The world championship challenge between Lewis Hamilton and Max Verstappen holds court in Austin, becoming the dominant theme of the American weekend. However, the recent discussions on the speed superiority of the Mercedes on the straight, combined with the deep and significant unevenness of the road surface of the Circuit of the Americas, focus on a practice already widespread among the teams, but poorly understood. The technical aspect in question is the control of the height from the ground of the rear axle, used to reduce drag and gain travel in extension. The theme becomes an opportunity to appreciate the engineering level of current Formula 1, as well as the multitude of mechanisms that contribute to overall performance.
A premise is necessary on therake angle, that is to say on the forward inclination of the car body, an attitude parameter which, although known to many avid technical enthusiasts, remains unknown to others. The basic principle is that the wider the outlet section of the diffuser, the more effective the extraction of air from the bottom becomes, with the flows flowing into the environment below the car gaining speed as their pressure drops. Therefore, the pressure difference between the external environment and the underbody increases, which translates into aerodynamic load.
The dimensions of the diffuser are limited by the geometric constraints of the technical regulation, an aspect that places limits on the outlet section of the extractor. To remedy the problem, the teams resort to the rake setting, tilting the car body forward so that the rear end rises, artificially widening the diffuser outlet area to increase the downforce generated by the bottom. Currently, some more or less, all the teams adopt a certain rake angle, with Red Bull traditionally preferring a more beaten set-up towards the front, while Mercedes opts for a less accentuated inclination.
A controlled movement
The rake angle does not remain constant throughout the lap, but changes as the riding conditions change. At high speeds, for example, the vertical thrust of the aerodynamic load compresses the car towards the ground. However, the engineers calibrate the suspension stiffness and kinematics to ensure that the rear is lowered more than the front, so that there is a rotation of the car body that reduces the rake angle. A similar behavior reveals itself useful for reducing the aerodynamic drag at forward movement and improve straight-line speeds for two reasons.
Firstly, as the rear axle lowers and the rake angle decreases, the outlet section of the diffuser also shrinks. With it progressively decreases the aerodynamic load generated by the bottom and therefore theinduced drag, that is the component of the resistance to advancement proportional to the load released. In parallel, it must also be considered that with a less accentuated rake the entire car body and the wing appendages are less accidents, thus also reducing the resistance generated by them.
It may then happen that in extreme conditions the rear of the car is lowered to the point of stalling the diffuser, with the distance of the bottom from the ground which is reduced to the point of causing a detachment of the flows from the underbody surfaces. In these contexts there is a sudden and significant loss of the load generated by the road surface which, although cornering can be harmful as well as dangerous, in the straight line it proves to be an advantage, as it also reduces resistance to forward movement. In detail, it plays a central role the third element of the rear suspension, used to control the height from the ground and to contrast the pitching movements. It is in fact calibrated with a predetermined preload so that, beyond a certain vertical thrust value of the aerodynamic load and therefore beyond a very precise speed, the element collapses, leading to a sudden lowering of the rear axle and causing the diffuser to stall.
The phenomenon recalls what is also exploited by Williams FW15C from 1993, an example of the maximum development of active suspension in Formula 1, on the steering wheel there was a button that the drivers could press on the straight to command an instant lowering of the rear axle and to stall the diffuser, gaining top speed. Contrary to the system developed by Williams for the ’93 season, the mechanism currently in use in Formula 1 does not however provide for an actuation by means of a hydraulic system. In fact, there is no active control of the third suspension element, but it is an automatic movement carefully calibrated to be triggered by external aerodynamic forces beyond a certain speed and therefore it is to be considered perfectly legal. Craig Scarborough, technical expert and direct collaborator with Formula 1 for in-depth programs, has also reported on his social profiles as the system has also been discussed over the years during the meetings between the teams and the FIA technical group, but that it has always been agreed to keep it.
After the performance of the Mercedes in Istanbul, where the W12 showed an evident superiority on the straight despite the particularly charged aerodynamic look, the rear height control and the diffuser stall were pointed out as the possible secret of the speed in extension of the Silver Arrows. However, as also pointed out by several insiders, the artifice is exploited by all the teams, while the differences would instead lie between those who dare to exploit the phenomenon to a greater or lesser extent. The speed performance of the Mercedes could therefore actually find an explanation in a more effective exploitation of this principle, but without further supporting elements it would be wrong to exclude other possible factors. The power unit, for example, benefited from an update in July in terms of hybrid delivery and management, while at the same time the aerodynamic developments introduced at Silverstone have improved the efficiency of the bottom, allowing the external aerodynamics to be unloaded. compared to the past. It is also not unrealistic to imagine that the three factors work together to make the W12 one of the fastest cars on the straight. Emblematic in this regard are the words of Christian Horner released to Sky F1 UK, which suggest a possible set-up work underway at Red Bull to replicate what was found in the Mercedes in the next appointments: “After more in-depth analysis, we understood how Mercedes managed to reach those speeds on the straight in Turkey”.
Delicate look in Austin
The differences between the teams in the controlled exploitation of the lowering of the rear axle are due to the risk that the diffuser may stall in the fastest corners, precisely where the maximum downforce is required. In the calibration of the third element and in general of the rear suspension unit, the teams therefore maintain a fair margin of safety, making sure that the diffuser does not stall below a speed, chosen to be above the travel of all curves on the track. Therefore, the more the track hosts fast corners, the more the stall will have to be induced at higher speeds, so that the surface can generate enough load in each corner.
Austin is a track full of curves with mileage over 200 km / h, which is why lowering the rear at high speeds must be more conservative. To further complicate the calibration of the suspension for the exploitation of the phenomenon there are also the numerous bumps and depressions of the Texan asphalt, in whose crossing the distance between the bottom and the ground varies continuously, influencing the aerodynamic behavior. In the rapid curves of the first sector, it was also seen how the bottom repeatedly collides with the ground, obstructing the flow path and significantly increasing the sensitivity to stalling. In addition to imposing a set-up with a greater ground clearance of the car, the context therefore requires a different rear suspension calibration, with a greater safety margin between the induction of the stall of the diffuser at high speeds in straight lines compared to the distances reached in curves. The road to victory in Austin, as well as to the title in general, also passes through here.
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