Frontal area v. chord ratio: better aerodynamics?

Triathlon Forum
1

hello,

I am currently building my new frame. It’s mostly Colubus Foco steel, but I am considering a different downtube. Here are my choices; my question will follow (minor diameter x major diameter/chord ratio/weight) :

Foco Megatube: 28x41mm / 1.46:1 / 236g

Foco standard: 28.6 round / 1:1 / 216g

Zona Megatube 1: 31x52mm / 1.68:1 / 362g

Zona Megatube 2: 28x43mm / 1.54:1 / 263g

All weights are for the same length tube, but not necessarily the length I will use on the frame. My question is; is it better to minimize the frontal area of the downtube (by using Zona Megatube 2 or Foco Megatube) or to push for the best chord ratio? I want the bike to be made out of steel and I realize that none of these chord ratios are equal to cervelo or tiemeyer in aluminum.

I ask these questions from a purely aerodynamic perspective. weight does not matter to me at this point, the weights are included for reference. So given the lackluster chord ratios as a whole, should I stick with a smaller minor diameter? Or, given that it is only a 3mm difference, should I go with the better chord ratio? Or lastly, split the difference and take Zona Magatube 2?

Thanks for any help. The bike will fit perfectly. I already have a disc wheel and an aero front wheel. I have slim integrated aerobars. What I really want is to focus solely on the frame since it will be the last one I buy for several years.

eddie

3

This is a fascinating thread and I would be interested to learn more about the role of chord ratio (tube or rim depth compared to width) on the drag coefficients of rims and frames. Any good resources? Aerodynamics fascinate me.

5

Great article Kraig.

Do you know of any bike manufacturers, besides cervelo, that use tubing shaped like the tubing on the “wheel strut” pictured in your fork study? Many bike companies claim to use airfoil tubing, but upon closer inspection, they don’t have a “teardrop” shape. At the speeds that bicycles travel at, how important is NACA “teardrop” shaped tubing?

On a related topic, can trip wires be used to increase a tube’s effective chord length regardless of tube shape?

I appreciate any insight that you may be willing to share.

6

“My bad, Tom, it looks like I didn’t link fork study right…”

One of the most informative sites on the web. Kraig, when are you gonna get back into the wind tunnel again?

7

One thing I’ve always heard is that the std teardrop shape is very important at airplane speeds (obviously), but for bicycles two factors predominate:

A) The more narrow the better - ie the old Hooker frame - from straight on it looked like a sheet of paper

B) The more side area the better due to the sail effect in cross winds - ie a disk wheel.

Aside from these two factors surface coatings (dimples) and tubing shapes (oval, teardrop, lens, etc) have a very small effect.