RoadBikeReview.com's Forum Archives - General


Archive Home >> General(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 )


Flexing and loss of energy(16 posts)

Flexing and loss of energyWoof the dog
Dec 11, 2002 2:04 AM
I think that the "loaded spring theory" is bullshit when applied to your flexing the wheel side to side when out of the saddle. Seems like the energy one spends to press the wheel to one or the other side is conserved and returns the wheel back to the normal position. But that of course does not do shit to help carry you forward.

True or not?

Peace

Woof, the green space alien dog.
It is garbagebigrider
Dec 11, 2002 4:43 AM
I thought the same thing when I read that theory on the Sheldon Brown site.

Lets think about flex in two directions. First case, you are cranking up a hill and the chain stays allow the wheel to flex out of their normal track. You put enrgy into the crank to flex the stays to move the wheel sideways, but wait, they move back to their original position. You spent energy to deform the chain stays but when they spring back the energy regained is in the horizontal direction, not the direction you want to go.

Secondly, you have flex in the vertical direction of your bike. You stand to crank up a hill and and you bottom bracket moves up and down with your pedal strokes. You are moving your entire body weight down with your force and then the bike responds to the bending and brings you back up to the normal position after the cranking force subsides. You used pedal force to move your body up and down, fighting gravity and none of that energy went in the forward direction.

That is my 02 cents worth.
Lost Energy?bsdc
Dec 11, 2002 5:40 AM
Where does all that lost energy go? It goes in the form of heat. Try this: Take a metal coat hanger and bend it back and forth in one place quickly for about 50-100 times. Now feel the place where the metal bent. It will feel hot. The "lost energy" you put into the metal took the form of heat.

Now do this with your bike: Stand up and crank as hard as you can up the tallest hill you can find then get off and feel for heat on the tubes around your bottom bracket. I don't think you'll find any appreciable warmth in any part of your tubes. Try checking it after a long hard ride. I've still never found any warm spots.

I'm a few years out of school, but I think this has to do with the First Law of Thermodynamics which states: Energy is conserved.

My point. The lost energy in a relatively flexible frame is extremely minimal.
If your bike didn't heat up?bigrider
Dec 11, 2002 5:58 AM
If you continue on that path then:

If the bike didn't heat up you didn't expend any energy to get up the hill. It was all conserved.

Think of potential energy and kinetic energy not just energy conversation.

I like my Archie Bunker slip so much that I think I will leave it.
No,Wayne
Dec 11, 2002 6:04 AM
you get hot going up the hill, because you expended the energy.
Correctobigrider
Dec 11, 2002 6:18 AM
Wayne,

I bet if we put our bikes in a trainer and didn't turn the wheels but made our bikes flex by stepping on the cranks that we would heat our bodies up a little bit and our bikes would have gone nowhere and returned to their normal position after each flex.

We would have expended energy and it all would have gone into flex.

If you throw a boomerang and it returns to your hand it doesn't mean you didn't work to throw the thing.
some of it is mental, tooDougSloan
Dec 11, 2002 7:14 AM
While there is no perfect spring (friction), the actual energy loss is likely fairly minimal. At the extreme, though, flexing of any parts can induce other problems, like brake rub, chain rub, or even ghost shifting. It also can make you feel uneasy, for a feel of flexing is not reassuring, and can even lead to handling problems.

I have a set of Velomax Ascent Pro wheels with Lew carbon rims, and the rear is so flexy that in hard corners it actually squirms around, and in hard out of the saddle climbs it is very noticeable. It robs me of "mental energy."

Doug
It's only valid at constant speedContinental
Dec 11, 2002 7:48 AM
The flexing spring theory assumes steady state conditions, that is, constant pedal cadence and pedal force. When you increase power to accelerate or climb hills you want all the energy to propel the bicycle. If the frame flexes while you are increasing power then part of the energy is used to increase the magnitude of the flex or to "wind the spring." Also, the deflections produce misalignments which also reduce efficiency.
Sure, the loaded spring theory has some validity, as doesbill
Dec 11, 2002 7:56 AM
Doug's observation that the mental sapping is probably more significant to our experience.
You can't say that the return of the spring doesn't do anything to propel you forward, or the motion you used to load the spring wouldn't have done anything, either. Obviously, it did, because you moved forward.
The frame as a spring loads because the force is being applied both vertically (through pedaling) and laterally (pulling up on the bars to be able to create the equal and opposite force of pushing your body down on the pedals). When the spring unloads, it supplies energy to the pedals in the opposite direction from the load, which happens to coincide with the other end of the pedal stroke, which is, after all circular. Because the pedal stroke is circular, with useful forces being supplied more or less through 360 degrees of the stroke (180 one side, 180 the other side), any energy left in the system is going to get used. I know that the force of your pedal stroke isn't balanced throughout the 360 degrees, but that doesn't mean that the energy isn't going back to the system.
Try to picture your bike as a realllly noodly spring. Don't you think that the frame's springing back against the intertia of your body would help propel you forward if you used the inertial resistance of your body to transmit the energy back to the spring?
BTW, just because the heat energy generated by twisting isn't perceptible by touching the frame doesn't mean that there isn't any. The frame is a pretty big heat sink, so that any heat build-up generated is going to be dissipated quickly. I think that it's probably right, though, that the heat energy lost is minimal.
A little more explanation Bill?djg
Dec 11, 2002 9:57 AM
I'm not sure I follow the analysis--even as an informal graphic suggestion--with respect to the business about the circular pedal stroke. I guess it's right that useful force is (or can be) applied through the 360 degrees of rotation, however well balanced that is. I'm just not sure how loading the "spring"--that is flexing/torquing/etc. the frame--could be viewed as creating potential, the realization of which is liable to be essentially (that is, almost entirely) productive. For one thing, it seems unlikely that the frame would sort out the recoil in a manner simultaneously helpful to moving both pedals (crankarms, feet, wherever you want to place the locus of analysis) in just the right way. In fact, I'd guess that the subjective feeling of a lively or springy frame signals something that is hard to reconcile with a largely productive return.

I agree with everything the follows the BTW. That is, on the one hand, given the size of a frame (and the amount of airflow) you might dissipate energy through the air without noticing that you are heating the frame (or making noise); and, on the other hand, that maybe the best pre-empirical guess is that this isn't all that big a deal in any case. Indeed, all of this may be a puzzle over something of a smaller magnitude than would justify much fuss, supposing we could agree on what THAT is.
okay, I'll bite. let's examine the system with your right footbill
Dec 11, 2002 11:16 AM
at about 3:00. That's probably when you are placing the most downward force on your right foot and therefore when you are pulling up the most with your right hand (to create the resistance that allows you to push down with your right foot). The frame is going to torque a little in response, loading the spring by flexing.
You then finish the power part of your right stroke, and you're starting the power part of your left stroke. The spring is then going to want to unload by extending back right. With your weight providing its own resistance, this frame extension is just what you need to force your foot down with just a little more energy without adding muscle energy.
How is it that the effect is going to transfer smoothly from one side to the other? Probably maybe because the torque creating the flexion is held through the power part of the pedal stroke, lightening only as the pedal power shifts to the other side. The unweighting, or the extension from the flexed position, is controlled/modulated by the very action of moving your pedal through the circle with gradual, as opposed to sudden, changes in the pedal force.
I agree that the effects are probably minute and that the subjective sense of using energy that is not going directly to the pedals is likely more significant. Hence the preference for stiff frames.
I don't know. I ain't no physicist, but it makes sense to me.
Don't buy it a bit. But then, I aint no physicist either. (nm)djg
Dec 11, 2002 11:26 AM
Me and Sheldon against you, Dan. at sunrise. nmbill
Dec 11, 2002 2:13 PM
Old school, brother Bill. 1982 HP programmable calculators atdjg
Dec 11, 2002 4:18 PM
20 paces.

No hiding behind Sheldon Claus either, and I don't just mean that metaphorically.

I hate to see it end like this. So, not to change the subject, but after you go down can I have the Marcello?
I'd loan him my HP-67, but it might be damaged ...Humma Hah
Dec 11, 2002 5:27 PM
... and I wouldn't part with my HP-15C for Doug's Colnago!
I'd worry more about the sidewall scrubbing ...Humma Hah
Dec 11, 2002 5:23 PM
... I used to do a lot of side-to-side pumping of the bike while climbing out-of-the-saddle, which has always been most of my climbing since I've only got the one gear. You have a lot of time to think about this sort of thing grinding up a long hill on a heavy singlespeed.

Straight ahead, you're riding a pair of the most efficient wheels ever designed, with very low rolling resistance. But lay it over, and you're scrubbing the sidewall across the pavement, leaving energy and tire rubber behind.

I taught myself to hold the bike straight and steady in a climb, and I'm convinced it's more efficient. Pumping does give some ability to throw extra muscle into the climb, but that should be reserved for all-out, spend-your-reserves, finish-line sprint stuff.