Archive:
Ride Smart / Not Hard
Today's Topic: Making Circles
By Peter Jensen
Long, long ago, I was a dirt-bike freak. I'm talking about the other kind of bike with a motor that make lots of noise and weighs more than you do. In those days, my motto (and everyone else's) was "When in doubt, give it gas." In other words, if severe pain and possible death appear certain within the next second or so, a quick twist of the throttle generally repairs the situation. I'll spare the details; but trust me, it works.
Now that I'm a bicyclist, I have a different, though still broadly relevant motto: "Ride smart, not hard." In other words, know your bike, know your body, and know how to get the most from both. For example, if you see a steep climb approaching, should you start breathing harder only after the climb drives you into oxygen debt? Or should you (be smart and) start breathing hard in anticipation of the hill? Well gosh, now that I think about it, a little planning goes a long way, so maybe I'll start breathing in advance of need.
More to the point, this column will focus on the physics and mechanics of bicycling. For example, if your bicycle is heavy, then you'll have extra weight to carry up hills, and your performance will suffer. If your hair is long, or if you're wearing a bunch of jewelry, then you will weigh more. If your body fat level is high, or if you carry two water bottles when one is sufficient, then you'll have more weight to carry up hills. Weight is the cyclist's nemesis, and more often than not, it slows you down.
Now lets move onto something less obvious. Today's topic: "Making Circles." Or, as is often heard in reference to pedaling technique, "think about making circles with your feet." Over the years I've heard this suggestion from a dozen sources, stated in a dozen slightly different ways. But I've never heard a meaningful physical explanation that reaches beyond the words. Yes, pedal in circles. Why? Well, 'cause that's what you're s'pose ta do.
So here's the real reason. The physical explanation.
Consider your crank arm, as shown in Figure A. When the crank is at the very bottom of the pedal stroke, does it do any good to push straight down? Obvious answer: no. All that can happen is an imperceptible stretching of the metal crank arm. Pushing down doesn't make the pedal go 'round. If you don't believe me, you can try it. Take your bike out on the street, position the pedal at the bottom of the stroke, and then push down as hard as you can. What happens next? Well, you fall over.
Now suppose the pedal is exactly at the top of the stroke, as shown in Figure B. Does it do any good to push straight down? Not so obvious answer: no. All that can happen is an imperceptible compression of the metal crank arm. Provided that you push exactly straight down, you will get nowhere, and all of your effort will be wasted.
You might ask why this isn't obvious based on experience. After all, as a child you learned to push down on the pedal from top to bottom of the stroke, and you have been making progress. Why, suddenly, is it wrong to push down on the pedal? The answer is simply that you haven't been pushing straight down. Instead, you've been pushing slightly forward too, and this forward force pushes the pedal over the top of the stroke.
So when should you push down on the pedal? Answer: in only one place, and that is when the crank arm is forward and horizontal, as shown in Figure C. When in this position, the crank arm cannot be compressed or stretched by a downward force. Instead, the force causes the crank arm to turn - which is what you want.
Similarly, when the pedal is at the back of the stroke, you should pull directly up. When at the top, you should push directly forward, and when at the bottom you should push directly back. This is shown in Figure D. (If you remember your high school geometry, you'll recognize the force lines/arrows as tangents to the circle.)
In other words, any force directed along the crank arm is wasted. Meanwhile, the force perpendicular to the crank arm is useful; it "makes the pedals turn." And, this force must track the rotation of the crank arm, changing direction in concert with the circle traced by the pedal.
In other words, don't waste energy compressing or stretching the crank arm. Instead, make circles with your feet.
|
 |
"Ride Smart / Not Hard" is a much anticipated regular column in the Eastern Idaho Bike Magazine. An alternate
heading is "Pedal Smart." Which one do you prefer? Or can you think of a better heading?
Let Us Know!
|
