Getting the balance right in Brazil 20 Oct 2004
Sector sensitivities key to perfect Sao Paulo lap
How do you determine the best car set-up for Interlagos? As at most circuits, its a balancing act. Finding time in one sector can often mean losing it in another. But how do you know when youve found the optimum balance? Renaults Pat Symonds explains.
When we engineers arrive at a given circuit, we do so armed with a database of facts and figures that allow us to evaluate the performance of our cars and devise the appropriate set-up. These figures are termed sector sensitivities and they detail how different factors affect car performance in the three sectors of a given circuit. While we study many different parameters, the four primary factors are downforce, drag, grip and engine power.
Looking at Interlagos, it is a track with two similar sectors comprising a long straight and few corners (S1 and S3) and a much twistier middle sector, which accounts for roughly half the total lap time. Sector 1 begins at the timing line, and includes three corners: T1 taken in 2nd gear at around 95 kph (60 mph), T2 taken at just under 160 kph (100 mph) in 3rd gear and T3 which we exit at around 240 kph (150 mph) onto the short back straight where the sector finishes. S2 includes eight of the circuit's twelve corners, which range from T5 taken flat out at 240 kph (150 mph) to T10 taken at just under 80 kph (50 mph). The final sector begins between T11 and T12, and comprises only one real corner (T12 at 110 kph or 70 mph) and the uphill acceleration out of this turn, through a number of flat out kinks, to the timing line.
When setting up the car at Interlagos, we must balance overall lap time with sufficient speed along the main pit straight that we can overtake other cars and also protect our position. In general, we aim to achieve a maximum speed of approximately 320 kph (200 mph) at the end of the pit straight. However, if we choose to alter this top speed, then the sector sensitivities allow us to calculate the impact this will have on our overall lap-time. This can be illustrated by looking at two extreme settings.
For example, if we were to reduce our top speed along the straights by between 5 and 8 kph (3 and 5 mph) by using more downforce, then the simulations show we would lose only a few thousandths of a second in lap-time, but the way this loss would be derived is very interesting. This change would lose 0.06s in S1 and 0.15s in S3 owing to the two straights, but gain back 0.2s in S2 through the numerous corners. At the other extreme, if we were to reduce downforce in order to improve our straight-line speed by 10 kph (6 mph), then we would lose 0.25s over the complete lap - losing just 0.005s and gaining 0.15s in S1 and S3 respectively, but losing a very significant 0.4s in S2. Lower downforce settings bring benefits in straight-line speed and lap-time in S1 and S3 respectively (the low number of corners in these sectors means the time losses through the turns are proportionally smaller) but come at the price of a high time penalty in S2. Altering downforce levels always involves balancing gains and losses owing to the increase or decrease in parasitic drag that this inevitably brings.
When we examine the contrasting nature of the circuit sectors, it is logical that an increase in grip would bring its main benefit in the second sector, which has the highest number of corners. A theoretical 5% increase in grip (which is much larger than could be found by switching between two raceable tyre compounds) would see overall lap time reduced by 1.25s, with 60% of this improvement (0.75s) coming in the second sector - three times the gain obtained in the first and third sectors (0.25s each). Contrastingly, increased engine power would bring greater benefits in the first and third sectors. A rise of 5% (around 45 bhp) would provide gains of 0.2s in S1 and S3, and just over 0.1s in S2.
So why are these figures useful to the engineers during a race weekend? Essentially, they provide the basis for comparing performance between the team's cars and our competitors during the race weekend. We know roughly where our level of performance is relative to that of our competitors and can therefore evaluate any differences in speed or lap-time using these figures. For example, if we are very quick in S2 and not in S1 and S3, with low straight-line speeds, then we are running too much downforce. If these speeds are competitive however and we remain extremely quick in terms of overall lap time, then this would indicate we are running lower fuel than our rivals. The database of performance sensitivities provides the race engineers with the reference points they require to make informed choices as they search for the most competitive and race-able set-up.