Does the coke bottle design create negative DF above the floor?

[Bandit1216]

Hello

I hope this is not covered before. I'm quite new at this forum.

I was wondering about the coke bottle design and how that relates to down force above the floor. Almost all teams use it now, some more then others. What I don't understand is the following. From the point where the side pods narrow to the gap between rear wheel and gearbox, I would think the air will speed up a lot. Increased speed will lower the pressure, right. This would create a negative down force because pressure is low above the floor. Am I missing something here?

[Just_a_fan]

The reason for the coke bottle shape is to ensure that good, fast air flows over the top of the diffuser. This is because the diffuser's job is to slow the air coming out of the underfloor region back to free stream velocity. By ensuring fast air flowing over the diffuser's exit, the teams reduce the speed difference between the flows above and below the diffuser. This allows the diffuser to be worked harder/more efficiently which increases downforce generated at the diffuser's throat (the point where the diffuser kicks up from the floor). The top of the diffuser also has devices that help to turn this fast top air flow upwards which helps again to improve the downforce production by effectively making the diffuser bigger.

The teams also manipulate the air flowing along the rear floor to help deal with rear tyre squirt, gain better flow to the rear brake ducts - more importantly the array of aero devices there which apply downforce directly to the rear wheels, and various other areas.

Another benefit of the coke bottle will be that air more easily flows to the rear wing too - although this is probably a minor issue.

[JordanMugen]

Am I missing something here?

Yes, the increased airflow over the top of the floor, when it passes the rear of the car, will help suck more air out from under the diffuser, thereby overall reducing the pressure under the floor.

[godlameroso]

The simple answer, more air mass over the floor. The amount of air flowing through the car is limited, especially at lower speeds, the fewer obstructions for that air, the greater the amount of air you can manipulate. There's also the element of drag to consider. Truck trailers are extremely draggy because of the box shape in the rear, so it would make sense that a sculpted rear end would be more efficient aerodynamically.

The current ruleset forces teams to run the narrowest body possible because there are a lot of aero surfaces that can channel air towards the rear of the car. Guiding airflow to the rear of the car is very important because of the performance benefit, and difficult to do because you have a whole car with spinning tires obstructing that airflow. Removing these restrictions, or mitigating them allows the aero of the car to work at lower speeds, which means more grip in the corners. Then you also have the issue of too much downforce at speed, and excessive drag, however I think teams are still finding a lot of downforce so eliminating drag hasn't been a top priority except for those with weak engines.