's Forum Archives - General

Archive Home >> General(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 )

aero wheels and cranks stiffness, are they all that?(11 posts)

aero wheels and cranks stiffness, are they all that?ishmael
Oct 30, 2002 3:03 PM
are aero RIMS worth it? Aero spokes make sense to me (how good are they by the way?), but rims arent really rotating into wind.

I keep hearing about certain cranks being stiffer than others(dura ace), but it doesnt jive in my mind. For one, a rubbery crank doesnt even sound like a bad idea, the energy wont disapear, just get released at the end of the stroke. It might even help pull through this dead zone. But I doubt they flex to begin with, and if they did, it would make sense that the crank on the left would be the flexiest of the two. The force on the left one would have to go through the bb and through the other crank while the right crank is more direct. I would think the chain, spokes, bb shell, or just about anything else would be much more likely to flex than short chunks of alluminum or carbon. I would think cranks would be the stiffest thing on the bike by far.
Oct 30, 2002 3:08 PM
What are you talking about?Kerry
Oct 30, 2002 4:36 PM
Of course aero rims are rotating in (and into) the wind. The idea is that air will flow more smoothly over the rim as it disengages after passing over the tire. The spoke is rotating into the wind, the rim is attached to the spoke, therefore the rim is rotating into the wind. In rough terms, about half of the drag improvement of an aero wheel is due to the rim, the other half due to low spoke count.

Obviously you have to treat the crank/BB/rings as a system when talking about flex, so you have to be careful to compare different cranks on the same BB, etc. When people talk about stiff cranksets, they're including the whole package. However, there's no question that there is a range of stiffness in crank arms, and this has been shown by testing and rider experience. Just because you think this is not an issue doesn't influence reality much. We're talking about beam stiffness here, and there have been many attempts at lightweight cranks that were noticeably flexy. If crank flex were not an issue, then we'd see a lot more variability in design, but you'll notice that all the top cranks have pretty much the same dimensions for a given material of construction. That's because mechanical engineers have done the calculations and the measurements and they know what the dimensions have to be in order to avoid the flex that you "think" is not an issue.
I'm talking about your soul, redemption and salvationishmael
Oct 30, 2002 5:29 PM
I still dont see how the rim is rotating into the wind. It seems to me that since the surface of the rim is smooth you might as well be moving along without the wheel turning as far as how much wind resistance the rim gives. yes, no?
About flex, to begin with, why is it bad since the energy is not lost but just expended later. I still dont "think" cranks flex. Imagine that you were to jump on your cranks and thereby apply pressure to both in opposite directions, such as when bunny hopping, you'd be putting way more force which would lead to flexing than if the cranks were rotating in the same direction. When hopping the force of both legs work against each other, and against the crank, as apposed to when pedaling, in which the legs are working together. I can't make them noticably flex when hopping
Thinking vs. scienceKerry
Oct 30, 2002 5:45 PM
Picture a non-rotating wheel cutting through the wind. On the front edge, the wind strikes the tire, flows around it and the rim, and then separates at the rim. The shape of that rim determines the separation drag, and that is where much of the drag is. A smooth separation (due to the aero rim profile) will reduce drag. What seems to you is not applicable here. The facts of aerodynamics are, and the science is well proven. Do your thought experiments trump proven science?

You can't make your cranks flex noticeably? How do you know. Are your eyes well enough calibrated that you can see deflections of less than a couple mm? This, while standing on your pedals and viewing your cranks in the same plane as any likely flex? Again, does your "data collection" trump the counless measurements and finite element analyses done by legions of mechanical engineers? The broad consensus is that mechanical systems for transmitting power should have a balance of light weight and minimum flex. It sounds like you've determined that consensus to be wrong. Let the world of engineering step back.
Thinking vs. scienceishmael
Oct 30, 2002 6:13 PM
so the non-moving rim cutting through the wind is the same as a spinning rim? Thats what I'm asking and you seem to be saying it's so. If thats the case I'm not impressed with aero rims, but dont take it personally. As far as crank flex, the amount of flex your saying cranks give is miniscule, so small I cant notice it. But I can notice the flex of my tires, bottom bracket, bars, stem, and just about anything else. So the legions of mechanical engineer's "data collection" seems to agree with my point that crank flex is insignificant compared to everything else. I still doubt they flex a millimeter or even half or even a quarter or even an eighth or even a sixteenth while pedaling.
Tell you what:cyclequip
Oct 31, 2002 5:49 AM
I'm 80kg and pretty strong. When I get up and stomp I can look down and SEE the crank flex. I've tried a few cranks - now with FSA on a DA BB and I can feel the difference.
I seeishmael
Oct 31, 2002 6:05 AM
being 150lbs is a different story. Does your left flex more than the right? I'm a sceptic, I'll have to see it to believe it. Not even experiencing it would satisfy me, there are too many other possible flexing bits to confuse it with.
Aero rimsBrooks
Oct 31, 2002 10:29 AM
The other side of the rim is important too. That is, after the wind has passed over the leading edge (tire and rim) and then through the spokes (the aero benefits which seems to be obvious to you) it will hit the rim on the other side. Although the wind now has an element of turbulence from the leading edge and spokes, the benefits of an aero rim vs a square-ish rim should also be obvious.

I'll leave the crank flex to others. I'm not sure what you are after there, but yes the crank is probably the stiffest part of the bike but does have some flex.
re: aero wheels and cranks stiffness, are they all that?tigermilk
Oct 30, 2002 6:30 PM
Do this little experiment. Take a rubber band a tie it to the handle of a coffee cup. Hold the free end of the rubber band and slowly move your hand up and down. See what happens? Now increase the speed with which you move your hand up and down. You get to a point where the cup is essentially staying in place and your hand is moving wildly up and down. There is little to no force/acceleration being transmitted to the cup. If you use a stiffer rubber band or spring, the point at which this occurs changes. You'll have to move your hand even faster to get to that point. If you have an infinitely stiff spring, all the motion/force at the hand is being transmitted to the cup. That's what you want. You don't want any power losses in the drivetrain. Cranks have a finite stiffness. By making them as stiff as possible you minimize this component of the equation.

Deep rims? I'm not an aerodynamicist. I leave that area to them. Perhaps you can code up an unsteady compressible flow CFD program and find the answer ;)
Oct 30, 2002 7:00 PM
but I think in the case of cranks the energy is released before the pedal is on the up swing(very stiff spring). Infact it might be released just at the bottom of the pedal stroke just when you need it most to help you pull over the dead zone. In my mind this theory works.
Dont know what an unsteady compressible flow CFD program is but I'll just blow smoke at the front of my wheel and see what happends.