I have to agree that even in the elastic state, a material will absorb some of the energy put into it, otherwise you'd be able to create infinite motion with a spring (in a vacuum at least). But, I also think it's irrelevant to the discussion. The amount of energy loss from frame flex is so insignificant to compared to all the other factors that rob power that there's no point in arguing about. The biggest issues I can see resulting from frame flex are derailleur rub and inefficient chain angle. The friction generated by either of those will sap way more power than the flexing frame ever will.

That second that Lance lost was probably also the equivalent of him moving his knees in 1mm or not picking his head up one time or dropping his shoulders a tiny fraction. Until bike racing is perfectly efficient machines racing against each other with races decided by a fraction of a second, it's never going to be about the bike.

I've heard of people saying the frame flexed so much the wheel was rubbing the inner chain stay and there are marks there. Frame flex, despite what some people say, WILL rob you of energy, especially on hard sprints and long climbs. But that's just a minor inconvenience. What you WILL notice is the bike going all noodly when you're trying to sprint or descend really fast. So what will affect you most would probably be safety and control over performance.
Like I said, it's all about what people deem acceptable loss. For most of the pro teams, it IS all about the bike as much as it is about everything else. That's why they stick people and bikes into wind tunnels and test the hell out of them. I can almost guarantee you that someone back at Trek stuck that bike back into the wind tunnel for 2005 thinking, "how am i going to shave that extra second off". Of course in the end, performance wise, tt doesn't matter for civilians, unless you want to squeeze every drop of speed possible out of a bike (which isn't unreasonable if someone's willing to pay $6-8000 for a bike).

The energy that goes into frame flex is on the order of 1% of each pedal stroke. However during the "dead spot" this energy is all returned to the drive train (minus extremely small histerisis effect). Please see my web site at http://www.bikethink.com/Frameflex.htm for further details on this.

That reminds me of a fisherman who was trying to convince me that a very flexy pole was much better for casting, as the energy is "built up" and then dissipated at the release of the lure...sure it is. He never could outcast me. At the moment that the lure is released, the pole is still flexed, meaning that some of the energy is not transmitted to the lure at launch. Somehow I think the same concept applies here. Lateral flex or any drivetrain flex is never a good thing on a bike, IMHO. Vertical compliance, on the other hand, is probably not a bad idea.

Here's another interesting article on flex. Makes some good points.

I think it comes down to numbers. How much energy is lost for how much flexing. With the existing 1-2" of lateral flex at the BB under high-power sprints, how much more speed can we gain by reducing that flex to just 1/2 of its current values? What other compromises would be introduced? Without actual numbers to examine, talking about flex is purely academic.

Glad we have so many scientists in our midst. Flex and deflection are a characteristic of the "whole" bike wheels and frame. A flexy bike is a detriment to a sprinter in a race and not too bad for the skinney high cadence hill climber. For anyone else probably doesn't matter unless like you like to descend like a bat out of hell down hills and the front end becomes whippy and hard to control.
The characteristis the is the most important for road comfort is how much vibration is transmitted up to the rider's hands and Butt. Dampening of the various high frequencies in riding is important. Pea gravel used as road treatment on the back roads is the worst.
Early Aluminum bikes were like riding telephone poles and about 35 miles was all I could stand. Early Ti bikes were very flexy and felt like riding a wet noodle. Steel has nearly always been my material of choice but the modern materials and great designs that the better bike builders have come up with have improved the ride on most bikes of Al, Ti, Carbon, and Steel so it is much more difficult to differentiate the materials of construction. I have one bike that is Ti with carbon seat stays and carbon fork. It is like riding a lexus and it is fairly stiff. Putting a carbon fork on one of my steel bikes took care of sleeping hands on the longer rides.
I find some of the modern wheels are not exactly the best for riding long distance. Conventional 3X 32 hole wheel sets are the best for road riding, especially if you are a bigger than normal rider. Have tried numerous combinations wheel sets and find the transmission of the road bumps up to me more pronounced in the lower spoke count very high spoke tension wheels.
Ride what makes you feel good and if flashy high tech sexy parts make you feel good then just do it. What is hard to take is the evangelical zeal of the lighter is better crowd. Lets all just have fun!

It's not only good, but a requirement for all bicycle frames to engineered to absorb (by flexing) much of the twisting and pulling forces induced by the rider; especially when they're out of the seat and whipping the machine from side to side. You migt wish to be more specific about the type of energy absorbing frame flexing that you're talking about.

Whenever people mention flex in frames, I think back to Sean Kelly sprinting like a madman on a Vitus 'screwed and glued' frame. Frame flex didn't seem to hurt his palmares any.

Man, I miss that guy in the peleton... but I digress. :)

When you hit your racing bike with your car. Like I did the other day to my Cannondale. Fortunately I noticed before I did any damage. The frame absorbed my supidity rather nicely : ).

A lot of people who believe that BB flex is inefficient hold that believe because they understand that the spring system returns the vast majority of the energy (minus some heat) but believe the energy return occurs while the crank arm is no longer in the power phase of the pedal cycle, i.e., isn't returned to the rider.

But if you think about it, the forces from the leg on the crank increase and decrease fairly slowy relative to the speed of the spring system. The spring's energy is returned during the second half of the power stroke when leg force on the crank is decreasing, and the spring system is so fast that by the time the power stroke is finished, the spring system is at neutral again. Other than some heat, all of the energy has been return prior to the end of the power stroke. The only measurable lost energy is heat.

The first time somebody explained to my why frame flex was bad, they cited the loss of energy from the spring system that occurred AFTER the power stroke. Only later did I realize that they were wrong. The energy is returned DURING the second half of the power stroke.

That could be the most brilliant argument against a scientific theory I have ever heard. You didn't pay enough for your textbooks, so you don't know what you are talking about. No wonder you think people in Texas are stupid. Our Education Agency is always trying to negotiate down the price of textbooks with the publishers. Therefore, your experience with 3 bikes is more valid than the textbooks we read. In Texas we call that anecdotal evidence and it ain't worth squat.
If stiffer correlated with better, all motorcycle racers would be riding Harley hardtails. Unfortunately, anyone who has seen a motorcycle road race knows that if the bike is too stiff, it can't deliver all of its horsepower to the road and starts hopping. This causes loss of power and loss of control.

Didnt the GP bikes go overboard with carbon frames that were too stiff? I think this around 96-97 or so. They broke traction and slid too easily. So they made the frames more flexible and easier to control. This was around the time the big-bang engines came into being as well.

I'm pleading total ignorance here, but these bikes have suspension, so I really don't see how that's probable. Besides, for the last 15 years every car manufacturer is touting how this year's chassis is 128% stiffer than last years. I'm beginning to think either the cars of the '70s were make of wet noodles, or they are making the unibodies of today out of unobtanium. Everything I've ever read about performance cars (especially racing machines) is how the engineers just hate a flexy chassis, as it totally throws off their setup. The stiffer the better.

I should clarify. I'm not trying to say flexy frames are better. I'm not trying to advocate any particular frame design or material. This study is not about ride quality, durability, responsiveness, or any other of the many important factors that go into a frame design. All I'm saying is that frame flex does not waste energy. Frankly, my original assumption was that frame flex did waste energy. It seemed to be the obvious conclusion. I set out to get an estimate of how much energy was wasted for various frame designs using FEA models. However, as I got into the detail of the FEA model, I came to realize that the energy is returned to the drive train.

Dissipation of the energy as heat is not an explanation. As the frame returns to its unloaded position each pedal stroke, it will dissipate a small portion of the strain energy as heat. But that small portion is a constant for the given frame material's histeresis property, and is extremely small for all materials used in bicycle frames. The vast majority of the strain energy of the frame is returned as work done, or a force times a distance. The distance is the same as the distance that the frame was flexed in the high force portion of the pedal stroke. So the question then is what is the force applied to. This is the fundamental topic of my article. (If you did'nt see my previous post please see http://www.bikethink.com/Frameflex.htm)

I am not a real expert on automotive design, but some history might help answer your question. Cars in the 70's had frames made out of steel rails. They were very stiff but heavy. They also had shocks and springs to give the flex necessary to keep the wheels on the road. When the oil shortage started, manufacturers were looking to save weight and created the unibody design. There really isn't a frame at all. Since then, the goal has been to get the stiffness of a steel rail frame with aluminum and plastic unibodies. This is totally the opposite of what we are debating and doesn't really have anything to do with materials, the issue is 2 entirely different designs.
Even Nascar mechanics try and tune the car to give it as much vertical flex as possible without the shocks bottoming out. There are 2 reasons for this: 1) to keep the tires on the pavement,and 2) to make the car lower at high speeds. On the other hand they use steel tube frames and sway bars to prevent lateral flex and twisting. So there is good flex and bad flex.
The same thing applies to bikes, I assume. All I know is I like my steel bike and my aluminum bike equally on smooth pavement, but I love my steel bike on chip and seal or other rough pavement. But I don't have the ability to get anywhere near the limits of the design of either bike unless I crash. That is anecdotal evidence and it ain't worth squat.

My 2 cents: the built up energy (spring effect) is recoiled or absorbed back into your leg (backwards from the direction of rotation...meaning its bad) when you go from the high-torque position (horizontal or 3 o'clock position) down towards the vertical dead spot. The energy that is built up in the flex as your legs reaches its maximum leverage is lost as it goes towards the dead spot. I'm only talking about the energy that is involved in flexing the frame and drivetrain.

if a frame and/or chainrings are too flexy, some elite sprinters can drop the chain off when they hammer

I'm sure how relevant this is, but many of the sprint world records still stand from the late 1980s, and were done on flexy, steel bikes........i think...............i'm sure.............i think :)

http://www.uci.ch/english/palmares/t...cord_index.htm

Heh, heh... I see Rory O'Reilly out on just about every ride I do. Interesting to see that he still holds the 500m record. The guy's pushing 50 now and he's still kicking everyone's asses... on a Vitus no less! :eek:

He doesn't.

25''850 ARNAUD DUBLE (FRA) 10.10.2001 LA PAZ (BOL)

His time is still one of the fastest ever though.

The UCI need to tidy up that page up and update it. The Aussie Teams pursuit record from nearly 18 months ago isn't on there yet!

The spring energy isn't "lost" or otherwise disappears. It's transmitted elsewhere. Where it the question.

Unless you suddenly stopped pedaling or started pedaling backwards, the torque from the spring system is transmitted back to the drive train. The only way it would be "absorbed" would if the leg was not applying enough pressure on the pedal to overcome the torque of the spring system.

That would be true if you kept constant torque all the way around the pedal stroke, but explain what happens at the deap spot.



Although you might be placing downward pressure, the rotational torque of the recreational rider is probably zero, or very close to it at the dead spot, and this is where the built up "spring energy" is lost.

The point of my first post is that the spring system is way, way faster than the cadence of pedaling.

At the deadspot nothing happens because the spring's energy has already been put back into the drivetrain. It doesn't just sit around waiting for the deadspot.

In fact, it the lack of constant torque that allows the spring to rebound during the second half of the power stroke when the torque is decreasing. Once pressure on the pedals starts decreasing immediately after the zenith of the power stroke, the spring's energy starts feeding back. The spring has sprung long before the deadspot.

Yours is a common misconception.

So if I were to use a set of bamboo cranks that really built up some energy, it would have no net effect on the power output to the cranks? If all this energy from flexing were not wasted, then why would all the component manufacturers be so eager to tout stiffness qualities?

Just like with designing a lot of things, it's about compromises. Notice that many bikes use shaped tubings for their bikes? They're shaped for a reason. Round tubes behave similarly in all directions, but shaped tubes might be extra stiff in one direction and flexy in another. For all the parts of the bike that are involved in power transmission, you want them to flex as less as possible. For example, you don't want your BB spindle to be doing a see-saw motion when you're sprinting-->you want that displacement to go toward turning the crank (that's why more and more cranks are going with outboard bearings). Similarly for the front area (if you care about the handling of the bike). You want the bike somewhat flexy when it comes to having shock/vibration transmitted from the road to the rider. It's unlikely you'll see suspensions on road bikes, so you address this issue by trying to address the shock/vibration closes to the source (the parts connecting the wheel to the frame)-->use carbon fiber fork on the front, and things like S-bends on the seat stays, carbon seat stays, etc. So, frame flex is good or bad depending on where it is on the bike.

Yes, but is the reason marketing or engineering? In the case of ti frames, that is not an idle question.

And exactly how many people would pay money for these bamboo contraptions? The purchaser, no matter how engineering challenged, has the final say in design.

Moreover, any point taken to its extreme can be perverted and thereby disproven. My remarks relate to existing, typical bicycles.

* * *

Ironic that you reference Keith Bontrager in your posts. He was one of the first bike personalities to question the received wisdom that frame flex is bad. His article on frame flex, that flex is not necessarily bad, is still floating around the net somewhere, last I checked.

Like these?

http://www.calfeedesign.com/images/bamboo.jpg

Anyway, I still have yet to see anything to convince me that the energy that goes into flexing the frame/drivetrain eventually goes back into it as you continue the pedal stoke.

BTW, if you're going to say "yours is a common misconception", you'd better have something better than your opinion to back it up.

Edit. Never mind.

http://66.102.7.104/search?q=cache:D...ame+flex&hl=en

Cached paged. Bontrager has apparently disowned Keith, except for marketing purposes. There are four previous pages. Anybody got a link to them?

BTY, where's the bamboo crank? And am I understanding you to say that is a typical bike? I was aware of Calfee's bike when I made my earlier statement, hence my use of the term "typical."

-------------->"Anyway, I still have yet to see anything to convince me that the energy that goes into flexing the frame/drivetrain eventually goes back into it as you continue the pedal stoke." <------------- Well I guess that about settles it. You told me, yessiree.

I don't know. I'm pretty comfortable with Keith Bontrager in my corner.