This is everything I have learned about fitting crank length after over 1000 attempts.
1. The only way to intuitively understand the benefits of shorter cranks is to have a competent fitter utilizing proper equipment take you to the limit of your current crank length before making a change. In other words, if your fit is not at least in the ballpark of optimized, you probably won't notice much, or even worse, you might have a different experience than if you were close to optimal.
2. When done as described above, about 80-90% of riders are going to prefer a crank length from .5 to 1.5 cm shorter than what would typically come on a stock bike in their size.
3. Many of the things previously mentioned are important, especially riding a lower / more aero position, but the first thing the fitter and rider need to determine is "Do you prefer this?". It is the same question asked when seat height, setback or drop is changed.... How does this effect your intuitive sense of pedaling? After which, we can look at the power, how much drop you can ride, etc. First is always rider feedback.
4. There is no perfect crank length, or if there is I don't know how to find it. I do know how to find an acceptable range for a given rider. Acceptable here is synonymous with appropriate, and NOT the same thing as aggressive. The #1 thing I look at is how close to the FIST described 100 degrees of hip angle can we achieve with this rider? Is their lack of mobility or excess belly fat getting in the way of this angle? If we have a lean and reasonable limber rider who just can't seem to pedal effectively when we dip below 105 degrees of hip angle, crank length absolutely becomes the prime suspect.
5. Crank length is often the most important metric I change for riders under 5'6" or so. The majority of these riders are what I term "massively over-cranked". These riders typically reduce crank length from 1-2cm, with the shortest commonly available length of 145mm (Cobb) being frequently used for riders under 5'3".
6. Generally, not one watt is gained or lost from a crank length change. Now if the crank was so long that it was basically contributing to a lousy fit, sure, we will probably find some power, but that is more a function of fixing the overall fit. This leads me into one of the final points...
7. Crank length is not fundamental to your bike fit. Seat height, setback, reach to bars, drop to bars ARE your bike fit. Nail those four and you've got a fit. Saddle selection, aerobar shape/tilt/width, crank length, cleat position etc are all secondary factors. Crank length is probably the most important secondary factor for the majority of riders, right up there with saddle selection. What this means is that if we determine 160mm is in your ideal range, your bike fit still exists on 175mm cranks. Saddle height will change, drop will change, set back will change and even reach may change with the less optimal length, but the fit isn't ruined by the wrong cranks. If cost is an issue, we adjust the saddle, ride a bit less drop and advise to change the cranks when you can.
8. Adaptation time for changes in crank length is about 3-5 minutes. Seriously, it just disappears. Which somewhat addresses the question, ‚ÄúDo I need the same length crank on my road bike.‚ÄĚ Generally, no. For ‚Äėcross training‚Äô triathletes, road bikes tend to be more of a laid back affair. Specifically, we are not trying to ride in our most bent over, reduced thigh to torso positions, as we do on the tri bikes. So it is not usually as big of an issue.
Crank lengths on road bikes have been around for over a century, and generally work well. The problem with crank length arose when we transferred those historical lengths to commensurately sized aerobar bikes, without realizing the inherent difference in those styles of bikes. For those riding a road bike set up low, in a racing position, an exploration of crank length could be useful. Keep in mind the one tangible drawback that I have found with shorter cranks is a reduced ability to perform out of the saddle climbing. This seems to be a situation where one lever (crank length) comes to the forefront, and diminishes our sense of the system of six levers* used in propelling the bike.
*Foot, crank, lower leg, upper leg, gears, spokes.
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