Crank lengths?

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Dirk

If 6 Was 9
Location
Watchet
I'm currently riding with an Ultegra Di2 groupset on my 'best' bike.
The crankset is 52/36 and I run an 11 - 32 cassette.
It's very hilly around here (North Devon) and I find I am on my limit at times on this gearing.
I'm thinking of changing to a compact, 50/34, chainset as I reckon this would solve my problem.
My current crank length is 172.5mm. This is what came as standard on the bike.
Looking around the internet, I can get a crankset with 170mm cranks for around £30 less than one with 172.5mm cranks.
Would it make much difference losing 2.5mm off the crank length? Would I notice?
 

Sharky

Guru
Location
Kent
No
Might need a minor adjustment to saddle height (2.5mm higher)

I switched from 170 to 150mm and after initial acclimatization, it becomes "normal".
 

Hugh Manatee

Veteran
How tall are you? I notice the difference between 170 and 175, or at least my knees do! I don't think anyone outside the pro peleton would notice 2.5mm
 

Hugh Manatee

Veteran
I'm slightly taller and actively prefer the 170 length cranks. I'll swap slightly less torque for happier spinning.
 
I use 170 and 175 mm cranks. I prefer smaller ones but it doesnt make a huge differene to a medium sized male.
Crank size is more of an issue for big or small riders so I was surprised to read of User46386' s experience. You need to adjust the bike position for different crank size. You also need some time to adjust to a new size if you have never used it before.
 

the snail

Guru
Location
Chippenham
It's about ergonomics, not gearing - about 1.5% difference in gearing compared to say 10% difference between gears. But your knees will be a bit closer to your chest, saddle a bit closer to the ground and your feet will be moving faster with longer cranks.
 
OP
OP
Dirk

Dirk

If 6 Was 9
Location
Watchet
Cheers guys.
After checking out other threads on the subject, I've ordered the 170 mm crankset.
Should have used the search function. :rolleyes:
 
It's about ergonomics, not gearing - about 1.5% difference in gearing compared to say 10% difference between gears. But your knees will be a bit closer to your chest, saddle a bit closer to the ground and your feet will be moving faster with longer cranks.
Longer cranks are part of the gearing system so you would naturally compensate by running longer cranks in a bigger gear to achieve the same pedalling force at a slower revs. Angular velocity would be the same but each rev of a long crank is a longer distance so mire work than a short crank. Since the rider power output is constant, something else has to give.
 
OP
OP
Dirk

Dirk

If 6 Was 9
Location
Watchet
Longer cranks are part of the gearing system so you would naturally compensate by running longer cranks in a bigger gear to achieve the same pedalling force at a slower revs. Angular velocity would be the same but each rev of a long crank is a longer distance so mire work than a short crank. Since the rider power output is constant, something else has to give.

Same amount of work surely - smaller force over longer distance vs. larger force over smaller distance. Still small beer though.

I reckon, for the type of riding that I do, the difference between the 50/34 chainset and the 52/36 one will more than offset any losses from 2.5mm off the crank length.
I'm a 63 year old, reasonably fit, leisure cyclist - not looking for marginal competitive gains.
 
Last edited:

S-Express

Guest
http://myworldfromabicycle.blogspot.co.uk/2010/08/dude-your-crank-lengths-fine-you-just.html

Summary: Common crank sizes are nearly all equal in efficiencies. Cranks size can be chosen for reasons such as ground clearance for cornering/obstacles (shorter), aerodynamics (shorter), or rehabilitation/flexibility (longer). Sprinting 120 rpm is best. 60 rpm is better than 100 rpm aerobically (generally lower cadences are more efficient than higher). Natural pedal stroke is best (do not pull up), crank length has no effect on fatigue, no effect on metabolic efficiencies and very small effect on maximum power. A big gear sprint is better than a small gear sprint (for 30seconds) .
 

BrynCP

Über Member
Location
Hull
I went 175 to 172.5 and on my first ride experienced pain in my knee. Minor adjustment to saddle height (increased by slightly more than 2.5mm) corrected it and otherwise no difference.
 
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