Williams FW18/19 vs Ferrari F310/B

Mark Hughes has a useful survey of Ferrari's F1 fortunes from 1996 to the present day in the May edition of Motorsport Magazine. At the beginning of the article, it's noted that "The secret to the speed of the [Williams] FW18 in '96 and the following year's FW19 was exploiting a regulation loophole that allowed Newey to take the diffuser over the top of the plank to get a much bigger exit area - and therefore a more powerful diffuser effect...This arrangement made its debut late in '95 on the FW17B but amazingly Ferrari - and everyone else - had not noticed and thus did not incorporate it into their '96 cars."

So let's take a closer look at precisely what this loophole was.

The images below of the FW18's diffuser and its counterpart on the 1997 Ferrari F310B, show that whilst both exploit the greater permitted rearward extension of the central region, they differ in the crucial respect that Newey opened up windows in the vertical walls of the central diffuser. This not only increased the effective exit area of the diffuser, but coupled it to the beam-wing, thereby increasing its mass-flow rate and its capacity to generate downforce.

 

How glaring was this regulation loophole? Well, let's study the 1997 F1 Technical regulations, which are available, pro bono, at MattSomersF1. The relevant propositions read as follows:

3.10) No bodywork behind the centre line of the rear wheels, and more than 15cm each side of the longitudinal centre line of the car, may be less than 30cm above the reference plane. 

This regulation permitted the central region of the diffuser to be 30cm wide. To give some idea of the relative dimensions here, the central box itself was only 30cm tall. So outside that central region, nothing was permitted to be lower than the roof the central diffuser.

3.12) Between the rear edge of the complete front wheels and the front edge of the complete rear wheels all sprung parts of the car visible from underneath must form surfaces which lie on one of two parallel planes, the reference plane or the step plane.

This effectively defined the kick-up point of the diffuser to be the leading edge of the rear-wheels. 

The surface formed by all parts lying on the reference plane must extend from the rear edge of the complete front wheels to the centre line of the rear wheels, have minimum and maximum widths of 30cm and 50cm respectively and must be symmetrical about the centre line of the car. 

All parts lying on the reference and step planes, in addition to the transition between the two planes, must produce uniform, solid, hard, continuous, rigid (no degree of freedom in relation to the body/chassis unit), impervious surfaces under all circumstances.

This seems to be the regulation which Ferrari mis-interpreted. Whilst 3.12 required all parts of the car visible from underneath to belong to a pair of parallel surfaces, and for the transition between those surfaces to be continuous and impervious, this applied only between the trailing edge of the front wheels and the leading edge of the rear wheels. Moreover, although the definition of the reference plane extended to the centreline of the rear wheels, there was nothing whatsoever in the regulations which required a vertical plane behind the rear-wheel centreline to be continuous or impervious.

(Ferrari F310B diffuser. Photo by Alan Johnstone)

As an observation in passing, another part of regulation 3.10 should cause some puzzlement:

Any bodywork behind the rear wheel centre line which is more than 50cm above the reference plane, when projected to a plane perpendicular to the ground and the centre line of the car, must not occupy a surface greater than 70% of the area of a rectangle whose edges are 50cm either side of the car centre line and 50cm and 80cm above the reference plane.

As written, this regulation is somewhat opaque, not least because it is impossible in 3 dimensions to have a plane which is both perpendicular to the ground and the centreline of the car. A plane which is perpendicular to the centreline is certainly a well-defined concept, but in 3 dimensions such a plane will intersect the ground plane along a transverse line, hence cannot be perpendicular to it...