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Crank Review #5

Viewing 14 posts - 16 through 29 (of 29 total)
  • #93947

    That’s the matter under discussion: whether there’s energy storage or power dissipation. If it’s for example, 1.6 joules (watt-seconds) of energy stored in the crank, and you’re riding at 320 watts, that’s 0.05 seconds of power wasted, assuming you have full force on the crank when you cross the line. I’m not worried about that, as I’m not a sprinter: I’m more worried about energy dissipation, which adds up to much larger accumulated losses.

    I’m really excited to see how the new SRAM crank scores, as well as the new Camillo.


    I think Jason does a great job explaining some of the challenges in measuring the energy lost when the crankarm is deformed. He makes the very conservative assumption that all of the energy used to deform the crankarm is lost but allows that some of it may provide useful work later in the pedal stroke.

    I think there is some confusion on when this “later in the pedal stroke” happens. Understandably some would think that this would happen on the upstroke and see little usefulness. The crank arm acts similarly to a very very firm springs and it springs back very quickly. Jason did not measure exactly how quickly and we can understand why, it happens continuously and very quickly as your leg reduces the force on the pedal during your downward stroke. Understand that the energy is not stored, held and then a tenth of a second later toward the bottom of the stroke (or worst on the upstroke) it is realized. As soon as the force on the pedal has reached a maximum, the crankarm is fully deflected. There is essentially no lag, and as soon as the force on the crankarm is reduced just a little the crankarm is now starting to release. Technically there is a lag, but my whole point is that it is so small that Jason cannot measure it. I suspect that it is something on the order of the speed of sound in aluminum or carbon fiber. Given the distance of deformation is less than a centimeter we can guess that the crankarms spring back quickly once the force is reduced from the maximum.

    So while Jason has not measured the precise time it takes to release this energy, a good portion of that energy was stored very near the maximum pedal force and is will also be released very near the maximum pedal force. By the time the downward force on the pedal is significantly reduced, most of the energy that was stored in the pedal is no longer stored, it was either returned to the drive train or lost as heat. How much was lost as heat of course depends of the crankarm and while these things are difficult to measure precise we can have a good idea on “when” they happen.


    There is no doubt that the maximum rotational crank/pedal deflection (as opposed to any lateral deflection) will be when the crank is in the forward horizontal position close to where the pedal force is maximum. All the rotational deflection doesn’t disappear until the pedal force producing rotation of the crank is zero. This should be near bottom dead center (unless you are an appostle of Frank Day and his Powercranks). So, it is over this quadrant that the stored energy in the crank is dissipated somewhere. I believe most of the energy is lost in diminishing the rotational speed of the lower leg while only a small amount provides forward motion of the cyclist. Why do I think this? I imagine this situation being somewhat similar to a small cart being driven and pushing a much larger cart. There is a spring between the two which is compressed and transfers a pushing force from the smaller cart to the larger cart. So what happens to the energy in the spring when the smaller cart reduces its pushing force to zero over a short time interval. This is somewhat like an elastic collison where momentum is conserved. IOW, the momentum gained from the push of the spring by forward motion of the larger cart must be the same as the momentum lost by the smaller cart (the force in the spring pushes the larger cart forward and the smaller cart backwards). Now if the ratio of masses is 10:1, then the decrease in velocity of the smaller cart will be 10 times the increase in velocity of the larger cart. The change in energy (increase) of the two carts will be equal to the energy returned by the spring. I’ve taken a few liberties in ignoring some things to better describe the first order effect. For a bicycle crank, the ratio of crank motion to forward motion of the bicycle will have a significant effect on the distribution of the energy in the crank. I say this because the force on the crank is reduced considerably by gear ratio and crank arm/wheel radius ratio. Assuming this ratio is 25mph for the bike versus 5mph for the crank, a ratio of 5:1, the distribution of energy will be a combination of the equivalent mass and speed ratios. Assuming a rider mass/lower leg mass ratio of 10:1 and a speed ratio of 5:1 then the distribution of energy will be close to 50:1. That is, 50 parts lost to the lower leg versus 1 part producing forward motion. IOW, this is so small it is easier to assume all the energy is lost. My 2 cents.


    Great article and discussion; thanks. Two comments and a question:

    1) All the discussion about whether the strain energy is returned as propulsive force or lost is, it seems to me, sidesteps the main issue–mentioned more than once early on–that any small differences in the crank mechanics is swamped by the messy issue of human kinematics. If one could measure it–difficult, but not impossible; with what level of uncertainty in that measurement is the question–I think it’s quite possible that much of the stored energy IS returned as propulsive force. Maybe some clever researcher could set up a controlled experiment using human subjects and cranks with variable stiffness as well as elastic and inelastic properties and see how the rider’s power output changes at a given HR? One advantage of such a scenario is that you could set it up so that the variations are large enough to detect–i.e., more than 1.6 W, the difference between the most- and least-stiff cranks in this test. OK, have at it.

    2) It’s been mentioned that carbon may have damping where metal (e.g., Al) has almost none within its elastic limit. To add to that, carbon is highly anisotropic, and even batch-to-batch (or arm-to-arm, talking about crank production) variations in carbon layup and resin content and distribution can cause significant variations in the stress/strain relationship. A perfect example of this issue is the new Stages crank arm-mounted power meters. Note that you can get them with a SRAM Rival arm (Al), but not a Red arm (CF). Why? Attaching a strain gauge to a CF arm doesn’t produce a consistent stress-strain output. Measuring the damping characteristics is similarly complicated by the material and part-to-part manufacturing variability.

    3) Where’s the EE crank?


    Many thanks for the excellent reviews, they are very useful and great fun to read! I notice that you set up the caliper at the loading rod’s anchoring end rather than at the ‘pedal’. Why this? It would seem to require the added step of calculating and subtracting the elongation of the loading rod and strain gauge from the measured strains.


    Dave, we did test a newer version of the EE crank than we had in the previous review, but decided to omit it since the production version is still slightly different. We hope to have a production version shortly to test in the next round along with a few others.

    I’d also like to point out that we’re looking at pretty much the stiffest cranks on the market. This review focuses on a an average deflection range of about 0.2 to 0.3 but other cranks tested have been in the 0.45 range. This upper end of the range is where square taper cranks tend to fall. I only mention this because square taper is still popular with a lot of track cyclists where wattages are higher and events are often decided by very small margins.


    In frictionless elastic collisions (which is essentially what the cart analogy is) the transference of momentum between bodies is proortionally greater than the transference of energy. Althouh tat is perfectly true, it is hardly relevant to the present discussion because there is no difference in the speeds of the foot and the pedal.


    I’ve often wondered why track cyclists put up with the dearth of integrated-spindle cranks. I’ll put my SweetWings (now that they sport Enduro ceramic bearings) up against anything currently on the market for the “board crowd”. By the way, Jason, Cudos for everything you guys do! I’ve worked in the bike industry for 38 years, have met most of the movers and shakers in it, and where do I go for insight and inspiration? Right here to Fair Wheel Bikes! (p.s. Do you guys need another fit and sales guru? I also turn a mean wrench…)


    Hmm, I just saw Francisco’s repsonse to my elastic collision cart analysis. I think Francisco is missing the point that the drive train (from rear wheel to chain ring) maintains near constant momentum but the foot and pedal end of the crank move backwards (relatively) when the crank arm springs back with reduced pedaling force.


    So, madcow, what do we trackies have to do to get the ee/Pacenti cranks with a track spider?


    I only mention this because square taper is still popular with a lot of track cyclists where wattages are higher and events are often decided by very small margins

    What about the Octalink BB interface? Would be personally interested how the track SRM would test…

    My question is what does this all really mean? From the flexiest crank in the test to the stiffest – is this something that you can really feel when riding? I am using an older style square taper bottom bracket SRM crankset – would I really feel a difference going to something else? Or is this just a fun math problem – I do like fun math problems.

    Have you ever wondered if there was more to life, other than being really, really, ridiculously good looking?

    @pritchet74…..are you familiar with the fairy tale about the princess and the pea? My take on it is in line with this story. Crank arm bending probably contributes to only a small percentage of the total drive train flex that the difference between the stiffest and flexiest crankarms would only be detectable by a princess :) Taking the insert soles out of your cycling shoes would be more noticeable. This is just my opinion. Mathematically however, the difference in lost power between the stiffest and flexiest crankarms is real and could make all the difference in winning or losing in a very close finish (a tire width in front).


    I am using an older style square taper bottom bracket SRM crankset – would I really feel a difference going to something else? Or is this just a fun math problem – I do like fun math problems.

    Same – SRM on my road bike is also Campag square taper…
Viewing 14 posts - 16 through 29 (of 29 total)

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