brasch wrote:
Of course the aero is relevant with crank length as much as anything. But it’s to stay within the context of things. If clearance between knee and chest/belly is an issue getting into your position, then yes, it’s relevant to address. If it isn’t and you Can still put out the power, then you’ll probably be less aero, as you change the position up and nothing else and then crank length is
probably not something you should address.....
From an aerodynamics perspective - no. I agree.
However aerodynamics is not the only, or even the main reason, that I think it's worth considering a change to crank length. I reduced the length of my tri bike cranks over 2 years ago. The main reason was not aerodynamics, although it has aided me a little there too. The primary reason was to open the knee angle at the top of the stroke as I felt it would mitigate long persisting knee ache that I've had on long rides. I was delighted to find that it did indeed reduce this problem dramatically. It also seems to have improved my power figures, at most durations except perhaps a short all-out sprint, although it's hard to be sure. It would be rather difficult to figure out if the cranks are responsible or to identify the mechanism of their contribution. My suspicion based on how it feels to ride the tri bike compared to my recollection with the longer cranks and my road bike which is still equipped with longer cranks, is that the dead spot at the top of the stoke is reduced and I'm applying useful force for a larger proportion of the stroke, but that's very subjective so I'm not going to claim it as fact.
brasch wrote:
....Maybe there are flexibilty issues than should be worked on instead?
But it’s hard to say, as bike/rider integration is hard to predict, so the higher position might be the faster.
But
changing you crankset to a shorter one, just for the sake of doing it because it’s “better” - don’t....
It's probably not worth spending money on shorter, or longer, cranks unless you have some reason to think they'll suit you better. But similarly there's no reason to think the size that came on your bike is likely to be best. Crank length doesn't seem to be hugely critical for most people, but the size supplied stock on bikes is pretty arbitrary, so there's equally no reason NOT to change, besides cost and convenience.
brasch wrote:
...As for leg speed, that’s offset by the lack of leverage, so I dont buy that argument. You’re effectively “gearing up” with a shorter crank, But not really. In the sense that in the same gear, at the same cadence, you’ll ride at the same speed, But you’ll have to
put out more torque to keep that cadence. However, you should be able to ride more comfortably at a higher cadence in a lower gear. But that’s a preferrence as Well. It’s say, go for the longest crank arm, where you’re comfortable with your position
That's not technically correct. If you stick to the same gear with a shorter crank, you don't need more torque. You do need more pedal force (at least on average).
Power = Torque * Rotational Velocity (i.e. Cadence)
Torque = Force * Radius (i.e. Crank Length)
So, since power is proportional to speed (assuming unchanged aerodynamic, rolling resistance and drive train losses), we can say power is fixed for equal performance. For equal output, we simply need the product of average pedal Force, Crank length and Cadence to remain constant.
Force_long * Crank length_long * Cadence_long = Force_short * Crank length_short * Cadence_short
you can already change the balance between force and cadence just by changing gear, no need to change crank length. Some prefer to use a higher cadence and lower pedal force, others prefer lower cadence and higher pedal force. However without changing the crank length, the product of these and the path of the pedal remains unchanged. The length of crank that best suits you from a power production perspective would therefore seem to depend largely on pedal speed, and stroke path preferences, not cadence, leverage, torque....etc. If you haven't enough "leverage" you just need more gears in my opinion. Incidentally, I tend to be a "grinder" compared to many. I tend to cruise at around 80rpm and like to climb at about 70rpm. I've reached 150rpm+ during brief sprints but typically the only time I'll stay over 90rpm is a short time trial at or above FTP. I've seen it said that shorter cranks are for people that like high cadences. I like shorter than typical cranks (160mm) and my cadence has not increased since I changed, nor has my power dropped. I think I apply force more effectively around more of the stroke rather than now using a higher peak pedal force. But again, that's an educated but still somewhat subjective best guess, not a fact.
