Tuesday, December 28, 2010

The Light Side of the Road

Since I don’t own a scale, I can’t tell you exactly how much my bike weighs. But rest assured, it is heavy. It’s a secondhand steel-frame model purportedly manufactured by Sears department store, probably in the 1970’s. Friends complain of its unwieldy mass (particularly anyone hoisting it onto their car’s bike rack) and more than one person has suggested “upgrading” to something more modern. But I’ve bonded with the vehicle and strive to paint it in a positive light. Sure, it sucks carrying the behemoth up a flight of stairs to shelter when it rains. But overall, it’s a solid bike. Sturdy and hardworking. “And a comfortable ride too”, I tell the skeptics, “not like those flimsy featherweight carbon-frame bicycles that retail for ten times as much.” And so I eagerly put aside any ambitions of doing a year-end top ten science innovations/headlines/etc. list (you’re surely as sick of them by now as you are of holiday cookies) to report on this merry little piece from the British Medical Journal's Christmas edition, in which an anesthesiologist attempted to determine if the newer, lighter bikes had any advantage over their clunkier predecessors.

Being fortunate enough to have two bicycles available for his daily commute, the good doctor designed a simple experiment to examine which of the bikes was the more efficient way to get to work. Over a 6-month period, (winter to summer) he chose his bike for the day by flipping a coin. The riding time for each round-trip journey, as well as the top speed, was recorded by a bicycle computer. During the experimental period, Dr. Groves made 30 trips on the steel-frame (809 miles) and 26 trips on the carbon-frame (711 miles). * At the end of month 6, he totaled the data and, wouldn’t you know it, the older, heavier steel bike fared no worse than the shiny new one, which had been purchased for what is almost a month’s salary to an average person without an MD.

The author discusses several physical forces that can affect the cyclist; rolling resistance, drag and gravity. Rolling resistance (the friction encountered by round objects, such as bike tires, moving on a flat surface) is minimal on paved roads, so the additional work needed to overcome it is slight. The effect of drag (aka air resistance) on the cyclist is significant. However, drag is an odd force. It is independent of mass and instead varies relative to velocity. More velocity results in more air resistance. It’s a drag, but not any more so on a heavier bike. This leaves gravity as the most relevant consideration. As you may vaguely recall from your first semester of physics, more work is needed to push a bike with greater mass up a hill. But since a round-trip commute can’t actually be all uphill both ways, things should even out a bit as you coast downhill. Unless, of course, you were crazy enough to purchase a fixed-gear bicycle.

On average, Groves’ commute was about 7 minutes shorter in summer than in winter. He attributes this in part to the poorer weather and bulkier clothing that plague winter cycling, but he also mentions that greater caution taken to avoid falling on the ice and snow may be an additional slowing factor. This raises an interesting question. Was the upper speed limit in the summer months a result of the physical limitations of how fast the rider could propel his bike, or merely the highest speed at which the rider could safely control the bike. If it was the latter, then one of the bikes might be less efficient and the rider may just be working harder to achieve the maximum comfortable riding speed on that vehicle. The author made no mention of whether he felt a greater desire for a cold alcoholic beverage following commutes made on the steel bike.

It’s understandable then if you still feel that a lighter bike would be easier to peddle. But keep in mind that you’re also hauling your own weight up those hills. Groves’ steel-frame bike was about 9 lbs heaver than his carbon-frame (the bikes were about 30 lbs and 21 lbs, respectively). This looks like an impressive weight difference until you add to each bike the weight of its rider. Couple this the frequently-made observation that lighter bikes are less comfortable (rumor has it one feels the bumpy road more on the newer bikes), and it’s hard to justify paying more money for less mass.

To be fair, I should note that the author of the bicycle paper does not claim that his single-subject study is a conclusive and exhaustive exploration of the subject. My willingness to generalize his findings to all bicycles on both sides of the Atlantic is a result of the human tendency to gratefully accept any data that supports one’s existing conceptions. ‡ I like my bike and have no plans to get a newer, lighter one. And as far as I’m concerned, there is now medical literature to back me up.

* That’s 56 total trips, so either he doesn’t work a 5-day week or he used other means of transportation more than half the time.

† This makes sense when you think about it. On a reasonably calm day, the “wind” blowing at you as you ride the bike is created by the forward motion of the bike. The faster you go, the windier it feels.

‡ That’s confirmation bias, for those with a fondness for psychological terminology.

1 comment:

  1. i vaguely recall a study or some kind of article describing how the weight of the bicyclist matters very little for biking long distances. all else being equal (bicycle, physical fitness, etc.), a very large biker will not have a harder time doing the same distance than a very small biker. the explanation was simple: unlike running, bicycles have almost zero up-and-down motion so the rider's weight doesn't matter much.

    i'm assuming they were only talking about moderately sloped tracks.