I want to use a bit slower cadence in ERG workouts due to my really long cranks. I ride a Clydesdale Draft with 210 mm cranks, (I’m 6’6"), which fits me perfectly. This results in excessive actual pedal speeds at 100 rpm, too much for my 70 year old knees.
I know that I can just ride under the recommended rpm and ignore the “Spin Faster” prompts, but is there a way to tell Zwift that I require a slower target cadence, due to my personal geometry?
Thanks - JIm.
The short answer is no.
You can however ignore the cadence prompts as it’ll not affect your workout stars.
210mm cranks? 
Jim, the idea that 210 mm fits you perfectly is an “old school” nonsense. Long cranks rarely bring any advantage. In fact, they are more likely to increase load on your knees, and they indeed will have negative impact on your cadence. Higher cadence means less force which is equal to lesser load on your knees.
I am 6’4" and after experimenting with crank length (very easy on Kickr Bike, you just move pedals into other pair of holes), I moved from 175 mm cranks to 170 mm cranks, both on the trainer and on my street bike. It feels so much better and smoother!
210 mm would kill everybody’s knees. Where did you find such obscene length? Is this from, like, 25 years ago? I never heard it exists in real life.
It’s part of the ethos of Jim’s bike builder. More info at: https://bikeclydesdale.com/draft/
They do cranks going up to 220 mm. I run 160 mm on my XS frame, so cannot imagine riding such a mastodon, but there has to be a market or they wouldn’t sell.
Long cranks will only result in more force if the cadence drops more than in inverse proportion to crank length, ie to 170/210 x 90 = 70rpm (in comparison to 90rpm on 170 cranks).
They will result in more knee bend but perhaps only to a degree commensurate with shorter riders on shorter cranks.
I used to ride 185 but have mostly given up as they are too hard to source. I think I could benefit from longer than the standard 175, the limited range of leg motion is quite obvious for someone of my height and is contrary to everything I’ve always been taught in all sports other than cycling (which is to basically make use of the full comfortable range of motion).
Basically once you accept the simple principle that very small riders (children) should use smaller cranks then it’s pretty ridiculous to claim that 175 must be optimal for everyone from 160cm up to 200cm in height. Might as well say a 52cm frame and size 7 shoes are right for everyone too.
You need to find what works best for you, but one should keep in mind that modern cycling “best known method” favors faster cadence over wider range of motion. The same principle possibly applies to runners - sprinters, who do not run with very large steps; they use smaller steps to be able to move their feet faster and aim for the most efficient combination. Full range of motion makes most sense when one needs to do a single motion with maximum control and power - throw of a ball, swing a racket. Smaller range of motion is beneficial when one needs to generate a steady flow of power over extended periods of time.
Long cranks favor low cadence and a lot of force generated by legs. The idea probably started at a time when bikes were single speed or triple speed; longer cranks meant longer lever and were helpful on climbs when the rider was out of gears. Yet, it became clear that legs loaded with high force efforts get tired quickly and lack endurance, and it is harder to reach and sustain maximum power with low cadence. Therefore, the sport moved to high cadence which moves part of the load from leg muscles to cardiovascular system and lungs, which can sustain longer loads without as much fatigue. Along this this, shorter cranks became preferred.
There is a good video on youtube from a smart bike fitter who explains the myths and misconceptions of the crank length, I will try to find it in the evening. He shows that there is no downside to shorter cranks (within reason). BTW, compared to the length of your legs, difference beween 170, 175, or even 210 mm is not that large. It is like running with large steps or a little shorter steps. At school, I had a tendency to run with large steps and because of that always was the worst runner in my class.
Long cranks are absolutely an advantage for standing starts, it has been proven.
Max power, a pretty good study I read found 20% of leg length.
You can spin faster with shorter cranks - to a point. There’s a window within which there’s not much change.
For many people, they can get into a better aero position on a TT/Tri bike with shorter cranks.
There are a bunch of considerations.
I’m a long-legged 6’0", and I’ve used cranks from 170 to 185 on different bikes for different purposes. Currently dropped the 185s. Honestly - they all work.
Longer cranks = higher torque. For the same gear and everything else being equal, Torque = length of lever * force.
This explains standing starts. You need high torque (not just power!) to accelerate quickly. We know it from the automotive world.
If we do not have a long lever, the only way to increase torque is to go to a lower gear and work our way up from there, as bike accelerates. Switching gears might cost a little bit of time, for those for whom it matters.
But we need high power to roll steadily up the hill or roll against wind resistance on the flats. It is important because we spend most of our time on the bike doing just that. This is where one has to find the right balance. Car engines generate peak power at relatively high RPMs. As it turns out, human engines also best at generating steady sustained power at relatively high RPMs. There is no fixed prescribed cadence, but, as far as I read, for most people the range is around 80 to 90 rpms or so.
If one just cannot into that range, e.g., cannot spin up to at least 90, it could indicate that cranks are too long. If one can spin with the target cadence, length of cranks, plus minus a centimeter or two, is a matter of bike fit and personal preference. It affects fore-aft saddle position required for a good balance, and indeed affects how much one can bend forward before legs start hitting the chest (or reach the limit how far up they can comfortably go, in older folks).
Regarding the video - I did not keep the bookmark, but it could be one of the better videos on this subject that I found because I remember it, and I like Cam Nicholls, he is smart and knowedgeable.
Jim here (originator) - I apparently started quite the conversion, certainly crank length is one of those topics, like carbon vs. ti, or tire width, that will be forever controversial. Anyway, thanks for all the very interesting comments, and here are a few points in response -
I can’t say even anecdotally if the long cranks give me more power, I would love to find a way to test that empirically against my old bike with 175 cranks. The bikes have way different tech (9speed triple vs. 11 speed Di2 etc), but I can check power on my trainer. So maybe if I tested both bikes at the same steady heart rate for 5 minutes (and in the same gear??) it might show a difference in watts. Would that work? Any suggestions?
Jim - If you can try both bikes with your trainer, by all means, do it and share what you found for yourself with us. My only recommendation is to ride for at least a few hours using your old bike before doing the test because you need to get used to the different crank length and a higher cadence available to you with shorter cranks. Otherwise, it will not be a fair comparison. In theory, you should expect to see no or very little change in power, but a big change in cadence. The problem which I foresee is that you got used to pedal slower, but with high torque, with your long cranks. Switching to a higher cadence with shorter cranks requires some work, maybe even some cadence drills.