Where the day was actually won and lost

When the race stops being decidable

At every checkpoint each finisher is ranked against the other finishers who reached it, and compared with where they ended up. The gap between the two is how much of the field they still had left to move through — from 8.14 points at Pied de col de Propiac down to 1.37 points by Chalet Reynard. Ranks at a checkpoint count riders who later abandoned, which would credit everyone with the places those riders vacated, so they are re-ranked among finishers only. Done that way the finish line comes out at exactly zero, which is the check that the arithmetic is right.

The line does not fall smoothly, and the bumps are the interesting part: position at the foot of a climb is worth much less than position at its summit. The field arrives at a col in whatever groups the valley assembled, and then the mountain sorts it out again.

the climbs do 62% of the sorting

The obvious way to ask this — take riders within a couple of minutes of each other and see how far apart they finish — turns out to measure the wrong thing. Early on the whole field is packed into a few minutes, so being level on the clock at Pied de col de Propiac means something completely different from being level on the clock at the top of a col. So this conditions on position instead: among riders sharing your place in the field at a checkpoint, how widely scattered are their finishes?

At Pied de col de Propiac the answer is 37.37 points of the field — knowing someone’s position there tells you very nearly nothing about where they will finish. Every section after it closes that number down.

Read it as a rider and it says something useful: your position at the bottom of a climb is worth far less than your position at the top. Across this course the spread at a summit is 24.51 points against 31.7 points on the approaches. You arrive at a col in whatever group the valley handed you; the climb then throws that away and sorts everyone on their own legs.