Cyclists are always debating which has a greater impact on speed: lighter weight gear or more aerodynamic gear. We set out to settle the matter for good. First, we're focusing on cycling gear, not bodyweight. In general, reducing aerodynamic drag by any given percentage will have between four to six times greater impact—depending on terrain and average speed—compared with reducing you and your bike's weight by the same amount. What does this mean? You can put your bike on a diet and save a few seconds, or you can get aero and save loads more time. Let's look at some specifics.
Climbing
The assumption when climbing is that the clear winner will be the lighter bicycle. But is it? Time savings are relatively small: Reducing your bike's weight by just half a kilogram will save you less than three seconds over a 1500m climb with a seven-percent grade. But a lighter bike may handle more precisely, so while it feels easier to ride, the time saved isn't as much as you'd think.
Descending
More mass while descending will allow you go faster, right? Not as much as the drag-reducing benefits provided by a wind-cheating frame, apparel, helmet and using an aero position on your bike. In this scenario, the winner is aero.
Flats
The clear advantage on flat terrain goes to aerodynamics. This is because more than 80 percent of energy expended while cycling is used to overcome wind resistance. The faster you go, the more aerodynamics come into play in reducing effort and increasing speed. Since a cyclist goes faster on flats compared to climbing, reducing your drag coefficient will have a greater effect on speed than reducing weight.
Mixed or Rolling Terrain
Once again, aero bikes and gear are faster than standard gear in situations where the road changes gradient. A cyclist is spending relatively less time climbing as a percentage of the overall time spent in the saddle, so weight is not as significant as drag reduction. Since aero is faster when descending and when on the flats, it's more efficient to decrease wind resistance compared to the small gains offered by reducing weight.
Bikes
It used to be the case that aero bikes weighed more than ultra-light climbing bikes because more material was required to create the aero shapes. Although this is still the case for time trial bikes, road racing bikes are now available as both lightweight and aerodynamic. And since aero is faster for all but the steepest and longest climbs, manufactures can blur the lines between an aero bike and a "climbing" bike.
Wheels
Once a wheel is up to speed, decreasing weight affects performance less than improving aerodynamics. This is why you see pros and cycling enthusiasts alike on deep carbon wheels, which weigh more than feathery light wheels but improve aerodynamics.
Helmets
You guessed it. Even with helmets, aero is faster, especially around your noggin. The caveat is that a lightweight helmet will be more comfortable on your neck and shoulders for long rides compared with a 300-plus gram time trial helmet.
Sure, aero helmets may weigh a little bit more than those with massive vents, but they can save you scads of seconds or even decrease your effort by an impressive amount: in excess of two minutes over 50 miles when riding hard solo.
Apparel
Those loose-fitting jerseys with ample pockets loaded with spare tubes, mid-ride snacks and your phone are fine for cruising along. But if you want to beat the clock, go aero. A skinsuit for road racing could save you nearly 90 seconds over 40K, according to Specialized's wind tunnel testing.
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