Stort tack för att du uppmärksammat oss på den texten, Bruno!
10 nov 2008 kl. 11.03 skrev B D:
Ett inlägg i debatten kring vevarmslängd från trikeslistan från en som akllar sig "25Hz" och som brukar ha intressanta och insiktsfulla synpunkter på liggisar, han bygger trikar även till andra:
For knee pain, as far as setup goes, gettingthe right leg length is no different in process on a recumbent than it is on an upright. It is also just as important to keep the hip - knee - 2nd toe alignment on a recumbent as it is on an upright. If mechanical issues are addressed, then the only other things are medical issues that need to be addressed medically.
Any benefit provided by short cranks is the same whether the bent is a two wheeler or a trike. It's the body orientation that allows one to take advantage of shorter cranks, not how many wheels it has.
Comparing crank length calculation to that on a bent is comparing apples to oranges. Uprights require a certain crank length based on bike fit and getting the knee over the pedal. The crank length is a by product of that process of fitting someone to a bike. People try to short cut that process by coming up with a myriad of reverse-engineered formulae based on leg length and a magic "contol" number.
Recumbents can sort out their own leg length calculators because our ergonomics are much simpler. Push on the pedals, it pushes you into the seat, and you go forward (LWB or low BB's being the exception). As a result, bent riders can go looking for an actual pedal length that is the most efficient for them. Unfortunately, there is no formula for that either. It has been narrowed down to a combination of minimizing the knee bend angle and factoring in leg length. Redtaildd at BROL and Warren Beauchamp at recumbents.com did some work on calculators and knee bend comparisons, but there is no hard and fast rule. A number of crank length tests have been done and all found that shorter cranks produced higher overall power, regardless of leg length, and in one instance, it even produced higher peak power.
Long cranks encourage lazy and poor pedalling technique. It's hard to spin properly (again, test proven that 80 - 100rpm range is most efficient) with long cranks because the knee gets bent to such an extent that it has extremely poor mechanical advantage and can do minimal work until the knee opens up to the optimum individual angle. People can grind because the long crank gives them enough leverage to pedal slowly, but the knees will complain if the load gets high enough or the cadence gets low enough, like on hills, or if you have poor alignment between your hip - knee - foot. People don't have to learn how to spin circles because long cranks usually give one leg enough leverage to only have to utilize half the leg power available to you. If you grind, and/or if you don't pedal circles, chances are good short cranks aren't for you.
Short cranks are not magic. They don't fix your pedalling technique. They don't teach you to spin. They don't teach you to pedal circles. In fact, short cranks will highlight how poor your pedalling technique is. If however you can spin around the 90rpm range on long cranks, and if you can pedal circles on long cranks, then you will likely enjoy the benefits of short cranks.
I have switched a little over 3 dozen bents to 155mm (or shorter) cranks, including 3 adamant upright riders. The vast majority of people who test ride these bents are completely oblivious to crank length. The ones who notice a different crank length are the ones who pedal in circles and the spinners. They notice an increase in average speed. They notice an increase in cadence into the 3 digits. They notice an improvement in hill climbing. Why? Because it has nothing to do with "leverage". Tests show that short cranks are more efficient, especially at overall average power, so that shows that the leverage has nothing to do with it. So if the bonus of long cranks and their leverage isn't an advantage, where is the disadvantage of long cranks or the advantage of shorter ones? Simple. Foot speed and knee angle.
People talk about cadence. Cadence can be counted. What they are actually talking about is foot speed. Foot speed is tougher to measure because no one knows how many miles per hour their feet are going when they're pedalling. It doesn't matter if you are pedalling circles, running, or sliding your feet back and forth, you are limited by what your comfort level is in how fast you can move your feet. Regardless of how long the cranks are, you are still moving your feet at the same comfort level and speed. With the speed being the same, long cranks mean your feet travel a longer distance to do a complete revolution AND it also takes longer for your feet to make that revolution. Shorter cranks mean your feet, still travelling at the same speed, are now travelling a smaller arc, and covering less distance to make a complete revolution AND they are also covering that smaller distance in less time. Short crankas making more revolutions in the same period of time than long cranks is one obvious source of better performance. So, if your feet are making more revolutions in less time, THAT is where the cadence increase comes from. You need to be spinning properly FIRST, then the short cranks will improve upon it. You can learn to spin properly on short cranks, but it is tougher. What some people complain about is they don't like the feel of the short cranks because it changes your "timing". Your feet and legs take less time to go out and back, so you just retrain yourself to transition faster mentally.
The other thing is knee angle. Everyone has a knee angle where they start producing their best power. At knee angles more closed than this optimum angle, the power drops off quickly. As the knee angle opens up past this optimum angle, the power also drops off, but not as fast as if it is too closed. With long cranks, the starting knee bend angle is more closed than shorter cranks. That means right off the bat, the long crank is going to take you more time, and the foot will need to travel further, before the knee joint opens up enough to start getting power down, compared to a shorter crank. Experimenting showed that from the dead spot to optimum knee angle, the curve was shallower and longer for a long crank, and steep and fast for shorter cranks. That meant that short cranks got you producing power fast in the stroke than long cranks did. As the cranks kept going in their rotation, the power curves of the short and long cranks was almost identical. So, the bottom line, is short cranks make the knee spend less time in angles where it produces low power. Add that to the point that short cranks also produce more rpms for the same foot speed, and the increase in performance isn't the occult phenomena that peopel like to think it is.
But wait! The short cranks give you less leverage!. Yeah, so what? The laboratory testing already showed that USUALLY long cranks produced higher PEAK power, and there is the leverage. Other than that, the short cranks produced higher OVERALL power, and in one test, 140mm cranks (I think it was) actually produced higher peak power than the long cranks. The fundamental problem is, that 99% of bike riders know next to nothing about what is actually going on when people pedal a bike. When a very small minority of people do actually look into the dynamics and actual physics of the forces at work in "simple" pedalling, they find it isn't "simple" at all. People think they're "spinning", but they aren't. People think they kow how to pedal circles, but they don't. People think they know about leverage and short cranks, but they don't. The biggest thing that stops people from reaping the benefits of short cranks is what is going on between their ears. As soon as they decide that short cranks aren't for them, they are 100% correct.
For me, I read anecdotal reports of pros and cons of short cranks. The ones I respected the most were not the arm chair physicists and nay sayers, but the ones who turned a critical eye on their pedalling technique, and what effect short cranks had on their riding. At the same time I came across a few test results that confirmed the anecdotal reports, so I started dropping crank lengths to 165, 155, 152, then 130mm and now back to the 140 - 145mm range. My knees are pretty busted up and after about 17,000km on 130mm cranks, I just can't turn them like I did on some of the hills I ride. If it wasn't for my knees, I think 130mm would be fine, and quite possibly even shorter, but for right now, 140mm seems to be the sweet spot. I still ride my 130mm highracer on rollers every noon hour when the weather is bad, and I also do at least an hour on 130mm cranks on my Speed on a trainer every Sat morning. Outside though, I use 140/145mm cranks on the road. I'm 6'2" tall and have a ~47" X-seam and a 36"/91cm inseam. I believe anyone (short of actual medical reasons) can reap the benefit of short cranks if they are open minded enough. I also believe that people will be able to calculate the optimum crank length using their optimum knee angle and their leg length, but I don't think there is a formula for that yet. At least to find out what your "optimum" knee angle is, you can do that pretty easily by sitting stopped, and getting ready to pedal off like you are in a drag race. Most people will instinctively get the pedals in the "sweet spot" and just measure that to get your angle. I also believe that people can use much shorter cranks than they want to believe they can.
Some "myths" that I have run across regarding short cranks are every bit as flawed as the "leverage" one. No one "has" to use lower gears because the cranks are shorter. Of the 3 dozen plus people I gave short cranks to, NONE of them reduced their chainrings or cassettes. More than a dozen actually got BIGGER gears because with the cadence and more open knee angles they found they could turn bigger gears than before. If people "want to", that's their choice, but using all the flawed logic and physics in the world won't support a "choice" and it tends to misinform and mislead others.
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