It doesn't matter that the pedals are inline, the truss will still be oscillating and giving the energy back when it returns to the 'normal' condition.

And on a truss oscillating - three words - Tacoma Narrows Bridge. (OK, cables...)

First point - although I'm betting you know this, the rider, or system of rider and bike, most definitely does conserve energy.

The question is whether the rider now somehow dissipates more energy because he is less spring-like than the bike. The rider is definitely another spring-mass-damper system, like the bike is, or the combination is. Of course we expect the damping of the the bike+rider to be greater than the damping of the bike alone.

On the other hand, the flexing of the bike in the lateral direction seems to me to be very unlikely to damped out by the rider. The oscillation is of extremely low frequency (about 1-2 Hertz) and of incredibly low velocities (perhaps .01 m/sec?)

As such, I expect the hips take up this flexing without much, if any, additional generation of heat (when compared to the 'stiff' frame of perhaps .005 m/sec).

Just my penny - which is worth two cents in copper these days.

Obviously, there are ways in which you'd want the frame to be more or less stiff. And I'm betting that sprinters/break-aways that are looking for a quick step away from the pack would probably want a stiffer bike than, say, a time-trialist would.

The real question is what deflections/energy storage mechanisms are appropriate at what point in the frame for a given use.

Spot me the SRM's and I promise I'll publish the data.

All I heard was 'Blah blah blah frame blah blah frame something blah blah flexing blah'. ;)

Who would like some apple pie with cream? Yum.

I think that this is a bad assumption. When I compress a spring, the energy I used to compress that spring is stored within the elastic deformation that the spring has undergone. This is not the case for a human body. Think of it this way, if I press a spring down, it will go back to it's original height, and probably pop up into the air too, without any more energy being added once it's compressed. If you are standing up and I press down on your shoulders until you're squatting on the floor, is there any potential energy in your squatting position like there is in a compressed spring?

Frame flex
 

Unless it's flexing so much that you are permanently deforming it (plasticly) then all of the energy you put into it is coming out. Or if it's flexing so much that, as the poster says, the wheels are rubbing or your pedaling is thrown out of whack.

The Tacoma Bridge is a suspension bridge, not truss-based bridge. Are you sure you're a ME (even though it's more of a CE thing)? How can you say that it doesn't matter if it's inline? When you're powering down on the pedal, you're subjecting the crank arms to bending at 6 and 12 o'clock positions and bending and torsion at other positions. You claim that the frame will oscillate. So show us what natural frequency such a big tube (like a seat tube) will oscillate at. Compare that with the frequency of the rider (cadence). If that energy is to go back and help the rider, it must be in some integer multiples of the other (and in phase). While you're at it, you might as well calculate and give us the amplitude of these oscillations that you claim. :D

the reason a mtb fs frame may feel a lot slower maybe because of standing pedaling style. most people, when climbing out of the saddle, dont in fact push down and pull up on the pedals at the same time. (as most do when sprinting hard) instead, what people tend to do is push on the pedal during the "deadspot" at the bottom of the stroke, and "stand up". they then transfer their lifted body weight to the other pedal, which forces the pedal down. the process repeats on the other pedal. seriously, go out and try it and you will realise this as true. on a full suspension, this "jumping' action will work the bike's suspension system, creating a feeling of induced lag. however, this lag does not, per say translate into a loss of energy, as long as the suspension is 100% efficient, which most arent. so you may lose a little bit of energy in a full suspension mountain bike when climbing out of the saddle using the specified method, but otherwise no. the reason mountain bikes in general are less efficient is because they have tires designed for traction. and increased traction means increased friction and slippage between tires and the ground. this is why you can go a lot faster on a road bike than even a rigid mountain bike. aerodynamic position also affects efficiency. but stiffness in the frame, does not.

this is an interesting theory. in my honest opinon (written out so you understand), the bike itself is very efficient. however , like stated above, the rider+bike combo may be ineffiecient. this could be because if the frame flexes LATERALLY while pedaling, say during the bottom of the pedal stroke, it will flex back releasing the same amount of energy that was stored. im not even sure if i am right, but it is possible that the human body cant react to this input/output of energy in the frame correctly, and will end up resisting, or pushing back slightly on the frame as it is releasing energy. this action takes extra energy used by your muscle fibers in order to slow the frame's flex back into normal position, eventually causing muscle fatigue if done repetitively. If the frame or crankarms flex vertically, or longitudinally, it will not waste any energy because the energy will still be put into rotational torque in the drivetrain, resulting in forward motion.

lets toss 2 more cents into that pile. doesn't lost energy result from friction? like a bearing in a derailleur rubbing against its casing. the energy changes form into heat, sound, and other forms, and also results in wear. does a frame "rub" when it flexes? not that i know of. so how can your frame lose energy when it flexes? it doesnt. remember when sean kelly won all those races on those noodle like early aluminum frames?

:beer:

I tried to follow this thread - I really did.....

Ummmm cream.....

The difference here is that when the suspension compresses, the energy is dampened, and not all returned. If you had the same pedaling form on a rigid frame, the extra push at the bottom would send your body upward. This upward velocity moves your center of mass higher -- storing that energy. You can then release it when your other foot gets on top of the power stroke. On an FS MTB, you just loose that energy completely to compression of the suspension.

Ok, so I've seen a lot of people claim that stiffer frames are faster based on feel and semi-educated guesses. There has been no hard evidence or study presented. I think we can conclude that this is just another marketing ploy. If stiffness was helpful the way people act like it is, there would some data to back it up.

I guess another consideration is systems that are designed to use flex to store energy and return it as motion. Think about a trampoline. You can jump much higher on a trampoline than on tarmac.

There's also the Slingshot bike frames (sprung cable for a down tube). They used to make a road racing frame that many people claimed was the fastest accelerating bike they had ever ridden. The reason it felt so fast is that it returned its compression energy as the stroke went dead, which smoothed out the power delivery. The beginning of each power phase doesn't require making up for the last dead spot. It's not like it had a motor or anything, but it was efficient.


http://img.epinions.com/images/opti/...Road-bikes.jpg

I would say flip it, but the stem might rub the front wheel. :D

Yea but on the Slingshot you are talking about a different flex than in the first post. The slingshot is flexing in the vertical plane and the horizontal plane as well as the BB twisting.

:roflmao:

True, but it all originates from a vertical force. Imagine about a "custom" diving board that is shaped like and "L" and fixed at the top. When you leap on it, the base of the L is moving horizontal, the end you're standing on is going sideways and down. But it's going to come right back at you and send you into the air.

OK, so then what about biking shoes? Why all the concern over carbon soles and stiffness? What happens if you ride in regular sneakers? Seems to me it would be pretty inefficient. Aren't your shoes and pedals an extension of the bike?

I just went to my bike and stood on one pedal, hopping up and down on it repeatedly, but the bike did not move one inch. Just to be sure I wasn't fighting an incline, I turned the bike around and tried again with no movement. A crude experiment, but I think it shows the lack of any significant energy converted into speed. Besides, if that worked I'd install a bowflex into my handlebars and compress and release that for extra speed! lol

The best example I can give you is a piece of bender board. Take that and bend it into a U, then relase it. It will spring back to shape, and not release all that much heat. The majority of energy was spent returning it back to its original shape, which only takes as much energy as was forced on it in the first place. That doesn't translate into any kind of propulsion without some mechanism to convert such a thing. Other than the chain stay flexing then springing back causing it to ellongate, pulling on the chain, I don't see any other mechanism that can convert such energy. With all you physics majors out there, none of you mentioned that the rebound energy could be causing DRAG just as easily as propulsion, and you all fail to point out how it would be the latter. Just like any other thread, this is full of half thought out theories without anyone backing it up with verifiable information.

Somebody send this question in to Myth Busters :D

Well, it was a suspension bridge, like I mentioned. It has trusses now. When it was a suspension bridge, it had a very stiff structure underneath, it just wasn't open like a truss.

I am sure I am an ME.

It doesn't matter in what direction the bike frame is flexing - it will return its energy, like a pendulum.

Instead of answering your inane questions about what the natural and forced frequencies are, and whether it matters if they're in sync (it doesn't) - consider this.

Assume for a second that the hysteresis in the bike is small (it is) and as such the heat generated in the bike frame is small. Put a control volume around the frame and wheel, but not the tires. Don't consider aerodynamic forces for a mometn. There is work going into the frame. Where does the work come out? How much work comes out at those places?

Exactamundo. When you neglect the heat generated in the frame (which is an appropriate assumption), there is no work lost to the flex.

This is what one of the world's foremost builders, Richard Sachs, has to say about frame flex:

GP: What are your thoughts on frame stiffness?

RS: I don't ever consider stiffness in my frame. I wouldn't even know how to define it. When people talk to me about it, I've no clue what they are talking about.

GP: What do you mean you have no clue?

RS: Well, the bike has to be forgiving and resilient so it can be ridden comfortably. I don't know what stiffness is. My notion is that when a rider feels what he thinks is a lack of it (stiffness) it's really the result of a poorly designed or constructed bike. I think the assembly methods also contribute to how the bike feels, and it's not simply the tube's gauge or cross section.

Rest of the interview is here.

Stop and think about how utterly inefficient your body (wasted motion, unnecessary tension) and imperfect riding style are, and frame flex suddenly looks real insignificant.....

I am smarter than you.

Yes, you!

We have a mechanism to convert vertical forces into propulsion: crankarms. That's how you pedal: push down, go forward. Do you think a frame flexes and just waits until you get to the bottom of the pedal stroke to return? It will always be pushing back exactly as hard as you are (other than the differences that result in temporary flex displacement).

If you are still pushing hard at the bottom of the stroke, then the frame will send you, the rider, upward as you release that pressure. Guess what? You get to come back down, and that energy goes into the other pedal.