What a Drag

Posted by Chad Orzel on April 26, 2008
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I went for a bike ride this morning, the first real bike ride of the year (I’ve biked to and from work a couple of times, but this was the first real ride just for the sake of riding). There was some pissy drizzle at the start, but by the time I got on the bike path headed for Lock 8, the sun came out, and it was a cool, pleasant, calm spring morning.

As I was tooling down the path, I realized that I had the bike one gear higher than I usually do on that stretch of path, but I didn’t feel like I was working any harder than normal. “Gee, I guess I’m in better shape than I thought…” I said to myself.

Then I reached Lock 8, and turned around.

It turns out, I’m in just about exactly the shape I thought I was. What I was wrong about was the “calm” part of the weather description. There was actually a very steady wind blowing, that had been at my back the whole time (blowing from east to west, which is a little unusual, and why it hadn’t occurred to me as an explanation). When I turned around, I spent most of the ride back one gear lower than usual for that stretch of path.

So there’s today’s lesson on Experimental Physics for Morons: Wind resistance is a significant problem. Particularly if you’re 6’6″, 250 lbs– I’m around half a meter across at the shoulders, and nearly two-thirds of a meter from waist to shoulder, meaning that on a mountain bike, I’m like a giant goddamn sail.

Or, to be quantitative, the drag force on me is:

Fdrag = -0.5 ρ D(0.3m2)v2 = One bike gear

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Comments

  1. #1 andy
    April 26, 2008

    Huh, you’re lucky – whenever I’m on the bike, the wind’s *always* blowing directly against me…

    …explain THAT mister science guy!

  2. #2 Adam
    April 26, 2008

    Sounds like my ride last Saturday. I happily rode north for 10-15 miles, got a little bit lost, and decided to head back when I finally got my bearings. Sure enough, there was a stiff headwind, and my 16-18mph pace turned into a 8-10 mph pace for the ride home, and my legs did not thank me.

  3. #3 6EQUJ5
    April 26, 2008

    I was once in an old Chevy Nova with a 4-cylinder engine southbound on Interstate 15, heading for Barstow. On the flat stretch just before Elephant Mountain I had the gas pedal floored as the 65-knot headwind brought the car to a stop.

    On the freeway.

    (Northbound traffic was screaming by on the other side of the median ditch.)

    The wind relented a little after a few minutes and the car began moving again, but I never got above 20 mph until I got halfway across the Mojave River and moved into the wind shadow of Main Street overpass and the railroad bridge.

    Worst gas mileage ever on that afternoon.

  4. #4 bigTom
    April 26, 2008

    I can recall taking a bike ride when I was a grad student in Boulder. Took off to the east, picked up a strong Chinook a couple of miles East of town (this was truly a stealth wind). Stupid me, I shoulda thought as I zipped past cars heading East, that I wasn’t that strong. But the delusion of superstrength was just too satisfying to question the why of it. When I reached the county line and turned around, then the awful truth was revealed (30mph plus headwinds). It was a very long difficult return trip.

    I guess Physics people are just vulnerable to this sort of mistake!

  5. #5 Markk
    April 26, 2008

    Yeah I think everyone who rides a lot starts to get suspicious of smooth, fast, easy, early parts of rides. Personally I have ridden up mountains and I would take that any day as opposed to strong headwinds. And I am 220 lbs plus so I am not a climber.

  6. #6 TimJ
    April 26, 2008

    Try running the Southern California Santa Anna winds sometime. :) A few weekends ago it felt like running through jello. At one point one gust blew me sideways slightly into the grass while airborne and a later sustained gust actually had me running in place for a few steps.

    Nice calculation on the drag force above; perhaps that could be a homework problem for your students :) .

  7. #7 natural cynic
    April 26, 2008

    Ah, the cyclist’s lament. Headwinds always outnumber tailwinds and uphills always outnumber downhills. Actually the first is true because a tailwind slower than your bike speed will be a slight headwind and we perceive the second to be true because we take wo much less time to descend than to ascend. The situation gets amusing when you use your bean-fed jet assist and you are riding exactly at the tailwind’s speed.

    6EQUJ5: Your Nova story gives credence to the reason that sales of that car didn’t do very well in Mexico – no va [doesn't go].