Crank length - just what difference does it really make??

Kirky

Hi all, crank length question for you - does it REALLY matter, and does 2.5mm (!!) really make that much of a difference??

I'm just a shade over 6 foot with a 33" inside leg (and 80kg - if that matters??), and I ride 175mm cranks on everything. But I'm changing my bikes over the winter and this has made me question the need for 175mm cranks instead of the more common 172.5mm cranks. I'm not sure why I currently ride 175's (other than I think I was told to by an old timer in the club some years ago!!) and I do wonder why I always tend to spend more and search longer to find the 175 cranks I want!!

Does it make a difference if I TT on 175's and have 172.5's on my road bike??

In terms of the type of rider I am (not sure if this makes a difference) I'm not a big gear man - in a TT my average cadence is around 102rpm.

Would appreciate some advice and thoughts to steer my decisions on new kit purchases over the winter months.

KIRKY

Gussio

You might find this, and the associated links, interesting: http://www.nettally.com/palmk/crankset.html

More stuff here: http://www.cptips.com/crnklth.htm

I use 175mm cranks, chosen along with other variables, to enhance the fit of my bike.

LangerDan

Kirky, I'm much the same build as yourself and I have everything from 170s to 180s on my various bikes. I did go through a phase of searching out 175s but it really isn't worth the hassle. Crank length vs. leg lenght IS an important parameter but only to a point. The variation in commercially available crank lengths (typically 170 to 180 mm) is far, far smaller that the variation in leg lengths. The difference between a 50th percentile and 95th percentile European male is about 125mm in height, which would probably account for a crank length difference of 25 - 30mm, if you use a simple multiple.

Someone put it to me this way - if you are the sort of rider who adjusts the saddle height to account for wear in cleats and whether or not you are wearing a pair of shorts under your bibtights, then a variation of 2.5 mm might be important. Otherwise don't bother.

iainf72

This article covers most of your questions.

dennisn

You will probably be able to spin a bit faster on shorter cranks if that's your wish, but
at 102 on 175 cranks you sound like you have a really good spin going already.
Shorter cranks are a bit easier on the knees because they don't have to bend guite
as much. I haven't found much in the way of literature and / or testing on this subject.
As I understand it shorter cranks are mainly for high RPM spinners and longer ones for
low RPM pounders. It's a matter of how you like to pedal.


Dennis Noward

maddog 2

yes.

Ste_S

No.

I have 170mm on one bike and 175mm on another. Doesn't make a perceivable difference for me.

Kirky

maddog 2 wrote:

yes.

?? Any chance you could expand on your answer?? Everything else seems to point me in the direction of 172.5mm cranks - and maybe stick to 175mm ones on my TT bike.

KIRKY

Monty Dog

Many track riders use 165mm cranks so they don't ground on the banking - so height isn't necessarily too great a factor. I have both 165mm and 170mm on my bikes - the only noticeable difference being on the track bike on rollers / turbo. I can get to over 220rpm on the 165s but struggle to get beyond 200rpm on 170's - but anything over 180rpm on the road is scary fast, particularly when decelerating as the bike jumps all over the place!

ScubaD

FWIW,

I am purchasing a new bike and was right between frame sizes. The 56cm frame is shipped with 172.5 cranks and the 58cm comes with 175.

I test rode both for about 4 hours each. I did notice a difference between the cranks. The 172.5 felt that I can spin faster but I lost leverage when sprinting up short hills and in the sprints. On one group ride, I lost the sprint that I normally win. I'm blaming this on the cranks versus my own weaknesses. That's my story and I'm sticking to it.

Most people I don't think it matters. It's personal preference. I felt a difference between them and I liked the 175's better. Try them both out on the same bike if possible and decide.

ScubaD

maddog 2

as said,

shorter = easier to spin, bit less leverage

longer = more leverage, bit less spin

Some people can't tell the difference. That's fine. I can tell within 20 yards.

Get the one that suits your riding, and your size etc. Experiment. Make your own mind up.

I use 172.5s. !75 are too long and 170s are too short.