Optimal Crank Arm length

I’ll preface this by saying that I’ve got short legs. My current cranks are Ultegra R8000 170mm.

That being the case, I have a difficult time setting a comfortable saddle height. I’ve had a professional bike fit but my resulting saddle height feels like a poor compromise due to the range of motion from top dead center to bottom. There doesn’t seem to be a happy medium where I’m comfortable. I’m either too flexed or too extended.

I was reading online last night about optimal crank arm length as one of a number of formulas that resulted in 160mm or 161mm depending on which one I used (Inseam x 1.25 + 65 or height x .95). My inseam is 30 inches (76.2cm) while I’m 5’ 7” tall (170.2cm).

And so I’m considering replacing my crankset with a 105 R7000 160mm set. I thinking this would allow me to have a better range of motion, where I’m not overly flexed or overly extended.

But I’d like some advice from others who have experienced the same thing before I replace an otherwise perfectly good crankset. Is it worth doing this?

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Similar to you, 5’8, 31" inseam, and I think I’m on Shimano Ultegra 172.5 currently, which I would think too long for me, but have worked just fine.

The 1-2x I’ve tried to resolve something with a crank arm change, it’s done absolutely nothing for me - my coaches/bike fitter once told me that’s literally the LAST place to try to address a problem.

Anytime I can be of no help whatsoever, just let me know!

:grimacing:

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Let me start by saying: I know nothing and have no recommendation.

However, my trusted bikefitter seems to think the crank length doesn’t matter so much.
His fitting resolved a lot of issues I had and I place my trust in him.

Based on this my vote would be: No.

He referenced Dr. Jim Martin as one of his sources. Maybe you would like to read/hear about his research on blog.bikefit.com.

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Hey.

So I have long legs for my height. Inseam was 33 or maybe even 33.5

I’m now on 170’s on MTB’s (down hill and XC) and road bikes.

That was based on a professional fit (actually two) which took into account my own personal (poor) range of motion.

I was in 172.5 before that.
I believe I could happily go to 167.5 but just haven’t got the financial will to do it.

FWIW - just last night I tried a bike that was 172.5 for the first time in years. Without thinking …. I spent the next 20 minutes feeling totally wrong then remembered.

Whereas when I dropped crank length in the past it was an instant easy move.
Don’t want to be responsible for your finances.

I think you’ve nothing to lose and possibly plenty to gain. Only question maybe is 165 or 160.

That’s interesting because Dr Martin’s research states that an optimal crank length would be 20% of leg length or 41% of tibia (shinbone) length. That would put me at 151mm cranks assuming my inseam, although he doesn’t specify in the abstract how leg length is measured.

He also stated that maximal power is relatively unaffected by crank lengths between 145mm and 170mm cranks. However, I’m more interested in lower flexion and a more open hip angle.

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For what it’s worth, I switched to 155mm cranks last year 5’9", 32" inseam) on my tri bike to be easier on the hips and because of reading it makes transition to running easier. I look like I’m riding on a clown bike, but I and as comfortable as can be with the shorter range of motion. I have not noticed any loss in power

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I switched to 165s almost five years ago and have never looked back. 172.5 were just long enough to close my hip up too much at the top of the stroke and cause me excessive pain. You wouldn’t think 7mm would make a difference but it’s made my hip happy as a little clam ever since doing it.

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Might be worth seeing if you can get to try a kickr bike as it has 5 crank length settings available to try out

@Sir_Brian_M I’m almist as tall/little as you. 172cm, legs 80cm.

I’m using 170mm on the roadbike, 172,5 on the Trainer and 175 on xc. Adjusted saddle high everywhere the same. All cranks feel fine to me.

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I came across this video a while ago. I’m 170cm tall, short legs and I’m ok on 170s but did a bike fit years ago that suggested 165s. Maybe one day I should try. On my mtb I have 175s, it is an old one and they all came with that length. I hate them. Should change really.
See if this video helps

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Seems that shorter cranks make it easier to spin higher cadences, whereas longer cranks are better for low cadences. Road bikes used to come with a 42:25 bottom gear now 34:28 or lower is common, my thoughts are that we used to need long cranks to mash those big gears at low cadence up hill, now we can sit and spin with compact chainsets and wide ratio cassettes.

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It may be age related, but my normal crank length is 170. I got a CX bike that had 172.5 cranks and I thought “No big deal, CX races are short”. Then I started doing longer gravel rides on that bike and eventually noticed I was having hip pain on the right side. I lowered the saddle after my buddies noticed I was rocking and still had hip pain. I remembered the longer cranks and replaced them with 170s and the hip pain is gone. YMMV.

I’ve found this thread very interesting in relation to some problems I am having with my bike fit.
Great topic.

Martin

Short legs long body - moved to 165 on my tt bike and have never looked back

I thought some fairly recent UK secret squirrel research suggested apart from the extremes crank length had no impact on efficiency or performance. I think I heard as a result of that Wiggo went back to 170’s on the track.

I would go shorter if it’s more comfortable!!

Ps. I know nothing but listen to a lot of podcasts!! :roll_eyes:

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I think this has caused some confusion though. Should we assume crank length is irrelevant because it doesn’t affect power or efficiency? I think the opposite is true. Namely that we are free to use crank lengths proportional to our bodies because we can remain just as powerful and efficient while optimizing our ergonomics.

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at 12 o’clock position, a shorter crank will reduce knee angle position (which can also help alleviate hip flexor issues). Only a bike fit will determine what crank length is needed to get correct angle

I think that secret squirrel research just provided evidence of the obvious, that the size of commonly available crank lengths eg. Dura-Ace 165-180mm accounting for just 3cm difference in leg length is far too small compared to the range of leg lengths of 50% of the population.
National Health Statistics Reports, Number 10, (October 22, 2008) (cdc.gov)
25th percentile female upper leg length was 36cm
75th percentile male upper leg length was 44.4cm
a range of 8.4cm!

On paper, your crank length should work, but if it’s not working for you, it’s wrong.

I’m fairly flexible, so a longer crank length works fine for me, especially in ultra events, where being able to utilize different muscle groups means less overall wear-and-tear.

I say try the shorter crankarms. The only thing you’ll really miss out on is explosive sprints. If you aren’t trying to blow-up crits, you’ll never miss it. But also work on your flexibility. You might find that some bodywork can allow you to extend another 5mm comfortably. When in doubt, it’s generally better to be a little compressed than over-extended. Just make sure you avoid knee pain if you drop the saddle a bit.

The issue for me is that the adjustment from my recommended crank length is 10mm - I’m on 170mm now and need 160mm. So while I can adapt, I’m not comfortable. With a 200 mile ride in 2 months, I’m looking to move to a shorter crank.

Problem is my bike comes with Ultegra and the shortest they offer is 165, unless I downgrade to a 105 crankset. That’s not something I’m too keen on, upgrading to an inferior crankset.