Crank arm length?

Discuss light weight issues concerning road bikes & parts.
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queylotrie
Posts: 1
Joined: Tue Oct 23, 2018 6:15 pm

by queylotrie

My inseam is 74~75cm.
I have used 170mm for two years, and i haven't been feeling nervous.
by the way, I'm going to change it to 165mm or 167.5mm.
is it good for me or i have to keep 170mm?

TiCass
Posts: 228
Joined: Thu Aug 04, 2016 1:13 pm

by TiCass

On a road bike, keep your 170mm.
On a TT, yeah try shorter.

by Weenie


mattr
Posts: 4673
Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr

Why are you looking to change? Just cos you have short legs?

I changed due to knee issues. My inside leg is 81cm for reference.

morganb
Posts: 667
Joined: Wed Mar 15, 2017 5:30 pm

by morganb

I went from 172.5mm to 150-155mm depending on bike due to knee issues. While there was an adaptation period with such a dramatic change, I can now put out the same sprint power and actually feel worse on all out efforts on even 160mm cranks. My endurance/threshold power were pretty much unchanged after one ride of adaptation.

hannawald
Posts: 586
Joined: Sat Dec 17, 2016 7:28 pm
Location: Czech Republic

by hannawald

What kind of knee isues does it help with?

AJS914
Posts: 3553
Joined: Tue Jan 28, 2014 6:52 pm

by AJS914

queylotrie wrote:
Tue Oct 23, 2018 6:21 pm
My inseam is 74~75cm.
I have used 170mm for two years, and i haven't been feeling nervous.
by the way, I'm going to change it to 165mm or 167.5mm.
is it good for me or i have to keep 170mm?
As others have said, there really isn't any downside. To me, there isn't a huge difference between 170mm and 167.5mm. I'm not sure it's worth spending the money there. What is the overal goal?

I went from 175 to 170mm. Mostly I liked the reduced hip angle. I left my bars where they were and raised my seat to compensate so I got a bit more of an aero position out of the deal.

mag
Posts: 414
Joined: Thu Mar 16, 2017 12:23 pm

by mag

I ride 170mm cranks having the inseam 84-85cm. Riding the same crank length with 10cm shorter inseam seems to be on the rather long side of things, but of course these numbers aren't everything and if you feel and perform well with those cranks even on long rides then why not.

I went for slightly shorter cranks (from 172.5-175) due to knee issues which hinder me when walking (and I can't run much because of that either), but luckily on bike I'm completely fine. Shorter cranks promote riding at higher cadences using lower gears which leads to knee having to deal with lower forces. And they reduce the range of knee movement which helps as well.

kma556
Posts: 4
Joined: Tue Oct 23, 2018 7:42 pm

by kma556

I am just over 5'6", inseam around 76cm.

Changing to 165mm cranks was the best change I made to my bike. You can raise your saddle by about 5mm to keep the extended leg angle the same, but what you gain is a bit more open hip angle at the top of the stroke. Knees are not as close to your chest meaning you can get a bit lower on the front.

I guess you get less leverage for a given gear which might feel like it's slightly harder work but it's not really, you seem to be able to 'spin up' a bit easier with your feet travelling less distance around the circumference of the movement they make.

It's hard to put it into words but I would say anyone around 5'6" / 168cm or less would be much better off on 165mm cranks. I know it's not just about height, more about leg length but anyway take at a look at a few pros measurements, take crank length as a percentage of their height and see how you compare.

Hexsense
Posts: 952
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

The change, biomechanically, could be big or unnoticeable, depend if the length changed shift crank length from just barely too long to now not too long, or change from not too long to also not too long.

I'm 169cm tall and having 79cm inseam. I like both 167.5 and 165 but prefer 165 because my ankle seems to stay horizontal to the ground (not toe down), therefore reduce my effective leg length compare to other people that do toe dipping when they pedal.

drivetrain mechanically (biomechanically of your leg excluded) changes from long crank to shorter crank are:
-reduce pedaling circle circumference --> faster cadence at the same linear foot speed.
-reduced torque --> same gear feel harder to push
-two of both cancel out and you get no change in power, providing that you are not anywhere near the limit of gearing (spin out or run out of gear). You may likely use one gear easier and spin at higher cadence on shorter crank.
-if you don't make change in gearing, your max speed will raise as you can now spin faster at top gear but your climbing at low cadence when run out of gear will suffer a bit because of reduced torque.
-all above are very minor shift, 165 vs 170 is like 3% difference afterall.
-to keep thing equal (which you don't have to), logical gear change would be to reduce your gearing slightly (like 52/36 down to 50/34 front rings) from longer crank to shorter crank.
Top speed maintained by automatically getting higher cadence, Climbing is still easy due to lower gearing. Again, you don't have to make it equal...

ooo
Posts: 822
Joined: Sat May 21, 2016 12:59 pm

by ooo

It may be more important to know femur length (not just inseam), comfortable crank arm usualy >41% of femur length

http://www.billbostoncycles.com/crank_length.htm

Image
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TobinHatesYou
Posts: 4310
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

If you haven't had any issues with 170mm, then stick with it. If you climb a lot of steep stuff, stick to 170mm. If you have cash to burn and want to try 165mm, go ahead... I'm 178cm tall with an 81.8cm inseam and ride 165s.

by Weenie


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