This isn't a public service announcement regarding your common garden 'Rake' but a feature on the nose down phenomenon used in F1 referred to as 'Rake' (The term however more than likely owes it's heritage to the garden rake as pictured above).
Creating downforce in F1 as we know is complex endeavor and requires one to think of the car as a whole rather than the sum of it's parts as by altering or achieving one effect can have a positive or detrimental effect elsewhere on the car. The word 'Rake' refers to the nose down pitch angle of the car and is most commonly referred to by the media as a sign of a team running with high rear end downforce.
The last time we saw 'Rake' used extremely was in 2011 when teams utilised Off Throttle Blown Diffusers (OT EBD). There are several reasons for this but the easiest way to explain it is that those cars had more rear downforce by virtue of the exhaust plume assisting in Diffuser extraction. This was achieved by releasing the exhaust plume directly into the gap between the rear tyre and floor of the car, nullifying the effects that the rear tyres have on the airflow in that region (Tyre Squirt) impinging on the Diffuser itself and is what the media refer to as 'Sealing' the diffuser. The exhaust plume creates an airdam or skirt whilst also helping to extract the vortex that trails off the edge of the Diffuser thus creating more downforce. (This is also the reason it's more difficult for a car to follow behind a car using OT EBD as the upward wake induced by the vortex is much larger)
Above: As a means of showing the appoximated path of the exhaust plume wanted by the teams I've highlighted it in red on the MP4/28. Obviously the Brake Duct and Suspension obscure the gap between the floor and the diffuser but hopefully this goes some way to explaining what the teams are looking to achieve.
So as I mentioned let's not try to think of 'Rake' in isolation and think of the other effects it has on the car. At the front of the car the nose down attitude created when raking the car drops the Front Wing into ground effect, as the wing gets closer to the ground the air accelerates increasing the downforce generated, whilst a vortex is generated at the joint between the wing and Endplate. This effect increases until such a point that the wing is so low that the ground and wings boundary layers combine reducing downforce and bursting the joint vortex. Obviously the rotating wheel behind the Front Wing has an impact of this effect and is the reason we see most teams turn out the Endplates to encourage the flow around the wheel. Meanwhile the Front Wing Cascades intercept some of the airflow above the wing flaps and by and injecting the airflow above the wing help to manage the airflow around the wheel.
Moving to the central section of the Front Wing and it's onward effect with Rake and the combined effect of the Splitter during this ruleset: The Y250 section is the central section of the Front Wing's mainplane that is mandated in dimension to 500mm in width. It's design/construction is also mandated and so the teams have to find ways in which to control it's aerodynamic effects as although it is designed to be neutral of course in terms of aerodynamics nothing is. The only difference here is all the teams have to manage the same aero problem, a vortice is shed from either end of the section as the section changes profile. This vortice encloses the central portion of airflow between the pylons making moving airflow either side of it more difficult, it does however mean that air spilling from the tyres when cornering has a more difficult time impinging on the centralised flow. Although teams don't have the freedom to modify that section of the car, many have however taken the opportunity to place the FOM camera's behind the section which undoubtedly changes the characteristics in the central portion of the wing. (The FOM camera's are also regulated in size and shape, so no one team can glean an aerodynamic advantage from their placement) Red Bull's movement of the FOM camera's to the hammerhead position on the nose further suggests the team have gone to in order to tilt the Front Wing backward under aerodynamic load with the camera's being placed behind the Y250 section it could have had an undesired effect on the aero aft of it.
Running with more Rake can become a problem for teams in qualifying and towards the end of the race as this is when the car is at it's lightest. The biggest issue comes under braking as the car pitches forward and can cause the Front Wing to either collide with the ground or Stall as it's range becomes too low to work aerodynamically. However moreover the problem comes from the Splitter as it is more likely to hit the track than the Front Wing and as we have already discussed, too much of this and it will wear the Skid Block resulting in a disqualification.
Above: I have highlighted the Y250 region in red on the underside of this great image captured of the underside of the Lotus E21. The blue arrow point to the element I'm describing as the Splitter whilst the yellow arrow shows the fence on the side of the Splitter that is used to direct the Y250's vortices
The section of the car that really regulates the amount of rake that a team can run is the T-Tray/Bib/Splitter or whatever other name you have heard it called. From here on I'll reference it as the Splitter as that's the job it is doing on the airflow that reaches it, whether it be splitting the airflow beneath or ontop of it or onward to the left/right Sidepods. The Splitter houses the Skid Block/Plank which is the FIA's way of mandating a flat floor, the piece of Jabroc that makes up the Skid Block must not be worn sufficiently during the race or the driver may be disqualified and so this limits the angle/rake the car can be raced at. The FIA also conduct a flex/load test on this area as during the mid 2000's teams exploited methods that allowed the area to deflect upward when it came into contact with the tracks surface. This deflection would allow a much more aggressive rake, meaning the then higher placed Front Wings were lowered toward the ground and the diffuser had a larger expansion ratio. Returning to it's relationship with the Front Wing and the Splitter is furnished vorticised airflow from the Y250 area from both the change in profile from the mandated section and also any other elements each team look to place in the area (With the prevailing trend for teams to place R Cascades and/or split their Wing profiles to generate vortices that lead onward to the Splitter area)
The Splitter itself generally has a section at it's outer edge that is designed to take these Vortices and send them onward along the edge of the transition between the Step and Reference planes. Whilst the remainder of the vortices (higher) are managed by the Sidepod undercut etc.
Above: a great shot of the RB9 showing not only the Diffuser but partially the Skid Block and Floor ahead of it
So why do some teams appear to have good downforce but don't run with high rake? A good question, there is a tipping point at which it becomes fruitful to have more or less rake and in reality only the team will know where that lies. A Diffusers performance has to work over a wide speed and pitch range which both contribute to the Diffusers expansion, get the rake wrong and you can end up with the Diffuser losing efficiency or some cases stalling. If we imagine that exhausts could not contribute to the sealing of the diffuser then peak downforce generated in the Diffuser would occur at a much lower height (due to Ground Effect) and so this brings us back to the effects of the exhausts in determining whether to run with more or less Rake. As mentioned earlier in the article when referring to the joint between the Wing and Endplate, the same Vortex exists in the Diffuser (just flip the logic) with the Outer most Diffuser Strake acting as the Endplate and the Sloped section of Diffuser acting as the Wing. The Vortex created at the edge of the Diffuser helps to extract the airflow and as we discussed before if we lower the rear end and in turn the outer Diffuser Strake(Lower Rake) peak downforce is generated at a lower range. (Ie less airflow can move from outside the Diffuser into the Diffuser destroying the Vortex)
So how does increasing the Rake improve downforce? This is all based on the intensity of the exhaust plume being pumped between the rear wheel and Diffuser Strake. The exhaust plume acts as an extension of the Strake extending the Diffusers height whilst stopping airflow entering the Diffuser from the side and destroying the Vortex thereby increasing the available area of expansion for the Diffuser.
Rake's effect on the rest of the floor: Inevitably by increasing the rake you affect all parts of the car and a large benefactor in this enterprise is the floor of the car, with the Splitter now more acutely angled to the ground the airflow will be pushed aside more effectively as it tries to get both under and over the Splitter. Due to the nature of the regulations the Skid Block/Plank resides on the Reference Plane whilst the floor to it's left and right is placed 50mm higher on the Step Plane with a Vertical transition residing between the two. This leads to more expansion being available in the outer portions of the Diffuser and by this virtue we must see the Diffuser as having three components, the two outer channels and the central one. This is why we see the centralised section of the Diffuser with Strakes that reach down much further than the outer ones as controlling and shielding the Vortices generated in this section are imperative to the balance of the car. The vortice that has been dispatched from the Y250 zone at the front of the car exits in the same place in the Diffuser guided by the second most inner Strake with the most inner Strakes regulating the size of the vortice so that it doesn't break down (especially as teams tend to use the Starter Motor hole to also dispatch air through)
The outer Sections of the Diffuser generally have their walls (outside edge) tapering outward to maximise the Difusers area meanwhile at least one more control Strake will sit inbound on the centralized channel in order to control the vortices that are being created at the outer edge.
With Rake increased the requirement for the outer channels to be fed airflow is increased with the exhaust plume creating the most obvious interaction in the outer section just as was done when the rules permitted EBD. The exhaust plume (if directed correctly) not only fills the void between the tyre and the floor helping to 'Seal' the diffuser but ingresses into the outward channel helping to extract the airflow more quickly by enhancing the Vortex giving the effect of that region being closer to the ground.
Airflow to the outer channels of the diffuser is also dealt with by the floor and so when using more Rake adaptations must be made here too in order to fully extract the benefits of larger expansion. This will lead to changes in the lower edge design of the Bargeboard (In Front of the Sidepod) along with other changes to the forward section of the floor. Furthermore changing philosophy to the way in which airflow migrates along the side of the floor will not only effect the car in yaw but increase the Diffusers performance too. Using Red Bull as an example (as they are a team that tend to increase the Rake in their cars) a new Floor Scroll has been added to the RB9 that features a perforated edge, this perforation will increase the areas efficiency and push aside the airflow that the wheels have dispatched of upstream.
In Summary
Rake can be identified as an increase in downforce, however the effects of Rake can either be scaled as marginal or large. The crossover point of running the diffuser in ground effect and at a higher height are relative to the 'Sealing' effect being created by the exhaust plume. It could be argued that it's a trade off unless you can successfully create a large enough effect from the exhaust plume to 'Seal' a much larger area of the Diffuser's edge or indeed enhance the vortex being created in the outer edge of the Diffuser aiding in flow attachment. With raised Rake you then also have to redesign / profile the rest of the cars design too.
The 2011 variant of Blown Diffusers owed their success to the now banned off throttle maps and so creating that same 'Sealed' effect when the driver is entering the corner is the most difficult aspect for the teams to overcome when utilising additional rake. In the case of Red Bull (RB8/9) and now Lotus (E21) and Williams (FW35) we see the teams utilising Cross-Under Tunnels beneath their exhaust channels.
These help to separate the airflow above the floor giving the exhaust plume a clearer pathway to the gap between the rear wheels whilst migrating the airflow coming around the Sidepods/floor into the more centralised coke bottle area via the tunnels. This allows a transitional period between on and off throttle, smoothing out the loss of downforce as the Diffuser is robbed of it's 'Sealing' skirt. Furthermore we have seen that throughout 2012 as Red Bull made revisions to the rest of their aero scheme they also changed the quantity/geometry of the Cross-Under tunnels which undoubtedly mirrors the other changes when considering centre of pressure.
These help to separate the airflow above the floor giving the exhaust plume a clearer pathway to the gap between the rear wheels whilst migrating the airflow coming around the Sidepods/floor into the more centralised coke bottle area via the tunnels. This allows a transitional period between on and off throttle, smoothing out the loss of downforce as the Diffuser is robbed of it's 'Sealing' skirt. Furthermore we have seen that throughout 2012 as Red Bull made revisions to the rest of their aero scheme they also changed the quantity/geometry of the Cross-Under tunnels which undoubtedly mirrors the other changes when considering centre of pressure.
I suppose the other plausible solution to smoothing out the transition in downforce would be if the team could find a way in which to adjust the ride height of the car as it enters the braking phase. Lowering the car at the rear could lead to the Diffuser re-entering 'Ground Effect' and recovering some of the losses associated with the loss of downforce as the exhaust plume dissipates. This is obviously a difficult task as when the car brakes it will dive forward and so the teams would need a method of circumnavigating this to either raise the front of the car under braking or lower the rear. This is a method used in the active suspension days in order to keep the car in it's aero window and so is extremely difficult to replicate now but I do have a feeling we may be revisiting this topic soon.
I'll leave you with a brilliant video from @PeterDWindsor where he interviews James Allison (Lotus, Technical Director) on the topic of blown diffusers and moreover the differing solutions teams used when utilising exhaust gases (obviously Renault's FEE isn't applicable anymore, but it does highlight the need to think about centre of pressure shift during on and off throttle moments)