Just why is overtaking so difficult? 18 Aug 2004
A look at the physics behind one car passing another
We all know that passing at Monaco or Hungary is almost impossible, but even on more conventional circuits overtaking remains a very tricky business. But exactly why is passing in Formula One racing so difficult? Renaults Executive Director of Engineering, Pat Symonds, takes a look at this problematic topic.
Many people feel that because modern F1 cars are such sophisticated aerodynamic devices, and derive much of their performance from aerodynamics, overtaking will be difficult to achieve until there are limits in this area. Let's think about why aerodynamics make overtaking difficult.
In simple terms, the aerodynamic forces on the car are designed to increase the load on the tyres and therefore give them more grip. In doing this, they create a large turbulent area behind the car called the 'wake'. This is an area of disturbed air that is no longer travelling the expected direction and one can easily see this when watching the cars running in the wet. The large rooster tails generated behind the cars are indicative of the wake area but in reality, as water is much heavier than air, the wake is vastly larger than is seen by this phenomenon. Approximately speaking, the length of the wake is proportional to the square of the car's speed, whereas the total magnitude of it is proportional to the drag of the car. At high downforce circuits such as the Hungaroring, the drag - approximately one third of the downforce - is of course also high and hence the magnitude of the wake is even greater. Of course, this wake also includes a small low pressure pocket which, when a car is close enough to the one in front, allows it to slipstream and gain straight-line speed.
At the sort of maximum speed we see in Hungary, of just over 300 kph on the main straight, the length of this wake is approximately 150m, which is over 20% of the length of the straight. The wake from the leading car has two fundamental effects on the car following it. Firstly, as an aid to overtaking, it reduces the drag of the following car and at extremely small distances, when one car is tucked right behind the leading car, the drag reduction is very large, bringing a gain in straight-line speed. Unfortunately, when the following car is further back in the wake, its downforce is severely affected, not just in magnitude but also in its front-rear balance.
For example, if two cars are travelling at around 200kph, when the following car is ten car lengths from the one in front, it will experience a downforce reduction of around 20% and the balance will shift about 4% to the rear. This alone makes it difficult for the driver of the following car, because not only does he lose grip but the car also understeers a lot. As it gets closer to the car in front, things get even worse. At three car lengths' distance, the following car may have lost one third of its downforce and be experiencing an aero balance shift of over 15%. This makes it virtually impossible for the following driver to stay close to the car in front through any reasonably quick corner leading onto a straight, and if he can't stay close on the corner leading to the straight, then it is very difficult to use any speed advantage for anything other than catching up, rather than actually overtaking.
Of course, if the cars had extremely limited aerodynamics, then this wake would be very small, but will never be non-existent: any body moving through the air will always generate some wake, no matter how streamlined it is, and in other formulae, one can see even touring cars slip-streaming each other.
There are other factors which will either directly or indirectly affect the likelihood of overtaking in an F1 race. Some of these are contradictory. For example, if we increased the grip of the car to the road, perhaps by having larger tyres, then teams would tend to reduce the downforce as the optimisation of the total package would move in this direction: corner speeds would be maintained, and thus it would be possible to increased top speed by removing downforce. This of course would reduce the wake, and may make overtaking easier. Conversely, it could be agreed that if we were able to reduce the grip in slower corners, by alterations to the tarmac, and yet keep grip high in faster corners by banking them, then some of these faster corners could be taken flat out which would have the effect of joining two straights and making the straights longer. Of course, this would be a radical proposal for the circuit owners and require much research to get the numbers right, but circuit design undoubtedly does affect overtaking, and the general premise of a slow corner followed by a long straight and another slow corner is conducive to overtaking by F1 cars, in their current configuration.
Having said that, even this type of circuit configuration cannot be taken as a sure-fire guarantee of exciting racing. The 'new' Hockenheim uses such a layout and although we had a very exciting race there three weeks ago, if we look at previous years, we saw just three competitive overtaking manoeuvres all race in 2003, while in 2002 the only overtaking came about as a result of teams making incorrect tyre choices: Barrichello, for example, spent 47 laps (two race stints) stuck behind a competitor during this race. So, even this kind of 'formula' for circuit design can produce poor racing. Equally, if we consider the British Grand Prix in 2003, it produced a fantastic race with a great deal of overtaking on a circuit that does not follow what has become the accepted layout, whereas 2004 produced a much more "normal" race. Circuit design is important, but there is no single effective formula for entertaining racing.
These facts may lead us to draw a more relevant conclusion. Essentially, if cars are of similar, consistent performance and leave the grid together, there is no real reason to believe they will ever overtake each other. Similarly, if their performance is vastly different, and they are pre-sorted by speed, then the leading, faster cars will simply pull away. If we believe overtaking is important, then what we need to do is devise a method whereby the cars will have different levels of performance, at different stages of the race. This sometimes occurs if people use different types of tyres, which have different performance profiles through a race. It has also been employed in some form with turbocharged engines, where a limited number of over-boosts are allowed in a race. But overall, it is this concept of a variable performance differential between the cars that is central to generating overtaking during a race.