Crank arm length

[Nicomrs]

Hi all, 

I recently statred to build a custom hardtail with some parts i had but added a few new parts here and there. 

Fitted a sram nx crank 165mm, diddnt know it was that short. 

Went for one or two rides and realised i need a lot of watts to turn the cranks. Ended up injuring my calve muscle. 

All my other bikes had 175 cranks with either 3/32/34 chainrings, this one a 36.

Does crank arm length play such a big role?

[Newbie321]

Shorter crank length and the 36 chainring is probably the issue. Here is a nice GCN video on the subject of crank length. Short of it is that for any given crank length your power is unchanged if you have a sufficient gear range, only cadence would differ. So you probably are just over geared.

 

[Mountain Bru]

I get what you mean, but your understanding of the relationship between power and crank arm length is slightly off. 250w is 250w regardless of crank length.
However....
Power=Torque x Angular Velocity     (Angular Velocity would be cadence in our world)
And 
Torque = Force x radius  (Radius here would be crank arm length, Force is how hard you're pushing the pedal around the BB)

So if power is the same, cadence is the same, and you reduce the crank arm length from 175mm to 165mm, that's roughly a 6% reduction, so you'll need roughly 6% more force on the pedals to produce the same power (not exactly, but roughly). 

That's the science behind the power, but many other things are at play. Remember, if you shorten the crank by 10mm, your saddle fore/aft and height needs to change too to keep your front knee over the pedal spindle at "3 o'clock-9'clock" (ie: horizontal cranks)  and also you've effectively increased the angle your knee is bending at the bottom of the pedal stroke by shortening the saddle to pedal length. Not sure if you set your bike up according to your "normal seat height" and "normal saddle tip to bar" measurements, but I'd guess one of these is probably to blame for your injury, especially if you're really used to 175mm cranks. I mean it's not like you've never pushed higher watts before.  

Edited August 8, 2021 by Mountain Bru

[MORNE]

Im taller than most and I’d ride 177.5/180 mm cranks if i could find some. But i just ride 175 because thats what you get locally. It feels comfortable enough. My vintage roadie came with 172.5 and i can definitely feel that I spin faster with them.  My wife is 1.68m and her nee bike i built off the internet - getting parts from EU so that meant i could find her some 170mm cranks for the first time…since all her bikes used to cone with 175 too. She loves them, says it just feels right.

what I’m getting at is….how tall are you?

Edited August 9, 2021 by MORNE

[NotSoBigBen]

My experience is slightly different, crank length hasn't really made much difference to me ... I in fact ride a few bikes with various lengths and they all feel fine but

Seat height - as mentioned I have always measured from the pedal not the center of BB as some do make sure it's correct. Worthwhile also again as previously mentioned to make sure the seat 'fore/aft' position is correct once this change is made

Gearing- again as some have mentioned going from a triple to specifically a 36 1x depending on the size of the cassette you are using, now that could hurt! Well me anyway ????

[NotSoBigBen]

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Edited August 9, 2021 by NotSoBigBen
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[Jewbacca]

And here I am trying to find someone who can mill and tap my crankset down to 150......

But on my full sized bikes I have 170mm. I won't go longer unless I absolutely have to. At 1.75m short with the inseam of a semi full grown hobbit I'm considering some of the new Lyne 165mm for the MTB

[buckstopper]

Even me. Just received, as a gift of course, a 165cm 105 set, and also a pair of Assioma duos. Very excited. I went to 170s 5 years ago, loved them,  and am looking for a more open hip angle and improved cadence. 177cm/85 inseam

[Nicomrs]

I am willing to swop my nx 165 crank for a 175 crank as long as its dub

[Jewbacca]

52 minutes ago, Nicomrs said:

I am willing to swop my nx 165 crank for a 175 crank as long as its dub

I am unfortunately too poor to have DUB/modern standard cranks and groupsets.

I have a set of 175 1st edition Lyne cranks which I will be milling down and re-threading to 150mm. Otherwise I'm a 24mm spindle old school hippy peasant.

The BBs are easier to get hold of, one third the price and tend to last a heck of a long time. If I had DUB cranks I would swap

[Mountain Bru]

13 minutes ago, Jewbacca said:

I am unfortunately too poor to have DUB/modern standard cranks and groupsets.

I have a set of 175 1st edition Lyne cranks which I will be milling down and re-threading to 150mm. Otherwise I'm a 24mm spindle old school hippy peasant.

The BBs are easier to get hold of, one third the price and tend to last a heck of a long time. If I had DUB cranks I would swap

Never heard of 150mm cranks... is that even a thing? 

Sounds like it could be pretty harsh on the knees though.... Gonna need to put some proper force on those cranks to produce watts, and I think the deadspot at the top/bottom of the stroke gets bigger with shorter cranks too. 

[ChrisF]

11 minutes ago, Mountain Bru said:

Never heard of 150mm cranks... is that even a thing? 

Sounds like it could be pretty harsh on the knees though.... Gonna need to put some proper force on those cranks to produce watts, and I think the deadspot at the top/bottom of the stroke gets bigger with shorter cranks too. 

Had to check ...

 

Titan Haydes 24" comes with a 155mm crank.

 

The 20" comes with a 140mm crank.

 

On their 26" (extra small frame) they go to 165mm crank.  The Mongoose 26" Medium has a 170mm crank (if I recall correctly).

 

 

Certainly some pattern to in seam measurement and crank length.  And yet the vast majority of 29" Medium bikes are sold with 175mm cranks .... I know a couple of riders that changed to 170 and swears by it.

[Chadvdw67]

Previously only rode with 175mm cranks, because thats what all my bikes came with, test rode a bike that had 170mm cranks, honestly couldn't really feel any difference, but my mate commented how I hadn't had any pedal strikes, I pedal strike a lot ???? only then did I find out they were 170mm cranks, so my new bike I specifically asked for 170mm cranks, it has been awesome, no more pedal strikes ???? I actually wouldn't even mind trying 165mm cranks, just to see how they feel....

[Eldron]

37 minutes ago, Mountain Bru said:

Never heard of 150mm cranks... is that even a thing? 

Sounds like it could be pretty harsh on the knees though.... Gonna need to put some proper force on those cranks to produce watts, and I think the deadspot at the top/bottom of the stroke gets bigger with shorter cranks too. 

Shorter cranks are better for the knees and hip flexors.

Do a little digging - there are a few paper that measure power, oxygen consumption etc and the scientific consensus is that crank length is largely irrelevant (even at the ridiculous extremes).

[Mountain Bru]

13 minutes ago, Chadvdw67 said:

Previously only rode with 175mm cranks, because thats what all my bikes came with, test rode a bike that had 170mm cranks, honestly couldn't really feel any difference, but my mate commented how I hadn't had any pedal strikes, I pedal strike a lot ???? only then did I find out they were 170mm cranks, so my new bike I specifically asked for 170mm cranks, it has been awesome, no more pedal strikes ???? I actually wouldn't even mind trying 165mm cranks, just to see how they feel....

Lifting your pedal up 5mm should theoretically make pedal strikes happen less, but in all honesty I think it's bad technique if you pedal strike a lot, not a crank length problem. Ride over the rocky stuff with level pedals. If you must pedal, judge the rocks nicely so you pass them with your "impacting pedal" at 12 o clock when you pass it, even if you've gotta free wheel a bit.

 

 

[Nicomrs]

Ok but my logic tells me that say i keep my cadence in a specific gear the same, do i have to put down a lot more power on shorter cranks? 

Thats how it feels, just want to make sure im digging in the right place.