Climbs are assigned a classification depending on their length and severity. The lower the number – from fourth to first category – the more difficult the climb. In some races, most notably the Tour de France, an additional classification – hors catégorie (beyond categorisation) – is applied to the most difficult ascents of all.
Although the biggest climbs of the day frequently occur earlier in the day (as in the example above), it is unusual to see a race-deciding attack by one of the favourites until the final climb of the day. More typically, as with flat stages a breakaway is allowed to establish a significant lead over the peloton before being gradually reeled in.
Mountain stages typically follow one of two patterns, depending on the nature of the climb and the race situation. Sometimes the contenders’ group is happy to ride tempo and effectively call a truce. This might occur because key rivals are still accompanied by teammates who can protect them, because there is an even more difficult day to follow, or simply because everyone is too tired after several days’ of hard racing. In the latter case riders are more concerned with not losing time than they are with the possibility of gaining it – a rider who attacks and then blows up can find themselves losing one minute per kilometre. On such days, any surviving breakaway will probably succeed, with the elite riders staying together until the final kilometre or so when someone might decide to launch an attack to gain a few seconds.
On other days, however, one or more riders will attack the contenders’ group. An explosive climber such as Alberto Contador or Joaquim Rodriguez will typically pick a section where the gradient suddenly increases, perhaps coming out of a sharp bend, and will typically kick hard from near the back of the group looking to catch rivals by surprise. Others whose acceleration is more gradual but sustained (Andy Schleck, say, or Cadel Evans) will instead attack from the front of the group, gradually cranking up the pace in the hope of wearing down their rivals’ resistance and ‘snapping the elastic’. When an attack succeeds, it’s common to see a tiring breakaway overhauled rapidly over a relatively short distance.
While many situations favour the chasing many over the leading few, there are some scenarios where the pendulum swings back in favour of the escapees. Stages with a closing descent even out the odds – frequently a good solo descender or small group can out-pace a less nimble larger group. And in instances where the chasing group closes down repeated attacks from within their number, the resultant quick-slow-quick-slow pace often plays into the hands of the break who are riding at a more even tempo. Whereas a sprint stage is stacked in favour of the peloton, mountain stages are much less predictable, which is what makes them so exciting to watch.