Global Cycling Networks Video on Frame Flex

Global Cycling Networks just published a video in which they did an experiment that many of us have been talking about: Load up a frame with flex, and then release that energy. The rear wheel turns as the energy is returned to the drivetrain. It’s nice to see it in practice…

Also nice to hear: “I wonder whether frame flex is going to be the new tire pressure. Go back 10 years, and we all knew that harder tires rolled faster. And you could feel it as well. Except that now, we know that lower pressures can roll faster.”

Watch the video above, or click here to see it directly on YouTube. Enjoy!

To read our recent post about how frame flex actually can contribute to making you faster, scroll down or click here.

About Jan Heine, Editor, Bicycle Quarterly

Spirited rides that zig-zag across mountain ranges. Bicycle Quarterly magazine and its sister company, Compass Cycles, that turns our research into high-performance components for real-world riders.
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22 Responses to Global Cycling Networks Video on Frame Flex

  1. Tom Hovan says:

    old experiment. no less valid though. see “Real World Example You Can Try” on https://web.archive.org/web/20140113125615/http://www.bikethink.com/bicycle-frame-efficiency/

    • Yes, Gary Houchin-Miller’s Bikethink web site pioneered many of the ideas we have about frame flex and why it doesn’t matter. Let’s see how long it takes for the next step to become widely accepted – that returning that energy to the drivetrain at just the right time can have rather large benefits.

  2. Max Sievers says:

    I bet Simon secretly studies BQ!

    • Not so secretly. They have mentioned Bicycle Quarterly in past episodes. That is how they were among the first to talk about ‘suspension losses,’ when all the other experts still were trying to explain how energy that made the bike go up on vibrations and bumps somehow was lost forever.

      • Archetype says:

        GCN began mentioning you Jan and BQ after I had posted info about the tires and tire pressure research a few years back on their youtube site in the comments section on subjects involving tire sized and pressures. Soon thereafter I noticed that they were looking in to your information. As well as taking specific insight from my LeanIn Cornering site and practices on cornering and countersteering. Kudos to GCN for embracing proven and useful information.

      • We are grateful to all our readers who’ve spread the word about BQ‘s research. It’s nice to see a mainstream program that is open to new ideas, and willing to look beyond the accepted wisdom, which isn’t always correct.

  3. Bryan Willman says:

    The thing to remember for all of these parameters (tire size, wheel size, frame flex, weight) is that there is a useful range. Flat tires are very slow. There IS such a thing as a too wide or too heavy tire. Decades ago I rode some bikes so flexy (and heavy!) that they weren’t really safe.

    None of that means that a 1mm wide tire at 500psi on an infinitely stiff frame is a good idea, just as a flat tire 100mm wide on a wet noodle frame isn’t a great idea.

    Predicition – the *interaction* between tire damping and frame stiffness will be the Next Big Topic – softer tires on stiffer frames? Harder tires on softer frames? The reverse?

    • mike says:

      You’re prediction is an experience, most of us already made.
      I had 2 cross bikes, an old and cheap and very (too?!) flexy one mode of steel and a new, bomb stiff one made of aluminium.
      I tried the same 2 wheelsets with the same 32mm tyres in both:
      – low profile rim, 32 DD spokes
      – deeper rim, 36 2mm spokes

      The low profile wheelset with less and smaller spokes was able to make the stiff frame more comfortable. Still agressive, but no longer ‘undestroyable’ hard. I rode much more ‘elegant’ with that setup.

      The deep rim 36 2mm spokes wheelset let the old, flexy frame feel like an old garden chair with wheels made of concrete. The bike was ‘dead’, even it was still flexy.
      But even the low profile wheelset couldn’t cure the weak frame. It was better matching and more comfortable, but the bike still had a weak handling.
      I transferred the whole set of components later to another (better) frame of the same size and now all feels good. Still different to the modern aluminium frame (more comfortable, less agressive), but harmoniuos.

      My conclusions:
      – There is a thing like a “to flexy for me” frame
      – Different types of wheelsets behave different (some wheelbuilder say, that a wheel is just stiff or underdesgned)
      – the stiffness of a frame and the wheelset both influence the way you ride and there are better and worse matching couples.

      Of course,
      tyres, handlebar, stem, seatpost, etc. influence the way you ride as well and ideally the whole setup matches your needs. There might be ways to cure a mismatch of one component with a countersteering other component …

      • Wheels can be a factor, especially with narrow tires. We once tested a Toei with thin fork blades, and the front seemed to be a little less supple over really rough pavement than my Alex Singer with similar old-style Reynold 531 fork blades – both bike with the Rolly-Poly tires we used back then on test bikes. The Toei had ‘aero’ rims. We switched front wheels, and now the Singer (with the ‘aero’ rims) had the slightly harsher ride.

        I suspect that if you repeated that test with 42 mm-wide supple tires, the tires shock absorption would drown out any influence of the rims. That is also why tubular tires make less sense with wide, supple tires, as their advantage will be too small to notice. (Tubulars are useful to avoid pinch-flats in cyclocross, but that is another issue, and we now can run our tires tubeless to reduce that problem.)

  4. alderbanks says:

    Jan,
    I believe that you have for a long time maintained wider tires roll faster, which is experimentally demonstrated and explained theoretically by contact patch shape and hysteresis. Tires run at a lower pressure having a lower rolling resistance and rolling faster are clearly two different things in the real world. Schwalbe has information on their website that states wider tires have less rolling resistance than narrow tires “at the same pressure”. I think that you can reason that lower pressure could result in lower rolling resistance if the testing is done on an “imperfect surface” due to suspension loss (?), but is that not very difficult to quantify? Michael Banks

    • Schwalbe’s information is many years old. It still shows that tires roll faster at higher pressures. They should update it!

      Yes, it’s the suspension losses that cancel out any reductions in hysteretic losses (tire deformation) at higher pressures. Suspension losses are easy to measure – we did that many years ago. What was interesting is that suspension losses occur even on very smooth roads, but then, you know how much the bike vibrates when bolts that aren’t properly tightened come loose almost immediately as you ride…

      You can read more about suspension losses here.

  5. Chris Goebel says:

    What about the chain? Before they release the brake the frame is flexing and the chain is stretching. When they release the brake everything goes back to a neutral state and releases energy into the bike. The chain is perfectly aligned to release energy into the wheel. The frame could be experiencing also sorts of losses and I’m not sure if the energy in the frame is going into the wheel or not. How do we know the frame is the dominant source of energy?

    • It doesn’t really matter where the energy is stored when you flex the bike, just that it returns to the drivetrain. However, when you look at their video, you can see that the frame flexes more than the other components. If we called it ‘bike flex’ instead, this would be more clear. What their experiment does is flex the bike in a realistic way (push on the pedal) and show that once released, the energy that has been stored in the bike (as flex) turns the rear wheel.

      Of course, the will be losses due to the frame heating up, but they are tiny. It’s easy to calculate these losses, and you’ll find that a metal frame is almost a perfect spring, with no significant damping. Carbon do absorb more energy, but even there, it’s infinitesimally small. I think everybody knows that carbon frames aren’t slower because they absorb more energy as they flex.

      The biggest issue that the ‘test’ doesn’t address is when that energy is released. It needs to be in sync with the rider’s pedal stroke, otherwise, it works against their legs. We have found some frame flex highly beneficial and other is very detrimental.

      • Francisco says:

        As an aside that planing also happens with other materials, I feel that syncronised feeling when riding my Time RXRS, a bike known for its moderately sized bottom bracket and headtube shells.

      • Absolutely! Most titanium bikes we’ve tested ‘plane,’ and so do the best carbon bikes. One good thing is that if you try to make a bike superlight, you use as little material as possible, so the bike inevitable gets some flex. And the best bikes do flex in all the right places, and only there…

      • Orin says:

        “The biggest issue that the ‘test’ doesn’t address is when that energy is released. It needs to be in sync with the rider’s pedal stroke, otherwise, it works against their legs.”

        Newton’s third law… What provides the reaction against which the flex unwinds into the drivetrain? I suspect that when you are in sync with the bike, it’s the inertia of your legs as they decelerate at the bottom of the pedal stroke.

        In the test, they held the pedal in place as the energy was released. As far as physics was concerned, no work was done by the leg, but as far as the leg muscles holding the pedal in place were concerned, it was an isometric exercise that required the muscles to expend energy just holding the pedal in place. (Assuming that the other foot was clipped in and they didn’t use the unbalanced weight of the leg to hold the pedal in place. Normally, with both feet in place, the weight of the legs tends to balance out; certainly, the symmetry of the cranks means that the same weight on each pedal will not produce a net torque on the cranks.)

        I believe there are some power measuring pedals that will map the forces around the pedal stroke – it would be interesting to see the results with a flexy vs a stiff frame.

  6. Grant McLean says:

    The video nicely demonstrates that if you change the relative position of the BB from the rear hub, it changes the distance along the chainline. This effect tugs the chain when they “release” the brake as the frame returns to the neutral position. The opposite is also the case – when loading the pedal, the frame deflects and shortens the chain length. I’m not an engineer, so i don’t know if this is a
    net effect or there is energy lost to other parts of the system that don’t make it into the chain.

    It reminds me of how the linkage in full suspension drive-trains can have a similar effect, changing
    the relationship between the crank and hub, mountain bikers call it suspension kick-back.

  7. Ian says:

    I’ve got one bike that’s fairly flexy, and one that’s fairly stiff, or at least stiffER. When I’m climbing in the saddle, the flexier frame feels “normal”, the stiffer frame feels like I have a motor! I can push a bigger gear with ease. But when I stand up, the flexy frame becomes a dance partner, spinning up the hill, and the stiffer frame dies, it refuses to go on unless I sit back down. Whats going on here?
    On either bike I go about the same speed. Of course the most important component is the legs, and I use the same ones on both bikes. The difference is a feeling, maybe a meaningless second or two.

  8. Paul Ahart says:

    I remember from past bike reviews in BQ that ultra-high performance bikes, like the Trek Madrone, performed best when pushed to their limits, which only very strong riders can do. For recreational riders, especially older riders like myself, a flexible frame offers considerable advantage, especially when climbing. I own two fairly new bikes, one with OS very light tubing, the other with 1″ top tube and 1-1/8″ down tube of ultra light tubing. On flat terrain, not much difference, but when climbing, the second bike outperforms the first. My wife has a similar bike, and commented, “I feel like I’m dancing with it.”
    I must say, the video shows brilliantly how frame flex works, and how having less leg strength allows the more flexible frame to perform well.

    • Absolutely! Back in the 1960s, racers used stiffer frames for road races, which are won with high-power accelerations, but more flexible frames for time trials, which require sustained efforts (at lower power outputs)…

      Andy Hampsten won that famous 1988 Giro d’Italia on a bike made from ultralight tubing (standard-diameter 7-4-7) against others on much beefier and stiffer frames – clearly, even very strong (and tall) pros don’t need ultra-stiff frames.

  9. David T. says:

    This does not show that a flexible bicycle is more efficient than a stiff bicycle.

    • You are right – that wasn’t the goal. GCN wanted to show that the energy that a rider inputs into the frame as flex isn’t necessarily lost, but can be returned to the drivetrain.

      Even with ‘planing,’ nobody claims that a flexible bike is more efficient (i.e., transforms more of your power into forward motion). Instead, the rider’s power output isn’t a constant, but varies depending on the flex characteristics of the bike. Optimizing the flex (rather than simply making the bike more flexible) allows the rider to put out more power, hence go faster.

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