It’s important to be able to judge the difficulty of the climbing on a stage – both to measure performance and to categorize each stage.
Utilizing length of climb, average gradient, and elevation at the summit, we can categorize the difficulty of climbs across Tours at a more granular and accurate level than the HC to 4th category system. More importantly, we can apply this model objectively across all races. This allows us to measure that a Category 1 climb in a small race is actually a weak Cat 2 climb when compared to grand tour climbs.
Our training data are the categorized climbs of the Grand Tours between 2013 and 2019 – as judged by the race organizers (this is based loosely off an objective system as well). Based on the common KOM points given out, we assign a difficulty value of 20 to HC, 10 to 1st category, 5 to 2nd category, 2 to 3rd category, and 1 to 4th category. That is our dependent variable.
I ran a number of regression models to obtain the model which explained the most variance. The best result was:
KOM_POINTS ~ gradient + length + summit + gradient:length
I then applied this model to all climbs across the full data-set of races.
The most difficult climb by this method is Col d’Finestre – often seen in the Giro – which rated out as a 22.7 point climb (beyond category for the already beyond category climbs). Finestre is 18.4km at 9.1% topping out at 2175m. The median HC climb in Grand Tours in the data-set was 14.0km at 7.4% topping out at 1860m.
Other toughest climbs are Stelvio (20.0) and Mortirolo (19.5) from the Giro and Rettenbachferner (20.0) from Tour de Suisse.
The toughest in the Tour de France are Col de la Madeleine, Mont Ventoux, and Col d’Portet at around 19.5 points. Angliru (17.5) is the toughest in the Vuelta.
Other notable climbs are Monte Zoncolan (18.0), Plateau de Beille (16.0), Col du Galibier (15.5), and Alpe d’Huez (15.0). Val Thorens – which could decide the GC in the final stage of the 2019 Tour de France is an 18.8 thanks to its 33.8km length and nearly 2400m summit.
To find the stage difficulty, add up the individual KOM_Points for each categorized climb. Summit climbs where the race ends with 2km of the summit of the climb are doubled to account for greater intensity on a final climb.
By this method, the toughest stage since 2013 is Stage 11 of the 2015 Vuelta which combined five climbs reaching over 1750m include a summit finish. The toughest Tour de France stage was Stage 12 in 2018 which combined two massive HC climbs of Madeleine and Croix de Fer with a summit finish on Alpe d’Huez.
When judging stages on this objective level, it becomes obvious how much the grand tours stand out in difficulty. Of 52 stages/races judged at least a 45 in climbing difficulty, 40 came in the Giro/Tour/Vuelta. 8 others came in the TDF warm-ups (Dauphine & Tour de Suisse). The rest of the racing calendar provided just four stages with that difficult level of climbing.
In addition, those 52 stages came at a median position of Stage 16 in grand tours, meaning the riders have over 2500 km in their legs already.
Looking ahead
By this method, the toughest stage for climbing in the 2019 Tour de France will be Stage 20 with the summit finish at Val Thorens. This stage starts with the Cormet de Roselend which at 13.9 on the KOM scale is the fourth toughest climb in the race. That’s followed by the strong Cat 2 Cote du Longnefoy (6.9). The climb to Val Thorens is nearly 34 km at over 5% – with about 6 km of slight downhills/flats (18.8).
Above is a plot of length and gradient for Tour de France categorized climbs.
Science of the Peloton 
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