The Lotus 79's sliding skirts
The ground-effect floor only sucks the car down if air can't sneak in from the sides. Along each sidepod of the Lotus 79 hung a spring-loaded strip — the skirt — whose ceramic edge rode the track surface and sealed the tunnels. Those humble strips carried the whole system, and their failure modes eventually got it banned.
The seal is the system
The Lotus 79 made its downforce underneath: two narrowing tunnels beneath the sidepods sped the air up and dropped its pressure, sucking the car onto the road. But low pressure under the car only survives if the high-pressure air outside can't rush in to fill it. The solution ran the length of each sidepod: a stiff composite blade in a channel, pushed down by springs so its ceramic rubbing strip stayed in contact with the track. A worn edge or a jammed skirt and half the car's grip evaporated between one corner and the next — mechanics checked the skirts the way other teams checked tyres.
A pressure vessel with a moving wall
Think of the floor, the road and the two skirts as a shallow box of low-pressure air, driven by the venturi tunnels. Any gap under a skirt is a leak straight from the outside world — and the suction doesn't fade politely as the gap opens. Air accelerates through the slot, feeds the tunnels with pressure they were supposed to be losing, and the downforce drains away steeply with every millimetre. A few millimetres of skirt wear could cost a measurable slice of total grip; a stuck skirt could cost half of it, instantly.
It had to slide, not just hang
A fixed strip could never work. The car rolls in corners, pitches under braking, and rides kerbs — a rigid skirt would either drag along the ground or lift clear and gape. So the skirt floated: roughly fifty millimetres of vertical travel in its channel, with springs holding the edge on the road through every movement of the body above it. It was, in effect, a suspension system for a strip of plastic — with the car's cornering grip riding on how smoothly it slid.
Banned, twice
The sport's regulators fought the skirts for years: restricted in 1981, dead by 1983 when flat floors were mandated after a series of accidents caused by sudden downforce loss. The idea outlived the part. When ground effect returned to Formula 1 in 2022, the sealing problem was still there — solved this time not with rubbing strips but with shaped floor edges and carefully-placed vortices that act as invisible skirts. The physics never changed; only the engineering answer did.
Go deeper: why millimetres matterfor engineers
The tunnels hold a pressure difference Δp against the outside air. A gap of height h under the skirt leaks air at a rate that grows roughly with h × √Δpper unit length — and the tunnels' pumping capacity is fixed by car speed. As the leak grows, the tunnels can no longer maintain Δp, which weakens the suction, which is itself what holds the car (and the skirt geometry) stable. The feedback runs the wrong way: each millimetre of wear costs more grip than the last, which is why teams measured skirt edges between sessions and why a jammed skirt read like an aerodynamic light switch.