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Bike weight and the myth of ‘fast’ bikes

  • By Jim Gourley
  • Published Aug. 6, 2014

Editor’s Note: This excerpt is adapted from the book FASTER: Demystifying the Science of Triathlon Speed by Jim Gourley and republished with permission from VeloPress. Learn more about the science of triathlon at freetrispeed.com.

Let’s clear something up. There is no such thing as a “fast bike.” Bikes are neither fast nor slow. Bikes are shiny or expensive. Bikes have a lot of mass or a little. Without a rider, they are stationary. Physics holds a bike in place until you get on it and start pedaling. Even then the bike may not necessarily be fast. Of all the equipment on your bike, your legs are the most critical component. There are plenty of nice bikes on the road that are being ridden slowly.

But more insidious than inaccurate vocabulary is a simple overestimation of how much bike weight matters for most riding.

In FASTER, I show the math that explains why just a degree or two of incline makes riding a bike feel so much harder. Riding up a hill, it may seem more important than ever to dump any and all extra mass we can from our bikes. That’s the allure of a carbon fiber bottle cage, an upgrade to carbon fiber cranks, handlebars, stem, carbon saddle rails, or wheel spokes. Five grams here, 10 grams there, it all adds up, right? Pretty soon, you’re 500 grams lighter. That’s half a kilogram!

True. But such upgrades could easily total $500 or more, which is also half a grand. Is it worth it?

Not exactly.

A good approximate difference between an entry-level aluminum bike with a decent set of components and a top-of-the-line carbon model with some of the lightest components on the market is just shy of 3.25 pounds.

Was the weight loss worth it?

Let’s find out. Take a hypothetical rider and have her ride two bikes up a hill at the same speed. The first bike weighs 15 pounds and the second bike will shave off the 3.21 pounds to weigh in at 11.79 pounds. For each test, we’ll have her ride at 15 mph. Everything is constant, except for the bike, so what we ought to see is a reduction in the power required to get up the hill. That’s the real test of your savings.

Refer to the second image, above, for a graph of the results.

If you’re having trouble telling what the difference is, save yourself the eyestrain, because there isn’t much — that’s the message.

But pro athletes use the lightest equipment they can, so there must be something to it, right?

Remember that professional athletes operate in an entirely different environment than the rest of us. They are all very close to each other in terms of fitness, and they are also all very close to being the absolute best a human being can be.

Beyond that, our result also makes intuitive sense: 3.21 pounds is just over 2 percent of the total weight of our 150-pound cyclist and 15-pound bike. Ten watts is 2 percent of the 500-watt power requirement to maintain speed up a 10 percent grade. Because the weight-to-power savings ratio is linear, we should expect that one-to-one relationship.

The implication is a bitter pill, though. If you want to reduce the power requirement by 1 percent, you have to reduce the total mass that’s moving up the hill by 1 percent. And because you’re moving both your body and the bike up the hill, a measly 1 percent equates to a whole lot of grams before you see returns on your carbon investment.

In short, you’re much better off upgrading your legs and dropping body fat through proper training and diet. In fact, losing unnecessary weight would have a dual impact on your power and speed. As weight decreases, the amount of power required to maintain a certain speed will also decrease. At the same time, the amount of power you are capable of generating should actually increase. This is because oxygen uptake is related to body mass and improves as fat is lost.

Wattage vs. time

If the power argument doesn’t quite satisfy you, we can look at it another way. Let’s answer the question you really care about: How much faster does it make me? After all, you win races by saving time, not watts. Let’s see what will happen when our hypothetical rider rides bikes of varying weight up different hills. We’ll hold power at a constant 200 watts and have her ride up a 1-mile climb at seven different grades (1–7 percent).

Let’s look at the difference between 15-, 16-, 17-, and 18-pound bikes, with the 18-pound bike serving as the baseline. Because of the complexity involved, we’ll eliminate air resistance and analyze the impact of weight reduction only. How much time do we save?

A graph of the results is in the third image above.

Read it and weep, weight watchers.

Look at the far right of the graph. Take 3 pounds off your bike, pedal at a constant rate of 200 watts, and you’ll get to the top of a 7 percent climb a whole 7.5 seconds ahead of the competition. A 1-pound advantage only puts you ahead by 2.5 seconds. Over the course of an hours-long race, a few seconds per climb is not a significant advantage.

Keep in mind that the advantage only holds when the climbs are long and steep. Courses with fewer and shorter ascents will keep the difference small.

FILED UNDER: Bikes and Tech / News

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