Wednesday, December 26, 2007

10 A Field Test for Calculating Aerodynamic Drag Area?

We all know and have seen professional cyclists with bent, flat backs over the aerobars or knees closer to the top tube while pedaling. The lore of aerodynamics is that drag increases as square of the velocity, so the faster you go, the double becomes the drag against you. The biggest area exposed to the wind is the cyclist himself, so the largest gains in speed are made if one minimizes or streamlines this exposed area. In mathematical terms, its called CdA.

Traditionally, aero drag area of a cyclist was measured in wind tunnels. But it has been found that the energy expenditures of operating the wind tunnels overwhelm the costs of the measurements. It is a big investment.

Ordinary folks (or extraordinary folks) sit on their saddles wondering how they can calculate/quantify this area. I have many times.

But if you want to sink your head deeper into numbers, there is a paper I discovered written by James C. Martin et. al. (from the University of Utah at Salt Lake) in 2006 describing in detail a field test that could be done by anyone with :

1. Microsoft EXCEL
2. A Power meter like PT or SRM
3. A nice stretch of road or a velodrome
4. Some knack for taking measurements and a pinch patience

Aerodynamic Drag Area of Cyclists Determined With Field Based Measures
can now be accessed on the peer reviewed website, SportSci.Org in the Tests and Technology Section under Biomechanics category. The results can have slight errors compared to a wind tunnel (duh) but according to the paper, they are negligible.

Along with the free paper comes the EXCEL spreadsheet that you can play with!

What I like about this method is the fact that you can immediately get an idea of all the factors that play a role in dictating CdA. And you can get the experience being a bike scientist as well..

Head over there, bike freaks. This could get slightly addictive...

*  *  *


  1. Since I don't have a power meter I did something very primitive one day. I was pedling into the wind on the hoods and wanted to see how much difference the drops would make. I focued on maintaining my pedaling force as best I could. As near as I could tell, I could do 2 mph faster when on the hoods given the same exertion. Rough but I believe the number.

  2. Chris,

    That has been the observation for 100 or so years :)

  3. I meant to say 2mph faster when on the drops, but I am sure you figured that out. Not insignificant at all. There are many many recreational cyclist who never ride the drops.

  4. Chris,

    No sarcasm intended above.

    I think some folks don't think about the drops during recreation. They just want to have fun you know. I have a feeling this is more an issue of discomfort, but I'm also believing that discomfort issues aside, the low speeds (~<18mph) involved in casual riding don't quite justify the use of the drops as much.

    I tend to think that above a certain threshold forward velocity and perceived effort, its better to go on the drops in order to be more efficient, i.e, getting more out of doing the same.

  5. Ron

    Interesting post

    I guess cycling uphill the wind drag is relatively minimal and its all about gravity.

  6. I believe that being in the drops also gives you better handling and control of the bike.

    Over a year ago I broadsided a big dog (Rottie mix) that charged our group. I was doing about 26 mph at the time. Rode over the pooch, spun the bike around, and although I thought I was going to be able to save it, high-sided the bike and landed on the grass (luckily). I thought the dog was going to come over and chomp on my leg but he just ran home.

    I tacoed the rear wheel but, again, was able to maintain better control of the bike in spite of the (large and moving) speed-bump.

    Later I heard the dog was fine, but I am sure he was sore for a while. I've never seen him again when we ride that road.

    Just my 2 cents.

  7. Luis,

    Interesting story. I think you ride for a club. Do you have any provisions for taking action against the owner, should the dog bite off, I don't know...your shoe adjusting lever or something?

    There was only once I was chased by an angry black dog, that was all uphill. No point of the hoods then. I wasn't in the best mood to dance the pedals like Pantani either. The best part is that I was doing hill repeats at the time, and I believe I broke my best time on the 2 mile climb. I'm sure the climb was tough on the dog's cardio system so he probably gave up.

    Having said that, its nice to carry a small hand pump with you. Should the dog make any nasty moves, you have something to scare away or wield against an attack...

  8. Hi Ron,

    we don't have any legal provisions per se, so it's every man for himself. Of course I could've sued the owners of the dog, but, as someone said, "You can become the proud owner of a trailer." It seems that these situations usually occur in those type of areas.

    Your encounter with a dog reminded me of American Flyers when Kevin Costner takes his "brother" out for a training ride.

    Oops, time to get ready for our New Year's Ride. Gotta run... err, pedal :)

    Happy New Year!


  9. hi Ron,
    me and my team are designing a 'Quad Rider' as a project.The seating arrangement of this rider is same as that of a normal car.The four people together provide the total power for cycling.This rider has no outer body like a car and the riders ride in open.
    I wanted to ask you if there is any way in your knowledge to calculate the optimum distance between the drivers seat and the seat beside it for minimum aerodynamic drag.

  10. Pranay,

    I would think quad bicycles have been made before, if not commonplace. Google tells me so, atleast. You can check it out yourself.

    I think you can focus on keeping the riders close to the ground, like in Recumbent designs, or somewhat reclined positions. But with four riders, you have to design with contraints.

    Spacing arrangments must be centered around seating and pedalling comfort, you don't want people too close to each other so they can smell each other's sweat and tears, you know that kind of thing. :)

    Brainstorm a few ideas out. You can send your concept to me or the bicycle design blog, if this is a student design.


Thank you. I read every single comment.