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I’ve been wondering what are the pros and cons of having a shorter crankset.
I’m looking into changing my crank length to a 165mm. I currently have a Shimano FC-RS510 172.5mm upgrading to Shimano 105 FC-R7000 165mm.
Thoughts on this idea? Thanks! I’m riding a Giant Contend AR 1.
There is little research on the impact of crank length on power output in cycling. There are some lab studies, and they seem to show that even for crank lengths significantly shorter or longer than the norm, the metabolic cost to maintain a specified speed or power output in the lab doesn't change. Importantly, some of these studies included lengths ranging from 120-160mm at the short end to 190-220mm at the long end. Sometimes, the very shortest or very longest cranks seem to be more metabolically costly. However, it could be that the human body is fairly adaptable to different crank lengths, and most people can accommodate a given crank length if it's not too far outside their optimal one.
That said, there may be some reason to prefer shorter crank arms. Professional road cyclists seem to be trending in that direction, and I believe that professional triathletes are doing so as well. One stated reason is that, emphasis mine:
According to Trek Precision Fit program manager Matt Gehling, shortening the crankarms and moving the saddle forward means the rider’s legs aren’t coming up as high at the top of the pedal stroke, which then allows riders to drop down lower up front for better aerodynamics, all of which can be done without losing power.
“The big reason the team wanted to try it, and this was at first focused on time trial bikes, was to get their front ends lower,” he said. “A time trial position is the worst thing you can do on a bike for your hip angle, and that is why triathlon bikes come with steeper seat tube angles and shorter cranks; both of these things open the hip up more.
Regardless of the needs of professional athletes, because shorter cranks reduce the range of motion required at the knee and the hip, they might make overuse injuries less likely, as discussed in the first Cyclingtips article I linked. This blog post by Ridefar and this blog post by a bike fitter both make that association, although do note that this is based in their personal experience.
One of the limits of lab studies is that they are a highly controlled scenario, and they only measure things in the short term. Expert opinion posts can be based on longer-term effects, but one of their weaknesses is that now, everything is uncontrolled - a rider's increased power or injury recovery might have happened for a myriad of reasons unconnected to different cranks. Moreover, experts and their clients are also vulnerable to the placebo effect.
One possible harm of shorter cranks the cadence to maintain a certain power is increased. This means that at very high power outputs, some athletes might have their cadence climb out of their optimal range. However, people can attempt to adapt to this, and most people don't absolutely need to maintain very high power outputs regularly. Riders should be aware that to keep all else equal, going to shorter cranks requires raising the saddle and moving it slightly backward, as discussed by Lennard Zinn (albeit he is discussing the opposite scenario).
I'd probably do the following:
Many serious cyclists benefit from periodic bike fits anyway. If you get one and the fitter indicates that you should shorten your cranks, then do that unless you can see a reason not to. Naturally, give yourself time to adapt, but if you have pain as a result, you should revert. Knee and back pain have many causes, but they sometimes result from cranks too long. If your thighs hit your stomach when in the drops, that might be another objective indicator that you'd benefit from shorter cranks.
In the absence of anything objective, there doesn't seem to be a strong reason to get shorter cranks unless you were changing the cranks for another reason, e.g. getting a power meter, getting a new bike, upgrading the whole groupset.
If you're at equipoise between two lengths, then the balance of upsides and downsides seems to indicate you'd favor the shorter cranks.
If you're very short or very tall, then the common range of cranks is centered around the meat of the bell curve. You might want to experiment with shorter or longer cranks as you are able. For example, I'm a 5' 5" male rider, and I've been successful on 170mm cranks, but I did get a deal on a set of 165mm cranks with a power meter, so I am trying them. Conversely, if you're much taller than the norm, e.g. over 6' 6", then you might not want to choose cranks shorter than the norm. In fact, Lennard Zinn makes custom long cranks (and bikes) for very tall riders, and his experience seems to be that very tall cyclists benefit from very long cranks. Nevertheless, height alone should not compel you to immediately switch.
A digression: the Marginal Gains podcast has an episode titled "Granola and Cargo Cults." The upshot is that in the absence of objective data, we tend to do what our opinion leaders do. This led to a bunch of teams at the Tour de France adopting granola for breakfast because someone in the top team was doing that. Amateur athletes should be wary of following professional athletes just because we want to emulate them or to be seen emulating them. They are a lot fitter and more flexible than us mortals, for one. Hence, adopting the slammed stem position is a choice with a lot of downside for many amateur riders. With shorter cranks, on careful consideration, the balance of upsides and downsides does appear favorable.
TLDR; cranklength is like saddles, when you find the right one you'll know, but there are no hard and fast rules about the right length.
Crank length is one of those things in cycling that seems to go in and out of fashion. Back in the dark ages, you road 170mm cranks on the road, 165mm on the track and there weren't many bike racers over 6 ft tall. Then mountain biking came in and cranks all got 5mm longer. ( A longer crank is effectively a slightly lower gear.) Niche crank manufacturers started making 180mm or longer cranks for tall people. All but the smallest size road bikes came with 172.5mm or 175mm cranks.
Shorter cranks came back into fashion during the fixie craze in the early 2000's, but for me at least most current road bikes and all MTB's come with overly long cranks.
My experience has been that cranks over 170mm make my knees hurt. I was pretty adamant about riding only 165mm cranks for a while, but over the last 10 years or so I've come to the conclusion that for me 170mm cranks work just fine. I still have 165mm cranks on my MTB which I like for the slightly greater ground clearance. The arguments relating crank length to leg length and knee and hip angles certainly make a lot of sense, but I haven't seen any that factor in the ratio of thigh to shin length. Zinn's ratio is probably a good place to start.
This is a very contentious subject. There's a formula for determining crank length that Lennard Zinn (who knows a bit about these matters) recommends:
crank length (in mm) = inseam (in cm) × 2.16
Note the shift in units. I'm currently riding 172.5s, but going by this, I should be on 175s, which surprises me. I am not going to rush right out and change them.
However, I've seen a study (which I haven't been able to dig up since) that crank length is ultimately irrelevant, and that people can get accustomed to extremely long or short cranks and be just as efficient with them.
There are reasons other than biomechanical ones for choosing a certain crank length. You might want a shorter crank for the track, or for riding off-road, although some BMX riders—already very low to the ground and on dirt—ride 225-mm cranks.
crank length=0.216*inseam to eliminate the shift in units.