During the 1982’ Championships there have been several accidents, including the one who killed Gilles Villeneuve, revealing of an increasing level of danger that resulted from the high performances reached by the wing-cars that exceeded the safety limits of the same single-seaters and of the race circuits.
Alarmed, the FIA reacted immediately and in order to dramatically reduce the ground-effect produced by the wing profiles of the lower surfaces of the side pods of the vehicles in the different categories of motor sport.
The flat under floor surface was thus imposed by the technical regulation in 1983. Since then, it is the most secret area of a race car.
All teams try to hide, as much as possible, this area from the competitors’ curious eyes, because it is the most important part in the aerodynamic efficiency of these cars.
The biggest loads of negative lift are created there, applied precisely where they are needed most, on the rear axle.
From a flat surface, mandatory by the technical regulations since 1983, the lower surface of competition cars became more complex in order to limit the growing in its effectiveness in the generation of negative lift. Indeed, the fatal accidents of Roland Ratzenberger and Ayrton Senna on the GP of San Marino’ weekend in 1994, led the FIA to establish a reference plan, the lower portion of the vehicle, above which then was defined the flat bottom or step plan, with a limited efficacy, due to the larger distance to the ground.
Article by António Eiras - 16/11/2014
When we talk about flat under floor surface, we refer to the entire bottom surface of a Formula 1 car (or of most other categories of Formulas cars and Sport-Prototypes or DTM cars, for example) and that the technical regulation of 2005, the Year when I made this work, corresponded, in the case of a Formula 1 car, to the area between two transversal imaginary lines located one 33 cm behind the front axle and the other 40 cm in front of the rear axle. Its width allowed by regulation, was a maximum of 140 cm, in the level of the step plan, who was 5cm above the reference plan, the width of which varied between 30 and 50 cm, positioned centrally and symmetrically along the longitudinal central line of vehicle and extending behind until the level of the rear axle.
Under the reference plan and along its entire length, is placed the "skid block", a wood device of 30cm wide, also imposed after the accidents of Senna and Ratzenberger, designed to prevent the cars to be tuned to run too close to the track, putting in risk the driver's safety, a situation that would be revealed by an abnormal wear of its surface.
This surface is conceived in a way that a Formula 1 car works as if it was a big inverted wing. It is discreetly lean, with the front lightly high, and so it forces the air that passes between this surface and the track, to a considerable acceleration, creating a negative pressure that is exercised in the area where the acceleration is larger, immediately before the rear limit of the step plan of the under floor, in it’s transition to the back diffusers.
Central rear diffuser operating:
a. Flux channelled by the lateral openings.
b. Step plan.
c. "Skid block".
d. Diffuser that extends the reference plan.
e. Vortex created by the flux splitter of the lateral walls of the central diffuser.
As we saw in the work about Wings, when we studied the air flux around an inverted wing, the force of negative lift created by the interaction of the wing with the air flux that crosses it, is due to the difference of pressures of the air fluxes that cross over and under the wing, provoked by the different speeds that those fluxes travel those surfaces, due to the asymmetry of the wing’s profile.
In the Formula 1 bodywork case, and imagining it as an enormous inverted wing, this differential of pressures is the result of the acceleration of the air flux that is channelled to pass under the vehicle, between the under floor surface and the track, and that flows faster than the air flux which passes over bodywork, which has a short distance to do.
As we can observe in McLaren MP4/20-Mercedes of 2005’ profile (drawings 1 and 2), probably the most effective of that year’s Formula 1 cars, the air flux, after crossing the front wing (m), will be guided, not only by the wing’s own drawing, with their flux deviators, placed in it’s inferior face, like also by the chassis and the flux deviators (j, l) and splitters (i) placed between it and the front wheels, to, for the side pods leading edge, have access to the step plan (f), and, under the chassis(k), to the reference plan leading edge(g).
a - Rear wing, biplane.
b - Presence and braking rear light.
c - Inferior rear wing’s profile.
d - Rear central diffuser.
e - Rear lateral diffuser.
f - Step plan.
g - Reference plan.
h - "Skid block".
i - Splitter with "slat" function.
j - Flux deviator.
k - Splitter.
The air flux will travel the whole space created between the under floor surface and the track surface, with these two levels, the reference plan and the step plan, separated by an unevenness of 5cm and of round boards.
This space is going to be narrowed light and progressively as the flux of air approaches the back axis, because of differences in the regulation in height of the front and rear suspensions, to reach the minimum height immediately before the beginning of the rear lateral diffusers (e).
This area, located about 40 cm ahead of the rear axis, acts as the throat of the Venturi tunnel, that we analyzed in the Wings article, and it is the place where the air flux reaches the biggest speed, and consequently, the place where the pressures are more negative, meaning that, the place where the largest load of negative lift is exercised on the bodywork.
After crossing this throat the air flux slows down, and the pressure increases, softly, in the passage by the rear diffusers, until joining, in the trailing edge of this enormous "wing", located, in theory, in the trailing edge of the inferior rear wing (c), to the air flux that passed over the bodywork.
The rear diffusers correspond to the divergent duct of the Venturi tunnel, and have as main function, allowing air flux, accelerated in it’s passage by the throat, retake, in a progressive way, the speed and pressure levels of the free air flux that circulates around the vehicle.
Since the under floor surface is a flat one, and the distance to the track pavement is very small, with the added difficulty that the reference plan, with the "skid block"(h), must reach the level of the rear axle, the inclination that the technicians can use in the under floor is quite limited.
From here it resulted that the technicians needed to discover another additional way of accelerating the air flux at this level that could increase the aerodynamic load created in that area.
It is known that the need sharpens the creativity, and like this the famous "step" was invented, in the beginning of the rear lateral diffuser.
When the surface that limits a certain air flux presents an angulations’ of 900 opened outside of the previous trajectory, the air flux is forced to do this straight angle, by suction, and, by doing it, suffers an immense acceleration.
This air flux behaviour has been used, since 4 decades ago, in the racing cars, through the use of the so called "Gurney flaps". These are small aerodynamic devices, in straight angle, that, when applied on the trailing edge of the wings, they provoke, due to the effect of additional acceleration of the air flux that passes under the wings, when being forced to do the straight angle of the "Gurney flap", a larger stability of this air flux, that stays adherent to the inferior surface of the wing for more time and in larger attack angles (angle created between the free air flow direction ahead the wing and the imaginary line that unites the leading edge to the trailing edge of the wing: the bigger it is, the bigger will be, at least theoretically, the aerodynamic load generated by the wing). With the "Gurney flaps" it is accomplished a simultaneous increase of the aerodynamic load generated and of the air flux stability that passes under the wing.
When applied on the rear limit of the step plan (drawing 5-b,e), as we see in the rear lateral extremities of the under floor surface, and in the rear limits of the splitters of McLaren MP4/20's lateral diffusers, these "Gurney flaps" will provoke an additional acceleration of the air flux in the passage under the rear extremity of the under floor surface and under the splitters of the lateral diffusers, increasing, consequently, the negative aerodynamic load in that area, in a similar way of what happens in a conventional wing.
In the early nineties, Ferrari tried, successfully, the application of this principle to the rear lateral diffusers, and, instead of a soft and continuous transition between the surface of the step plan and the lean plan of the rear diffusers (see drawing of the Venturi Tunnel on the Wings article), it started to present a “step” in straight angle, causing an air acceleration, similar to what happened with the "Gurney flap."
The “step” (drawing 5-a) went being tested successively, in wind tunnel, until it reached a quite complex form that, as it was told in confidence to me by an old Ferrari engineer,”...nobody realised the reason why that worked better than a straight angle, but there were not doubts that it was more efficient in the tests that we performed"!.
After being accelerated in it’s passage by the “step”, the air that flowed along the step plan will be aspirated and drained by the divergent duct of the rear lateral diffuser, witch develops to the limit, allowed by regulation, represented by the rear axis, where it will unite, smoothly, with the air that flowed, first over the flux separator created immediately ahead the rear wheels by the narrowing in the "bottle of Coke" side pods, and later, by the superior surface of the lateral diffusers.
One part of the air that flows under the step plan will be aspirated by the central rear diffuser, which efficacy is limited by the compulsory extension of the reference plan until the rear axis level.
a - Safety structure of rear impact energy absorption.
b - Small rear central diffuser, in the continuity of the "skid block."
c - Flux deviator of the rear lateral diffuser.
d - Lateral splitter of the rear lateral diffuser.
e - "Skid block."
f - Reference plan.
g - Step plan.
g - Splitter attached to the flux deviator placed backwards ahead of the side pods.
h - Superior splitter of the advanced placed flux deviator, ahead of the side pods.
i - Suspended flux deviator of the front wing’s inferior face.
j - Front wing’s main profile.
k - Splitter attached to the end plates of the front wing.
l - Inferior splitter of the advanced placed flux deviator, ahead of the side pods.
m -Flux deviators that operate together ahead the side pods.
n - Splitter that extends ahead the step plan and helps to channel more air under this plan.
o - Splitter that works as a "slat".
p - Rear lateral diffuser.
q - Small splitters in the inferior boards of the lateral walls of the rear central diffuser.
r - Superior rear wing’s main profile.
s - Inferior rear wing.
t - Rear wing’s end plate.
However, the lateral openings of the central diffuser, that allow it to work in continuity with the lateral diffusers, will drain not only part of the air that flows under the step plan, as well as aspirate some of the air flux that passes under the reference plan and the "skid block".
In McLaren’s MP4/20 and immediately behind the level of the rear axis, we can find a small diffuser, that continues backwards the inferior surface of the "skid block" and that will allow some small additional earnings of aerodynamic load.
The central rear diffuser (drawing 6) was limited, by 2005’s technical regulations, to the 15 cm on each side of the central line of the car, and it could be extended until the level of the trailing edge of the rear wing. This rear limit was rarely used, because, since the interaction of the rear wing’s inferior profile with the air flux drained by the rear diffusers, had a frankly positive effect over the diffusers function, powering significantly it’s air aspiration capacity, it was verified that the central diffuser extension until the allowed rear limit, had, as a result, the loss of this interaction with the rear wing, with consequent loose of rear diffusers efficiency.
The rear diffusers drawing and it’s interaction with the inferior profile of the rear wing is extremely delicate, specific of each car, because in aerodynamics, as in all the other operation areas of these cars, a good solution for a vehicle may not be a good solution when applied in another vehicle.
And it is extremely important, for the good operation of these cars, that the diffusers allow a good drainage and aspiration of the air that flows under the under floor surface, because, as it told, in confidence to me, the same engineer: " ...it is important to channel the largest possible amount of air to go under the car, but never again more air that the one the rear diffusers can get to aspirate."
Like all the other areas in Formula 1, as in almost all race cars categories, the flat under floor surface is one of the technicians' constant concerns, and it is under a permanent study and evolution.
With the efficient improvement of the rear diffusers, the technicians will be able to guide a larger amount of air under the cars, with the creation of larger aerodynamic loads in their central part, among the axis, turning these race cars less dependent of the front and rear wings, consequently more stable in all points of the circuits, and under the different race conditions. Maybe its development can be a solution to allow a larger number of overtaking in all circuit motoring categories, helping to increase the emotion in motor racing.