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Flexing and Loss of Energy The Truth(20 posts)

Flexing and Loss of Energy The Truthbigrider
Dec 13, 2002 7:32 AM
When your frame flexes and the bottom bracket moves closer to the ground and then raises back up to the original position YOU ARE WASTING ENERGY.

When your bottom bracket sways when pedaling hard and causes your rear triangle to flex and your rear wheel to track out of line with your front YOU ARE WASTING ENERGY.

I checked with two experts, one who doesn't cycle and the other an excellent cyclist and the developer of analytic web site devoted to cyclists.

We discussed and agreed to the following:

Your body uses energy to flex the frame and/or your bottom bracket out of position. When the frame flex returns to its original position it doesn't propel the bike forward, it doesn't cause the pedals to turn faster, and your body can't absorb energy. In fact you use energy not only flexing the frame but also in slowing the frame's return to its's normal position.

Unfortunately, Sheldon Brown's web site has the following info which is false:

This is mainly related to the stresses generated by the forces you create from pedaling. Any frame will flex around the bottom bracket a bit in response to pedaling loads. This flex can be felt, and many riders assume that it is consuming (wasting) pedaling effort. Actually, that's not the case, because the metals used in bicycle frames are very efficient springs, and the energy gets returned at the end of the power stroke, so little or nothing is actually lost. While there is no actual loss of efficiency from a "flexy" frame, most cyclists find the sensation unpleasant, and prefer a frame that is fairly stiff in the drive-train area. This is more of a concern for larger, heavier riders, and for those who make a habit of standing up to pedal.
You may be right, but this is where I lose you. If the motionbill
Dec 13, 2002 7:56 AM
that causes the spring to flex is created by pedaling, which, is not only a circular motion but two-sided, so that whatever one foot is doing, the other is doing the exact opposite vector-force-wise at the exact same time, how is it that the energy is lost? Where does the energy go?
If you start out by asking that question -- where does the energy from the returning spring go? -- the concept doesn't seem so preposterous.
As I'm sorting through this, I'm starting to agree with you, though. I had been thinking that your body weight, which is a constant, provides resistant inertial force so that the force of the returning spring would make it back to the pedals. But the returning spring is going to impart force not to your entire body as a solid, immobile system but as a loose, complicated system of levers of varying weights (your arms and legs) that takes energy to maintain as a more solid, immobile system. I guess that's where the energy goes -- sapped by your arms and legs.
HeatEager Beagle
Dec 13, 2002 8:01 AM
the energy from the return spring goes in heat. If there was no energy lost, you'd be onto a perpetual motion machine.
Flexing and pedaling motionbigrider
Dec 13, 2002 8:11 AM
The energy causing the flex is the downward force on the pedal that does NOT go into the rotational movement of the crank. That makes sense because if it went into the rotational movement of the crank there would be no force causing the flex. It makes no more sense assuming when the displacement of the frame returns to it's original position that the energy is transferred INTO rotational movement of the crank
I guess if you assume that the flex is created only at thebill
Dec 13, 2002 8:39 AM
bottom of the stroke through a downward force that is at ninety degrees from the motion of the pedal at 6:00 you'd be right, but, because we're all supposed to be spinning in circles, aren't we? If the maximal downward force is at three o-clock or even five o'clock, and we have started to shift our weight to our saddle or other pedal by the time the pedal is at 6 o'clock, pushing the pedal only backward at that time, I'm not sure that you're correct in that part of the analysis.
Which makes me think that part of the reason why a smooth, circular pedal stroke is more efficient is not only because you are applying power through more of the stroke but you very well may be losing less energy to bottom bracket flex, because spinning in circles means that you are not heavy on the pedal at the bottom of the stroke. Any takers?
I'll biteEager Beagle
Dec 13, 2002 8:46 AM
Unless you are a monster masher with a bad bike, you aren't losing any energy to BB flex.

A smooth round stroke is more efficient because you are transmitting more of your energy to the cranks, rather than using it wobbling your body from side to side. You are both pushing and pulling the cranks round, rather than your back foot/leg riding up on the effort of the front one's down-stroke.

It's not about using less energy to ride, but about using the energy you expend more efficiently.
I've always thought there is far more flex in the tires & wheelsMB1
Dec 13, 2002 8:58 AM
Frame flex is pretty minor compared to what happens in your wheels and tires. A lot of the flexing you feel while riding comes from there and the energy is indeed lost forever.

It is called comfort.
that's because you areMJ
Dec 13, 2002 9:09 AM
reasonable and sensible
I don't know. We spend a lot of time talking about "noodly"bill
Dec 13, 2002 9:43 AM
frames and which is stiffer and how and paying big bucks for the most sophisticated iteration of balanced stiff and comfortable. Don't you think it's a legitimate discussion about whether any of it makes a difference? Or don't you have those considerations?
I don't disagree that it's a lot easier to dissipate energy through cushy tires. Not really the question though.
I don't know. We spend a lot of time talking about "noodly"MJ
Dec 13, 2002 9:52 AM
frame and how they ride are obviously a huge consideration - but is it the frame that's noodly or the tyres and wheels - I think the obvious gets overlooked in favour of riders believing they're bending metal rather than compressing their tyres

would I buy a lightweight carbon race frame? - no

would I think a steel/alu frame is going to flex before the tyre and wheel flexes? - no

that's where the line is for me - not in a mech. engineering lab testing steel and welds and coming to theoretical conclusions
well, then, why is the frame a huge consideration? is it onlybill
Dec 13, 2002 10:07 AM
weight and comfort? then why wouldn't you go out looking for the lightest, wimpiest frame you can buy? it would be the most comfortable, wouldn't it? and then spend the rest on solid tires and 42 spoke wheels.
Look, it's an esoteric, largely theoretical discussion that has some translation to the real world. In trying to be the voice of "reason," you are denying that (a) we think that we know all sorts of unexamined truths that we don't really know, (b) this IS the place for nerdly esoteric discussions about bikes, and (c) if you weren't a nerdly bike geek yourself you wouldn't be here.
Good points all...MB1
Dec 13, 2002 3:38 PM
It is pretty easy to build a test bed for frame deflection. Therefore a lot of folks have done it and attempt to draw a scientific conclusion on what to look for in a good bike.

Bicycle test beds are much harder to build and the variables go through the roof. That is where the arts of riding, writing, observing and interpeting collide. Often violently.....
A frame builder looks at the frame, a cyclist should look at the whole.MB1
Dec 13, 2002 10:02 AM
When you put a bike together you get to choose any 2.

Price
Weight
Stiffness
I'm starting to wonder how much we lose flexing muscles ...Humma Hah
Dec 13, 2002 11:05 AM
... the biggest energy losses tend to occur not when bending springy materials like metals, but squishy materials like tire rubber, clothing, and, I suspect, body tissue.

I've noticed a big difference in perceived work when switching from short cranks to longer cranks. Switching from 160's to 175's, for me, made me feel like I was pedaling thru oatmeal. I was suddenly aware of how much my knees resisted bending.

That suggests to me that all sorts of unnecessary motion you guys are suspecting waste energy in the BIKE are actually wasting considerably more energy in the MOTOR.
not even thatJofa
Dec 14, 2002 10:47 AM
wheels are vertically rigid like frames and don't even radiate the minute amount of heat that a frame does. It is tyres alone, ignoring infinitesimals. And that loss from the tyres is virtually undetectable when compared to the energy required to push a heavy human body through the air or lift it against gravity. All this talk of efficiency that manufacturers love sells expensive bikes, nothing else.

I prefer comfort as well and also looking at the view, not my bike.

Jofa
A little heat goes a long way.Continental
Dec 13, 2002 9:19 AM
A few watts is a significant amount of power for a cyclist. But a few watts would not generate any noticable heating of the frame in a 20 mph air stream. I'm don't think that any energy of flexing is returned to the crank to propel the bike forward. The flex motion is in a plane perpendicular to the rotational plane of the crank. It must be completely wasted energy. Here's a thought experiment. Sit on a bike that is not moving. Now deflect the bottom bracket. Release the bottom bracket. Can the bike move forward with no energy except the energy from the bottom bracket deflection? No.
What I want to knowMel Erickson
Dec 13, 2002 12:28 PM
is where does all the wasted energy go? I don't think it all goes to heat so where else does it go? When you find out and tell me I'm going to go there and retrieve it and store it. I should be rich in no time! (If only I can figure out the storage part)
Sheldon Brown is rightKerry
Dec 13, 2002 4:20 PM
Sorry, but your thinking experiments are pretty much wrong, and Sheldon is pretty much right. In order for you to lose significant energy to frame flex, you would have to be deforming the metal (the examples given about heating metal due to flexing are all about deforming the metal, not about simply deflecting it a bit). If you read Sheldon's comments carefully, he is basically saying that when you flex the frame, you are winding up a spring, and when it flexes back, it is returning the energy. Your pedaling forces cannot be 100% in the plane of the frame, as the pedals are not in the plane. However, the force "wasted" in deflection is returned as you go through the pedal stroke, and the only place for that force to go is either into your legs or into the drive train. If it goes into your legs, your legs opposing the force sends it back into the drive train as well. You can't have it both ways, to say that your body can't absorb the energy and then say you use energy to prevent the frame from returning to neutral. I know you WANT a flexy frame to be a big problem, but don't tell that to Sean Kelly, who won the TdF points jersey on what could arguably be the flexiest frame of all time. Likewise all the TdF climbers who dominated on that same flexy Alan frame. Both practical experience and physics do not support what you WANT to be true. You've talked yourself into it, but it ain't so.
Sheldon Brown is rightupandcomer
Dec 13, 2002 8:52 PM
I think you might be right. I started thinking about it and came up with this. If the right foot is pushing down from the 12 o'clock to the 6 o'clock and causing the bottom bracket to flex to the left, the left foot then starts to push down and causes the bottom bracket to flex to the right. Since you've preloaded your "spring" (BB) the left foot's job to push the BB back to the right is now easier because the spring back is helping you. Thus energy is conserved. Right?

Altough intuitively, I would still want a stiff frame in a a sprint.
re: Flexing and Loss of Energy The Truthnonsleepingjon
Dec 14, 2002 9:00 PM
ok, since we are on the topic of frame flex and wasted energy, take a look at this site and post your comments. would this system work, or is the idea bunk?

www.slingshotbikes.com/