175mm rather than 172.5mm cranks ... will I notice the difference?

C3PO

LollipopJimmy wrote: »

Chatting in a LBS and they've said it's near impossible to source Shimano parts at the moment. As for the question in the OP... I don't think it makes a massive difference unless you're looking for marginal gains

Yep - my LBS told me to source what I can and he’d fit it for me as he can get nothing! Ordered a SRAM Force groupset today rather than wait for 4-5 months for Shimano. 2 weeks delivery for 170mm .... 14-15 weeks for 172.5!

fat bloke

I have two mint cranks for sale for ages and no interest whatsoever in them!

theunforgiven

My good bike has 170mm cranks and I upgraded my winter bike about a year and a half ago to 105 but all I could get were 172.5mm cranks, (was a great deal from Merlin) and to be honest I can't tell any difference.
I'm a short arse, 5"6, and the 2.5mm is not noticeable.
You should be fine.

C3PO

fat bloke wrote: »

I have two mint cranks for sale for ages and no interest whatsoever in them!

Brand new build so keen to use new parts ... while since I’ve had a new bike!

grogi

C3PO wrote: »

Thanks guys - any research I've done seems to suggest that cranks that are too long are more likely to cause issues than shorter ones so I might be better going with 170mm?

Yes, it's safer to go shorter than longer.

There is hardly any downside to too short cranks, while too long cause many issues. I currently rife 170mm and I'm 194cm...

grogi

dom_1000 wrote: »

Longer cranks and gearing can both positively affect climbing.
Hip angle is negligible unless you are going for an extreme aero position.

The difference in angles is certainly not negligible, especially in the knee angle.

cletus

I've been looking at some studies and websites discussing crank length, hip and knee angle etc.

None of them discuss length changes of 2.5mm. the norm seems to be 7.5 to 10mm. Last page I looked at suggested that a 7.5mm reduction could reduce maximum knee flexion by 3-4°, and if the saddle is raised by the same amount, another 3-4° can be gained, leading to less pain at the knee, for a total of between 6 and 8°

If we assume that the correlation of figures is linear (which would give the 'best' possible score for a 2.5mm crank difference), shortening your crank by 2.5mm would gain you between 1° and 1.3°. Combining this with a 2.5mm saddle height adjustment would double this for a 2-2.6° reduction in flexion of the knee.

I would strongly argue that it would be next to impossible to notice a difference in knee flexion of between 1 and 2.6°

Lumen

The video I posted near the beginning of this thread deals with the dynamic effects. It's not all about angles.

I get the impression almost nobody has watched it. :pac:

velo.2010

Lumen wrote: »

The video I posted near the beginning of this thread deals with the dynamic effects. It's not all about angles.

I get the impression almost nobody has watched it. :pac:

I did! Even downloaded a free CAD package from the internet.;)

Eamonnator

Lumen wrote: »

The video I posted near the beginning of this thread deals with the dynamic effects. It's not all about angles.

I get the impression almost nobody has watched it. :pac:

I did and have been worrying about it ever since.

cletus

Lumen wrote: »

The video I posted near the beginning of this thread deals with the dynamic effects. It's not all about angles.

I get the impression almost nobody has watched it. :pac:

I watched it, but didn't refer to it in the last post I made, because it doesn't support the idea that a 2.5mm change in crank length would be noticeable.

It's an interesting video, but the crank length change he makes to his 4 bar linkage is 15mm, from 175 to 160.

This change causes a reduction in hip flexion of 2°. I didn't bother downloading a free cad package, but we can extrapolate these figures like I did above.

2.5mm decrease in crank length gives a 0.167° reduction of hip flexion, albeit for his particular setup. You'd have to put your own numbers into the cad drawing to see how inconsequential it would be for you

Lumen is right to point out that there is much more going on than just crank length, but a lot of the time you'd need to be an actual engineer to work it out.

Having said that, nothing so far presented, or that I've read, convinces me that anyone other than maybe the top riders in the world, could detect, in a double blind test, a crank length difference of 2.5mm

Lumen

velo.2010 wrote: »

I did! Even downloaded a free CAD package from the internet.;)

Bravo! You win the internet.

Eamonnator wrote: »

I did and have been worrying about it ever since.

Me too, and I was hoping to inflict that misery on as many people as possible. :pac:

cletus wrote: »

that there is much more going on than just crank length, but a lot of the time you'd need to be an actual engineer to work it out.

That video is interesting to me because he focuses on the kinematics, which is (I think) exactly where the good bike fitters and physios focus also.

Pedalling dynamics has been a particular concern for me recently as my return to cycling has seen a steady ramp up in intensity accompanied by various niggles around the knee, and I'm trying to figure out some combination of strength & conditioning, bike/cleat fit and intensity to fix them.

As I sit on the turbo for interminable hours I look down at my knees and think "are they tracking properly?". And where I detect a bit of a wobble is at the top of the stroke.

Obviously it is possible to fit a bike well with various different crank lengths, but if by choosing shorter cranks I could (for instance) get another 10rpm in cadence without my form going to crap (and risking attendant injury) that would be worth it, if I was in a position to buy various lengths and so the choice didn't cost anything.

FWIW I'm 183cm with short legs and use 172.5mm cranks, and I have no particular desire to spend a load of money on shorter cranks, but I think it's an interesting area of discussion nonethless, and I very definitely would not go longer in order to "get more leverage".

Large bottle small glass

Eamonnator wrote: »

I did and have been worrying about it ever since.

You'll be grand, to quote a rather hardy Australian miner I met one time "if you don't wear out you'll rust out"

CramCycle

Long story short, saddle height is of far more concern than crank length for 99.9% of posters here, if you have a choice, go shorter but if you have to adjust your saddle height to accommodation your cranks, it most likely means your seat post was at the wrong height regardless.

sbs2010

I think the video guy has made a couple of errors in his analysis,

He mentions ankle angle but says it's pretty much constant for most people and he sets it as 90 in his CAD. I don't think that's right at all. Ankle angle changes all the way through the pedal stroke. >90 at the down stroke and <90 at the up stroke. There's no one cycling with a constant 90 deg. shin to foot angle.
And by my reckoning it only takes 0.5deg difference in ankle angle at the upstroke to allow you keep the same hip and knee angles if you change your crank by 2.5mm

And secondly, he has the ankle pivot in the same plane as the pedal axle. But your ankle is maybe 8-10cm above your heel. Thats got to make a difference to all the angles he's worked out.

So I reckon it makes damn all difference, except to give you maybe a tiny bit more leverage, essentially you're riding a light gear if you've longer cranks. But I have 177.5 cranks so what would I know

CramCycle

There are studies out there that show differences that don't go into the reasons why, just what the hard data says. Long story short, evidence indicates that shorter is better in regards the lengths supplied by most manufacturers but that difference is small, so small in fact that most of us will never notice. Also, as said before, a saddle height being off by the same difference as a crank length change would make a far greater difference (+/-).

Eamonnator

CramCycle wrote: »

Long story short, saddle height is of far more concern than crank length for 99.9% of posters here, if you have a choice, go shorter but if you have to adjust your saddle height to accommodation your cranks, it most likely means your seat post was at the wrong height regardless.

I'm gonna go with this.
I'm 186cms with long legs. I have 175mm cranks on all 4 of my bikes. I have been using 175mm cranks since records began about 35 years ago.
I'm not going to switch at this stage and I'm going to try and stop thinking about it.

grogi

CramCycle wrote: »

There are studies out there that show differences that don't go into the reasons why, just what the hard data says. Long story short, evidence indicates that shorter is better in regards the lengths supplied by most manufacturers but that difference is small, so small in fact that most of us will never notice. Also, as said before, a saddle height being off by the same difference as a crank length change would make a far greater difference (+/-).

One won't notice performance difference between 2,5mm or even 5,0mm. I didn't see feel any difference when I jumped from 175mm to 170mm.

However, even 2,5mm might make the difference for injuries - I don't get knee pains anymore.

harringtonp

Lumen wrote: »

Bravo! You win the internet.



Me too, and I was hoping to inflict that misery on as many people as possible. :pac:



That video is interesting to me because he focuses on the kinematics, which is (I think) exactly where the good bike fitters and physios focus also.

Pedalling dynamics has been a particular concern for me recently as my return to cycling has seen a steady ramp up in intensity accompanied by various niggles around the knee, and I'm trying to figure out some combination of strength & conditioning, bike/cleat fit and intensity to fix them.

As I sit on the turbo for interminable hours I look down at my knees and think "are they tracking properly?". And where I detect a bit of a wobble is at the top of the stroke.

Obviously it is possible to fit a bike well with various different crank lengths, but if by choosing shorter cranks I could (for instance) get another 10rpm in cadence without my form going to crap (and risking attendant injury) that would be worth it, if I was in a position to buy various lengths and so the choice didn't cost anything.

FWIW I'm 183cm with short legs and use 172.5mm cranks, and I have no particular desire to spend a load of money on shorter cranks, but I think it's an interesting area of discussion nonethless, and I very definitely would not go longer in order to "get more leverage".

Welcome back to boards. Was wondering where you had been.