Seventy-two years ago, it no doubt seemed like an innocuous change to speed up the circuit layout. When the teams arrived for the 1939 Belgian Grand Prix at the end of the June, the fiddly Virage de l'Ancienne Douane (Customs House Hairpin) early in the lap had been bypassed with a new, steep uphill section linking to the long straight towards Les Combes. The new section probably merited little special consideration in comparison with the rest of the 14-kilometre circuit, which included sections like Burnenville, the Masta Kink and the run from Stavelot to La Source. The old Spa disappeared from the calendar in 1970, to return in 1983 using a layout that was half the length and had been made significantly safer in order to meet the needs of modern motor racing.

But that piece of tarmac, Le Raidillon de l'Eau Rouge, remained and is now taken flat-out in a Formula One car. Furthermore, it has also come to symbolise the unique, majestic challenge of one of the last remaining classic road circuits in the sport: Spa-Francorchamps.

What are the key numbers to describe the challenge of Eau Rouge and the Raidillon?
Le Raidillon de l'Eau Rouge is reduced to three simple corner numbers on the official map of the Spa-Francorchamps circuit: Turns Two, Three and Four. Turn Two, the left-hand kink which crosses the bridge over the creek named Eau Rouge, is taken at 306 kph, with a lateral G-force of 2.4G; Turn Three, the right-handed uphill sweep, at 303 kph with a lateral G-force of 4G; and Turn Five, the left-hander over the crest, at 296 kph with a lateral G-force of 2G. The cars also undergo significant vertical loadings through this section: a vertical force of -1.7G in the compression at the bottom of the hill and +1G over the crest. Although the section is taken flat-out, the cars lose approximately 10 kph through the sequence. The series of corners is 535 m long (7.6% of the lap distance) and is negotiated in 6.4 s (6.1% of the 2010 pole time). The sequence from La Source to Les Combes, including the Raidillon, lasts for 23.5 s and is the longest full throttle sequence of the entire season.

Do the vertical accelerations present any particular challenges?
This upward vertical acceleration of 1G effectively means the car is weightless as it goes over the crest; contact with the road is therefore assured by downforce alone, and not the weight of the car. However, at such speeds, the downforce generated is approximately 2.5 times car weight. The high levels of vertical acceleration could also compromise engine and gearbox reliability if not accounted for in system designs and installations. The pick-up points in the oil tank must be accurately placed to ensure the pumps are continuously primed during these phases.

Which other significant high-speed corners are there on the circuit?
The other major high-speed challenges are Blanchimont (Turn 17), which is taken flat-out at over 300 kph; and the double left-hander at Pouhon (Turns 10 and 11), which are taken in fifth gear at 240 kph, with a lateral G-force of 3.75G. Pouhon is the longest corner on the circuit, lasting for a total of 7.8 seconds.

The circuit presents contrasting sector profiles. How different are they?
There is a marked contrast between the profiles of Sectors One and Three, and the profile of Sector Two. Sector One feature just one braking event (for Turn One) and the rest is spent flat-out: of the 2205 m in this sector, 2050 m (93 %) are spent at full throttle. Sector Three is similar: of its 2080 m, 1750 m (84%) are spent at full throttle, and the only braking event is for the chicane at the end of the lap (Turns 18 and 19). In contrast, Sector Two contains nine of the circuit's 19 corners and features six braking events, with just 60% of the sector spent at full throttle. By way of comparison, last year's pole position time was set at an average speed of 238 kph: the average speed in Sectors One and Three was 259 kph and 262 kph respectively, while the average in Sector Two was 211 kph.

How important will DRS be during the weekend in Spa?
Of the circuit's total lap distance of 7004 m, drivers will be able to use the DRS system during practice and qualifying for 4400 m - equivalent to 63% of the lap distance. Only Monza features a higher potential usage of the system, at 74% of the lap distance. DRS is likely to eliminate some of the need to find a set-up compromise between low drag for Sectors One and Three, and higher downforce for Sector Two, as it will offer the best of both worlds for qualifying - and enable teams to run higher downforce levels for the race, which will help to protect the tyres.

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