Technical analysis of a high speed run-Part 1
I’d like to give you a better understanding on the technical aspects of a high speed run, especially in terms of aerodynamic measurements.
In this article I’d like to give you a better understanding of the >300mph run of the Bugatti Chiron SS300+ (this name is way too long, so from here onwards only SS300), with special attention on the aerodynamic aspects and the reasons/mechanics behind them.
Putting aside the whole discussion about how official or not the record is, the technical achievement of the run is extremely impressive and specially to see which measurements they have taken for it. It also is to point out, that a lot of the following presented aerodynamic principles are used in other cars of this league as well. To not excess the length of the article too much I will split the article into two parts (starting at the front and going towards the rear) and release the second part a day after.
This was something stated in several articles in the media, but what does it mean, because it sounds fairly easy to produce no net-lift or zero net-downforce. The word to look out for is thereby “net”. If you look at the profile of a car like in the picture with the Pagani Huayra  below, add a bit of fantasy (or bad photoshop) and you can see that it strongly resembles the profile of an aircraft wing. And we know all that the wing of a plane creates lift. The same is also true for a car, every car produces lift simply due its basic shape.
Representation of the wing shape on an "ordinary" car
Most sources I read so far, citate that the SS300 produces 2’000kg of lift at a speed of 490km/h (the speed is an important factor with aerodynamic numbers, if you are interested in this I’ve written an article about this topic some time ago). To have now a net zero lift the car has to produce an equal amount of downforce.
“But the more downforce the better”, you now might say. That’s true...if you want to take a corner fast, break in a short distance or have maximum acceleration out of faster corners. But to achieve high top speed you need to create as less drag as possible. The production of downforce is correlated to the redirection of airflow and by this always with a drag force. The keyword here is “conservation of momentum” for the ones interested in physics, but for everyone sane I will leave the proof open (or you can ask in the comments). Another reason for it are the tires, if you increase the downforce you also increase the load on the them. This means they will be stressed even more and will create more heat within them, which can lead to a failure due overheating. On the same time, you don’t want generate lift, otherwise the car will start to feel literally light and unstable. This is nether enjoyable for the driver nor safe as he will lose controllability of the car. As mentioned by Andy Wallace (which also drove the current high speed run) in an interview, this was a problem of the McLaren F1, when it drove its record run in 1994.
There are two important types of flows in fluid dynamics: attached and separated flows, an attached flow follows exactly the curvature of the surface, whereas a separated flow will not flow along the surface, but even have flows in the “wrong” directions and curl up. You always try to avoid having separated flows, because they create turbulences, energy losses, lower pressures and noise. On a car you will have several locations where the flow separates, one of it is at the front corners due the sharp turn the air has to take. To avoid this, you can place a small deflector at the corner to guide the air around more smoothly. In the picture below you can see a simple visualisation of the principle and how it was applied on the SS300.
Simplified airflow around the front of a car in top-down view, with and without air curtains
This so called air curtains are used on nearly all newer sports cars and on heavy lorries/trucks for even longer (you see them often quite prominent at the corners of the driver cabin). Newer regular cars started to apply this principle too, but a bit more hidden. On cars this curtains have an additional use to redirect or/and seal the weak/vortex coming from the front tires/wheels.
Speaking of them, the wheels were something I was a bit surprised. For the record run they used a fairly standard wheel design, but the presented production version showed a more covered flat wheel design (in the picture below the two wheels can be seen with an optimized wheel design of an BMW i3  as reference). Optimally for a high speed run you would use a completely covered wheel, to avoid the turbulences generated by the rotating spokes and the air passing through the wheel. This design is recently more applied in a more subtle manner on several road cars, especially more eco-friendly ones to reduce drag and increase fuel efficiency or range. Fully covered wheels are rarely used on sports cars however, firstly due their design which isn’t quite appealing to the costumers I suppose and secondly because the air flow passing through, is needed to extract the hot air generated by heavy break manoeuvres on the track.
Comparison between the wheels of the prototype, the production version and an optimised wheel design on an eco-friendly car as reference
Further considerations one has to take into account at this speeds are that the wheel-tire-combination has to be perfectly balanced, even small deviations would lead at this rotational speeds to strong vibrations and impair the directional stability (something you most certainly don’t want when covering over 130m a second), further this also would increase the load/stresses on all the suspension components. The high rotational speed also will affect the tires. I’m sure everyone of you already experienced a carousel where you go around and around and around, there you will feel a slight force trying to push you outwards, away from the centre of it. This force (actually imaginary force, but I won’t even start on this topic) has a quadratic dependency on the rotational speed, also two times the speed means four times the force, and will start to tear the tire apart. The tires for this kind of speeds are already reinforced and for the run Michelin developed an updated version of their tire with further reinforcements made from carbon fibres to assure it will retain its shape even with these stresses . The generated heat mentioned before also will deform the tire, so the circumference would grow. As you will know, most materials expand if they are heated, with some exceptions like aramid fibres, which are weaved within the tire compound to counteract the heat growth . To develop a street tire which can withstand those forces at a speed of over 500km/h operating within the design range and still maintain the street usability like a reasonable lifetime and water displacement is quite impressive. Of course, there were other people driving high speeds with this kind of tires, but these never were certified by the tire-manufacturer or were very specific tires for speed runs/drag races only without street usability.
If you hold a bag in the wind you will experience how the bag will start to blow up and you have to hold it more firmly due the force it generates. The same principle applies with the wheel arches of a car. It is a cave which fills itself up with air and generating a significant amount of drag (up to 20% of the overall aerodynamic drag ). Additionally this backed up air also creates a high pressure zone, which pushes the wheel arch (and the attached car) upwards, generating again lift. The cut outs behind the wheel arches and over them shall release this pressurised air and even create downforce. To mention is also that the upper cut outs are shaped in resemblance to the legendary air intakes of the EB110 SS, which I personally think is a nice detail. On the rear wheel arches the air is similarly released, but the cut outs are place behind the fence on the rear, at least on the standard car.
Simplified aerodynamic flow around the front wheel arches
The red arrow in the picture above represents the front diffusor on the car, which can be adjusted supposedly by flaps, similarly to the ones Ferrari uses since the LaFerrari. On high speed mode the car will close them to reduce the downforce and drag they produce. The exit on the lower rear side of the arch is also on the standard car and I only can assume about it. It is either a normal exit of the wheel arche or additionally the air coming from the front diffusor is guided inside the arche to this exit, similar how it is done on the Ford GT. This kind of cut outs you only find on very track focused cars (like on the Porsche GT3 RS), high end cars and race cars and are often called "Louvres". Even if the arches produce drag and lift on normal cars, is this acceptable at the legally allowed speeds and the drawbacks of having them outweighs the benefit, like a more complex and cost intensive production and specially the soiling due the dirt and water coming through them and being directly guided on the bodywork or windscreen.
This marks the end of the first part, in the next part the focus is put on the rear of the car and the safety. I hope you enjoyed this little technical insight and if you have questions please put them in the comments, I will then try to answer them as good as possible. I’m sorry for grammatical mistakes, I’m not a native english speaking person. Following are the sources of the pictures and other sources used (the simpliefied pictures concerning the airflows are made by myself):
5) Vehicle Propulsion Systems, Guzella/Sciarretta, Third Edition; Chp.2, p.29
6) Selected books: Fahrzeugaerodynamik, Thomas Schütz; Race Car Aerodynamics, Joseph Katz; Rennwagentechnik, Michael Trzesniowski
7) Pictures of the Bugatti Chiron SS300+ from netcarshow.net
8) Further: several interviews with Andy Wallace on Youtube and general public media information