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Back 2.20.2017

The Great Weight vs. Aero Debate - a Mathematical Approach

How do you make a bike go faster? It’s a question we ask ourselves every day. Our best answers result in our latest wheels and components. We then go back to the drawing board – sometimes literally a white board or even a pad of paper – to come up with even better answers.

It’s all part of bike science. Speed on a bike comes down to a battle between two things, propulsion and losses. Propulsion comes from one thing – the rear tire on the road, powered by your legs.

Losses are all the things that slow you down. The biggest are:

  • Gravitational losses (weight)
  • Aerodynamic drag (wind)

Which of these components of loss can we combat the most effectively? At Zipp, we’ve found that in most situations, aerodynamics has a bigger influence on speed than weight. We’ve reached this conclusion over decades of testing on the road, in the wind tunnel, as well as real-world experience working with some of the world’s best cyclists and triathletes during races and in training.

What follows are basic steady-state comparison examples based on some simple physics and wind-tunnel data. This force-balance technique is a glimpse into how we can cross-check CFD, wind tunnel, and field test results. Our examples below use the following assumptions:

  • a rider-and-bike system weighing 90kg
  • traveling 40km per hour (24.85mph)
  • the road with a 3 percent gradient
  • the crosswind averaged 7.5 degrees of yaw, a common situation out on the road

Weight Savings
Let’s start off by looking at weight savings. Any cyclist who’s weighed or picked up a modern light-weight bike knows the feeling that a light-weight machine can provide – feels fast, right? Let’s say you’re able to drop 3 pounds (1.361 kg), or about two full 24oz (710ml) bottles, from the bike’s weight.

In this scenario, that 3 pound weight reduction results in a power savings of 4.31 watts.

Aero Savings

What about improved aerodynamic efficiency? Imagine installing a pair of deep-rimmed carbon wheels instead of wheels with shallow aluminum rims. Looks fast, right? We’ll begin here with our same rider traveling 40km an hour, but this time the rider achieves a 4 percent drag reduction by adding a pair of Zipp 808 NSW Carbon Clinchers, which are 82mm deep carbon wheels.

In this scenario, that 4 percent reduction in drag results in a savings of 17.49 watts.

Another issue to consider is the weight of the wheelset. Deep-rimmed wheels often are a bit heavier than more shallow wheels. For our example, a very high end set of aluminum clinchers comes in around 1,520g. A set of 808 NSW Carbon Clinchers weighs 1,810g.

That 290g difference results in a 0.92 watt advantage for the aluminum wheel due to weight in our scenario. Even after that adjustment, the 808 NSW provides a decisive wattage advantage.

Wattage-saving calculations such as the one presented here provide cyclists of all types and abilities with information for making decisions when it comes to wheel choice. Zipp provides a full range of rim depths for every terrain and rider type with our NSW and Firecrest and lineups:

- 202 Tubular (32mm depth)
- 303 NSW or Firecrest (45mm depth)
- 404 NSW or Firecrest (58mm depth)
- 454 NSW (Hyperfoils undulating from 58 to 53mm)
- 808 NSW or Firecrest (82mm depth)

When it comes to wheel choice, look at terrain, personal goals, and your discipline. But in most riding conditions aerodynamics plays a more important role to your speed than does weight.

Learn more about Zipp wheels.