Carbon Frames less efficient?
09-03-16, 07:39 PM
#26
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09-03-16, 07:42 PM
#27
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In the strictest sense, yes: there will be some losses when a frame deforms. However, in-frame losses should be extremely minimal, especially in metal frames that don't dampen much. Assuming significant losses happen, it's probably in the rider's leg due to kick-back. But this hasn't been well-quantified. And it's entirely possible that flexy cranks have some (possibly frequency-dependant) benefits with respect to smoothing motion and biomechanics. The point is, there haven't been detailed, rigorous results in any direction. Considering that people don't seem to actually seem to suffer on flexy frames, and that the big manufacturers haven't published anything on the matter despite screaming about the benefits of stiffness for the last several decades, it seems likely that ultra-stiff cranks have very minimal performance benefit if it exists at all.
09-03-16, 07:53 PM
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Crank flex, frame flex ... luckily I am in that group of riders who need never fear these things.
09-03-16, 07:54 PM
#29
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Energy transfer with a bicycle is pretty simple---the rotating crank transfers energy to the rear wheel and to the ground to propel the bike forward. Any motion which is not in the direction of rotation is not moving the bike forward.
If the shaft of the golf club is not sufficiently rigid, it will flex back from the ball at the moment of impact, wasting energy---imagine a golf club with a thick stiff rope shaft ... not quite going to get those 275-yard drives. If the bottom bracket of a bike flexes side to side, the amount of energy it absorbs---the amount of energy needed to make it bend---is Not getting through the crank arms to the chainring to the chain to the cassette to the ground to propel the bike forward.
Think of it this way---would you wear loose, soft-soled shoes with big thick cushions on the sole when biking? Why not? Because compressing all that cushion and flexing the sole would waste energy---not magically "return energy to the system." The enrgy needed to fully compress all that adding would not drive the bike. The energy absorbed by the bending of the sole would not drive the bike.
Which is why cycling shoes have super-rigid soles ... energy transfer is maximized when flex is minimized.
If you can explain precisely how there is a "trampoline effect" in the bottom bracket of a bicycle frame and how this helps move the bike forward ... I will have learned something. But do consider---- a golf club striking a ball is a direct impact, and the compression and subsequent expansion of the ball is used as a spring to increase distance.
There is no spring in a bicycle's drive train---as with stiff-soled shoes and stiff cranks and a chain which hopefully doesn't stretch, there is no spring. there is no one-time impact, no controlled compression and expansion is response to that impact. The only "impact" as such is the pressure of the feet on the pedals, and there the impact should be continuous force, not a sudden impact.
If there is any compression, it would involve the foot, ankle, and leg---and the idea there is for the foot, ankle and knee to be pushing through the stroke, not springing back ... when your feet, ankles and knees can no longer stop from compressing, when you can no longer extend them to pedal, you have hit max power. The golf ball is supposed to compress and expand---your legs are not supposed to go backwards from the force of pedaling.
Again, consider a very flexy golf club shaft---it might whip on the downswing and snap forward, but if it flexed Backwards or sideways upon impact, you would redesign the club. That would be lost energy.
If the shaft of the golf club is not sufficiently rigid, it will flex back from the ball at the moment of impact, wasting energy---imagine a golf club with a thick stiff rope shaft ... not quite going to get those 275-yard drives. If the bottom bracket of a bike flexes side to side, the amount of energy it absorbs---the amount of energy needed to make it bend---is Not getting through the crank arms to the chainring to the chain to the cassette to the ground to propel the bike forward.
Think of it this way---would you wear loose, soft-soled shoes with big thick cushions on the sole when biking? Why not? Because compressing all that cushion and flexing the sole would waste energy---not magically "return energy to the system." The enrgy needed to fully compress all that adding would not drive the bike. The energy absorbed by the bending of the sole would not drive the bike.
Which is why cycling shoes have super-rigid soles ... energy transfer is maximized when flex is minimized.
If you can explain precisely how there is a "trampoline effect" in the bottom bracket of a bicycle frame and how this helps move the bike forward ... I will have learned something. But do consider---- a golf club striking a ball is a direct impact, and the compression and subsequent expansion of the ball is used as a spring to increase distance.
There is no spring in a bicycle's drive train---as with stiff-soled shoes and stiff cranks and a chain which hopefully doesn't stretch, there is no spring. there is no one-time impact, no controlled compression and expansion is response to that impact. The only "impact" as such is the pressure of the feet on the pedals, and there the impact should be continuous force, not a sudden impact.
If there is any compression, it would involve the foot, ankle, and leg---and the idea there is for the foot, ankle and knee to be pushing through the stroke, not springing back ... when your feet, ankles and knees can no longer stop from compressing, when you can no longer extend them to pedal, you have hit max power. The golf ball is supposed to compress and expand---your legs are not supposed to go backwards from the force of pedaling.
Again, consider a very flexy golf club shaft---it might whip on the downswing and snap forward, but if it flexed Backwards or sideways upon impact, you would redesign the club. That would be lost energy.
Also regarding the golf club, the shaft does flex backwards, and twists sideways then returns back to its original position. The manufacturers can design/manipulate all of these properties of the shaft to suit any type of swing and produce most any type of ball flight.
Last edited by seypat; 09-03-16 at 08:10 PM.
09-03-16, 07:56 PM
#30
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In the strictest sense, yes: there will be some losses when a frame deforms. However, in-frame losses should be extremely minimal, especially in metal frames that don't dampen much. Assuming significant losses happen, it's probably in the rider's leg due to kick-back. But this hasn't been well-quantified. And it's entirely possible that flexy cranks have some (possibly frequency-dependant) benefits with respect to smoothing motion and biomechanics. The point is, there haven't been detailed, rigorous results in any direction. Considering that people don't seem to actually seem to suffer on flexy frames, and that the big manufacturers haven't published anything on the matter despite screaming about the benefits of stiffness for the last several decades, it seems likely that ultra-stiff cranks have very minimal performance benefit if it exists at all.
09-03-16, 08:32 PM
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Fortunately, people who don't understand physics are not the ones designing our bikes. Those people work in the advertising department (or for the movies, or in politics).
If physics was a democracy, then we would all still be riding 19mm tires inflated to 140psi. Common sense is a very poor guide.
If physics was a democracy, then we would all still be riding 19mm tires inflated to 140psi. Common sense is a very poor guide.
09-08-16, 01:14 PM
#32
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Actually, both steel and titanium are near perfect spring materials and lose almost ZERO energy from the flexing. So yes, nearly all of the energy does go back into the system.
That said, it IS debatable (and hotly debated) whether that energy returning into the system actually goes back into forward propulsion. For example, one theory is that the frame flex transfers deformation in the tires, which rub against the ground slightly to cause (increased) heat loss.
However, none of the bike manufacturers have released any studies or data that proves (or even suggests) that a stiffer bike is faster.
That said, it IS debatable (and hotly debated) whether that energy returning into the system actually goes back into forward propulsion. For example, one theory is that the frame flex transfers deformation in the tires, which rub against the ground slightly to cause (increased) heat loss.
However, none of the bike manufacturers have released any studies or data that proves (or even suggests) that a stiffer bike is faster.
Last edited by link0; 09-08-16 at 01:18 PM.
09-08-16, 01:27 PM
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Are red carbon frames the most efficient of all carbon frames?
09-08-16, 01:39 PM
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This gets us back to the weight debate .... yes a lighter bike is faster given the same applied power, but at what point is it meaningful. Save a gram? a pound? People will fight over that for dozens of pages ... in fact, they have.
Crank flex, frame flex ... luckily I am in that group of riders who need never fear these things.
Crank flex, frame flex ... luckily I am in that group of riders who need never fear these things.
Now rolling resistance is a very low percentage of total resistance at speed on good tires. Bearing resistance is far lower than rolling resistance. Added chain resistance is a small percentage of those low numbers (like 1%). Weight weenies will realize virtually no gain for their best efforts on flat ground unless: their tires are poor and/or the bearing and chain in a bad state of maintenance. Uphill is a different story. Steep enough and total weight of bike, rider, clothes and gear rules. A 10% change there means 10% faster. (10% of a 175 pound rider with 20 pound bike, 5 pounds of shoes and clothes and 2 pounds of gear and waterbottles is 20 pounds 3 ounces. Our weight weenie has work to do.)
Ben
09-08-16, 01:50 PM
#36
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Actually, both steel and titanium are near perfect spring materials and lose almost ZERO energy from the flexing. So yes, nearly all of the energy does go back into the system.
That said, it IS debatable (and hotly debated) whether that energy returning into the system actually goes back into forward propulsion. For example, one theory is that the frame flex transfers deformation in the tires, which rub against the ground slightly to cause (increased) heat loss.
However, none of the bike manufacturers have released any studies or data that proves (or even suggests) that a stiffer bike is faster.
That said, it IS debatable (and hotly debated) whether that energy returning into the system actually goes back into forward propulsion. For example, one theory is that the frame flex transfers deformation in the tires, which rub against the ground slightly to cause (increased) heat loss.
However, none of the bike manufacturers have released any studies or data that proves (or even suggests) that a stiffer bike is faster.
Pedaling smoothly enough to be efficient on a flexible frame is, for most of us, a learned skill. I suspect that a very stiff CF frame suits the vast majority who have not trained for this skill better by minimizing the loses from frame flex and poor pedaling style. Good marketing. Engineers design what marketing calls for, because what marketing can get behind is what sells.
Ben
09-08-16, 02:08 PM
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09-08-16, 02:47 PM
#38
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Interesting discussion. I have steel, ti, and CF bikes; and I love them all for different reasons. I'll only say two things: 1) it seems, to me, much easier to get up to speed on a CF bike, and 2) I would much rather ride one of my CF bikes when I do a lot of climbing, which is most of the time since I live in the Sierra Nevadas (rarely do my rides incorporate less than 4-5 thousand feet of elevation gain with steep gradients; most times, much more than this, on a fairly routine basis). I tend to think that the more you climb, the more you appreciate CF (of course, steel and ti are mighty nice on long descents on rough roads, too!).
09-08-16, 02:57 PM
#39
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09-08-16, 03:21 PM
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That certainly made me go faster. No debate needed.
09-08-16, 09:51 PM
#43
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Good humor, and true.
The energy you expend moving the frame sideways does not go towards moving the bike forwards.
Sure, the frame springs back ... but it doesn't transfer power back into your leg --- seeing as that power is the product of metabolism. It's not like you can bend a spring and regain the energy you spent bending it ... if that worked, we'd never need to east, just go to the gym and bend springs for a while---we'd get stronger and refuel at the same time.
The energy you expend moving the frame sideways does not go towards moving the bike forwards.
Sure, the frame springs back ... but it doesn't transfer power back into your leg --- seeing as that power is the product of metabolism. It's not like you can bend a spring and regain the energy you spent bending it ... if that worked, we'd never need to east, just go to the gym and bend springs for a while---we'd get stronger and refuel at the same time.
09-08-16, 10:16 PM
#44
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No, it doesn't transfer power back into your leg - but that doesn't mean the power is wasted. Let's consider what happens during half of a crank rotation, i.e. one pedal downstroke by your right leg. You start with the pedal up and the frame is relaxed. Now you push down on the pedal and most of your foot motion is used to turn the crank and move the bike but a little is used to bend the frame toward the left and therefore doesn't immediately help move the bike forward. But as your foot approaches the bottom of the pedal stroke you begin to ease up on the pressure and the frame starts to move back toward the right. Now all of your foot motion results in turning the crank and the crank also turns just a bit more due to the rightward motion of the bottom bracket thereby lifting the right side of the BB spindle. So you're getting back at least some of the energy you had 'lost' during the initial part of the pedal stroke. If the frame acts as a perfectly elastic spring then there's no loss of energy due to the frame flex, but I'd expect some minor loss in an actual physical system. But there's clearly going to be some return of the energy lost during the first half of the pedal stroke by the energy gain in the last half.
09-09-16, 12:09 PM
#46
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Suppose the frame flexes right as you pedal down with your left foot and flexes back to the left before you get the pedal around? Have you timed the degree and period of flame flexure? How does it vary depending on how much power you use?
Can you honestly say that you are so perfectly in sync with your frame flex that no matter what gear you are in, no matter whether flat, incline, or decline, at any speed, you know exactly how quickly and how hard to pedal to be in tune with your frame flex?
Can't we stick with ... i don't know, Logic or something?
Can you honestly say that you are so perfectly in sync with your frame flex that no matter what gear you are in, no matter whether flat, incline, or decline, at any speed, you know exactly how quickly and how hard to pedal to be in tune with your frame flex?
Can't we stick with ... i don't know, Logic or something?
09-09-16, 12:26 PM
#47
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It seems to be generally accepted that carbon is good in forks, handlebars, and seat posts because it adds comfort by absorbing road vibration.
Surely this is basically saying it is damping and absorbing energy?
So why isn't it said that the frame is damping and absorbing the energy the rider is putting through it and is less efficient for transferring power?
Surely this is basically saying it is damping and absorbing energy?
So why isn't it said that the frame is damping and absorbing the energy the rider is putting through it and is less efficient for transferring power?
If you are asking if it is more or less efficient to deflect the bottom bracket by .050 inches or .025 inches during this 10 inch pedal movement... The answer is "negligible".
09-09-16, 12:39 PM
#48
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One of the advantages to carbon fiber is that the maker can use any shape. Some shapes absorb shocks, and some don't, so you can have stiffness where you want it and flexibility where you want it.
...says a guy who has no carbon fiber anything. I suppose I will one day.
...says a guy who has no carbon fiber anything. I suppose I will one day.
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