Climbing in a bunch: where to sit before the road goes up

Ask a group of amateur racers where they lost the race and a striking number will say "on the climb". Ask where they were when the road went up and the honest answer is usually "somewhere near the back, moving up". The climb did not lose them the race; the two kilometres before it did. This post is about those two kilometres: where to sit, when to move, and why the physics of drafting and the physiology of your anaerobic reserve both point to the same answer.

What the draft is worth, and where it stops being worth it

On the flat, sitting in a bunch is a spectacular bargain. Kyle's wind-tunnel and track work in the late 1970s showed that a rider drafting closely behind another cuts wind resistance by roughly a third at racing speed, and Blocken and colleagues' simulation and wind-tunnel study of a full peloton found that a rider buried in the middle of a large bunch can face as little as five to ten percent of the drag they would face alone. Those numbers are why the bunch stays together on the flat regardless of who is strong.

The saving depends on speed, because aerodynamic drag rises with the square of it. When the road tips up and speed falls from 45 km/h to 20 km/h, the share of your power going into the air shrinks and the share going into lifting you and the bike against gravity grows. Padilla and colleagues' profiling of professional riders makes the point plainly: uphill ability is determined by power relative to body mass, while flat ability tracks power relative to frontal area. On a 4% drag, the wheel in front is still worth having; on a 9% wall at 14 km/h, it is worth very little, and the race becomes a set of individual efforts that happen to be close together.

Two practical consequences follow. First, the draft is most valuable on the approach and the first ramps, which is exactly where the bunch is fastest and the fight for wheels is hardest. Second, once the climb is steep, your position is your position; you cannot "sit in" your way up it.

The base of the climb is a sprint you did not sign up for

Watch what happens to a bunch in the last kilometre before a climb. Speed rises, because the riders who want to be at the front for the climb are moving up, and everyone else is trying not to lose the wheel. The bunch narrows into the corners of the approach, and the riders at the back go through a concertina of braking and sprinting. Then the road goes up, the front of the bunch sets a pace it can hold, and the back of the bunch, which entered the climb at a lower speed with a gap to close, has to ride above the front's pace just to stay in contact.

Sanders and Heijboer's analysis of grand-tour stages shows how much harder the decisive parts of a mountain stage are than the average: the demands concentrate in short, very high-intensity periods around the key climbs, with the rest of the day comparatively steady. The amateur version is the same shape at lower numbers. Riders at the back of the bunch on the approach to a climb are asked to produce a short, near-maximal effort before the climb has properly started.

Why that effort is so expensive: W′

The reason a surge at the base hurts out of proportion to its length is that it comes out of a fixed reserve. Above your Critical Power, or in practical terms above roughly your FTP, you are spending W′, a finite quantity of work you can do before you have to slow down. Skiba and colleagues modelled how W′ is spent and how it comes back: it recovers only when you drop below Critical Power, and it recovers slowly, on a time constant of minutes rather than seconds, faster the further below CP you go. On a climb ridden near threshold there is almost no recovery at all, so W′ spent closing a gap at the base is simply gone for the rest of the ascent.

Put the two pictures together and the positioning argument is complete. The rider at the front of the bunch enters the climb at the front's speed, with a full reserve, and spends it only when the racing demands it. The rider at the back enters slower, spends a chunk of W′ to close the gap, and then has less than everyone around them for the attacks near the top. Same fitness, different race.

Where to sit, concretely

• Front third, not front row. On the front you are in the wind and doing the work for everyone behind you. Two to ten riders back, you have the draft, you see the road, and you have a short line to the front if the climb opens up. The classic advice to be in the "first fifteen" for a decisive climb is a rule of thumb rather than a finding, but every study of bunch dynamics points the same way.
• Move up early, on the flat, on the sheltered side. Moving up costs power. Do it where power is cheap: in the draft, before the approach gets fast, along the side of the bunch that is out of the crosswind. Moving up in the last 500 metres, into a headwind, on the exposed side, costs the most and is what most riders do.
• Hold position through the corners of the approach. Every corner into a climb strings the bunch out, and the rider who gets pushed wide loses ten places for free. Look at the course beforehand, know which corner is the last one, and be where you want to be before it, not after it.
• Sit on the wheels of riders who climb like you. A wheel that is going to go backwards on the climb is a trap. In a race you know, that is experience; in one you do not, watch who was comfortable on the earlier drags.
• Do not pull on the run-in. Unless you are working for a teammate, the approach to a climb is not the place to do a turn, however uncomfortable it feels to sit in. Swain's and Atkinson's models of optimal power distribution both show that spending extra power where speed is high and drag dominates buys almost nothing; spend it where it counts.

On the climb itself

Once the road is steep enough that the draft has faded, the game becomes pacing. If the front's pace is above what you can hold, the question is not whether you will be dropped but how: gradually, riding your own tempo and losing a metre a minute, or all at once, after a heroic effort to hang on that leaves you with nothing. The first is almost always the better race, because groups reform on the descent and the flat and a rider who lost twenty seconds on the climb with a full head and half a reserve can chase back. A rider who blew has nothing to chase with. We cover pacing a climb by power in how to pace a climb with power; the short version is that your target is the number you know you can hold, not the number the wheel in front is doing.

If the front's pace is below yours, sit in the top five and wait. Attacks on a climb are for the final third, where the reduced group is committed and the gap you make will stick; an attack in the first third against a bunch that is still fresh is a gift to everyone behind.

Fitness that positioning cannot fix

Positioning is free speed, but it is not infinite. Van Erp, Sanders and Lamberts found that what separated the best professional riders was not their fresh power but how well they held it after thousands of kilojoules of work: durability, in other words. In an amateur race the climb often comes after two or three hours of riding, and a rider whose power falls off a cliff after 1,500 kJ will lose the climb from the front third as surely as from the back. Long rides with hard efforts at the end are how that is trained, and we go through the detail in durability: your power in hour four. If you are not sure which side of that line you fall on, the Power Profile shows your rider shape and your W′ estimate from your own data, and a puncheur with a big reserve should race a climb differently from a diesel with a small one.

Practising it

The skill of holding position in a fast bunch is only learned in a fast bunch, which for most riders means the club chain gang and low-category races rather than any training session. What you can train alone is the physiology: repeated short surges above threshold with incomplete recovery, followed by a sustained effort, which is the base-of-the-climb pattern in miniature. Thirty-second surges into five minutes at threshold, repeated, is a session that is unpleasant in exactly the right way. Indoors, ERG mode makes the surges honest, since the trainer will not let you soften them; a session like that from the Moveee Indoor library takes twenty minutes to set up once and none thereafter. Our post on group riding skills covers the wheel-following basics, and your first road race the rest of the day around the climb.

The point to take away is that a climb in a bunch race is decided by a decision made before it starts. Make it early, make it on the flat, and arrive at the bottom with the reserve you will need at the top.