Saturday, October 31, 2009

Pit Stops Considerations

Here are the vital pieces of information the teams will take into consideration:
  • The way you qualify. Because fuel cannot be added or subtracted between Saturday qualifying and the race, the cars have to qualify with enough fuel on board to do the first stint of the race. Obviously a car planning to three-stop can qualify lighter than one on a two-stop strategy. The team needs to trade off the importance of qualifying position with the optimum race strategy.
  • The weight of fuel. This varies according to how dense the fuel is, but the regulations specify the density must be between 0.725 and 0.77kg per litre. The largest tanks hold around 150 litres and brimmed to capacity will therefore weigh around 120kg.
  • The amount of lap time this costs at the track in question. A half-tank car is quicker by around 2.4 seconds per lap around Suzuka than a full tank one, whereas over the similar distance of a Monza lap – with long straights and not many corners – the difference is only around 1.6 seconds. Suzuka therefore errs towards a two-stop strategy, Monza a one-stop.
  • The consumption of fuel. How much fuel (and therefore weight) needs to be put into the tank and how much the car sheds as it races tends to be higher at a track with lots of accelerating, braking, and use of the lower gears. The Hockenheim track, as revised from 2002, induces heavier fuel consumption than any other on the Formula One calendar. Although lap times at the track are only averagely sensitive to weight changes, the big reduction in weight (because of the high fuel consumption) means that the total effect is large. It’s therefore another track favouring a two-stop strategy. At the other end of the pole is Nurburgring where there is relatively gentle fuel consumption but where lap times are highly sensitive to weight. The net result is very similar, and again a two-stop is favoured.
  • Your qualifying position. If your grid position places you in an attacking position – meaning that you need to overtake other cars – you are more likely to opt for an aggressive two-stop strategy, even if the maths tells you that a one-stop is quicker. If it’s a track where pretty much everyone can be guaranteed to be on the same number of stops, you have a choice: depending on whether your strength is early or late in the stint, you can either short-fuel for the first stint, using your lower weight to pass cars and then hope to pull out enough of a time cushion before you stop to keep you ahead after they have stopped. Or you might choose to take on board as much fuel as you dare and hope you can keep up with cars that are lighter early in the race; when you stop later it will take less time as you will not spend as much time refuelling to get you to the end.
  • Other cars and overtaking opportunities. The more pit stops you make, the more you are at the mercy of traffic – of getting caught behind slower cars after your stop but before they make theirs. At a track with plenty of overtaking opportunities this is less of a consideration than at a place like Monaco where passing places are few and far between. The timing of your stop might also be influenced by traffic – for example, you might come in earlier than planned so as to avoid encountering a bunch of lapped cars.
  • How the brakes are wearing. At some circuits – such as Imola, Montreal and Monza – brake wear is extreme. A heavier fuel load places extra strain on the brakes, due to the extra weight and momentum that they are working against.
  • The skill of the driver. The handling balance of the car changes a lot from the beginning of a stint to the end. This change will be larger the longer the stint. Some drivers can cope with handling changes better than others, so it is something that has to be taken into account.

Choosing the number of stops

Pit stops are not actually compulsory in Formula One. In theory, a team could build a car with a big enough fuel tank to do the whole race without stopping, and the tyre manufacturers could easily produce a compound tough enough to make tyre changes a thing of the past. But in reality, such a car would be hopelessly off the pace. Not only would it be slow because of its weight and hard compound tyres, but the bulkiness arising from its big fuel tank would make the car aerodynamically inefficient, slowing it yet further – and all the other teams would laugh at them. All current Formula One cars are therefore designed around tanks that are too small to hold enough fuel to complete a full Grand Prix.
At most tracks, a pit stop – including slowing down, stopping, re-fuelling, and accelerating back up to speed – takes around 30 seconds. The race distance is specified as the least number of laps exceeding 305km (190 miles). (For more on what constitutes a “complete” race, head to “race stoppage” later in this chapter.) Over that distance, and with that time penalty, it did not use to be worthwhile stopping more than twice. But since the 2003 regulations prohibiting fuel loads being changed between qualifying and race (see Chapter 8), stopping three times in a race has become a viable option once grid position is taken into account. Stopping just the once, or even twice, during a race has fallen out of favour after the introduction of these regulations. At some tracks an extra stop is quite feasible, more than making up the extra 30 seconds of pit stop time with a lighter fuel load. Magny Cours, in France, with its very short pit lane and high tyre degradation, is a good example of this. At other tracks the tyres perform close to their peak for longer and the pit lane can be longer. Silverstone in Britain is a good example of this. The most common strategy in Magny Cours is three-stops, whereas for Silverstone two stops is more popular.
Aside from tyre behaviour, it’s all to do with fuel consumption and how much lap time the extra weight costs you. In other words, a team needs to know how sensitive the car’s lap time is to changes in weight and how fast that weight is changing.

Strategy on the hoof

Rarely has the importance of race strategy been so well demonstrated than at Monaco in 2002. Michelin, the tyre suppliers of Williams and McLaren, arrived with a tyre that was of supersoft compound, making it very quick over one lap in qualifying but less so over a race distance. The reasoning was to get at least one Michelin car to outqualify the dominant Bridgestone-shod Ferraris and then let the tight confines of the track aid them in keeping the red cars behind for the race.
It worked brilliantly, demoting Ferrari to the second row. In the race, McLaren’s David Coulthard soaked up pressure from Ferrari’s Michael Schumacher to take victory. Even though the Ferrari was capable of lapping more than 1 second per lap faster than the McLaren, there was simply no way by. In desperation, Ferrari brought Schumacher in a few laps early for his pit stop, in the hope he could use his speed to get and stay ahead when Coulthard stopped. When he rejoined, now on a clear track, he immediately set a stunning fastest lap. This alerted McLaren, who realised that, at this rate, Schumacher would indeed be able to pass when the McLaren stopped. Before any further damage was done, they brought Coulthard in early too, getting him out still ahead of Schumacher. It sealed the result.