Deconstructing F1: The underfloor

During the course of this series of articles detailing different aspects of Formula 1 car design, I have banged on about how important it is to get good quality airflow to the leading edge of the underfloor. And also how the vortex structure induced by the front wing then helps the performance of the underfloor by acting as a skirt to reduce the amount of air spilt under the sides of the floor.

The underfloor is responsible for around 35 per cent of the total downforce produced by the car and the only way that it can do this is by the diffuser accelerating the airflow under the large flat area of underfloor - creating a low pressure over this area.

The size and shape of the diffuser is very well controlled by the regulations and because of this it can only ever do a certain amount of work to that airflow.

If the flow at the leading edge of the sidepods is poor, or there is a lot of spillage under the sides of the floor, then the underfloor will not produce as much downforce as it does when the airflow is of high quality.

From the rear all the diffusers look very similar. That is because the of size limitation in the regulations - a maximum of 1000mm width and 200mm height, and from the reference plane underneath the car it also has a maximum height of 175mm. This extra 25mm is to allow for a gurney flap or similar to trip the airflow on the upper surface to help the airflow remain attached to the under surface.

As for the vertical strakes, inside of 250mm from the car's centreline they must adhere to the reference plane height. Outside of that they have to be to step plane height which is 50mm higher. Other than that you can do what you want with them.

The detail of the coke bottle sculpting inside the rear tyre is also very important as the airflow through this area flows on the upper surface of the diffuser, trips on whatever detail is on the diffuser trailing edge and helps extract the airflow from underneath the car.

Red Bull

Much as with the rest of the RB7, it's the detail in the diffuser that allows the team to extract that little bit extra than the competition can. Instead of just having a right angle gurney flap on the trailing edge, Red Bull has a small wing section with a slot gap. This turns the airflow more efficiently and makes the diffuser work that little bit more.

The outer wall of the diffuser turns very aggressively and the lower part of the rear wing endplate helps connect this area to the low pressure behind the rear tyre, again, helping extract more airflow from underneath the car.

The twin splitters inside the 250mm from the centreline going down to the reference plane help control any airflow separation problems by containing the air in a smaller area and the small fillet radius where the splitters join the diffuser surface reduce the risk of separation problems. All these are small details, but like a good jigsaw, all the small bits add up to produce a great end product.

McLaren

In this picture you can see McLaren has tried to understand the airflow characteristics of this area of the car using a fluid called flow-vis. The only problem with this is that the airflow on the diffuser behaves differently at different ride heights so the flow detail is (as you can see) very messy.

The team has similar detail to the outer parts of the diffuser and rear wing endplates as Red Bull, but that is where it ends.

McLaren has only a 25mm gurney flap all around the trailing edge and only a single splitter either side going down to the reference plane height which has no fillet radius on the intersection with the diffuser surface. This lack of detail creates airflow separation problems and when that inevitably happens, the problem is greater because of lack of the extra splitters

The larger right angle section in the middle is to help the centre section of the diffuser work harder. This area, 75mm from car centreline, is free to allow central wing mounts so this type of thing is acceptable if not exactly elegant.

Ferrari

Like McLaren it is the small details that lets Ferrari down. It has twin splitters like Red Bull but no fillet radius where they join the diffuser surface. The outer foot area turns out very aggressively and does not have any airflow trip detail. It would connect to the low pressure behind the rear tyre better if it had less curvature and a small gurney flap or wing section. Other than that this area is very simple, perhaps too much so.

Looking at the central section of the diffuser, where it comes up from the lowest part of the car, this area is the most critical for airflow separation. Ferrari has a very square corner where the lower section of the under surface meets the sides, and, as airflow goes across this it will induce separation which in turn will render the diffuser inconsistent.

Toro Rosso

The Toro Rosso still keeps its family resemblance to the Red Bull in that the detail of the outer foot section of the diffuser is very similar and the team also uses the twin splitters, but lacks the detail of the fillet radius.

The central section of STR's diffuser has this arched splitter arrangement to help extract flow from the lower floor area. As I said above, this is the most critical for airflow separation when the car is at a low ride height and anything that can be done to improve this will help with the consistency of performance through a wider range of speeds.

Williams

This picture shows just how small the Williams gearbox really is. On this car everything was done to optimise the performance of the airflow over the top surface of the diffuser.

Minimising the blockage at the rear of the car is no bad thing, but then Williams put two rather large radiator exits just where it needed good high energy airflow to help the diffuser extract performance.

This small and very low gearbox also requires the driveshafts to be at a greater angle than other teams' concepts, driveshaft joints running at these angles will have higher losses so a small reduction of power to the rear wheels is the compromise. Again diffuser and splitter detail is nothing to get too excited about, but as I have said before, it is actually difficult to see why the Williams is not a car that is consistently in the top 10.