Any real advantage to internal cable routing?

[Former_User]

With those skinny cables, I can’t believe that they really are that much of a factor aerodynamically, either, but I could be wrong.
Chet Kyle gives the drag of cable housing as 18 g/in (yes, he used those mixed units).

Cable housing has an OD of less than 0.200". Assuming a Cd of ~1 for a cylinder perpendicular to airflow with Reynolds number of 3000, a 1-inch long cylinder has a CdA of 0.000130 M^2. Using your ROT, this should be worth only 130mg of drag, not 18g. Is that Kyle figure off by a factor of 10?

CdA = (1.0 ) * ( 0.005m x 0.0254m ) = 1.27e-04 m^2 (check)

F = CdA * 1/2 * rho * v^2
= ( 1.27e-04 m^2) * ( 0.5 * 1.2 kg/m^3 * 12 m/s * 12 m/s ) @ 12 m/s
= 0.0109 N

We have to remember that the grams reported are at a tunnel speed of 30 mph (i.e. 13.4 m/s). Also, I get an Re of ~7000 and assume a cylinder Cd of ~1.2, meaning a CdA of .00016 m^2…

In that case (and using the same rho) I get a drag force (@30 mph) of .017 N, or .017 kg-m/s^2.

Not sure why one would want to translate this to grams…

Yeah…that’s one of those “tradition” things…anyway, to check on the original question we divide the force value in Newtons by g, or 9.81 m/s^2

.017 N / 9.81 m/s^2 = .0018 kg = 1.8 g

Looks like AC was off by a factor of 10 when quoting Mr. Kyle, not 100.

… but multiplying by velocity will give the power lost:

P = 0.0109 N * 12 m/s
= 0.132 W

The ROT says at race speeds the 1.8g per inch would be worth ~ 0.2W per inch of exposed housing.

Double checking with the CdA calculated above:

Power = 1/2 * (1.2 kg-m/s^2) * (.0016 m^2) * (12 m/s)^3 = 0.17W per inch of exposed housing.

This leads into a related question regarding housing-less center-pull brakes. Does eliminating, say, 4 inches of housing in a brake setup (a la Hooker) have a measurable CdA reduction? (Using the .000130 M^2/in number, the CdA reduction is only .0005 M^2 – worth maybe 0.5W

For 4 inches of exposed cable, it looks more like closer to 1W…but, 4 inches seems a bit short for most folks’ setups…

Of course, none of this takes into account (for a front brake) the reductions in drag gained by eliminating the large brake arms hanging out in the breeze and the smaller overall frontal area and “smoothness” of something like a Hooker brake.

[chicanery]

The other figure that gets thrown around is 1W/in. My recollection is that it came from John Cobb.

Chris

[MOP_Mike]

In that case (and using the same rho) I get a drag force (@30 mph) of .017 N, or .017 kg-m/s^2.
…
.017 N / 9.81 m/s^2 = .0018 kg = 1.8 g
…
Looks like AC was off by a factor of 10 when quoting Mr. Kyle, not 100.
The ROT says at race speeds the 1.8g per inch would be worth ~ 0.2W per inch of exposed housing.
…
Power = 1/2 * (1.2 kg-m/s^2) * (.0016 m^2) * (12 m/s)^3 = 0.17W per inch of exposed housing.

For 4 inches of exposed cable, it looks more like closer to 1W…but, 4 inches seems a bit short for most folks’ setups…

Of course, none of this takes into account (for a front brake) the reductions in drag gained by eliminating the large brake arms hanging out in the breeze and the smaller overall frontal area and “smoothness” of something like a Hooker brake.

Thanks, Tom. I was trying to figure out how much of a penalty the housing was causing on my Egg brakes. (I was cooking a hairbrained naked cable scheme for 'em.)

[Former_User]

In that case (and using the same rho) I get a drag force (@30 mph) of .017 N, or .017 kg-m/s^2.
…
.017 N / 9.81 m/s^2 = .0018 kg = 1.8 g
…
Looks like AC was off by a factor of 10 when quoting Mr. Kyle, not 100.
The ROT says at race speeds the 1.8g per inch would be worth ~ 0.2W per inch of exposed housing.
…
Power = 1/2 * (1.2 kg-m/s^2) * (.0016 m^2) * (12 m/s)^3 = 0.17W per inch of exposed housing.

For 4 inches of exposed cable, it looks more like closer to 1W…but, 4 inches seems a bit short for most folks’ setups…

Of course, none of this takes into account (for a front brake) the reductions in drag gained by eliminating the large brake arms hanging out in the breeze and the smaller overall frontal area and “smoothness” of something like a Hooker brake.

Thanks, Tom. I was trying to figure out how much of a penalty the housing was causing on my Egg brakes. (I was cooking a hairbrained naked cable scheme for 'em.)

AC sent me the following in an email on this subject:

…on page 158 of Burke’s “High Tech Cycling”, he gives the drag of sheathed cable perpendicular to the airstream as being 0.106 lbs/ft (at 30 mph). Multiplying by 454 and dividing by 12 yields 4 g/in. So, I don’t know where I came up with the 18 g/in, but 'tis interesting that Kyle’s number is ~2x what people have estimated based on basic aerodynamic principles (interference effects, perhaps?).

[AndyF]

AC sent me the following in an email on this subject:

…on page 158 of Burke’s “High Tech Cycling”, he gives the drag of sheathed cable perpendicular to the airstream as being 0.106 lbs/ft (at 30 mph). Multiplying by 454 and dividing by 12 yields 4 g/in. So, I don’t know where I came up with the 18 g/in, but 'tis interesting that Kyle’s number is ~2x what people have estimated based on basic aerodynamic principles (interference effects, perhaps?).

Free Coggan!!!

[Former_User]

AC sent me the following in an email on this subject:

…on page 158 of Burke’s “High Tech Cycling”, he gives the drag of sheathed cable perpendicular to the airstream as being 0.106 lbs/ft (at 30 mph). Multiplying by 454 and dividing by 12 yields 4 g/in. So, I don’t know where I came up with the 18 g/in, but 'tis interesting that Kyle’s number is ~2x what people have estimated based on basic aerodynamic principles (interference effects, perhaps?).

Free Coggan!!!

He *is *free…he just freely chooses not to participate. Well…directly, at least…

[garytingley]

[
Free Coggan!!!

x2

[AndyF]

[
Free Coggan!!!

x2

Before this gets out of hand, my comment was tongue-in-cheek. I do wish Andy would come back – we’d all be smarter for it. I’m sure he has his reasons.

[rbuike]

There are inline barrel adjusters you can get as well.

Other question might be if you are going to design an aero bike why not put the cables through the frame?