Drag is the amount of air resistance and airflow, and how it affects car performance while downforce is how the car sticks to the floor, and the gravity acting on it.
To be faster, you need power, but there is a limit to how much power and torque you can transfer to the wheels and put to the ground.
To increase this limit, the wheels must press down further into to the ground therefore increasing the contact patch and speed. Putting more weight could do this, but makes handling worse and requires more power.
The term downforce is 'negative lift', which pushes the car down. Both drag and downforce are proportional to a car's speed.
Component: Total Downforce:
Front Wing - 25%
Rear Wing - 25%
Undertray + Diffuser - 45%
Bodywork - 10%
Downforce also allows tires to transmit a greater thrust force because friction can be greater than the contact force between 2 surfaces making the tires stick to the floor, increasing the maximum possible acceleration.
At max speed, a F1 car can produce 5G of downforce, meaning it can push 5 times its weight onto the track. More downforce improves tire grip, but induces more drag. It is impossible to have downforce without drag, but and engineers job is to get as much downforce as possible with little drag effect.
Downforce has to be balanced from front to rear, because the flow from one affects the flow to the other. Too much front downforce induces oversteer and too much at the rear equals understeer.
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