Crank Length Differences?

The "formulas" would say that it basically depends on height, but really it comes down to preference. I use 175s on my road bikes, but 172.5s in my tri bikes (just feels better when layed down low over aero bars), and 170s on my track bike (easier to spin at high cadence).

If you can swing it, try different lengths and see what works best for you.

There's a big advantage to ride the *right* crank length, but no advantage of a longer or shorter... most people tend to ride cranks that are too long, especially shorter people.

The right crank length comes from your femur length and hip flexor flexibility. The more flexible you are, the more closed the hip angle can be, which means that you can tolerate a longer crank and still produce useful power over the top. If you close up the hip angle too much, then you loose the ability to push over the top which results in loss of power and usually a pretty sub-par run... not exactly what a triathlete (or duathlete for that matter) would be looking for...

The trend to shorten crank length among triathletes is to allow you to raise your seat (effectively lowering your front end... or maybe in conjunction with lowering your front end) without closing your hip angle too much (which would result in a loss of power).

In other words, shorter cranks will allow most people to obtain a more aerodynamic position on the bike without much modification to their cadence (a shorter crank will require a very slight increase in cadence with the same gearing).  You will lose a tiny bit of leverage with shorter crank arms, which generally is not an issue when pedaling at normal cadence... however some people report a decrease in climbing efficiency with shorter cranks for this reason.

Most important to remember - changing crank length will change bike fit... you can't just swap on new shorter cranks and not adjust the rest of your fit... unless your fit was bad in the first place (e.g., your hip angle was too acute) and the shorter crank is a correction to that.

Cadence x gear = speed regardless of crank arm length.

You're right, I was sloppy with my metrics.

This quote from this article might be a better explanation:

If you keep or make a crank arm short, an incrementally higher cadence is required to offset the lower torque generated by the shorter lever. But a higher cadence seems not only justified, but required, by timed racing. Alternatively, a longer crank arm is incrementally harder to spin at that higher cadence, all things equal. In other words, there is an elegant confluence with shorter cranks and higher cadences in timed racing; these elements are not in conflict, rather they complement each other

The article, in general, captures my views.  Heck, it probably informed my views as much as anything else.

Had to re-read that to catch what the author's point.

Don't have the references in front of me, but more than one study has shown that riders adapt very readily to small differences in crank length to put out same TT power.  Boils down to personal preference.  I'm 6ft/34" inseam & find no difference between my 172.5 & 175mm cranks.

Yes, but won't your feet have to travel faster and longer distance with a longer crank in order to achieve the same cadence with a shorter crank?

Yes the foot speed will change for a given cadence but a specific cadence and gear will still give the same speed.

There is a change with mechanical advantage as one changes crank length; this means that for a given cadence, the force required will change.  With longer cranks, a rider would apply a smaller force over a larger distance while with shorter cranks, a higher force will be required but over a smaller distance.

Shane

For me, it's largely comfort and laziness based. As in, unless I'm uncomfortable on what is on the bike for some reason, I'm too lazy to change them.

I've had 172.5 on all my tri bikes and road bikes. I have a 172.5 on my 26" MTB; and 175 on my 29er. I had 170s on my track bike, but put on 165s, b/c 170s are just too close to clipping. 165 gives me a bit more clearance.

Hmmm....my new tri bike (Jamis Comet) has a standard gearing with 175mm length.  I have stuby legs (31" inseam).  I'm thinking I'll switch out to a compact with 172.5mm length.

Anyone want to trade a brand new FSA Gossamer standard & 175mm crank with a compact 172?

My friend, about 3 years ago, was writing up a study in favor of longer crank length and its benefits.  However, his focus was narrow, considering crank length can not be the deciding factor with all the other angle measurement adjustments and getting aero/fitted.