The unwanted effect of airflow being diverted by the rear tyre under the floor has been understood for some time. The trend towards needing a lower Lift\Drag ratio and higher rear ride height has brought this issue into greater focus in the past seven years. Last season saw the slots used to offset this effect, known as tyre squirt, grow increasingly in significance and number.
Tyre squirt is the effect of airflow near ground level, being pushed laterally by the tyre under the car. This occurs with the front and rear tyres. As the front wing deals with its effect at the sharp end of the car, this effect is less noticed by the typical fan. At the front it’s the under wing fences that direct airflow to offset tyre squirt choking the airflow in the area between the tyre and the chassis. That is not to say this is an un-researched area, teams devote a lot of time to the problem and even have specific wing tunnel models to research the effect. Marussia had this front end only car model to run in the McLaren wind tunnel for its front tyre squirt research.
At the rear, the problem of tyre squirt manifests itself as a lateral airflow spilling off the tyre and into the diffuser. The squirt is a low energy flow and disrupts the efficiency of the diffuser, robbing it of downforce and forcing the top rear wing to be worked harder resulting in a poorer LD ratio from the inefficiency of adding more top rear wing angle. The problem at the rear gets greater as more of the rear tyre is exposed below the step plane floor level due to increasing rear ride height from steep rake angles.
Looking back, we can see Red Bull’s first steeply raked car, the 2009 RB5 had solutions to offset tyre squirt. The scalloped floor sections inside the rear tyre push high pressure air from above the floor into to the gap between the tyre and floor. A year later the exhaust was being used quite far inboard to drive the diffuser on the Red Bull, once this was banned, the exhaust outlet was moved outboard to point at this same gap for a combined blown diffuser and tyre squirt offsetting effect.
Once this direct blowing was banned in 2012, teams did not return t Newey’s scalloped floor, but instead simple slots in the tyre deck ahead of the rear tyre. This again fed air from above to below the floor, to straighten the airflow twist when and diffuser, to keep the squirt away from entering the diffuser.
Come 2014 and the new rules with no exhaust blown effect at the floor whatsoever, the detail of these tyre squirt slots grew in scale and importance. Especially with the shift towards lower drag set ups, to maximize top speed from the new power units. Last Year Ferrari multiplied its solution to none slots and a few “L” shape slots at the corner of the floor. McLaren soon upped this with eleven slots, clearly realising there was a gain from multiplicity in this area.