Fun With Numbers

I was hoping to write a happy post about the glorious turn of the seasons and the joy of pedaling through the changing colors of the leaves and cool, crisp Autumn air.    Instead, I think I will share my thoughts on the science of wind resistance.

FUN FACT:  On flat ground, 70-90% of a cyclist’s power is spent overcoming the effects of wind resistance.  Only climbing hills requires more energy.

So off I went this morning on a short 17-mile ride, the first nine miles of which were uphill and into a 25 mph wind.  In short order, I was subjected to the invariable truths of science.  These truths proceeded to kick my arse.

Wind resistance is a really, really big deal.  As a cyclist tries to go faster, he encounters more resistance – the square of the increase in velocity, to be precise.  This requires dramatically increased amounts of power.  I used this site to get a sense of how much power I was using.  To climb a relatively moderate hill of 3% grade at 15 mph, I use approximately 400 watts of power.  To climb this same hill in the face of a 25 mph head wind requires 800 watts – TWICE AS MUCH POWER.  Your mileage may vary.

Bicycle companies work hard to make bikes light, but on flat terrain the effect of wind resistance is far more significant than reductions in weight.  For example, adding a pound of weight to a bike has the same effect as placing a pencil-sized object somewhere on the bike.  Obviously, the lighter bike has a bigger advantage when you’re trying to lug it up a hill (and we’ve already mentioned that hill climbing is a bigger energy drain than wind resistance , so this is nothing to sneeze at) but it’s still interesting to see that the square of velocity has far more effect than changes in mass.

This is why I was in an ill humor after 45 minutes of plowing into this wind.  I had used up as much energy as I typically do in an hour and a half of riding.  I therefore was not terribly interested in appreciating the beautiful colors of the foliage or the crispness of the Fall air.  I just wanted to get home in decent enough shape to take my kid to his Saturday bowling league without looking like an invalid. 

Life became noticeably better for me after I turned away from this headwind.  While I struggled to maintain a 15 mph average into the wind, I instantly began pedaling at 20+ mph with it at my back.  And now I was going downhill to boot.  However, it is important to note that even when wind is blowing perpendicular to the cyclist, performance is degraded.  In fact, it is only when the wind is somewhere between “four o’clock” and “eight o’clock” that the rider is aided.  When traveling in a circle, the cyclist is impeded by wind 66% of the time.

Which is why I don’t like wind.  Not one little bit.

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8 thoughts on “Fun With Numbers

  1. Nice post, in my ride today it seemed like whenever we changed directions and started heading back the wind would change. It was headwind all day long.

    • Don’t you hate it when the wind follows you? 🙂 At one of my Army postings, everyone was absolutely convinced that the wind shifted on the Physical Fitness Course to stay in front of the runners, despite the fact the route was up and back on the same road. Wind sure can be funny!

  2. Your train of thought when riding is obviously on a far higher plane than mine.. :o) I do seem to do a lot of thinking when riding, but strangely not a lot of it seems cycling based..

    • My train of thought often revolves around the amount of suffering I am currently experiencing: why it is occuring and how to lessen it. This takes me into all sorts of fields, from the sciences to metaphysics! 🙂

  3. Sounds like I rode where Matt rode. Headwind no matter which way we turned. AND hills. Ah well, tomorrow is another day. And I am never thinking about such intellectual things on my bike, either.

    • Armed with information such as this and IF I was so inclined to bicycle across the country, I would most certainly go from West to East, with the prevailing winds behind me. The difference in required energy over 3,000 miles would be incredible. Of course, you intend to do a round trip, so it really doesn’t matter for you, does it? 🙂

  4. Pingback: The Streak Is Alive | There And Back Again

  5. Now I know why it hurts so much going uphill and into the wind. Doesn’t make it any more fun. But I can try doing the math to distract myself as I pedal up the next hill or into the next gale. (Great post!)

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