To flex or not to flex....that is the question.
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  1. #1
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    To flex or not to flex....that is the question.

    There are a lot of stiff carbon frames out there today.
    Think about this- When do you have a frame that's too stiff?

    If you look at the motorcycling world, they've had to de-tune their chassis' to keep them hooked up and prevent chatter. This resulted in a more fluid and connected feel which was better for the rider.

    So we all want a "vertically compliant" frame but what about laterally? The marketing is "stiffer, stiffer, and more stiffness. Eventually won't there come a point where you want to program in a little torsional flex to ease the knees and provide a "live feel" in the frame-rather than one that doesn't work with you?

    thoughts?
    "People ask me what I'm on. What am I on? I'm on my bike - busting my ass - six hours a day. What are you on?"
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  2. #2
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    If your bottom bracket and crankset flexes, its robbing power. If your frameset is twisting when your standing/sprinting, you're losing power. If your seatpost/saddle doesn't have flex, it tires you out prematurely, because the road shock is getting transferred directly to your core. Builders and engineers have been designing bike frames around this criteria since forever. Your legs and knees have a ton of compliance built into them already.

  3. #3
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    Motorcycles were so powerful (500cc two strokes) that they could not put the power down to the pavement which led to chatter. When the delayed ignition Honda's came out, that worked more like twins, they realized they could retain most of the stiffness.

    I believe that cycists will want "feel" and torsional stiffness- a sweet spot where maximum forward movement occurs with each pedal stroke but does not feel like a "pipe."
    :It tastes like burning.

  4. #4
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    Framebuilder Dave Kirk has an interesting take on this... and it's different from the usual 'you can't ever be too stiff' philosophy/marketing:


    Frame Flex by David Kirk

    I'd like to share my thoughts about frame flex with you, as I believe this is a widely misunderstood topic. I'll point out at the onset that this is not an engineering study and that I have no hard data to support my conclusions. Rather, these are thoughts and conclusions that I have come to after 15 years of being a professional builder and 25 years as a serious cyclist.

    I'll start off by defining a few terms. As I see it there are 3 basic types of frame flex. They are:

    Vertical
    Lateral/torsional
    Bottom bracket/drivetrain


    We'll get deeper into these definitions in a moment. But first I think it's important to understand that these different types of frame flex are more or less related to each other.

    That is, given traditional construction techniques and materials, it is not likely that a frame would be very soft in one direction and very firm in another. For example, making a frame stiffer torsionally usually makes it somewhat stiffer vertically and in the bottom bracket area. So it is difficult to separate the three flexes.

    That said, a builder can consider the tubing material, diameter and butt lengths, as well as each tube's placement in the frame. Each of these factors will allow the builder to reach the best solution and make the most of a design.

    Let's have a look at how the three types of flex influence what you feel out on the road.

    Vertical Flex
    A frame with the proper amount of vertical flex will have a very smooth ride. More importantly, it also offers better control and handling. A frame that is too stiff for its rider will tend to have its tires lose contact with the road surface. When the tire skips off the road the rider has no control of the direction or speed of the bike. Even though the tire might lose contact with the road for only a brief instant, it is significant in the handling of the bike. A rider feels this most often during high speed cornering or descending. You might feel as if you need to use the brakes to maintain control while others just carve through the turn… then you get to chase back onto the group.

    In my opinion it's hard to design and build a bike that is too soft vertically. Long before that happens the excessive lateral and drivetrain flex will rule out the design.

    Lateral/Torsional Flex
    For the sake of simplicity I'll refer to this as torsional flex. Torsional flex is when the head tube and seat tube are twisted relative to each other, and thus are no longer in the same plane. All frames, regardless of material or quality have a good deal of torsional flex. Try this sometime - sit on your bike with it pointed straight at a mirror. With your butt on the saddle and your hands on the brake hoods, push your hands to the left and your butt to the right. Watch the frame twist. Fun, isn't it?

    Is torsional flex good? No, not really, but it is nearly unavoidable. If a frame has too much torsional flex for the size and weight of the rider, then it will feel vague and unstable at most speeds. A builder could, if he wanted to, all but eliminate torsional flex from a frame. But because the three flexes are intimately linked, the bike would ride very poorly due to inadequate vertical and bottom bracket flex.

    Bottom Bracket/Drivetrain
    I believe that this is the most talked about and least understood of the three flexes. You often hear people say that the stiffer the better and that any bottom bracket flex results in a loss of energy. I firmly believe that to be untrue. Let's look at what causes bottom bracket flex and what it leads to.

    When a rider pushes down on the pedal he also, for better or worse, pushes sideways. This is because the pedal is off to the side of the bike and is not in the bike's centerline. When the rider pushes down with the right foot the bottom bracket flexes to the left. The common belief is that the energy used to push the bottom bracket sideways is lost forever.

    I contend that it's not lost, but stored to be returned later. Our physics friends will remind us that energy cannot be created or destroyed. It can be converted to different forms such as heat or light, or it can be stored. In this case the majority of the energy that goes into flexing the frame is stored in the frame itself.

    When the bottom bracket is pushed to the side, it stays there until the force that was holding it there is released. So at the top of the pedal stroke the bottom bracket starts its sideways move and at the bottom of the stroke it returns to neutral. In returning to neutral it applies that returned energy to the drive train and then to the road. This flex and return smoothes out our power transmission to the ground, making acceleration smoother and optimizing traction.

    So what happens if a frame is too soft or too stiff in the bottom bracket for a given rider?

    If the bottom bracket is too soft, it will deflect so far that it doesn't have time to return back to its neutral position before the next pedal stroke on the other side. This will feel inefficient as well as unstable… not a rewarding experience.

    If the bottom bracket is too stiff for the rider something very interesting happens. We've all ridden with someone whose rear tire makes lots of scuffing noise when they are out of the saddle. What does that noise come from?

    When the rider pushes down (and unavoidably sideways) and the frame can't flex and store the energy, something has to give. So the tire loses hold of the pavement, resulting in a sideways scrub. In this case energy from the pedal stroke is lost as the tire slides sideways on the road, neither propelling the bike forward, nor returning energy into the next pedal stroke. So it's easy to see why a frame that is too stiff would be slower.

    So what are the ways the designer and builder can control and determine the amount of each flex a frame will have? This is done by choosing the proper tubes. The best tubing is available in a variety of diameters, wall thicknesses, and butt lengths. Let's take a look at these things separately.

    Tube diameter has the largest effect on frame stiffness. The bigger the tube the harder it is to bend or twist it.

    Wall thickness has a larger effect on bending that it does on twisting. Therefore changing wall thickness has less effect than changing diameter and is used to fine tune ride quality.

    Butt length and placement is an area that few builders explore. Outside the one-man shop, almost no one considers it. Each end of the tube has a butt, or a thicker section of tube. By carefully choosing how much of the tube to cut off each end to get the correct tube length, a builder can fine tune the ride and road feel.

    Now, for the wrap-up.

    What does all this mean to the rider? In my opinion it means a given rider needs a frame with the right combination of the three flexes. And because all riders are unique their bikes should be too. Take the time to really communicate with your builder and he'll take it all in and put together a dream bike. A bike with the proper amount of flex will help you ride faster, more comfortably, and will create that sweet feeling of a finely tuned machine.

    You've been reading too long now… it's time for a ride.

    Thanks for reading.


    http://www.kirkframeworks.com/Flex.htm
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  5. #5
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    The notion that flex takes away power is only true if:
    1. The flex turns primarily into waste heat, and does not go back into the system in a useful way.

    2. The lack of flex does not damage the power plant.

    One has only to watch someone use something like this to understand that some sporting goods store energy and give it back:
    http://www.youtube.com/watch?v=R1O1YaD-Gpw

    The damage I mention is no different that the reason engines are mounted on isolation blocks, and is why we don't all ride Cannondales. On top of that, a UW Madison researcher found that BB flex smoothed the pedal stroke, increasing efficiency.

    Perfectly rigid bikes are "more efficient" only in specifics. Overall, they are more fatiguing and may transmit less actual wattage to the road than a bike with more natural spring to it.

  6. #6
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    I think there's way too much focus on BB/drivetrain stiffness (with thanks to Dave Kirk).

    I'm not sure what use MORE lateral flex would do for me, although I suspect it might help keep wheel traction on high speed corners such as in criteriums.

    As long as the frame flex doesn't cause any handling or safety issues, it's likely stiff enough. And frames have been stiff enough for decades.

  7. #7
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    Not really

    Quote Originally Posted by turbogrover
    If your bottom bracket and crankset flexes, its robbing power. If your frameset is twisting when your standing/sprinting, you're losing power. If your seatpost/saddle doesn't have flex, it tires you out prematurely, because the road shock is getting transferred directly to your core. Builders and engineers have been designing bike frames around this criteria since forever. Your legs and knees have a ton of compliance built into them already.
    Your statements are not really true to any substantial extent, and certainly cannot be backed up by any evidence. In fact, there is evidence that if a frame is too stiff, it is actually slower when the road gets a little rough. As others have noted, in order for you to lose energy due to frame or drivetrain component flex, the energy would have to be turned into heat. In most cases, flex results in a sort of "pushing the spring" effect, and that energy is returned as your foot goes around the pedal revolution.

  8. #8
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    The "fix" for torsional flex was invented by Frank W. Schwinn who was issued patent number 2,151,533 in 1939. Schwinn called it the "cantilever" frame, and the seat stays were carried forward to the down tube in an arc, so that if dynamic forces on the frame attempted to twist the head tube out of alignment with the seat tube, one of the seat stays would go into compression and the other into tension, resisting the twisting forces. In fact, the first "mountain bikes" used on Mount Tamalpais in Marin County were built up on old Schwinn cantilever frames because of this resistance to torsional flex. The problem, of course, is that extending the seat stays forward adds weight, and in normal road racing frames torsional flex is not a significant problem.



    Dave Kirk is absolutely right about conservation of energy. Pedalling energy that is sufficient to flex the frame in the BB/drivetrain is simply stored in the frame itself and returned as motive energy when the frame unflexes; it isn't lost.
    -Stan
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  9. #9
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    Quote Originally Posted by Kerry Irons
    Your statements are not really true to any substantial extent, and certainly cannot be backed up by any evidence. In fact, there is evidence that if a frame is too stiff, it is actually slower when the road gets a little rough. As others have noted, in order for you to lose energy due to frame or drivetrain component flex, the energy would have to be turned into heat. In most cases, flex results in a sort of "pushing the spring" effect, and that energy is returned as your foot goes around the pedal revolution.
    Maybe you should try reading more closely then. I simply responded to the OP's questions. I didn't invent any new concepts here. I'm not even disagreeing with you.

    btw, from an engineer's viewpoint, a frame also acts like a shock damper. The force you transmit into frame flex doesn't all return to pedal force energy again. Some of it gets lost, just like you said. Otherwise we'd be riding full suspension road bikes.
    ...And just like I said, builders and engineers have been designing bikes around the parameters of stiffness and comfort since forever.

  10. #10
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    Quote Originally Posted by Scooper
    Pedalling energy that is sufficient to flex the frame in the BB/drivetrain is simply stored in the frame itself and returned as motive energy when the frame unflexes; it isn't lost.
    Sure it is! Energy is definitely lost. Its just that we've found a level of how much we're willing to sacrifice for ride quality. That also changes a little depending on the individual just like Dave Kirk said.

  11. #11
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    Quote Originally Posted by turbogrover
    Sure it is! Energy is definitely lost. Its just that we've found a level of how much we're willing to sacrifice for ride quality. That also changes a little depending on the individual just like Dave Kirk said.
    Only if the frame is so stiff that the rear tire scrubs instead of providing drive traction (like Dave Kirk said).
    -Stan
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    Quote Originally Posted by turbogrover
    Sure it is! Energy is definitely lost. Its just that we've found a level of how much we're willing to sacrifice for ride quality. That also changes a little depending on the individual just like Dave Kirk said.
    The only energy that is lost is what is converted into heat energy as a result of friction, and that amount is minuscule. This is 5th grade physics here, not rocket science. What people are believing is energy loss is simply an alteration of impulse, not a reduction of energy, and marketing departments seem to pray of this ignorance (as they tend to do).

    Feynman Lectures on Physics, 4-4

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  13. #13
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    I think this is a very interesting question - I recently built up a full carbon bike that isn't as stiff as for whatever reason I hoped it would be, and it got me thinking about this question.

    I think one can definitely sell a bike on the idea that great BB stiffness feels faster - stomp on the pedals in a climb and the difference is clear. I think plenty of folks who buy Madones and SuperSixes have been sold on them this way. But maybe it's just that your acceleration is more uneven, as opposed to a somewhat more flexy frame that spreads your power over the pedal stroke a little bit.

    I also know that after a couple of hard gravel races this spring, my new bike has been very comfortable and I've been very happy with that.

  14. #14
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    Energy is never "lost" or destroyed. Just a semantic error it seems.

    First law of thermo here:
    http://en.wikipedia.org/wiki/First_l...thermodynamics
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  15. #15
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    I find it interesting that people are trying to argue against me, while I am agreeing with them. Its also interesting and naive to think that the losses due to flex are minimal. There's a complex dynamic involved where the flex is needed in the frame. The losses are acceptable. Its just been controlled and refined over the years. I'm not talking about industry buzzwords.

    If the frame returned as much energy as people would believe, it would be very springy, and unrideable. The bb would be snapping back and forth, not just flexing. The damping characteristics in the bike and rider interface keep it from returning as much force as is put into it. The legs and body are acting as suspension dampers. This is an acceptable loss that adds to the ride quality, and performance of the bike. It just doesn't sound good to say it from a marketing standpoint.

    Watch how much your legs shake as you pedal over some rough pavement.
    Last edited by turbogrover; 03-20-2011 at 10:28 PM.

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    While it is fun to argue where "energy" goes, one has only to ride a thin tubed aluminum frame like a Vitus to encounter a frame where energy is lost to flex, compared to a flexy steel or Ti frame.

    I imagine that materials like aluminum that fatigue with every bending cycle has some bearing on where the energy goes, but I think that idea would take more than scans of textbooks or wiki links to demonstrate.

  17. #17
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    I think I'm a prime example of this.

    My first road bike... a 2005 KHS Flite 900 Team.. Basic entry level aluminum aero frame.. Carbon fork and stays. Rode the hell out of it and raced it. I picked up better wheels last summer with a powertap and immediately noticed a difference in the stiffness of the bike. Huge improvement.

    This winter I treated myself to a new bike.

    Built myself up a 2010 Cannondale Supersix.
    Rode it all winter on the trainer and just took it outside on it's maiden voyage this past Thursday. What a huge improvement!!! Compared to my old bike this bike it impossible to flex. Smoother ride and zero loss of power from twisting or flex. I feel a lot faster on this bike and more comfortable. My engine is happy to apply it's power to a stiffer transmission.

    I can't see wanting any discernible flex in a bike now that I've experienced this.
    ~ Long Live Long Rides~

  18. #18
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    Quote Originally Posted by Kerry Irons
    Your statements are not really true to any substantial extent, and certainly cannot be backed up by any evidence. In fact, there is evidence that if a frame is too stiff, it is actually slower when the road gets a little rough. As others have noted, in order for you to lose energy due to frame or drivetrain component flex, the energy would have to be turned into heat. In most cases, flex results in a sort of "pushing the spring" effect, and that energy is returned as your foot goes around the pedal revolution.
    but this neglects the fact that the energy may be returned to your food/leg, but your muscles cannot convert mechanical energy into glucose. Take a spring and compress it against a wall, then allow it to return that energy to your arm. What can your arm do with that energy? Nothing - in fact your arm will be fatigued and the energy will be lost as heat. The fallacy of the ideal spring view of frame flex is the notion that the returned energy can be converted back into usable energy. If this were true, you could compress/decompress a spring against a wall all day without fatigue.

  19. #19
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    My favorite bike, the one I go fastest on and the one I still enjoy riding after 80 miles is my Serotta NHX, designed by the aforementioned Mr. Kirk.

    It's a lugged steel SPX frame with shaped chainstays and pencil-thin seatstays.

    It is NOT stiff. It's like riding a coiled spring. It's pretty amazing.

  20. #20
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    David Kirk is genus. I have had several bikes he has designed, and he knows what his is talking about.

    For what its worth, I have owned over 40 top end bikes in the last 4 years. I hate all the super stiff carbon race bikes. Lets face it they ride like crap. Some of the highend custom jobs are making a good product, Parlee, Crumpton, Calfee. etc., mostly tube on tube construction, but the mass produces mono stuff is another story.

    If you are spending 15-20 hours a week on a overly stiff bike it really sux. The stiffness of the bike is not going to put you on the podium and its not fun to ride.

    I can produce more power on a very stiff bike over 5s sprint but its only a very small amount. Once you hit 10s, all bikes are about the same. On rough surfaces, power is decreased and the stiffer the frame the more your power is decreased. I am not saying a noodle bike is good, but somewhere between the super stiff and noodle is the sweet spot.

    Of all the bikes I have owned that you see the Pro Tour guys on, I can say they provided a less than desirable ride. I think the designers have went too stiff.

  21. #21
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    Quote Originally Posted by rockdude
    David Kirk is genus. I have had several bikes he has designed, and he knows what his is talking about.

    For what its worth, I have owned over 40 top end bikes in the last 4 years. I hate all the super stiff carbon race bikes. Lets face it they ride like crap. Some of the highend custom jobs are making a good product, Parlee, Crumpton, Calfee. etc., mostly tube on tube construction, but the mass produces mono stuff is another story.

    If you are spending 15-20 hours a week on a overly stiff bike it really sux. The stiffness of the bike is not going to put you on the podium and its not fun to ride.

    I can produce more power on a very stiff bike over 5s sprint but its only a very small amount. Once you hit 10s, all bikes are about the same. On rough surfaces, power is decreased and the stiffer the frame the more your power is decreased. I am not saying a noodle bike is good, but somewhere between the super stiff and noodle is the sweet spot.

    Of all the bikes I have owned that you see the Pro Tour guys on, I can say they provided a less than desirable ride. I think the designers have went too stiff.
    I can agree with what you're saying, but I also think the amount of "sweetspot" stiffness will be different for different riders or riding styles. The mass produced bikes have to find an average "good" spot which might be a little softer than what the pro tour guys need.

    Another justification for buying a custom if you can afford one.

  22. #22
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    One of the reasons I like my BMC over some of the other super stiff bikes. I can feel some compliance when I pedal hard, and some give when I crank on the handle bars. Under normal riding though it feels fine, no twist or flex evident. I think they got the frame just right on that one.
    I didn't really think about this topic when I made my decision, but I'm glad I read it now as it confirms a belief I have even though I was ignorant as to why I held that belief.
    "I felt bad because I couldn't wheelie; until I met a man with no bicycle"

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    Quote Originally Posted by stevesbike
    but this neglects the fact that the energy may be returned to your food/leg, but your muscles cannot convert mechanical energy into glucose. Take a spring and compress it against a wall, then allow it to return that energy to your arm. What can your arm do with that energy? Nothing - in fact your arm will be fatigued and the energy will be lost as heat. The fallacy of the ideal spring view of frame flex is the notion that the returned energy can be converted back into usable energy. If this were true, you could compress/decompress a spring against a wall all day without fatigue.
    I think the point is that a flexed frame coils up - the wheelbase shortens during torsional flex, so the frame doesn't return the energy to your body, it returns it to turning the wheels as the frame uncoils. Nobody's saying that the energy goes back into your body... and it's not like you can sit there all day doing pushups against a floor that doesn't move without getting fatigued, either.

  24. #24
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    Quote Originally Posted by thefutureofamerica
    I think the point is that a flexed frame coils up - the wheelbase shortens during torsional flex, so the frame doesn't return the energy to your body, it returns it to turning the wheels as the frame uncoils. Nobody's saying that the energy goes back into your body... and it's not like you can sit there all day doing pushups against a floor that doesn't move without getting fatigued, either.
    Wha??

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    eh, delete

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