We always say that the front wing of a Formula 1 car is the single most important aerodynamic component because it has two roles. As well as producing downforce in its own right, it dictates the quality of the airflow to all the aerodynamic surfaces following it. Coming a close second to the front wing is the area inside the rear tyres known as the ‘Coke bottle’. Since Alan Jenkins came up with the initial concept for McLaren in the mid-1980s, this is the area that has made single-seater racing cars much more open-wheeled.
This area, and how it functions, is critical to the amount of airflow that is displaced either inside or outside of the rear tyre. If the majority of the flow has to go around the outside of the tyre the drag levels will be increased. In effect this makes the car wider the further rearward you go, which in turn will reduce efficiency of the complete car.
However, persuading more air to flow to the inside is not as simple as making as big a gap as possible by narrowing the bodywork. If the profile of the sidepods is not correct it is very easy to get airflow separation on the inner surface when the car is mid-corner. If this happens it will affect rear downforce and cause the car to snap into oversteer.
Optimising this area is all about understanding the airflow regime coming around the undercut area of the front of the sidepods and any influence that the front wheels will have on that airflow – especially when the steering angle changes. Add to this the effect the hot-blown exhaust exits have on this airflow and it is easy to see why some teams have taken most of this season to get a grip on it.
It is an area where, as with the front wing, the teams have evolved different concepts. Let’s have a look at the variations and the reasons behind some of these different solutions.
Like most of the car, Red Bull’s Coke bottle is very tightly packaged and all the different surfaces work well with each other. The undercut on the front of the outer sidepod corner is maximised to get the best possible airflow to the Coke bottle area, which gets flow from around the side and over the top of the sidepod.
Accelerating this flow too much can give a little more lift on the top of the leading edge of the sidepods; however, the benefit of the extra mass flow into the Coke bottle area far outweighs the negatives. Having this extra flow coming off very well-detailed surfaces, and then adding the high-energy hot exhaust gases, increases the velocity of a huge percentage of the airflow that is going between the rear tyres and the inner body work.
The increased velocity helps move more airflow over the top of the underfloor surface, reducing the leakage that goes underneath. This enables Red Bull to get reasonable benefits from running higher rear ride heights and simply allows everything in this area to work better, creating more overall downforce.
McLaren has taken a very different approach to getting the airflow to this area. The inner section of the top of the MP4-26’s sidepods is much lower, allowing more airflow through to the rear of the car without it having to accelerate over the top of the sidepod’s leading edge.
This reduces the amount of lift in this area, but it also means the radiator inlet has had to be repositioned, and this in turn affects the undercut in the outer forward corner of the sidepod. If the Coke bottle area can ‘talk’ to this undercut area at the front of the sidepods it helps scavenge the airflow out from underneath the front of the chassis, which in turn improves the performance of the leading edge of the sidepods and the front wing.
As Ferrari has admitted, its 2011 car was not radical enough. The Coke bottle area still has a radiator exit just between the rear wishbones; cooling is important and having the exit placed in this area will be very efficient for that, but it will dramatically reduce the effect the Coke bottle will have on the leading edge of the sidepods and in turn the front wing.
It will also mean that the potential benefits of the exhaust exits will be reduced, and because of this the increase in downforce at higher rear ride heights will also be negligible.
Mercedes started the season with a bit of an ugly duckling and I am afraid it is still hanging around. I wrote in my season preview that its Coke bottle area looked a bit clunky, and to improve the cooling Mercedes added a bigger radiator exit duct in an already compromised area.
As with the Ferrari, having a radiator exit duct in the Coke bottle area improves cooling but any increase in velocity through this section of the car will be lost to overall car performance because it will just drag more airflow through the radiator. Also, because of having to shoehorn some bigger radiators into the sidepod, the designers have lost the connection between the Coke bottle and the undercut on the leading edge of the sidepods, in effect making the front and rear of the car two different components working independently.
Mercedes has admitted that the car has a fairly short wheelbase and that because of this, packaging has been difficult. The higher centre of gravity has a detrimental effect on tyre degradation, and I would suggest that the packaging has compromised its overall aerodynamic performance. If you have a size 11 foot it is better to fit it into a size 11 shoe; you will get it into a size 10 and it might look better, but at some point during the day it is going to hurt. I think Mercedes’ results this season show it is now suffering that pain.
Renault’s outward-facing exhaust exits at the front of the sidepods makes for a very different overall car concept. At the start of the season Renault looked like it had a package to potentially take the fight to Red Bull; but as the Red Bull exhaust concept started to filter down the pitlane and the other teams optimised their systems, Renault got left behind.
What Renault started with left very little room for development around the Coke bottle. Without the exhaust gases trained in this area there was nothing to be had.
Renault tried a very basic Red Bull-style system mid-season, but was forced to accept that the complete change in car packaging required to optimise this would take a long time and be prohibitively expensive. It was better to bite the bullet and try to develop the areas of the car that would give some performance return.
There has been a lot of head scratching at Williams. I don’t think that anyone in the team could actually tell you why this year’s car is so far off the pace.
It looks neat and tidy and has all the bits in the right places, but has got left behind as the team frantically tries to find speed. It is never a good idea to keep changing components in the hope that you will fix the problem; sometimes it is better to stand still on development and isolate the problem, then introduce a package to rectify it.
The Coke bottle is like the rest of the car, very tidy. However, I would investigate the sidepod undercut area where it continues into the Coke bottle, since in its current form it pulls more airflow over the top of the sidepods, giving more lift on the sidepod tops. The undercut area below and just in front of the rear lower rear wishbone can be prone to airflow separation.
Williams has also struggled with only using cold-blown exhaust flow, which is nowhere near as good as being able to use hot exhaust blowing. I think the team’s problems are a little bit of everything.
Lotus uses the complete Red Bull rear end package and by doing so should be able to have all the benefits. This includes the packaging potential around the gearbox and pullrod rear suspension and hot exhaust blowing from the Renault engine.
Like the Mercedes, the Lotus car has a very bulbous area just in front of the rear wishbone legs. This will affect the airflow in this area. Also, like some others, there is a radiator exit here which negates any potential from this exhaust blowing.
The initial concept is what makes one car stand out from the others and no matter how big a budget a team has, if it starts with a flawed concept it is virtually impossible to turn it around over the course of the year. Once the season starts, every team works flat out to develop their own individual package and the concept that has the most potential will give the biggest returns.
An example of this is the exhaust blown diffuser; the team that had this concept when the cars first hit the track had the Coke bottle package to get the best from it. All the others have had to compromise something else to get this concept working. In other words they have been playing catch up, which is never a good place to be when you are competing for a championship.