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Re: Shorter crank experiences. Then back to longer ones!

Posted: Fri Dec 15, 2017 4:47 pm
by 11.4
Lots of confusion here about what happens with different crank arm lengths.

First of all, stormur, everyone else is correct. Regardless of pedal and shoe stack height, it simply lifts your foot up higher but transcribes exactly the same circle. You simply have to raise your saddle by the same height as the stack height increase and it'll be the same.

Second, when changing by 2.5 mm, you're doing two things. At the bottom of the stroke you are stretched out farther by that distance and at top of the stroke your knees will be that much higher into your chest. You basically need the leg length to support that extra range without your thighs simply interfering with your abdomen and your breathing. The other thing that happens is that the point in your pedal stroke at which you are applying maximum force will change, and the anatomy in the knee and hip that you are bearing on will change. This happens regardless of whether you adjust your saddle up, down, or sideways. It's a bigger circle and the force vectors of your pedaling as expressed through your knee joint will change. This is why inappropriate crank arm changes can cause joint problems. And yes, the full rotation difference is 5 mm for a 2.5 mm change in crank arm length.

Going shorter rarely hurts anyone because you are simply reducing range of motion. However, you are also prone to less flexibility, which is its own problem, and you need to maintain (or ideally increase) your cadence so you need to have lower gearing available. Truthfully, if you're in your lowest gears on a steep hill, you are probably out of the saddle or in a contorted position on the saddle, and you will simply adjust to the different crank arm length that way. Your power application becomes more and more asymmetric through the pedal rotation in such circumstances. So you may not really need that gearing difference. More important is usually that you may find yourself cross-chaining more, and want to consider a different gearing setup to avoid that.

There's a lot of discussion and analysis, when what's really needed is simple testing. The issues that make a different crank arm length better or worse for you include knee anatomy and injuries, how much gut you have and whether it gets in the way of your hip rotation, impingement or other anatomic issues with your particular hips, foot length (which affects ankling, which allows you to compensate for a longer crank arm and allows you to use a shorter crank arm more efficiently), and so on. Lots of theory here but it's actual testing and practice that is essential.

Re: Shorter crank experiences. Then back to longer ones!

Posted: Fri Dec 15, 2017 4:47 pm
by Weenie

Re: Shorter crank experiences. Then back to longer ones!

Posted: Fri Dec 15, 2017 5:29 pm
by russianbear
I'm 192cm tall so bikes my size always came with 175mm cranks. Except for some reason my crappy single speed CX bike (my guess is they specced 170 on all sizes to save $$). After riding that around town I became kind of interested in trying a shorter crank on my race bike. I haven't gone back since. I have a fairly aggressive set up so for me the shorter cranks solve some practical problems: my knee hitting me in the chest when I'm low on the bike and pedaling cornering clearance - I can pedal through corners easier or be on the gas earlier in a crit.

Some other observations I have made since: it's easier to engage your hamstrings with the shorter cranks (not sure if this is symptom or a cause); my calves get a little more sore than before; I never get hip flexor soreness anymore with the shorter cranks; accelerations from high cadence and in general are a little faster and easier - i'm an awful sprinter, but this helps during surges in the race more than anything. I kept my gearing the same and only really regret it during our annual hillclimb. Cadence wise, i'm fairly comfortable with anything between 60-130 depending on situation (well as comfortable as grinding 60rpm is anyway hah), BUT i have noticed i'm spinning maybe 5rpm more than before. Again i'm not sure if this is a symptom of shorter cranks or just something i'm doing subconsciously because I know the shorter cranks are on. I've had knee pain on 175mm cranks before (but weirdly only with Look cleats) and i think this was mostly from overuse - too much too fast kind of thing during training.

As everyone says, best thing is to try it! I think shorter cranks can be treated as a fitting tool to solve some pain or practical issues on the bike.

Re: Shorter crank experiences. Then back to longer ones!

Posted: Fri Dec 15, 2017 7:32 pm
by wingguy
AJS914 wrote:Depends on whether you are talking radius or diameter and which number you think is more important.
I've never met anyone who only kept their foot on the pedal for half the rotation so yeah, I'm sticking with 5mm :wink:

Re: Shorter crank experiences. Then back to longer ones!

Posted: Fri Dec 15, 2017 8:47 pm
by stormur
wingguy wrote:
stormur wrote: 1,429% is the difference in length between 175 and 172,5 mm crank arm length. Some want me/you to believe that such difference makes such sensation.... Well ;)
The difference between 170 and 172.5mm cranks is 5mm. Go change your saddle height by 5mm and I’ll guarantee you notice the difference.
Nope; difference between 170 and 172,5 is 2.5 , not 5. Math for 7y old ;) . Difference in radius at the center pedal mount will be 5mm. Still same 1.43% of diameter of the circle. 5 from 350 for 175 crank.

If 2.5mm higher feet position makes to someone breath issue.... Does it , really ? TWO AND A HALF OF MILIMETER ??? Come on.... :)

Somehow I can't see anyones saddle height measurements done from top of the pedal, not even from pedal/ arm bolt center. Always from the center of the bottom bracket... regardless pedals, shoes, insoles.... what can make more than 5 mm difference...

And no, 5mm of saddle height is virtually non noticeable for me. Different bibs have different pad thicknesses, different saddles ( and position on it ) , I'm using road and mtb pedals ... have same saddle height for all setups. And for all type of shoes. I'll bet difference is more than 5mm... to not mention measuring accuracy for different saddle profiles. I can ask also, are all scales "certified" ? ;) again : saddle height 755mm + crank arm 175mm = 930mm / 5mm is equal to 0.54% of length,,,,,

I'll bet in blind test literally no one using 172.5 would notice change for 175.

In last two years I used 172,5, 170 and 175 cranks ( 182 / inseam 84 ) ; only difefrence I noticed between shortest and longest was reduced levarage forcing to use higher cadence. That's all. And about knee and its pain - after 4 reconstructions and few years between hospitals and physiotherapist - I know ... everything.

What cause knee pain is its position against center of motion ( much more complicated, connected to hip position, angles... ) , not 1,4% difference in leverage arm.

Disclaimer : for some ( mostly pro's ) using same few bikes any chang ein setup will be immediately noticable. No doubts. For the rest....

Re: Shorter crank experiences. Then back to longer ones!

Posted: Fri Dec 15, 2017 8:54 pm
by slowK
Lelandjt wrote:I'm totally happy with the feeling of 175 cranks and experimenting with 165 and 170 on my MTBs has me wure that 175 or 172.5 is the right size for my road bike but....
When I get really low the limitation is my thighs tapping my chest. I could be more comfortable in that low position if they weren't tapping with each pedal stroke or get even lower. Has anyone changed crank length purely because of thigh-chest contact? I already have a longer reach and more forward saddle than "normal" but my saddle-handlebar drop is much greater than normal.
I changed from 170 to 165mm cranks mainly because I'm very short. But I noticed one of the benefits was exactly as you describe.

Re: Shorter crank experiences. Then back to longer ones!

Posted: Fri Dec 15, 2017 10:48 pm
by wingguy
stormur wrote:Nope; difference between 170 and 172,5 is 2.5 , not 5.
I'm sorry, I don't think anyone who is unaware that cranks have two arms is ready for an in-depth discussion of cycling biomechanics. :|
If 2.5mm higher feet position makes to someone breath issue.... Does it , really ? TWO AND A HALF OF MILIMETER ??? Come on.... :)
A) It's not 2.5mm, it's 5mm. Get your 7 year old to teach you some maths.

B) I didn't say anything about breathing issues. In fact the word 'breath' does not appear and is not even alluded to in any post in this thread apart from yours. So what are you talking about? WHO ARE YOU ARGUING AGAINST ??? :lol:
In last two years I used 172,5, 170 and 175 cranks ( 182 / inseam 84 ) ; only difefrence I noticed...
Umm, so... you did notice a difference? Stormur, meet Stormur. You two kids go off and argue between yourselves, yeah? :thumbup:
between shortest and longest was reduced levarage forcing to use higher cadence.
That's interesting. Because, see, although the difference between a 170 and a 175 crank is less than 3%, the smallest ratio jump on a Shimano 11-25 cassette is around 6%, and on an 11-28 it's around 8% (biggest is 12%). So according to your own evidence that 3% change in crank length leads directly to a much bigger change in how you actually ride the bike. Food for thought, perhaps?

Re: Shorter crank experiences. Then back to longer ones!

Posted: Sat Dec 16, 2017 7:37 am
by thewinais
11.4 wrote:
morganb wrote:I went down to 165 then 160 then 155 then 150 from 172.5 after joint problems and general health problems left me almost unable to ride and feel faster on 150-155 than I did on 172.5 even after missing almost two years of riding. My peak sprint power is about 100-150W off but my 5s power is much closer and my peak has been steadily going up. My peak speed seems to be pretty close as I already had a very low position and now can get even lower.
That's going to a real extreme in crank arm length unless you are extraordinarily short. There's a range of crank arm lengths that typically are acceptable to a particular rider, with one or two offering peak performance. Those lengths tend to taper off around 160-165 mm, and that's for riders with shorter femurs. (It's the femur that's the determiner of crank arm suitability, if anything is. Your calf is just there for the ride, while vertical displacement is driven pretty much entirely by the femur. It's why fitters who are suggesting different crank arm lengths aren't doing their clients any favors when they are matching crank arm lengths to full leg length or, bizarrely in many cases, to calf length.) You may want to revisit issues such as cleat placement and cleat tilt, because joint issues across that wide a range of crank arm lengths are typically caused by other issues and no crank arm length will do it for you.
I've gone from 170 to 172.5 to 175 and by far 175 is a good all round crank. Get your set up right let the placebo affect die and with some tweaking you'll eat up the road with high cadence , sprints , hills a ease dont get caught up in all the shit they feed is ( company's ) most changes with tweaking take at least 1000kImage

Sent from my SM-G850Y using Tapatalk

Re: Shorter crank experiences. Then back to longer ones!

Posted: Sat Dec 16, 2017 8:54 am
by zefs
I've tried 172.5 then 175 and now 165. I have a femur injury and the 165 cranks eliminated the pain I was getting after rides. I have 82cm inseam and my femur is long, so I want to go to 170 or 172.5 next time, not sure.

Re: Shorter crank experiences. Then back to longer ones!

Posted: Sat Dec 16, 2017 6:29 pm
by Rubik
xcnick wrote:
However, for some time I've felt my sprint isn't as powerful as it used to be (put this down to getting older!) and when trying to ride at tempo or a sustained effort I just can't seem to push the pedals with force properly. Also my calves often cramped during a sprint which I never had before. I read a comment on a forum where a guy said if he went too short on the cranks he got that 'shoes tied together' feeling and I couldn't put it better myself!

Anyway, I stuck my 170's back on, lowered the saddle a tad and instantly I feel like I can push the pedals better. Sprint is better. I notice my thigh closer to my chest again but somehow I feel more stable and tend to bend my arms more to relax on the bike.
I'm 5'11 and run 165mm cranks. A 5mm crank length difference isn't affecting your power output in any measurable way.

That's all psychological.

Fit is another aspect, though, and whatever improves that is a plus.

Re: Shorter crank experiences. Then back to longer ones!

Posted: Sat Dec 16, 2017 6:34 pm
by Rubik
Lelandjt wrote: When I get really low the limitation is my thighs tapping my chest. I could be more comfortable in that low position if they weren't tapping with each pedal stroke or get even lower. Has anyone changed crank length purely because of thigh-chest contact?
That's exactly why I went from 172.5 to 165. Am able to get significantly lower and hold it much easier, now.

Re: Shorter crank experiences. Then back to longer ones!

Posted: Sat Dec 16, 2017 7:15 pm
by ohhyeok90
if crank arm length go 2.5mm shorter, you should up saddle height 2.5mm. so yes, it could 5mm differ at some point.

Re: Shorter crank experiences. Then back to longer ones!

Posted: Mon Dec 18, 2017 2:03 am
by 11.4
stormur wrote:
wingguy wrote:
stormur wrote: 1,429% is the difference in length between 175 and 172,5 mm crank arm length. Some want me/you to believe that such difference makes such sensation.... Well ;)
The difference between 170 and 172.5mm cranks is 5mm. Go change your saddle height by 5mm and I’ll guarantee you notice the difference.
Nope; difference between 170 and 172,5 is 2.5 , not 5. Math for 7y old ;) . Difference in radius at the center pedal mount will be 5mm. Still same 1.43% of diameter of the circle. 5 from 350 for 175 crank.

If 2.5mm higher feet position makes to someone breath issue.... Does it , really ? TWO AND A HALF OF MILIMETER ??? Come on.... :)

Somehow I can't see anyones saddle height measurements done from top of the pedal, not even from pedal/ arm bolt center. Always from the center of the bottom bracket... regardless pedals, shoes, insoles.... what can make more than 5 mm difference...

And no, 5mm of saddle height is virtually non noticeable for me. Different bibs have different pad thicknesses, different saddles ( and position on it ) , I'm using road and mtb pedals ... have same saddle height for all setups. And for all type of shoes. I'll bet difference is more than 5mm... to not mention measuring accuracy for different saddle profiles. I can ask also, are all scales "certified" ? ;) again : saddle height 755mm + crank arm 175mm = 930mm / 5mm is equal to 0.54% of length,,,,,

I'll bet in blind test literally no one using 172.5 would notice change for 175.

In last two years I used 172,5, 170 and 175 cranks ( 182 / inseam 84 ) ; only difefrence I noticed between shortest and longest was reduced levarage forcing to use higher cadence. That's all. And about knee and its pain - after 4 reconstructions and few years between hospitals and physiotherapist - I know ... everything.

What cause knee pain is its position against center of motion ( much more complicated, connected to hip position, angles... ) , not 1,4% difference in leverage arm.

Disclaimer : for some ( mostly pro's ) using same few bikes any chang ein setup will be immediately noticable. No doubts. For the rest....
You might actually get your math right and get the terminology right. What you describe, respectfully, isn't making a lot of sense and several people are trying to explain that to you.