The time trial start: how hard to push in the first seconds, and why a fast start is worth so little

A standing start costs 9.7 seconds compared with a flying start: at 300 W it takes 49 seconds and 474 metres to reach 98 % of cruising speed. That is the starting loss, and the only question is how much of it you get back by spending more at the beginning and a little less afterwards.

How much is a fast start worth?

Little. With the same total energy over the 40 km, starting at 500 W for 10 seconds and then holding 299.6 W saves 2.1 seconds and costs 2 kJ of W′. Starting at 400 W for 30 seconds saves 1.5; at 350 for a minute, 0.8; at 330 for two minutes, 0.5. The longer the hard start, the less it saves and the more W′ it spends, because after the first 20 seconds you are already almost at cruising speed and pushing there does not accelerate anything, it only spends⁵.

At 400 W, the launch takes 22 seconds and 196 metres; at 500 W, 15 seconds and 132 metres. Past that point, the only thing high power does is burn reserve that will be missing in the final stretch or on a climb, where every kilojoule of W′ buys far more time than on a flat start⁵.

And an easy start?

It also costs little: a first minute at 250 W made up with 300.8 W for the rest loses 1.6 seconds. What really costs is not the pace of the first seconds, but riding above sustainable power for several minutes, because then you pay in the last third: pacing studies show that starts that are too fast end in a final pace below the constant one^3,4.

• First 15 seconds: seated or standing, 400 to 500 W to reach cruising speed. That is what is worth it.
• From second 15 to minute 2: drop to target power already. This is where most people overdo it.
• The reserve: every kilojoule of W′ you do not spend at the start is worth double on the climb or in the headwind at the end.

Rule of thumb: 15 hard seconds to launch the bike and then target power. A good start is one you cannot tell apart after a minute.

What is known and what is not

The cost of the launch and the small value of extending a hard start are straightforward physics^1,2, and a slightly fast start with a steady rhythm afterwards being the best strategy for 20 to 60 minute events is well documented^3,4. What each rider has to measure is their own cost of starting too hard: how much W′ they spend and how long it takes to rebuild it, which varies a lot between people⁵.

Frequently asked questions

How hard should you push at the start of a time trial?

About 15 seconds at 400 or 500 W to launch the bike, then target power. Extending it saves tenths and spends W′.

How much do you lose with a standing start?

About 10 seconds on a 40 km time trial compared with a flying start. A good start gets about 2 of those seconds back.