Crank arm sizing

Mickyg88

I am looking to change my non series shimano crank which is 170mm for a 105 or ultegra compact chainset, but looking on eBay etc the usual offer is for a 172.5 minimum, unless I'm prepared to pay full price, which I don't really want to do.
If I go for the 172.5 what are the implications, at the moment I'm totally comfortable on my bike, but will I have to start changing the seat height, which I suppose means my knee will be higher on the upstroke too, is it worth it or should I keep looking for a 170mm.

Danny87

Now i suppose it would not be unreasonable to think 'It's a quarter of a cm difference which sounds like nothing'.

But I recently read Graham Obree's new e-book (highly recommend it btw) and I learnt more than I previously knew about the importance of crank length (amongst other things).

Basically he says to calculate your correct crank length a good estimate is to take 9.5% of your total height. I am 5"8 which is 172.5cm. 9.5% of that is 16.38cm or 163.8mm which means rounded up I'd say 165mm is best fit for me.

Anyway won't give away any more secrets from the book but have a stab with the old calculator and see what you come up with.

DavidJB

Or just move your saddle 2.5 higher.

displacedaussie

DavidJB wrote:

Or just move your saddle 2.5 higher.

If you get longer cranks you may need to drop the height of your saddle, not raise it. Think about the bottom of the pedal stroke when the cranks are longer than they were!

You _probably_ won't notice any difference if you move from 170 to 172.5mm cranks

smidsy

But if you drop the saddle to compensate on the down stroke it makes things more crunched on the top stroke so it's not that easy.

on-yer-bike

Danny87 wrote:

Now i suppose it would not be unreasonable to think 'It's a quarter of a cm difference which sounds like nothing'.

But I recently read Graham Obree's new e-book (highly recommend it btw) and I learnt more than I previously knew about the importance of crank length (amongst other things).

Basically he says to calculate your correct crank length a good estimate is to take 9.5% of your total height. I am 5"8 which is 172.5cm. 9.5% of that is 16.38cm or 163.8mm which means rounded up I'd say 165mm is best fit for me.

Anyway won't give away any more secrets from the book but have a stab with the old calculator and see what you come up with.

Interesting. I bet that method puts most of us on shorter cranks than we're currently using. Puts me on 170 and Im on 172.5. The Retul bike fitter said this was my size.

cougie

I've ridden 167.5 up to 175. TBH I didnt notice any difference.

Monty Dog

smidsy wrote:

But if you drop the saddle to compensate on the down stroke it makes things more crunched on the top stroke so it's not that easy.

Bike fit 101 fail! The power is on the downstroke and therefore saddle position needs to be optimised for the extended leg

sandyballs

I had a bike fit with AdrainTimmis and he spoke about a recent study on crank length. various cyclists were tested on routes with various lengths and whether they affected the body or performance and the results never indicated any difference in performance and the riders never noticed any diserable difference in stroke or effort. I would not change to the extremes but 1 size up or down will make very little change IMO.

Mickyg88

Ok guys thanks for the positive response, I think I will go for the 172.5 unless a 170 comes up in the next week, a couple of good priced 172.5 ultegra are on offer.

FransJacques

Why go for 172.5? Why not go for 175s or 180s or 165s? How do you know you need 172.5 is the point I'm making.

Why not do the proper thing and post your inseam length (at least) so we can have a conversation about what makes sense and why. I love Obree but he is a bit of a quack and I wouldn't take his advice on anything to do with bike fit. He also suggest to base shoe size on your hand size.

How much reading have you done on the subject?

If you've not read it yet you owe it to yourself to check out Leonard Zinn's article on proportional crank length. Now Zinn is also rather extreme but not as much as Obree b/c Zinn is a frame builder and a very tall dude so has done a lot of studies. His formula will defo suggest something on the longer side (with a 89cm inseam I should ride 188s!) but you need to read up on this and have a better approach than just "add 2.5 mms".

DavidJB

displacedaussie wrote:

DavidJB wrote:

Or just move your saddle 2.5 higher.

If you get longer cranks you may need to drop the height of your saddle, not raise it. Think about the bottom of the pedal stroke when the cranks are longer than they were!

You _probably_ won't notice any difference if you move from 170 to 172.5mm cranks

Yeah sorry I miss-read...I'm going from 172.5mm cranks to 170mm cranks (2nd hand purchase) Spoke to 2 bike shop owners and my coach who all said it makes no difference just move your saddle to compensate.

smidsy

If it actually made no difference they would only make cranks in one length.

Now it might not matter for 2.5mm but it does matter