Is an aero frame beneficial? (light or aero bike faster?)

Discuss light weight issues concerning road bikes & parts.
Courant
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by Courant

ultyguy wrote:You ever see a sprint from overhead cam shot? Especially the likes of Greipel and Cav throw their bikes back and forth so much that any 'aero' given by the frame surely gets washed up in all that churn.


A couple of sums...

The change in yaw angle on the frame due to the rocking is something like 10degrees (back of envelope calculation... 120rpm, 20cm amplitude motion gives max sideways velocity). This is likely well below the stall point of a frame - the motion alone is not enough to cause any upset in the drag response of one frame vs another.

The unsteady effects come into play when the time period of the motion of the geometry starts to approach the time period of the flow structures going past the geometry. In other words, 20m/s bike, roughly 1m long - a blob of fluid passes the bike in 0.05s. 120rpm is 0.5s period. So, 0.1 ratio of bike to fluid. This is just on the limit where unsteady effects would begin to play a role - it's fair in this circumstance to treat the bike as being in steady flow.

Sprinting is one occasion where aero can definitely play a role - high absolute drag, big power, slight differences are the margin between winning and loosing.

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andy2
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by andy2

@Franklin
Very simply stated:
An overly flexy frame will loose power transfer because the rear wheel will 'snake'.
As you are applying force on the downstroke the mid to upper
portion of the down tube will come under strain and flex and so will the inside front of the chainstays. This will cause the rear wheel to veer right and left with the pedalstrokes thus leading to a loss of effiency.
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by Weenie


goodboyr
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by goodboyr

andy2 wrote:@Franklin
Very simply stated:
An overly flexy frame will loose power transfer because the rear wheel will 'snake'.
As you are applying force on the downstroke the mid to upper
portion of the down tube will come under strain and flex and so will the inside front of the chainstays. This will cause the rear wheel to veer right and left with the pedalstrokes thus leading to a loss of effiency.


I'd love to see numbers and info to back up this hypothesis. Hard to believe that the amount of movement in a "flexy frame" will make this occur. My understanding is that we are talking mm difference here.

http://materials.open.ac.uk/bikeframes/bikeframe.htm

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andy2
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by andy2

The science is pretty clear :noidea: . I will post an illustration of a semi dynamic test case based on EFBe/Zedler tests in the morning. Hope that will be interesting:-)!
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justkeepedaling
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by justkeepedaling

There are typically mm of deflection and that force goes straight into the tires. There is very very very little power actually "lost." It's a very common misconception. This wouldn't be seen on typical road bikes.

thisisatest
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by thisisatest

power wouldnt be lost if the flex was in the direction of motion so that the spring return would return the stored energy. but riders dont pedal in circles, no matter how hard we try. and it's not the frame that is getting hot. it's the rider's body.
with an infinitely stiff frame with 170mm crankarms, from the top of the pedal stroke to the bottom, vertically, is 340mm. imagine a frame that flexes sideways so that the pedals move an extra 5mm. that would be top and bottom, so the distance traveled is 350mm, but with the same effort, and only 340mm of movement accomplishes rotating the wheel. 10mm wasted over 350mm of travel is 2.8% loss.
another thing i think of that typical physics equations dont account for is that it takes energy to push on something that isnt moving. you can push on a brick wall, that thing doesnt move a mm. you can end up exhausted but technically accomplished no "work". energy was dissipated, into heat, created in your body. thats why youre panting and sweating.
the frame doesnt have to heat up for energy to be wasted.

Franklin
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by Franklin

andy2 wrote:The science is pretty clear :noidea: . I will post an illustration of a semi dynamic test case based on EFBe/Zedler tests in the morning. Hope that will be interesting:-)!


But once again the question remains, where is the flex located? you are clearly talking about drivetrain flex, whereas the focus is usually on the main triangle.

It certainly makes the ever so popular hourglass stays and ultrathin stays look bad. I'm unconvinced that Aero is a tradeoff for stiffness in the rear triangle, which seems tio be the place where it counts.

Franklin
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by Franklin

thisisatest wrote:with an infinitely stiff frame with 170mm crankarms, from the top of the pedal stroke to the bottom, vertically, is 340mm. imagine a frame that flexes sideways so that the pedals move an extra 5mm. that would be top and bottom, so the distance traveled is 350mm, but with the same effort, and only 340mm of movement accomplishes rotating the wheel. 10mm wasted over 350mm of travel is 2.8% loss.


Nonsense.

1. the pedal travel is determined by the crankset, not the frame.
2. Flex means that a large part of the power will be returned, so power arrives with a tiny delay.

Power loss will have to translate into heat. In Andy2's example it would seem to go into "tire friction", but even then it's debatable, as tire friction is just as possible with an infinitely stiff frame.

Everyone knows that a massive rim is nowhere near as efficient as an air filled rubber tire. So it is debatable at best that stiffness automatically translates into better efficency. Indeed we can expect that a bit of flex is a neccesary thing to go fast.

goodboyr
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by goodboyr

Damon Rinard did studies trying to quantify this and concluded that any power loss due to frame flex was such a small amount, it was below the detectability of power meters. Unless a test is devised to measure this "power loss", its just a hypothesis. But of course, its still seems to be more believable to people than the actual and easily detectable power numbers measured experimentally due to aero frames. Go figure.

http://www.bikethink.com/bicycle-frame-efficiency/

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kgt
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by kgt

I agree all this power loss is bs. People prefer stiffer frames sometimes (myself included) not because these frames "waste" less energy but because they give you a specific feel.

The same is the case with aero wheels, frames etc. All that has little to do with measurable savings but a lot to do with the way these parts (actually) make you feel faster.

tanhalt
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by tanhalt

kgt wrote:I agree all this power loss is bs. People prefer stiffer frames sometimes (myself included) not because these frames "waste" less energy but because they give you a specific feel.

The same is the case with aero wheels, frames etc. All that has little to do with measurable savings but a lot to do with the way these parts (actually) make you feel faster.


I'd actually rather BE faster than FEEL faster. I've done a few tests of my own where if I'd gone by my honest human perceptions of what "felt" faster, I would have ended up actually slower.

It's a good thing that a stopwatch and a power meter (properly maintained and calibrated, of course) don't "lie" :thumbup:

Human perception is easily fooled.

BobSantini
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by BobSantini

I think you're right with the first bit KGT. I remember the days of Sean Kelly winning sprints on that noodle of a Vitus Duralinox.
However the virtues of aerodynamic wheels and frames are not just illusory.
r o y g b i v

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kgt
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by kgt

No, this is neither illusion nor placebo. I am just saying that how a bike feels is always (to us, riders) more important of whether a bike is actually 1% faster or not.

tanhalt
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by tanhalt

kgt wrote:No, this is neither illusion nor placebo. I am just saying that how a bike feels is always (to us, riders) more important of whether a bike is actually 1% faster or not.


Speak for yourself :wink:

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andy2
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by andy2

http://s730.photobucket.com/albums/ww30 ... movie1.mp4

Above is a link to an illustration of a load case of our frame with an overly soft layup for illustration. Here it is loaded 100 kg applied at the drive side pedal.
We are using Altair Hyper Works FEA, with is the leading industry analysis tool as used by MacLaren and others.
You are viewing the frame from the rear nds. The flex is magnified, again for illustration, but the impact is clear. The frame will flex in a manner that will not be contained or harvested by the drivetrain, resulting in forward propulsion. The work done flexing the carbon spring/frame is essentially lost and dissipated through heat in the engine/pilot/cyclist. If we could harvest this flex in the manner of a trampoline or bow as suggested by some we would have an instant ban by the UCI on our hands...
The other effect that is clear from the illustration is how the rear wheel will 'crab' under load. This is akin to the crabbing of an airplane and is of course resulting in a loss of efficiency.
To infuse the discussion with a measure of rocket surgery: Rockets are fired straight up for this very reason. :up:

/a
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by Weenie


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