Aero or weight: what saves more time on a flat course and at IRONMAN Nice?

By Kristoffer Visti Graae, physicist, former professional triathlete and founder of AeroGain. Published 10/11/2026.

The short answer

In our model, on a flat Ironman bike leg, saving 1 W of drag is worth as much time as losing about 1.5 kg. On the IRONMAN Nice course, with about 2,400 m of climbing, the same watt is worth about 130 g. Weight matters about ten times more in Nice than on the flat, but aero still counts: a 5 W drag saving is worth as much time in Nice as losing about 0.6 kg, more than most lightweight upgrades take off a bike. These are results of a model, not of a test: we calculated them for three riders over the Nice course profile, and every assumption is listed below.

Key findings

• Flat 180 km: 5 W of drag saved (measured at 45 km/h) saves 84-98 s; 1 kg less saves 10-15 s.
• IRONMAN Nice, 180 km and about 2,400 m of climbing: 5 W saves 54-63 s; 1 kg less saves 70-121 s.
• On the flat, 1 W at 45 km/h is worth as much as 1.3-1.8 kg. In Nice, it is worth about 100-150 g, and about 70-180 g across every assumption we varied.
• Most of the aero saving in Nice comes on the rolling roads and gentle drags; on climbs steeper than 4 % it is only a few seconds.
• The slower rider saves more time from the same drag saving, and more again from the same weight saving in the hills.

Why the answer depends on the course

A rider's power goes to three things: pushing air aside, rolling the tyres and lifting the bike and body up the road. Air drag grows with the cube of speed, so on the flat, at 36-43 km/h, it takes most of the power, and a small drag saving turns straight into speed. Weight only costs on the flat through rolling resistance, which is small.

On a climb it turns around. At 15-20 km/h the air takes a small share of the power and lifting the weight takes most of it. A kilo less is a kilo less to lift 2,400 m, every time the road goes up. Downhill, a heavier rider rolls slightly faster, but on the technical descents to Nice the brakes set the speed more than the weight does.

Nice is about half climbing: the riders in our model spend 42-48 % of their bike time on gradients above 2 %. That is why weight matters so much more there, and why aero still matters: the other half of the time is spent on rolling roads, false flats and descents, where the air is again most of the resistance.

How we calculated it

We used the standard power equation for road cycling (air drag, rolling resistance and gravity) and applied it to every 100 m of each course. The rider pedals at a constant power, eases off above 50 km/h downhill, and brakes at 60 km/h. We compared each rider against the same rider with a drag saving, and against the same rider with less weight, and counted the time difference over the whole bike leg.

• The riders: pedalling at 180, 230 and 280 W (in Nice they coast downhill, so they average about 160, 200 and 235 W over the whole ride); drag area (CdA) 0.24 m², a good age-group triathlon position; 85 kg for rider, bike, bottles and kit.
• The conditions: no wind; air density 1.18 kg/m³ at sea level (about 25 °C), falling with altitude; rolling resistance 0.004 (good race tyres); drivetrain efficiency 97 %.
• A drag saving is given the way we report test results: watts saved at 45 km/h. 5 W at 45 km/h is a drag-area change of 0.004 m². The time it is worth is then calculated at the rider's own speed on every metre of the course, so 45 km/h is only the label, not the speed the rider rides.
• The Nice course: the official IRONMAN France bike route (the GPX file on ironman.com, checked against the 2026 bike map), with its elevation taken from the European terrain model (EU-DEM, 25 m) and smoothed to about the official 2,400 m of climbing; the route file is 171 km, so the last 9 km to 180 km are added as flat road near Nice. The flat course is 180 km with no gradient at all.

Results: aero against weight, flat and Nice

Does aero matter uphill?

Some, mostly on the gentle drags. For the 230 W rider in Nice, a 5 W drag saving gives back 58 s: 39 s on the rolling roads and false flats, 8 s on the descents and 11 s on gradients above 2 %. Of those 11 s, only about 4 s come on climbs steeper than 4 %, where the rider is down at 15-20 km/h and the air is a small share of the resistance. Aero is won on the fast parts of a course, even a hilly one.

What this means for your race

1. On a flat or rolling course, choose aero. A watt of drag is worth well over a kilo, so almost no weight saving can match a real drag saving.
2. On a hilly course, compare the two with a rule of thumb: 1 W at 45 km/h is worth about 100-150 g in Nice. A change that saves 5 W and adds 300 g is still about half a minute faster for the 230 W rider (58 s saved, about 27 s lost to the weight). A change that saves 1 W and adds 300 g is slower.
3. Know your own numbers. The watts a helmet, a cockpit or a bottle position saves differ from rider to rider. A drag saving you have measured on yourself is worth more than one from a catalogue.
4. Your position matters more than either. A position you can hold for the whole bike leg beats a faster one you sit up out of after two hours, on any course.

What the model does not include

• Riders do not pedal at a constant power. Most push harder on climbs and ease off on descents, which shifts some of the value towards aero (a watt nearer 110-180 g).
• No cornering, braking or accelerating out of hairpins, so the bike times are probably on the fast side.
• No wind. Wind adds to the value of aero on exposed sections and changes the yaw angle the air meets the rider at.
• The descent speeds are the biggest assumption. On slower, more technical descents (braking at 40-45 km/h), a watt in Nice is worth about 70-95 g; descending faster (70 km/h), up to about 180 g.
• Changing one assumption at a time (CdA 0.21-0.28 m², 75-95 kg, rolling resistance 0.005, the descent speeds, how much the terrain is smoothed), the value of a watt in Nice ranged from about 70 to 180 g across the three riders, and stayed at 100-150 g in the central case.

Frequently asked questions

Is weight ever more important than aero?

Per unit, yes, on a hilly course: in Nice a kilo is worth more time than a watt of drag. But typical aero changes save several watts, and typical weight changes are a few hundred grams. For a change to be slower overall in Nice, it has to add more than about 130 g for every watt it saves.

Does the slower rider gain more from aero?

Yes, in time. The 180 W rider saves 98 s on the flat from the same 5 W drag saving that saves the 280 W rider 84 s, because the slower rider is on the course longer. As a share of the bike time it is the same, about 0.5 %.

Why give drag savings "at 45 km/h"?

It is how we report test results, so they can be compared. A saving at 45 km/h is a fixed change in drag area (CdA); the time it is worth is calculated at your own speed on every part of the course.

Keep reading

• Which time trial helmet is fastest? Velodrome data from real riders
• How to hold your aero position for a full Ironman bike leg
• Behind-the-saddle bottle mounts: a 25-rider velodrome test
• Book a velodrome aero test