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Discussion Starter · #1 ·
I'm looking for reliable data regarding the aero advantage of internal cable routing.

I'm planning to build a new TT bike and want to get as close to state of the art as my budget will allow. So far my shopping list has me a bit perplexed with regard to cabling.

The Scott Plasma Ltd is my current bike of choice (when it comes available in the next couple of months) but I'm considering others. The Scott, the Cervelo P3 and Trek TTX route the derailluer cables in the down tube but expose the rear brake cable along the top tube.

The Kuota Kaliber and the Orbea Ordu route the rear brake cable in the top tube but expose the darailluer cables along the down tube.

My intuitive reaction is to believe the top tube cabling is less aerodynamically intrusive than the down tube cabling, but experience has shown that intuition isn't reliable when we're talking about aerodynamic reality. I'd really like to understand the aero consequences of these designs considerations.
 

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from what i've read / understand, cable routing will have next to no effect whatsoever on aerodynamics. i was reading an article about team testing in either velonews or pez where the engeneer on call at the wind tunnel mentioned that an aero TT helmet had more total effect (i believe the number was 30% more) on wind drag than switching from 32 spoke box rim wheels to zipp 404 in the front and disk in the rear. from what i understand your time / consideration would probably be best spent on wheels and helmets as opposed to things like cable routing and shoe covers.
 

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The top tube cabling won't matter at all to the aerodynamics, as it's parallel to the airflow and a tiny cross-section. Matter of fact, a buried top-tube cable could (possibly) be worse, because the angle needed to get in and out of the tube might make the ends stick up farther.

As for down tube cables, I've seen "evidence" that suggests that it is both better and worse, so you get to choose. The "better" argument falls along the lines that it creates a bit of turbulence that helps out with the aerodynamics of a round downtube. I'd expect to see a buried cable on a bladed downtube. Note that I'm not necessarily saying that a bladed tube is inherently better or worse than a round tube in any particular set of conditions. I don't have data to support either side of that argument. In the end, I think this decision is more aesthetic than anything else.

It's worth noting that both Shimano and Campy explicitly recommend against internal cable routing, as the added drag and sponginess (from the additional bends) can compromise shifting performance. I've also seen bikes that have had the internal cables twisted around one another, such that they can telegraph shifts front-to-rear. Fixable, but a pain in the butt.
 

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You'll get a lot more from concentrating on what bike gives you the best aero-position and minimizing the total frontal surface. Wheels are important too. All the little aero things like special chainwheels and whatnot are good for blinging up the bike but make no real difference in performance, cable routing falls in that category.
 

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80% of the drag is from you, 20% is from the bike. Of that 20% damn near all of that is from the front end (fork, head tube etc.). Cable routing might make about a 0.000004% difference.

This being the case if you did a 2000 Mile TT and averaged 20 MPH over the entire ride the cables would save you 1.5 seconds. Of course if there was a significant head wind or very long downhill sections it might save a full 2 seconds.

Many TT are won by less than 2 seconds.
 

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Koop said:
I'm looking for reliable data regarding the aero advantage of internal cable routing.

The Scott, the Cervelo P3 and Trek TTX route the derailluer cables in the down tube but expose the rear brake cable along the top tube.
My P3sl has internal routing for the rear brake cable, not external... fyi

saying that, it probably doesn't make a bit of difference.
 

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Lifelover said:
80% of the drag is from you, 20% is from the bike. Of that 20% damn near all of that is from the front end (fork, head tube etc.). Cable routing might make about a 0.000004% difference.
As an upper bound on the drag of a cable, consider a cylinder in crossflow. The drag on a 1.2 mm cylinder (derailleur cable) at 25 mph will be about 2..25x10^-2 lbf per meter of cable. So the most you could save with internal routing would be about 4.5x10^-2 lbf and in reality the savings would be less. Compared to the total drag on bike and rider of about 5.5 lbf from analyticcycling, internal cables give less than 0.8% savings in total drag. While this is an upper bound on the drag, I doubt it overestimates by 5 orders of magnitude.
 

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A body moving through a fluid experiences a drag force, which is usually divided into two components: frictional drag, and pressure drag. Frictional drag comes from friction between the fluid and the surfaces over which it is flowing. This friction is associated with the development of boundary layers, and it scales with Reynolds number as we have seen above. Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the body.

This drag is associated with the formation of a wake, which as an example can be readily seen behind a passing boat (fluid dynamics), and it is usually less sensitive to Reynolds number than the frictional drag. Formally, both types of drag are due to viscosity (if the body was moving through an an inviscid fluid there would be no drag at all), but the distinction is useful because the two types of drag are due to different flow phenomena.

Frictional drag is important for attached flows (that is, there is no separation), and it is related to the surface area exposed to the flow. Pressure drag is important for separated flows, and it is related to the cross-sectional area of the body.

When the drag is dominated by friction drag, we say the body is streamlined, and when it is dominated by pressure drag, we say the body is bluff. Whether the flow is viscous-drag dominated or pressure-drag dominated depends entirely on the shape of the body. A streamlined body looks like a fish, or an airfoil at small angles of attack, whereas a bluff body looks like a brick, a cylinder, or an airfoil at large angles of attack. For streamlined bodies, frictional drag is the dominant source of air resistance. For a bluff body, the dominant source of drag is pressure drag. For a given frontal area and velocity, a streamlined body will always have a lower resistance than a bluff body.

Cylinders and spheres are considered bluff bodies because at large Reynolds numbers the drag is dominated by the pressure losses in the wake. The variation of the drag coefficient with the corresponding Reynolds number, and the corresponding flow patterns. We see that as the Reynolds number increases the variation in the drag coefficient (based on cross-sectional area) decreases, and over a large range in Reynolds number it is nearly constant.

At a Reynolds number between 10^5 and 10^6, the drag coefficient takes a sudden dip. The size of the wake decreases, indicating that the boundary layer separation on the cylinder or sphere occurs further along the surface than before. What has happened? The phenomenon is related to the differences between laminar and turbulent boundary layer. The boundary layer and its interaction with the local pressure gradient plays a major role in affecting the flow over a cylinder. In particular, near the shoulder, the pressure gradient changes from being negative (decreasing pressure) to positive (increasing pressure). The force due to pressure differences changes sign from being an accelerating force to being a retarding force. In response, the flow slows down. However, the fluid in the boundary layer has already given up some momentum because of viscous losses and viscous friction, and it does not have enough momentum to overcome the retarding force. Some fluid near the wall actually reverses direction, and the flow separates.

A laminar boundary layer has less momentum near the wall than a turbulent boundary layer, as shown in figure 4, because turbulence is a very effective mixing process. More importantly, turbulent transport of momentum is very effective at replenishing the near-wall momentum. So when a turbulent boundary layer enters a region of adverse pressure gradient, it can persist for a longer distance without separating (compared to a laminar flow) because the momentum near the wall is higher to begin with, and it is continually (and quickly) being replenished by turbulent mixing .

The boundary layer over the front face of a sphere or cylinder is laminar at lower Reynolds numbers, and turbulent at higher Reynolds numbers. When it is laminar (Re < 10^5), separation starts almost as soon as the pressure gradient becomes adverse very near the shoulder, and a large wake forms. When it is turbulent (Re > 10^6), separation is delayed to a point about 20^o past the shoulder, and the wake is correspondingly smaller.

It follows that, if the boundary layer of a sphere can be made turbulent at a lower Reynolds number, then the drag should also go down at that Reynolds number. This is the case, as we can show by using a trip wire. A trip wire is simply a wire located on the front face of the sphere and it introduces a large disturbance into the boundary layer. This disturbance causes an early transition to turbulence, and it effect on the size of the wake, and the total drag is quite dramatic.

excerpts taken from Princeton University - Engineering Dept.


Here is useful website:
http://www.exploratorium.edu/cycling/aerodynamics1.html

I think most if any gains can be made in skin suit/bib applications,
dimple or riblet tape applications, for bike and helmet/bibs.

http://home1.gte.net/pjbemail/RibletFlow.html#3b


Some previous comments on Nike Swift and Descente Vortex suits
http://www.msnbc.com/news/693675.asp
 

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p9group said:
A body moving through a fluid...[/url]
I feel like I just sat through an engineering lecture. Very impressive info. Is it right or wrong? No idea, but I could sure turn some geeky girl heads if I could memorize this...
 

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Discussion Starter · #11 ·
OK...if the downtube is airfoil shaped, then laminar flow is desireable. The cables are acting as trip wires creating turbulent flow over the airfoil. The question remains, is the increased drag quantifiable at bicycling speed?
 

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Koop said:
OK...if the downtube is airfoil shaped, then laminar flow is desireable. The cables are acting as trip wires creating turbulent flow over the airfoil. The question remains, is the increased drag quantifiable at bicycling speed?
No.

A reality check for those who pretend to understand aerodynamics. The first thing that's taught in any worthwhile scientific curriculum is the concept of mathematical significance. That is, if a calculated effect gives a result smaller than the accuracy of the least-accurate measurement, it's not an effect but a rounding error. Apparently some folks skipped that class to go riding.

The Reynolds number for a 1.2mm cable moving at 40kph is roughly 800, not the 10^6 or so mentioned above. Therefore, for drag calculations, the flow is essentially laminar and surface drag is prevalent.

Consequently, calculations of total drag based on the classic cylinder models are not correctly applied in this instance, as they assume much higher Reynolds numbers.

Whatever the calculation derived, only a small portion is additive to the total drag of the bike, as the cables don't add to the frontal area of the bike. Put another way, whatever part of the cable is in the airstream, it's sheltering that portion of the downtube.

The airflow around the downtube isn't laminar in any case. There's that pesky eggbeater called a front wheel in front of it. For off-angle airflows, the aero tube represents an increase in surface area and a shape that is closer to a 2x4 than it is a cylinder, much less an effective aero shape (a bit of hyperbole, deal with it.)

People who make their livings at low-speed, small scale aerodynamics are pretty much of a consensus that in the real world, aero downtubes and buried cables are more aesthetic and psychological advantages than consistent, measureable, proveable advantages.

Make you a deal. Buy the bike with the aero downtube and the buried cables, because it's dead sexy and it's what you really want. Never mind that it won't really help, that it won't brake or shift as well, and that it will be harder to maintain. After you get it, get a friend to help, and put on a blindfold. Have the friend hot-glue a set of mock cables on the front of the tube - or not, without telling you. Ride it around the block without knowing which is which. If you can tell or measure the difference, I'll buy the bike for you.

Matter of fact, if you can make it around the block with the blindfold, I'll buy you the bike. :D

Point is, it doesn't matter. Buy the bike that whispers in your ear, "Lets' go." That's the one you'll be fastest on, no matter where the cables are.
 

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asgelle said:
As an upper bound on the drag of a cable, consider a cylinder in crossflow. The drag on a 1.2 mm cylinder (derailleur cable) at 25 mph will be about 2..25x10^-2 lbf per meter of cable. So the most you could save with internal routing would be about 4.5x10^-2 lbf and in reality the savings would be less. Compared to the total drag on bike and rider of about 5.5 lbf from analyticcycling, internal cables give less than 0.8% savings in total drag. While this is an upper bound on the drag, I doubt it overestimates by 5 orders of magnitude.
So if I understand you (which I probably don't), you would contend that all the cables on a bike account for as much as 1% of the total drag of both the bike and rider?

I don't need to be an engineer (even though I am) to know that aint the case.
 

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Lifelover said:
So if I understand you (which I probably don't), you would contend that all the cables on a bike account for as much as 1% of the total drag of both the bike and rider?

I don't need to be an engineer (even though I am) to know that aint the case.
I wrote that 1% is an upper bound on the drag from cables along the downtube.
asgelle said:
Compared to the total drag on bike and rider of about 5.5 lbf from analyticcycling, internal cables give less than 0.8% savings in total drag. While this is an upper bound on the drag, I doubt it overestimates by 5 orders of magnitude.
When you say this isn't the case, are you saying that drag from cables along the downtube contribute more than 1% of the total drag? How do you justify that? Because if you're saying the drag is less than 1%, you're agreeing with me.
 

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Koop said:
Thanks asgelle and P9group. This gives me something to chew on.

Your welcome...

for sure concept v reality/application is a whole different story sometimes.

If all else fails - install a big Flux Capacitor! :D



Marty: "Time circuits on, flux capacitor...fluxing!"
 
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