IndyCar Downforce Explained: How the Floor Makes the Grip

Welcome to the Happy Hour Racing IndyCar Tech Breakdown - where we take one confusing part of the NTT IndyCar Series and make it make sense. No engineering degree required.

The Short Version

Every car in IndyCar is the same Dallara IR-18 chassis wearing the same universal aero kit, and that bodywork is what turns roughly 700 horsepower into cornering speed. The trick is that most of the grip does not come from the wings you can see. It comes from the shaped floor underneath the car. When IndyCar designed the current kit, it moved the work downstairs on purpose, and that single decision is why these cars can run nose to tail without falling apart in traffic.

What Downforce Actually Is

Take an airplane wing and flip it upside down. Instead of lifting the plane into the sky, it shoves down toward the ground. That downward shove is downforce, and it works like invisible weight that only shows up when the car is moving. The faster you go, the more of it you get.

Here is the part that surprises people. Downforce does not build in a straight line with speed, it builds with the square of speed. Double the speed and you get roughly four times the downforce. That is why an IndyCar feels loose and nervous at 60 mph in the pits and feels glued down at 180 mph in a fast sweeper. The grip literally shows up as the car goes faster.

The catch is that downforce is never free. Everything that pushes the car down also drags it backward, and drag costs you straightaway speed. The entire art of racing aerodynamics is buying grip at the lowest possible price in drag.

The Secret Is Under the Car

Look at an IndyCar and your eye goes to the wings. Your eye is wrong. Under the sidepods, the floor is not flat. It is shaped into two long tunnels that get wider toward the back, called the underwing. Air squeezes into the narrow front of those tunnels, speeds up, and the pressure under the car drops. Higher pressure on top, lower pressure underneath, and the whole car gets sucked toward the pavement. At the very back, the floor kicks upward into the diffuser, which lets that fast air expand and slow back down cleanly so the suction keeps working.

When IndyCar unveiled the current car, it said that in road course and short oval trim, 66 percent of the car's downforce is generated at the bottom of the car, an increase of 19 percent over the bodywork it replaced. Two thirds of the grip, made by a part you cannot see from the grandstand.

One Kit for Everybody

Before 2018, Chevrolet and Honda each built their own bodywork, and the cars were covered in winglets, flicks and wheel guards. The current universal aero kit ended that. Every team now runs the same Dallara-built bodywork, and IndyCar deleted the rear wheel guards and the winglets attached to them, lowered the engine cover, and shrank the wings.

Cost was one reason. Racing was the bigger one. Wings make downforce out of clean air, so when a car is buried in the turbulent wake of the car ahead, its wings stop working and it washes out. A floor is far less picky. It is working against the racetrack, not against the sky, so a car running in traffic keeps much more of its grip. That is the whole reason IndyCar pushed the downforce downstairs.

Same Car, Two Completely Different Shapes

The universal kit is not one shape, it is two. On road courses, street circuits and short ovals, the cars run the high downforce package with tall wings and every grip-making part turned up, because the corners are slow and tight and straightaway speed matters less. At Indianapolis and the other superspeedways, teams strip the car down to a low drag package with small, flat wings, because at 230 mph the enemy is air resistance, not cornering grip.

The Little Parts That Do the Fine Tuning

Inside those two packages, engineers still have dials to turn. Wing angle is the obvious one: steeper for grip, flatter for speed. Then there are wickers, also called Gurney flaps, which are small tabs standing up along the trailing edge of a wing. A wicker an eighth of an inch tall adds a surprising chunk of downforce, and a matching chunk of drag. Inside the diffuser, teams can run strakes, which are fins that split the underfloor airflow into cleaner channels and add suction. Ride height matters too, because the tunnels only work if the floor stays close to the pavement, which is why you see sparks under these cars.

What Changed for 2026

The biggest aero news this season was about safety, not speed. After Colton Herta flipped during Indianapolis 500 qualifying in 2025, IndyCar and Dallara added superspeedway tire ramp flaps. They are carbon fiber panels sitting on the bodywork just ahead of the rear wheels, hinged at the front so they can pop up when a car spins. A spinning car turned backward is basically a wing pointed the wrong way, and the flaps break up the airflow that tries to lift it. IndyCar's simulation work showed a car at speed was nearly 9 percent less likely to lift after a 180 degree spin. They became mandatory at Indianapolis starting with the April 28 and 29 open test, and the Dallara engineers behind them, Dominic Coffey and Andrea Bongiovanni, won the 2026 Louis Schwitzer Award for the design.

Bigger aero changes are coming with the all new car in 2028, which IndyCar says was shaped from the start to clean up the wake behind it so passing gets easier.

NEWS: INDYCAR has provided a status update on the new car set to debut in the 2028 NTT INDYCAR SERIES season. Dallara will design and manufacture the chassis, blending bold, modern aesthetics to engage a new generation of fans while preserving the timeless, iconic look

— NTT INDYCAR SERIES (@IndyCar) June 21, 2025

What You Will See on Track

Now watch a restart with this in your head. On a road course, the car behind stays close through a fast corner instead of sliding wide, because its floor is still doing two thirds of the work even in dirty air. On a street course, you see the car bottoming out and throwing sparks over bumps, because the floor is running as low as the team dares. At Indy, you see cars with almost no wing on them trailing each other at better than 220 mph, where one small wicker is the difference between holding a run and losing it. And when a driver complains that the car "will not turn in traffic," he is telling you his aero platform is upset.

The Bottom Line

The wings get the attention, but the floor wins the races. IndyCar deliberately built a car that makes most of its grip underneath, where dirty air cannot steal it, then handed every team the exact same bodywork so the racing comes down to setup and driving. Next time you see an IndyCar squat down and rip through a corner, remember the part doing the most work is the part you will never see.

Want the aero kit on your shelf instead of your screen? The Colton Herta #26 1:18 scale IndyCar shows every wing and sidepod up close. For race-day gear, the IndyCar checkered flag tee, the 109th Indianapolis 500 in-motion tee and the Snake Pit tee are all in stock, or browse the full IndyCar collection. New tech breakdowns drop every week, so bookmark the blog and never get lost in the jargon again.