Road Cycling - Help on Crank length

Hi,
I would like some advice on my correct crank length, I know all these things should be checked properly, but I am fascinated to see if anyone has some advice on this as I am looking to upgrade my chainset from Tiagra to Ultegra and its difficult to test ride lots of different sets, so I was wondering if there was any rule of thumb to be applied.

My vitals are:
Age 31
Height 5'7"
I ride a 52 cm cannondale
Inseam 31"
Double chainring set up
Ride for personal fitness, solo and in club group rides.
Solo averages approx 18mph for an undulating 40-50 mile ride

I havent got weak legs but I certainly dont have particularly powerful legs either, I am more of a natural spinner. My natural cadence without realising it or consciously working on it is about 95rpm.

170mm or 172.5mm should fit you nicely. if you're more of a spinner, the 170mm would be the better of the two.

I am just an inch shorter than you.. And I ride a 49~50 frame.

I use 170mm cranks.. Those are easier to spin, w/ our short legs, than the 172.5s, 175s...

:)

To totally confuse the issue. My wife is 5'6 max. and rides & demands 175 's. I'm 6' and prefer 172.5's.

So my advise is you gotta ride some different lengths.

An older LBS sometimes has some junker cranks in the back that you can borrow, try keeping an eye out for fellow riders with near same size bikes and check the cranks lengths and beg a ride if they are different that what you have rode.

Hmm. Is 31" your trouser inseam, or your saddle height inseam (i.e. floor to crotch)? If it's floor-to-crotch, stay with the 170s.


I have pretty similar specs in terms of height and trouser inseam, except my bike is a 55cm, measured C-T. I'd always considered myself a masher, but last winter I got a computer with cadence. Turns out my natural cadence is in the 95-100 rpm range with 172.5 cranks. IMO, crank length may be more of a limiter for max rpms.

Do you have any knee issues? The 2.5 mm difference will require you to lower your saddle 1/10", but at the top of the rotation your foot will be 1/10" higher than with 170mm's. More knee bend. If it's a potential problem, don't risk it.

Finally, I found going from 170s to 172.5s a bigger transition than I expected. 2.5mms isn't much, but you sure notice it. It just takes a week or two to get used to.

mr. 1oldroadie, I have 175mm on my MTB and 170mm on my roadbike...

The difference is my riding style.. When MTBing, I like the extrapower the 175s can give me. And I cannot maintain a 90+ cadence when MTBing... oh, I don't want to maintain a 90rpm cadence when I'm MTBing.. :D

But on the roadbike, I tend to spin a lot.. 90+ is normal to me, w/c is untrue with the MTB.

just a thought...

I then to agree with what is being said; higher spin, use 170s. I went from 170s to 172.5 about 5 years ago, but lately I've found it harder to maintain a fast spin. And I use to be able to get a real fast spin going for the sprint. A friend of mine put on some 165s on his fixie and said that his spin was much smoother, but could get enough power for the hills. I'm considering compact cranks with 170 lengths for a project I'm about to start.

Anyone know of a good deal for compact cranks?

Stronglight do a model with quite a small BCD. You can work them as a single, double or triple, down to 28T. Not the stiffest, but they are for commuting rather than racing.