Floor & ground effect

Ask a casual fan where an F1 car's downforce comes from and they will point at the wings. They are mostly wrong. On a modern car, around half of all downforce comes from the flat-looking underside — the floor — and it makes that force with far less drag than any wing. The fastest surface on the car is the one you cannot see.

How the floor sucks

Squeeze a flow and it speeds up — and faster air is lower-pressure air (Bernoulli's principle, covered properly in the Aerodynamics course):

The floor exploits this with the road itself as one wall of the duct. Air enters under the car's leading edge, is accelerated through the shallow gap between floor and tarmac, and its pressure falls below atmospheric. Normal pressure pushes down on top of the car; weakened pressure pushes up from below; the difference is a hand pressing the car into the road. Shaped channels in the floor (venturi tunnels, returned in the 2022 rules) narrow to a throat where $v$ is highest and $p$ lowest, then flare out through the upswept rear — the diffuser — which eases the fast air back to normal pressure without tearing the flow apart. The diffuser doesn't just recover the air; its expansion is what allows the whole underfloor to run so fast in the first place.

Cheap downforce

A wing makes downforce by shoving air upward, and pays for it with drag and a turbulent wake. The floor makes its force largely by pressure difference in a confined channel — far less air is violently redirected, so the downforce comes cheaper in drag. That is why designers will trade almost anything for floor performance, and why teams arrive at races with new floor edges more often than any other part.

The knife edge

Ground effect strengthens as the gap shrinks — until the boundary layers on floor and road merge or the diffuser flow separates, at which point the suction collapses. Run too low and the floor stalls unpredictably; this is one ingredient of the bouncing (porpoising) that plagued 2022's cars: suck down, stall, spring up, reattach, repeat — several times a second. It is also why F1 polices ride height and floor flexibility so hard, and why the sport once banned ground-effect designs outright in 1983 after cars cornered on rails until, suddenly, they didn't. The full history — sliding skirts, fan cars and all — is a story on this site.