’s and put the 165’s on the GF’s bike.
She apparently doesn’t mind them at all.
jaretj
5’7.75"
Unless you say you are a hot chic online then I’m 6’2
Tim
.
I gave it four months, it was starting to get warm enough to ride outside longer and I didn’t want to waste time screwing around with cranks. Possibly could have felt better if I’d gone longer.
I suppose I could have started earlier but have lost interest now.
5’7.75"
Unless you say you are a hot chic online then I’m 6’2
Tim
Made me laugh out loud at work.