Red Bull: Floor Hole Legality

Its been announced today that the FIA have issued a Technical Directive clarifying the issue that emerged over the Monaco weekend around the Red Bull floor hole. This TD-13 outlines that the area 650mm outboard of the cars centreline cannot now exploit fully enclosed holes. As a result Red Bull will have to change the floor design before the next race, the Canadian GP. Although their design has now been deemed to be illegal retrospectively, so they are allowed to keep their results from the three races in which the design has been raced, including the win in Monaco.
Having introduced a “tyre squirt” slot into the floor ahead of the rear tyres at the Bahrain GP, Red Bull had completed two complete GPs before rival teams raised questions about its legality. On the morning of the Monaco GP, several teams started a discussion regarding the slots legality, as it did not follow the practice of Sauber or Ferrari in linking the hole to the edge of the floor. No formal protest was made, but the Technical Working Group (TWG) wanted the rules around holes in the floor clarified.

Tyre Squirt Slots
The hole is set into the floor ahead of the rear tyres is to aid airflow in the diffuser. It’s used to offset an aerodynamic phenomenon called “tyre squirt”, this the effect of the rear tyres splitting the airflow sending a bow wave of air into the diffuser. This is an unwanted effect, as the tyre squirt is of low energy and has a detrimental effect on flow through the diffuser, this robs the diffuser of downforce.

Tyre Squirt is when the airflow under the car, hits the tyre and passes into the diffuser

The tyre squirt slot offsets this problem by taking the high pressure from above the floor and directing it along the flank of the diffusers footplate. This adds extra energy into the flow along the edge of the diffuser, redirecting the low energy tyre squirt away from the diffuser. Having a bad tyre squirt effect will rob the diffuser by as much as 50% of its flow, getting it right merely maximises the potential of the diffuser. Having a slot to negate tyre squirt will gain some downforce and hence lap time.
Tyre squirt slots were common in 2010, when the problem became better understood and Exhaust positioning was well inboard of the outer flank of the diffuser. In 2011 the rules regarding openings in the floor were changed, to combat the double diffusers. At the same time teams placed their exhausts further outboard of the diffuser, blowing along the flank and footplate, which had the same effect as the tyre squirt slot, so the slots fell out of fashion in 2011. For 2012 with the exhaust rules altered to ban blowing low near the diffuser, so these slots have become useful again. However the rules that came into force in 2011 are making the slots difficult to engineer.

Three rules are obstacles to these slots, covering: openings, enclosed holes and continuous surfaces.
3.12.5 The main floor rule sets out that the floor must be rigid and impervious
All parts lying on the reference and step planes, in addition to the transition between the two planes, must produce uniform, solid, hard, continuous, rigid (no degree of freedom in relation to the body/chassis unit), impervious surfaces under all circumstances.

Rule: 3.12.5 says that enclosed holes are allowed in the front section of floor (shaded)

3.12.5 (cont) This explains enclosed holes are only allowed 450mm forward of the rear of the cockpit template (the very front of the sidepods). This implies enclosed holes are not allowed elsewhere.
Forward of a line 450mm forward of the rear face of the cockpit entry template, fully enclosed holes are permitted in the surfaces lying on the reference and step planes provided no part of the car is visible through them when viewed from directly below. This does not apply to any parts of rear view mirrors which are visible, provided each of these areas does not exceed 12000mm² when projected to a horizontal plane above the car, or to any parts of the panels referred to in Article 15.4.7.

Rule 3.12.10 allows discontinuous surfaces int he outer 50mm of floor

3.12.10 this last rule opens up the outer 50mm of floor for discontinuous surfaces.
“In an area lying 650mm or less from the car centre line, and from 450mm forward of the rear face of the cockpit entry template to 350mm forward of the rear wheel centre line, any intersection of any bodywork visible from beneath the car with a lateral or longitudinal vertical plane should form one continuous line which is visible from beneath the car.”

Saubers solution clearly shows the slot joins the edge of the floor

However there remain interpretations that can allow these slots to be used. Sauber came up with their solution before Melbourne; they formed an aerofoil section at the trailing edge of the floor ahead of the rear tyre. This has the effect of injecting the energy into the airflow running along the flank of the diffuser. It was legal as the aerofoil section was formed on the outboard 50mm of the floor, the slot could not be inside of this area as the continuous surface rule applies here Additionally the slot ran to the edge of floor and formed part of the periphery of the floor and thus was not an enclosed hole. Their solution gained a degree of interest in Melbourne from the other teams. One this design was accepted, other teams were open to develop their solution.

Ferrari’s three smaller slots are still joined to the edge of the floor by a tiny slots

Ferrari soon followed suit with three small scoops set into the floor ahead of the rear tyres. To make these legal, again they say in the outer 50mm of floor and to prevent them being enclosed, each scoop is joined to the periphery of the floor by a thin slot.

Having the slot joined to the floors edge, makes the slot a continuation of the floors periphery, which is clearly legal

Red Bull have created a single larger scoop set into the floor, joined at one side to a vertical fence. However unlike the other two aforementioned teams, Red Bull did not keep the slot open, thus they feel that this is not a requirement in this area.

Red Bulls slot is not joined to the floors edge, so the hole is enclosed and not part of the floors periphery

It appears Red Bull feel that the rules do not explicitly say that enclosed holes are not allowed in this area. Presumably because enclosed holes are only explicitly allowed in the front of the floor, the rule implies that they are not allowed in other areas. So with no explicit ban on enclosed in this outer 50cm of floor Red Bull feel justified to do so. The new Technical Directive has clarified this to explicitly ban enclosed opening in any area of the floor other than the aforementioned area.
As Red Bull have had a counter case that the holes are not explicitly banned, there is a difficult case for the FIA to argue that they are in clear contravention of the rules. So the team are allowed to keep their results, but change their design before the next race. Red Bull can now either remove the tyre squirt slot, or more likely add a simple thin slot to join it to the edge of the floor as Ferrari and Sauber have done. This will lose a small percentage of the slots efficiency, but overall the effect of the slow will still be a benefit to the cars performance.
This case had the potential to be a far larger and messier affair. With F1 having an entertaining season so far, perhaps its best that the saga has ended quickly and quietly.