Road Cycling - Crank length question

I'm getting ready to purchase a group set for my new bike and was wondering how much difference will I notice in crank length. I believe I would be looking at a 172.mm or a 175mm. Are they so close I can get either set?

Thanks

The general rule of thumb is:

* below 5'10 use 170mm cranks on for general (road) racing and
training,

* 5'10 to 6'1 use 172.5mm cranks,

* 6'1 plus use 175mm cranks,

And modify for specialized events:

* subtract 2.5mm for track racing (or more on short steep tracks
or short events),

* subtract 2.5 to 5mm if you have frequent knee problems,

* add 2.5mm for time trials and hilly races, (and MTB races),

* add 5mm for pure hillclimbs.

Originally posted by bman
Are they so close I can get either set?

I'm gonna say no.

Here's my nickel's worth:

What size are you riding now, and why are you considering changing? You asked for advice, but didn't give us much to go on. I'm guessing you've measured your inseam, and you land in the grey area, where either might be acceptable. Rules of thumb are that smaller cranks lend themselves more to spinning, larger to mashing. What's you're predilection? Longer cranks give you a little more leverage when climbing, so you can generate a little more torque for the same amount effort. I've never done the math, I don't know if it's enough to matter.

The longer cranks are each about 1/10th of an inch longer. Total difference between top and bottom of the stroke is a little less than 1/4". Doesn't sound like much, but it took me a long time to get used to it. You also have to adjust your seat a little to accomodate a length change (whichever direction). If you have knee problems, a longer crank will force a little more acute knee bend angle... just another thing to keep in mind.

Finding the right crank arm length is determined by your riding style and body style. Smaller crank arms are for the riders that like to use smaller gears and pedal at a higher cadence, while longer crank arms provide you with more leverage to push the larger gears normally used in climbing.

A plug in formula to determine your crank arm length is shown here.

http://www.cranklength.info/cranks.htm

With articles here at these sites.

http://www.cranklength.info/experiment.htm

http://isb.ri.ccf.org/biomch-l/archives/biomch-l-1996-11/00056.html

OK, here is more info:

Right now I'm riding a '99 Allez Sport 56cm and very happy with the fit. Has factory length crank. Not sure on the size, but I can measure it..i imagine it's C/C length.
Yesterday I purchased a new '03 S-Works 56cm frame. So I'm getting ready to purchase a group set and wasn't sure what length I needed on the crank arm. I'm 5ft 11 and have never had knee probs. I'll be doing mainly training at this stage...hills and flats mainly under 60mile rides.

Thanks everyone for the info and sorry for the lack of detail in my question.

Originally posted by bman
Not sure on the size, but I can measure it.

The size should be stamped on the inside of the crank arm. (I would guess it is a 172.5mm).

East Coast, you quote some recomendations for size vs crank length which are commonly seen in fitting guides. As riders change in size between 5'10 and 6'1", they should vary their crank in small increments. Above and below this small height range, there are no changes. Most female riders are below 5'10, and some are below 5'0". Why does a rider 3" taller than 5'10 need to change their crank, whereas one 3" shorter does not?

Cranks are available in sizes from 140 to 190 is you know where to look. For any size rider (or rather, femur) there is a recomended "normal" length, which may be altered to a long or a short depending on the riders requirements.

If you really want to get technical about crank sizing here is a web site that goes into lots of analysis and formularizes the crank length: http://www.nettally.com/palmk/crankset.html

It only makes sense that the total height of a person is not as relevant to crank length as is leg length, or more specifically femur length.