See the Light—A Quick Physics Lesson

Every once in a while I read something spurious on all of these athletic sites I visit that just irks me, that leaves me thinking, “Where did this guy get his information? Has he done any research or talked to anyone with any kind of experience?” Today’s post is about one of those.

I came across an article recently where someone suggested getting a lighter bike as a key to a better bike split in a triathlon. Lest you believe I totally disagree with that idea, I would like to state that, yes, a lighter frame can sometimes make you faster—but sometimes, it makes no difference at all. It really depends on the topography and a few other things. Allow me to explain:

The whole idea behind the bicycle was that you let the machine carry your weight rather than hauling it yourself. On climbs, weight plays a huge role as you have to push the whole package (machine, man, clothing, hydration, etc.) up the hill. But most triathlons don’t take place on daunting climbs. Most of them, while perhaps featuring the occasional climb, take place on relatively flat roads. And on flats, it’s all about aerodynamics—weight isn’t even a secondary concern (I’d place it pretty far down the list, actually). The truth about riding on flat roads—as any roadie from the Tuesday-night group ride can tell you—is that the bike renders weight a virtual non-factor. The bike is carrying all of your weight for you, and once that body gets in motion, it tends to stay in motion—so you really just need to get it moving.

Now, granted, spinning weight is a different subject, though it does function on the same physical principles. The drive train and wheels, if lighter, can improve your speed somewhat. But even then, you don’t want paperweight wheels because they won’t carry as much momentum. In a flat time trial, momentum is what allows a rider to ease off from time to time—it’s a crucial determiner of speed and exertion.

Even bodyweight has only a minimal effect on speed on flat roads. Take a look at the current World Time Trial Champ—Fabian Cancellara (hang in there, Dave Z, it’s your turn next year!): Does Fabian look light to you? His web site reports his weight to be about 80 kilos or about 176 pounds (and you can bet that’s a conservative estimate). That means he’s about 16 pounds heavier than Lance Armstrong was. It’s his weight—the muscle power in his thighs—that allows him to be the world’s best time trialist.

One piece I read in Joe Lindsay’s blog at suggested that a full 90 percent of the resistance against a cyclist comes from aerodynamic forces—from the wind and air the rider is moving against—and two-thirds of that 90 percent (so 60 percent, I suppose?) comes from drag produced by the rider. Even drivetrain resistance accounts for a mere one percent. Granted, as the road becomes steeper and the rider moves slower, aerodynamics factor less and less into the equation.

So what’s the take-home principle? Improve your legs and your lungs BEFORE you go drop a bunch of cash on a more expensive bike, improve your own aerodynamics, and make only smart upgrades. What would be a smart upgrade for a triathlete? Well, maybe some slightly lighter, more aerodynamic wheels, perhaps an aero helmet, and of course a set of aero bars. Save the 15-pound frame for when you plan on racing up 10-percent gradients at high altitude (where aerodynamic resistance is less of a concern).


One thought on “See the Light—A Quick Physics Lesson

  1. Boz

    Why is it that power to weight ratio applies only on the flats. I could always put the boots to greyhounds on the flats or short grades, but when the climbing starts, I\’m off the back right away. Seems that gravity has a say in the equation. So, a thin, light guy has an aero-edge on the big, strong guy, but it doesn\’t matter on the flats, but the stick-man can climb all day. A big auto engine uses a lot more fuel, but can go uphill and on the flats better than a small displacement engine in the equal size car. Different rules aplly to human power than to mechanical power, evidentaly.
    Just a thought.

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