Forum Replies Created
June 18, 2013 at 12:51 am #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.June 17, 2013 at 6:09 pm #93942
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.June 17, 2013 at 1:44 am #93938
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.
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.
June 13, 2013 at 10:03 pm #93807
- This reply was modified 5 months, 3 weeks ago by djconnel.
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.May 3, 2011 at 11:19 pm #93124
SRAM Red is 16 grams saved?