Interlagos is a circuit of contrasting extremes, combining slow hairpins with one of the longest straights of the season. Sitting in a natural bowl, it undulates throughout its 4.309 km length, and is notorious for its bumpy surface – although this was improved by resurfacing in 2004, and again last year.
The physical demands of the bumpy circuit are intensified by the fact that it runs anti-clockwise, subjecting the drivers' necks to the opposite loadings experienced at a normal clockwise track. It is a circuit where overtaking is possible, particularly on the entry to turn 1, and the set-up compromise therefore tends to favour straight-line speed over optimum lap-time, to ensure the drivers can make up positions, and defend them, during the 71-lap race.
The contrasting nature of the Interlagos circuit makes very different demands on the cars. The first and last sectors are made up primarily of long straights, where good top speed is necessary to maintain competitiveness and protect position; this means a low level of downforce is required. However, the middle sector requires the opposite: high downforce to ensure good grip under acceleration, braking and cornering through the twisting series of hairpins. Balancing these requirements gives an optimum downforce setting for achieving the fastest possible lap-time. However, this optimum is then skewed by the demands of racing with other cars. To do so successfully requires competitive end of straight speeds – and achieving these may drag teams away from their optimum downforce to a slightly lower setting which allows the drivers to overtake and defend their position into turn 1. This means teams use downforce levels similar to a circuit such as Bahrain.
The combination of high and low-speed corners means it is hard to find a suitable mechanical compromise at Interlagos. As with the choice of aero level, teams prioritise certain sectors of the circuit over others. The most important corner at Interlagos is turn 12, as it determines your speed along the uphill main straight – a full throttle period lasting over 15 seconds. Teams therefore pay special attention ensuring the car gets a good exit from this corner, even though this can generate some slow-speed understeer in the middle sector. However, any losses incurred with this understeer are outweighed by the benefits in lap- time and competitiveness achieved in sector 3. The second important factor for the mechanical set-up is the track surface. This was traditionally very bumpy, but the resurfacing in 2004 allowed teams to run lower ride heights, and the situation improved again last year. The circuit is relatively easy on the brakes, with just three major braking events, and overall braking energy similar to somewhere like Barcelona.
Interlagos includes relatively few high-speed corners with high lateral loadings on the tyres. Coupled with a track surface that is not particularly abrasive, this means teams can use relatively soft tyres. Consequently, Bridgestone has made available the medium and soft compounds from its 2008 range for this final race of the year.
The long main straight at Interlagos means engine power is a critical factor at this circuit, and the longest single period at full throttle is over 15 seconds. All the engines, though, must contend with the effects of running at altitude, as the circuit is situated around 800m above sea level. The reduced atmospheric pressure costs the engines around 7% of their power output; as a result, the 60% of the lap spent at full throttle is equivalent to 56% at sea level. While this reduces the demands on some components such as the pistons, other parts of the engine, such as the crankshaft, are still subjected to significant loadings. Driveability is also an important factor, especially through the winding middle sector. The drivers run in the lowest gears at this point on the circuit, with sudden changes of direction and significant brake and throttle inputs. Smooth power delivery can make a real and significant contribution to maintaining a stable balance, and optimum driving lines, in this part of the circuit.