How is that a useful approach? Would you suggest designing a racing car transmission without including the engine characteristics? If the Pro cycling world has "no idea" then they clearly haven't chosen to use an analysis that's so blatantly over-simplified as the one you propose. It can yield very little, if anything, to help towards getting 'the perfect bike'.Let's put the rider out of it for this topic as there are too many variables out of our control. What's your theory. I'm interested because in Pro Cycling world. I think they have no idea....
That is noble enterprise, but you need to do some work:How is it useful? Well. I guess I am trying to start from the basics. Google crank length and I am empty handed in proper information. When I hear that having a shorter crank will require more Nm then a longer crank and changing to a smaller gear will offset that I role my eyes.... because an easier gear means you're going slower.... when Nm is the same. Nm is the amount of force the rider is pushing on the cranks.
I've done courses in bike fits and unless a crank is too long reducing you hip angle flexition too much then I haven't been taught anything from a performance point. Just shorter is better. I'm trying to understand why....
Their arguments from the best bike fitters are. You can have a higher cadence(rpm) but in my opinion rpm is only the rotation number. Rpm doesn't equal to leg speed. Further away from the bottom bracket(pivot point) you increase your leg speed. If two riders have the same leg speed and one has a slower RPM because of longer crank then that rider has less time at 12 o'clock and 6 o'clock times when you produce zero power(not really but you get my point) and have a longer(time) period over the down stroke. If that makes sense.
I really want someone to argue with me so I understand it from a mechanical point of view. That's why I am here.
Better?So.... which is better in your opinion?
The Physics of this question is not a matter of feeling. You seem to be requiring an answer in your terms but the only way you will get a meaningful answer from PF is to learn what the proper terms mean. Yes - it's an annoyingly exclusive club and you can only benefit if you play by the rules and have common terms. You want to play football and you cannot use the laws of cricket.To me I feel the longer crank require less energy because torque is equal to force(being applied by the cyclist) x be perpendicular distance(170mm vs 180mm) and as the longer cranks have a longer perpendicular distance to the pivot point I understand it to be easier for the cyclist.
In general it doesn't. You have seen a kid on his first bike with legs going so fast it's a blur. He cannot deliver much power because he has only one 'gear' available.I'm just failing to see why shorter cranks means more powe
Everyone is asking the same thing, the user(rider) is the same, same energy output, but the bikes are different. Only difference everyone is asking is crank length. Same rider, different cranks. Which produces more power?Better?
This is your problem. The usefulness of the machine depends on the user.
You're right, it does feel like.... as you say in your words... an annoying exclusive club, everyone seems to not point me to the rules for me to play by. I see responses but no answers the OPs topic or even someone willing to help and point me to the rules that I must play by to get an answer of a question 3 people have asked on a forum site where people are coming here for help from smarter individuals. I'm here for help. Not to disturb, upset anyone. You even said to RipVanCalcNoob that they should say their views. I have, I feel from a mathematics point of view, same energy being applied a longer crank is better, but people say otherwise and I came here hoping that a smarter person will tell me why I am wrong so I understand the science of it, not just because John Smith said so....The Physics of this question is not a matter of feeling. You seem to be requiring an answer in your terms but the only way you will get a meaningful answer from PF is to learn what the proper terms mean. Yes - it's an annoyingly exclusive club and you can only benefit if you play by the rules and have common terms. You want to play football and you cannot use the laws of cricket.
Here's all the specific physics of bicycle locomotion:What's your views?
But you cannot specify that the rider is producing the same energy output or the same force or the same pedalling rate. In fact, even replacing the rider with an electric motor that's somehow regulated to satisfy those conditions would be problematical.Everyone is asking the same thing, the user(rider) is the same, same energy output,
Thanks Hutchphd for your response. Let's take #2 From what you said,Here's all the specific physics of bicycle locomotion:
There is no optimization to be made without further specific information about humans.
- Take the force of the foot on pedal
- multiply by (crank length/tire radius)
- multiply by (sprocket teeth/chainring teeth)
- That is the propelling force of tire on road
- Apply Newton's laws of motion (and aerodynamics if you wish).
If one desired to attach a "leg" with "muscles" to the model, and characterize these appropriately you might start towards your idea.
I am happy just to pedal 10 miles every other day and not fall off...
Not a badly stated question...!What's your opinion?
Yet in the Pro's, there is no clear answer. This graph doesn't say much. Is this for a 1 second all out? Or a 10min effort? Plus while their peak is understandable for a 1 to 5 second All out Sprint. I fail to understand what the minimum represents, because even for a 10 min effort, those minimum numbers are off the charts..There is a really good book on this subject: Bicycling Science, by David Gordon Wilson. It's in the third edition, and is still available on Amazon. It has 477 pages of bicycling science goodness. Highly recommended. Here is a plot of power vs crank length:
View attachment 246590
Opinions on crank length have been around since bicycles were invented. This book has the available science on the subject.
Maybe the question asked should be if a robotic bicycle rider. Because when talking about cranks in different length, what is more efficient for the rider and when we are talking about energy, it's not energy at the crank that's important. It's energy the rider is spending to produce the force on the crank arm. If he/she pushes the same amount of force at different points on the crank arm it must have an impact in performance. Changing crank length in a ICE makes a different in power output. So I fail to understand why people think it doesn't make a different on a bicycle.All three supposed criteria cannot be satisfied at the same time if the back wheel receives the same torque.
Energy in == energy out.
Power == Energy / time.
If the bikes are travelling at the same velocity, the frictional resistance forces would be the same, then crank length makes no difference if energy input is the criteria.
Maybe the question should be:
For a robotic bicycle rider, with pneumatic legs, how is his riding affected by pressure, crank length?
Of course it makes a "difference" but the optimum is different for all users. If you cannot characterise the user then a single, two dimensional graph is unlikely to be of use. I would say that the only way to find the best crank length would be to do a vast number of experiments with different crank lengths and different gear ratios at different speeds and inclines. And that would only apply to ONE rider.So I fail to understand why people think it doesn't make a different on a bicycle.