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Crank Review #5

Viewing 15 posts - 1 through 15 (of 29 total)
  • #93747
    Keymaster

    Welcome back to the 5th installment of the crank shootout. We are not going to include all of the previously tested cranks in this review but will keep the most relevant/current ones. We’ll also be writing new descriptions as many of the cranks have changed over the years. You can see older cranks here: Perhaps the biggest change in this round of testing is the addition of a new contributor. We’re really thrilled to be able to introduce Jason Krantz. Jason has his masters from the University of Wisconsin in Engineering Mechanics and Astronautics, with a focus on the intersection of composite materials and finite analysis. Jason  has worked for some of the best bicycle companies in the industry and never fails to amaze me with the depth of his cycling related knowledge.

    Disclaimer: A lot of typing and numbers have gone into this article and we apologize in advance for any typos, but would warn that the possibility of mistakes is present.

    About the testing method: Each arm was preloaded with 50lbs to take up slack and then all calipers were zeroed out. Then another 200 lbs was added and the difference was measured in inches. Each arm was tested twice and an average of those two measurements is the result. A lower number represents a stiffer crank. These will be labeled as Deflection-D(Drive side deflection) and Deflection-ND(non-drive side deflection.

    Stiffness/Weight: Is determined by: ((1/average deflection)/weight)x100

    Notes about stiffness: According to received wisdom, pedaling stiffness is good. Stiffness implies efficiency, confident handling and gratifying response. As wonderful as all of those traits are, they’re only subjectively good—many people feel more efficient on a stiff bike. But it’s far from clear whether that aesthetically pleasing, from-my-quads-directly-to-the-road effect is actually faster. This is the question: is stiffer actually faster, or does it just feel faster? Looking at the numbers, we can see that average deflections range from roughly 0.20 inches to 0.30 inches. From this, we can generalize and say that the most flexible crank is about 50% more flexible than the stiffest crank. It’s easy to imagine that the stiffer cranks feel better, or have better “power transfer,” which is a particularly vague and ill-defined concept. But how can we move beyond “feel” and attempt to quantify whether a stiffer crank is better? The answer is, strain energy. Strain energy is simply the energy stored by an object as it is loaded. Quantifying how much energy stored by a given spring under a particular load is a basic problem that works perfectly well for understanding whether a stiff bicycle crank is better than a slightly less stiff crank. That is, you can think of a bicycle crank as a very stiff, oddly-shaped spring. The equations for calculating stored energy in a beam under bending are fairly simple. The following figure is taken from a Creative Commons-licensed engineering textbook by Piaras Kelly at the University of Auckland: strainenergy In the equation above, U is strain energy, M is applied moment (torque), L is beam length, E is the Young’s modulus (stiffness) of the beam material, and I is the moment of inertia of the beam cross-section. A crank is somewhat beam-like, but it’s not really a beam. And we’re not applying a pure moment but rather a force at a distance that creates a moment. This bending example is somewhat close, but it’s still not a very good approximation of a crank on a bicycle. We can get a very good approximation of a crank on a bicycle by using finite element analysis (FEA). To find out how much strain energy a typical crank stores, we can solve an FEA model to find the strain energy of each of the constituent elements. We can then add up all of those strain energies to get the strain energy of the whole crank. This strain energy can then be converted into absorbed power by assuming a cadence; we used 100 RPM for this example. In this way, we can determine exactly how much power goes into crank flex, which can then tell us how much crank flex matters to total power output. We used ANSYS, a well-regarded FEA program, to model a generic aluminum left crankarm (172.5mm) and half an attached 24mm steel bottom bracket spindle.The remote force works out to 250 pounds of force (lbf), and it is applied 60mm to the outside of the center of the pedal threads. This simulates applying the force through a pedal.

    Most of the action is happening on the inside of the crank; the interior elements report a higher strain energy value than the external ones do. But things get a lot more interesting when we dump the strain energy values for each of the elements to a spreadsheet. By summing the elements’ strain energy values, we can get the total strain energy for the entire crank and half BB spindle. The total strain energy of this crank under a 250-lb pedaling load is 4.604 Joules. As a unit, Joules don’t do much for most cyclists. We can convert them to a more useful unit by assuming a cadence of 100 RPM. At that cadence, this half-crank soaks up 7.67 Watts. The right-hand crank is usually stiffer than its left-hand counterpart, so it stores correspondingly less strain energy. So rather than doubling the left-hand figure, we’ll round down a bit to 14 Watts for the entire crank/BB axle system. 14 Watts might sound like a lot, and it is. But let’s keep in mind that this is a 250-lb force applied 1.67 times per second. A rider applying this force over 160 degrees of crank rotation produces an average power of 880 Watts, which few of us can sustain for long. And 14 Watts out of 880 is 1.6% To put this in perspective, if you were pedaling along at a steady 300 Watts, your crank would be absorbing 4.8 Watts of your effort. But those 4.8 Watts go into winding up your crank “spring,” which will spring back with nearly all the energy that was spent winding it up. Some of that spring-back energy probably helps turn the drivetrain while some of it may behave in a negative manner. However, there’s a fair amount of debate about how much energy is returned. The answer to the energy-return question involves kinematic analysis far outside the scope of this article. For now, we’ll assume that all of those 4.8 Watts spring back in a way that doesn’t help turn the drivetrain nor hinder it. As mentioned before, the most flexible crank in this review shows about 50% more deflection than the stiffest crank. Our FEA crank is quite flexible, and it absorbs 4.8 Watts of a 300-watt effort. Strain energy, roughly speaking, is inversely proportional to stiffness. We can use these relationships to calculate that at 300 Watts, a our flexible crank absorbs 4.8 Watts, or 1.6% of total power output. Meanwhile, a 50% stiffer crank absorbs 3.2 Watts, or 1.07%, in strain energy (technically, strain power). That’s a difference of 1.6 Watts (or 4.7 watts at our tested 880 Watts). And remember, this assumes that no strain energy is returned to the drivetrain. That’s not to say that crank stiffness is irrelevant, there is a measurable difference. It also provides all the psychological and “feel” benefits described at the beginning of this section. A stiff crank also incrementally improves efficiency by keeping bearings aligned, keeping the pedals more directly beneath your feet, etc.

    Notes about Crank Length: Over the years a lot of different arguments have been made about the benefits of longer/shorter cranks. None of which has really been thoroughly tested until Jim Martins study.  Martin showed that length didn’t statistically matter when it came to power, once power was averaged around the entire pedal circle and not just in the forward position, it turns out that shorter cranks (down to 145mm) produced more average power than a longer crank. This conclusion however considers only average power and not other factors which definitely have a bearing on real world use. Damon Rinard followed up the Martin study with some of his own testing comparing the aerodynamic differences in crank length.  In almost every case there was an aerodynamic improvement with the shorter crank and without a loss in power. So the power advantage and aerodynamic advantage, combined with shorter cranks generally allowing for a more aggressive or more comfortable position on the bike and less chance of repetitive motion injury we feel that shorter cranks are something most people should consider. We’re not saying they’re right for everyone, but if you’re on the fence as to which size is best for you, we suggest that you go for the shorter. If you’re interested in more on crank length we suggest reading the above articles as well as this article written by Frank Day for USA Cycling:

    Notes about BB standards: We’ve tested several different bb standards and have seen no coorelation between the type of bb and the stiffness of the crank, with one exception. True bb30 cranks do seem to produce slightly stiffer results than their traditional counterparts, however this difference seems to be pretty insubstantial.

    Notes about weight: Some cranks include rings and bb in their complete weights while others do not. To give an even comparison we’ve done each set using a pair of Praxis rings (124 g) and their own production bsa bottom bracket. This comparison we’ve labeled as corrected weight. So on to the review. These are the cranks we’ll be looking at in this installment of the review.

    1. Campag Record UT
    2. Extralite QRC2
    3. Kcnc Proto
    4. Lightning SL
    5. Lightning HD
    6. Rotor 3d
    7. Shimano Dura Ace 9000
    8. Sram Red 2011
    9. Thm Clavicula
    10. Thm Clavicula M3
    11. Tune Smart foot

    Data & Charts

    weightavgdefl s_w2

    Campagnolo Record Ultra Torque

    record

    Claimed Weight 690g w/bb
    Actual Weight 702g w/bb
    Corrected Weight 694g
    Price $590
    Available lengths 170, 172.5, 175, 177.5, 180.
    Q-factor 145mm
    Spindle 25mm
    Deflection-D 0.163
    Deflection-ND 0.279
    Average Deflection 0.221
    S/W 0.644

    Likes: Aesthetics, it’s a very simple and clean looking crank. Looks good on almost any bike. Q-factor is 2nd narrowest in the test which is great for most people. The rings shift quality and durability is also near the top of the charts.

    Dislikes: Setup. The design prefers tight frame tolerances, it’s often not a plug and play kind of crank and can sometimes have issues with creaking noises. PF30 bb has a known bearing/cup migration problem. Also the proprietary bcd on the compact is annoying in that finding aftermarket rings is almost impossible. The proprietary chainring bolts that are ridiculously priced are well beyond explanation but probably do produce a stronger spider tab.

    Extralite QRC2

    qrc2

    Claimed weight 485g no rings
    Actual weight 479g no rings
    Corrected weight 603g
    Price $660 without chainrings
    Available Lengths 170, 172.5, 175
    Q-factor 138mm
    Spindle 27mm
    Deflection-D 0.222
    Deflection-ND 0.439
    Average Deflection 0.331
    S/W 0.501

    Likes: This is a lot like the original QRC and has all the same likes, it’s light, with super easy setup and a great preload adjustment. Add in the new lighter weight and stiffer design and it’s an improvement over an already nice crank. I also really appreciate the self extracting mounting bolt as a nice upgrade over the previous. Also new for 2013 they’ve added this crank in more lenghts while it had previously only been available in 1 length it’s now available in 3 lengths.  At 138mm the QRC2 is the narrowest of all cranks.

    Dislikes: Again I don’t like that it is only available in compact. I would still like to see it gain a little more stiffness. There also seems to be an issue with the hidden bolt tab needing to be filed with some rings but not others in order to ensure they run true.

    Kcnc K2 Ver2

    kcnc_proto_cranktest

    Claimed Weight None
    Actual Weight 742g
    Corrected Weight 738g
    Price Est. $440
    Available Lengths 165, 167.5, 170, 172.5, 175, 177.5
    Q-factor 149mm
    Spindle 24mm
    Deflection-D 0.196
    Deflection-ND 0.353
    Average Deflection 0.275
    S/W 0.492

    Likes: Stiffer and with easier setup than a previous version of the K2. Simple 3 piece design with much improved looks over the last version. Available in wide range of lengths and chainring sizes. The new Cobweb rings which can be included with this crank shift well and look great in my opinion. At around $400 for the complete crank this is one of the most affordable in the review. Customer service with Kcnc has always been really good.

    Dislikes: A little heavier than the previous version and a proprietary spindle.

    Lightning SL

    LightningCrank_LRG

    Claimed weight 445g no rings
    Actual Weight 448g no rings
    Corrected Weight 572g
    Price $670 Without chainrings
    Available Lengths 160, 162.5, 165, 167.5,
    170, 172.5, 175, 177.5,
    180, 185, 190, 200
    Q-factor 150mm
    Spindle 30mm
    Deflection-D 0.260
    Deflection-ND 0.355
    Average Deflection 0.307
    S/W 0.542

     

    Lightning HD

    Actual Weight 592g w/bb
    Deflection-D 0.264
    Deflection-ND 0.361
    Average Deflection 0.313
    S/W 0.519

    Likes: The latest version have come along way in appearance and are really looking much better than earlier versions. I like that they are available in gloss or matte finish and with or without logos. It’s light. It’s reasonably priced compared to it’s carbon competition. Like the Extralite crank the Lightning has a bb that is preload adjustable, a big plus in my book. Setup is easy and performance is good with a lot of available bb options. Customer service with Lightning has always been reliable. And with the newer version the recessed pedal insert has been moved flush with the arm. One of the best features is the very wide range of lengths from 160 to 200.

    Dislikes: In fixing the recessed pedal insert Lightning has increased the q-factor 6mm wider than previous versions. Reports over the last few years do sometimes mention that the cranks creak more than some others. SL version does have a weight limit.

    Rotor 3d

    rotor3d

    Actual weight 724g
    Corrected weight 724g
    Price $400 without Chainrings
    Availalbe lengths 167.5, 170, 172.5, 175
    Q-factor 145mm Spindle, 24mm
    Deflection-D 0.176
    Deflection-ND 0.309
    Average Deflection 0.243
    S/W 0.563

    Likes: Looks (this refers to the shape not to the graphics.) Removable spider that can take a power meter. Stiffness. Saab bottom bracket. Separate preload/pinch bolt design that has proven as a concept to be trouble free.

    Dislikes: The main dislike for me is the graphic package, graphics like these were only half way cool on a trapper keeper back in the 80′s and for me lack any retro appeal. But if my biggest complaint about your crank is the graphics package, that can’t be a bad crank. Note: I’m hoping to have a set of the new 3D plus for the next review as they seem very promising and come in a great range of lengths and bottom bracket styles.

    Shimano Dura-Ace 9000

    dura_ace_9000_cranktest

    Claimed Weight 683g
    Actual Weight 711 g w/bb
    Corrected Weight 711g
    Price $750 complete
    Available Lengths 165, 167.5, 170, 172.5, 175, 177.5, 180
    Q-factor 147mm Spindle, 24mm
    Deflection-D 0.145
    Deflection-ND 0.31
    Average Deflection 0.227
    S/W 0.619

    Likes: The 9000 has shaved some more weight from the previously tested 7800. I like the universal spider concept and being able to swap compact and standard rings on one crank. The looks are also improved over previous versions, but best of all is the stellar shifting rings.

    Dislikes: One of my dislikes is also a like. The universal spider. This currently prevents users from replacing their rings with aftermarket and forces them into the very expensive OE rings.

    Sram Red

    sramred

    Claimed Weight 760g
    Actual Weight 755g
    Corrected Weight 754g
    Price $500 complete
    Available lengths 165, 170, 172.5, 175, 177.5
    Q-factor 150mm
    Spindle 24mm
    Deflection-D 0.150
    Deflection-ND 0.278
    Average Deflection 0.214
    S/W 0.619

    Likes: It’s a good looking, stiff crank at a good price. The Red crank had one of the lowest average deflections of all cranks we tested. When you add price into the equation it becomes a very balanced crank set. I’d call this crank the sleeper of the test.

    Dislikes: Rings. While the rings shift ok, they could definitely stand for some improvements. I’d also like to see the q-factor reduced by at least a few mm, but preferably more. It was also one of the heaviest cranks in the test. Notes: We hope to test the Sram Red22 crank in the next installment.

    Thm Clavicula

    thm_calvicula_crankset_gloss

    Claimed Weight 420g no rings
    Stock Weight 406g no rings
    Corrected Weight 524g
    Price $1250 w/o rings
    Available Lengths 170, 172.5, 175
    Q-factor 151mm
    Spindle 30mm
    Deflection-D 0.169
    Deflection-ND 0.315
    Average Deflection 0.242
    S/W 0.788

    Likes: Looks, this crank is gorgeous. Good stiffness and fantastic weight give this the best S/W of all the cranks in this round. The attention to detail is fantastic. There is a built in wear indicator in these cranks. Under the outer layer of carbon is a layer of yellow carbon fiber. If you have heel rub and eventually wear through the outer layer you’ll begin to notice the carbon turns yellow indicating it’s time to replace your cranks. It’s these details that make this the carbon crank that other makers should look at as a bench mark. It also has one of the largest selctions of available bottom bracket standards.

    Dislikes: What’s not to like about this crank? Well, since nothing is perfect how about the price, and the 151 q-factor. This crank does also have a weight limit and a fixed lifespan.

    Thm Clavicula M3

    thm_clavicula_m3_cranktest

    Claimed Weight 450 g with spider and bb
    Corrected Weight 581 g
    Price $850
    Available Lengths 170, 172.5, 175
    Q-factor 148mm
    Spindle 30mm
    Deflection-D 0.205
    Deflection-ND 0.357
    Average Deflection 0.281
    S/W 0.612

    Likes: There’s a lot to like about this crank. It improves on the q-factor and reliability of the original. It has an interchangeable spider which also allows for the use of a SRM and the ability to switch from standard to compact and back again. It has a better price. It has one of the largest selections of bottom bracket standards as well.

    Dislikes: It’s not quite as light or as stiff as the original.

    Tune SmartFoot

    tune_smartfoot_cranktest

    Claimed weight 630 grams no bb
    Corrected Weight 734 grams
    Price $700
    Q-factor 152mm
    Spindle 30mm
    Deflection-D 0.208
    Deflection-ND 0.334
    Average Deflection 0.271
    S/W 0.521

    Likes: A definite improvement over the previous Tune crank. It has a really nice look and a lot of available bb options. In my opinion this is perhaps the nicest looking crank in the review.  Setup is super easy and preload adjustment is simple and secure. Overall a very nice crank with a bolt it and forget it design.

    Dislikes: A little heavier, a little wider than some other cranks.

    #93792
    Moderator

    Thanks for sharing this.

    #93795
    Participant

    Thanks for the review.

    Would it be possible to include the same crankset with different BB spindles? For example, the new SRAM RED crank in both BB30 and GXP variants.

    #93796
    Participant

    Nice review.

    Any idea of the deflection differences between Super Record with Ti axle and the tested Record with steel axle?

    #93799
    Keymaster

    Nice review.

    Any idea of the deflection differences between Super Record with Ti axle and the tested Record with steel axle?

    We can’t say too much but we decided to wait for a short period to re-test Campagnolo cranks. The steel version here is from the last test before the Ti version was released. We’ll have our new testing facilities up and running by then too.

    I run the Facebook Page, Twitter Account and write code the rest of the day.
    #93807
    Participant

    Excellent! I’m glad to see the crank test is active. I look forward to new cranks.

    The one issue I have is with the assumption that all elastic strain energy is dissipated as useless heat. There’s one obvious relaxation path, and that’s to drive the chain, which represents a power delay but not an energy loss. I suspect the vast majority of deflection energy ends up being dissipated propulsively.

    #93809
    Moderator

    Hi DJ,

    I’m the author of the strain energy section. (We’ve interacted before; I’m on the Weight Weenies forum as youngs_modulus). You’re right that I assumed that all strain energy was lost; I made that assumption because I wanted to find an upper bound for energy absorbed by loading the crank.

    However, I did not assume that the strain energy was dissipated as heat. While some of the strain energy is certainly dissipated through hysteresis, I’m confident that it’s a fairly small percentage of the total. The remaining energy is dissipated when the crank springs back as it’s unloaded. The obvious question: how much of the returned energy is propulsive?

    I intentionally left that question unanswered. Some of the energy that goes into bending the crank is dissipated in a non-propulsive manner; some of that energy may well help turn the crank. This question gets complicated when you consider that any propulsive spring-back gets reacted out through the legs.

    These complications are why I didn’t attempt to answer the energy-return question. As I said before, I only wanted to find an upper bound for dissipation. It turns out that even if we assume all the energy is lost, the losses are still very small. We’re talking about ~1.6 watts’ difference* between the stiffest and most flexible cranks. In my opinion, it’s not worth worrying about. More to the point, it would be very hard to measure in the real world.

    That said, if you can think of a way to measure propulsion from crank spring-back, I’m all ears! :)

    Jason

    * That’s 1.6 watts at 300 watts total output.

    [Edit: specified total output for claimed power loss]

    • This reply was modified 10 months, 1 week ago by  Jason Krantz.
    #93831
    Participant

    Love it. A question regarding units for S/W: based on your Eqn your units are in^-1 * gm^-1 which is confusing to interpret the data, is it a mixed unit inversion relating to torque? For example, Sram Red and DA9000 scored equal using those units. But if you were to compare mm/kg you get 7.2mm/kg for Sram and 8.1mm/kg for DA. Would that mean Sram deflects about 1mm less per kg compared to DA?

    #93905
    Participant

    Having read through the article and the above posts, I take the “upper bound” explanation applies to using an analysis of a beam with pure end moments instead of a closer approximation of a simple cantilever with an end point load (pedal). The difference between these two is fairly significant in determining stored energy. The conclusion however doesn’t change; the amount of stored energy in the crank arms is small compared to the drivetrain as a whole and the rider output. I agree that most of the stored energy is lost as it will retard the momentum of the legs on the upstroke rather than propel the rider forward (25lb leg versus 200lb bike + rider). Good article though and I would favor a lightweight crank and sacrifice a little bit of stiffness although the actual numbers for such a tradeoff would have to be scrutinized.

    #93907
    Moderator

    Hi Phil,

    Having read through the article and the above posts, I take the “upper bound” explanation applies to using an analysis of a beam with pure end moments instead of a closer approximation of a simple cantilever with an end point load (pedal). The difference between these two is fairly significant in determining stored energy.

    Not quite. The pure moment condition is just one example of how to quantify the strain energy in a deformed body. If you click on the textbook link, you’ll see several other closed-form solutions, including one for torsional loading and another for axial loading. But none of these is very close to a crank under a pedaling load–even when you combine them–which is why one would use FEA. The FEA solution is much closer than even a linear combination (superposition) of all the applicable closed-form solutions.

    A well-executed finite element solution can be very, very accurate. I’m not sure that this is the place to expound on how the finite element method works. But if you’re up to it, Google “FEA.” It’s very, very well understood. It’s inherently an approximation, but it can be an outstanding approximation. Correspondence between the FEA solution and the real world depends mostly on the analyst’s skill.

    I used the phrase “upper bound” to explain why I assumed all strain energy was lost. The question of how much strain energy is returned to the drivetrain is kinematically murky and difficult to prove either way. I used strain energy to establish an upper limit to the amount of energy a crank can absorb.

    While we can’t answer the question of drivetrain energy return, we can address whether the most flexible crank absorbs appreciably more energy than the stiffest crank: it does not.

    Does that clarify things?

    Cheers,

    Jason

    #93913
    Participant

    I appreciate you and this pretty tests.

    I’m looking forward to join the other new cranks as soon.
    For example Campy TT crank, Ciamillo Gravitus crank or EE’s production model etc…

    I’m waiting a set of special Ciamillo crankset.
    It may arrive untill end of this month though this is just a schejule.
    The crankset will have special pedal holes for Vista Integral unit.

    #93938
    Participant

    Jason:

    Ah — good to connect real names with aliases. Your posts on Weight Weenies are certainly excellent.

    It’s a funny difference in perspective: you say “only 1.6 watts” but I see “a whopping 1.6 watts”. That’s around 5 seconds up Old La Honda road, as much effect as 350 gram difference in crank mass.

    By “heat” I am referring to conservation of energy. Assuming the leg can output the same power, aerobically limited, then power goes to either propulsion or to heat. The heat can be in the bicycle or in the human body. However, the leg sees a certain force-versus-position trajectory: one with an infinitely stiff crank, a slightly different one with a flexible crank. Assuming no power is lost in the bicycle itself, then it’s not obvious to me that the flexy-crank trajectory is going to yield more internal dissipation in the leg than the other. True, the crank may be pushing less against the leg during the upstroke, but this is balanced by the fact the leg is pushing less on the crank on the downstroke. Power transmitted to the crank would be reduced, but the aerobic system is worked less, and therefore the body is able to adjust and push harder. It spreads out the force pattern more over the distance of the pedal trajectory, perhaps, but the integral of force dot distance with respect to distance is constrained by the power.

    Forget about the human leg: that’s hopelessly complex. I could assume the crank is hooked up to a lossless machine which is programmed to deliver a fixed amount of mechanical power to the crank such as the one Metrigear built to test the Vector. In that situation, the flex must be relieved either propulsively or via heat in the bicycle. I know in this situation my money’s on the vast majority of those 1.6 watts going into the drivetrain. It’s the obvious energy sink.

    It would be interesting to see FEA of this.

    Dan

    P.S. There’s a general confusion (not from you, but in the subject as a whole) about the difference between elastic compliance and inelastic compliance. For example, if I put a thick insole in my shoe, it is easy to understand than when I compress it, then allow it to expand, I will get only a fraction of that energy back, and therefore it’s better to not use thick insoles. The insole will deform one way, then un-deform a different way, yielding hysteresis which results in heat generation. With highly springy materials like Al, and to a lesser extent carbon fiber which is more lossy, the direct hysteresis will be far less and so energy loss becomes more dependent on the nature of the load.

    • This reply was modified 10 months, 1 week ago by  djconnel.
    #93941
    Moderator

    Hi Dan,

    Thanks for your insightful comments. It’s also nice to hear your perspective on the 1.6-watt magnitude. I admit that it seems to me that 1.6 watts gets lost in the noise of chain lube, jockey wheel bearings and tire rolling resistance. On the other hand, when you put 1.6 watts in terms of time and weight, it certainly seems more significant.

    Thanks for clarifying your use of “heat.” It’s broad–all losses end up as heat, eventually–but I see what you mean. That said, consider out-of-plane bending of the crankarm (as when you stand on the pedal at BDC). When you unload the bottom pedal, the crank returns to its unloaded position without adding to forward propulsion and without literally heating the arm. (We agree that there’s a very small amount of hysteresis; I suspect some of the remaining losses are as low-frequency sound waves). That bending absolutely happens in a “real” pedal stroke, and that’s one component of the total strain energy that isn’t returned to the drivetrain.

    But beyond these overly simplistic sub-cases, we’re just speculating. While I have some intuitive guesses about the subject, I don’t know the answer to the energy return question. Finding the answer would require either really well-controlled physical testing or, as you suggest, a more involved FEA model.

    In order to properly address the power loss question with FEA, I would need highly detailed force data from a complete pedal revolution. You mentioned Metrigear, and it seems pretty likely that the Metrigear guys have those data. But if the boundary conditions (including force input and direction vs. time) are properly understood, such an analysis should give pretty good results. It would require a full transient solution with many load steps, but it’s doable. Sure, you could use a constant force input, but you’d still need to do a similarly involved analysis; why not go whole hog?

    Your postscript hints at another point: carbon fiber cranks will certainly damp more power than aluminum ones, although how much more is unclear. Regardless of the degree of damping, there are situations to which a 700-gram aluminum crank would be much better suited than a 600-gram carbon crank. If I were facing off with Mark Cavendish in Tour de France field sprints–I can dream, can’t I?–I’d definitely want a stiff aluminum crank.

    Thanks again for your thoughtful feedback.

    Jason

    #93942
    Participant

    Thanks, Jason! This would perhaps be a good test for FrictionFacts, who have the best publicly available testing of drivetrain power efficiency at present. Unfortunately the crank is part of an integrated system, including frame and rider, so I wonder how relevant a bench test would be. Despite that, it would be better than nothing, and measuring deciwatt differences in power loss isn’t easy.

    #93945
    Moderator

    Yeah, who would want 1.6 Watts left in his crank after having allready crossed the line? It would have to be energy anyways, not power, right?

    "Nothing compares to the simple pleasures of a bike ride," said John F. Kennedy, a man who had the pleasure of Marilyn Monroe.
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