Introduction

Side skirts are among the most common visual upgrades on modified cars, but their function goes far beyond aesthetics. Often overlooked by casual enthusiasts, these panels that run along the rocker panels—just below the doors—play a key role in aerodynamic management. One of their most impactful yet underappreciated benefits is helping to lower the car’s center of gravity (CG). A lower CG means better stability, sharper handling, and improved safety. In this article, we’ll explore exactly how side skirts contribute to lowering the CG, the science behind it, and what you should know if you’re considering adding them to your vehicle.

Understanding Center of Gravity in Vehicles

The center of gravity of a car is the theoretical point at which its mass is evenly balanced in all directions. In a static car, think of it as the balance point. For example, a front-engine car typically has its CG somewhere near the dashboard, just above the floor. The height of the CG relative to the wheelbase and track width is a primary factor in rollover resistance, cornering grip, and weight transfer during braking and acceleration.

Why a Lower Center of Gravity Matters

When a car turns, centrifugal force tries to tip the vehicle outward. If the CG is high, that force produces a larger moment (torque) around the outer wheels, causing more body roll and reducing the tire contact patch. This leads to less grip and slower cornering speeds. A lower CG reduces this lever arm, so the car stays flatter through corners and transitions weight more predictably.

In practical terms, lowering the CG by even a few centimeters can dramatically improve a car's handling. Race cars are built with ultra-low CG targets through techniques like engine placement, seating position, and suspension geometry. For road cars, lowering the CG is often achieved by lowering the ride height via springs or coilovers. Side skirts complement this by enabling a lower ride height without severe aerodynamic penalties.

How Side Skirts Work Aerodynamically

To understand how side skirts affect the CG, we first need to look at their primary job: managing airflow. When a car moves, air flows underneath the vehicle and along the sides. The underbody is a turbulent, high-pressure zone that creates lift. Lift pushes the car upward, reducing tire traction and making the car feel light and unstable at speed.

Side skirts bridge the gap between the front and rear wheel wells, sealing the sides of the underbody from the outside air. By blocking air from entering the area under the car, they help reduce the high-pressure zone under the chassis. This reduces lift and can even create a small amount of downforce—pressing the car downward. Less lift means more stability, especially at highway speeds.

Moreover, side skirts smooth the airflow along the car's sides, reducing turbulence and drag. This aerodynamic efficiency is why side skirts appear on everything from supercars to track-day specials. They are often paired with front splitters and rear diffusers to form a complete underbody aero package.

The Connection Between Side Skirts and Center of Gravity

Side skirts themselves don’t directly lower the CG like a weight reduction would. Instead, they enable a lower ride height—and it’s the lower ride height that actually reduces the CG. Here’s how:

  • Allowing Lower Ride Height: When you lower a car with springs or coilovers, the body sits closer to the ground. Without side skirts, the exposed rocker panels and underbody create a parachute effect, trapping air and generating lift. Side skirts seal the sides, so lowering the car doesn’t create an aerodynamic penalty. In fact, lowering combined with side skirts actually improves downforce or reduces lift.
  • Adding Low-Mounted Weight: Side skirts are installed low on the car, near the bottom of the chassis. If they are made from a dense material like steel or heavy carbon fiber, they add mass at a low point, which mathematically lowers the overall CG (since CG is the weighted average of all mass positions). While the weight is small relative to the whole car, every bit helps in motorsport.

In short, side skirts are an enabler: they allow you to drop the ride height—and therefore the CG—without ruining the car’s aerodynamic balance. For a car already lowered, adding side skirts can further lower the effective CG by cleaning up airflow, reducing lift, and adding a small amount of downforce. The net effect is a car that sits lower, turns better, and feels planted.

Measuring the Impact

On a typical sports car, lowering the ride height by 30 mm can drop the CG by roughly 10–15 mm, depending on weight distribution. Side skirts may contribute 1–5 mm of additional CG reduction via added low mass (if heavy) and aerodynamic downforce that effectively “virtualizes” a lower CG by increasing tire load. Professional race teams use computational fluid dynamics (CFD) to optimize this, but aftermarket enthusiasts can feel the improvement by seat of the pants.

Selecting and Installing Side Skirts for Optimal CG Reduction

Not all side skirts are created equal. To maximize the CG-related benefits, you should consider the following:

Material Choice

  • Fiberglass: Lightweight and affordable, but may crack. Adds minimal weight low-down, so little direct CG benefit.
  • Carbon Fiber: Very light and strong. Expensive. Adds almost no weight, but can be shaped for maximum aerodynamic efficiency.
  • ABS Plastic or Polyurethane: Durable and flexible. Cheap. Good for daily drivers, but heavier than carbon fiber.
  • Steel or Aluminum: Heavy and rare. Rarely used except in custom builds where additional low mass is desired to lower CG.

If your goal is to actually lower the CG by adding weight low down, choose a moderately dense material like thick FRP or even steel if you can afford the weight. However, most enthusiasts prefer carbon fiber to keep total weight low; the CG reduction from aerodynamic downforce outweighs the tiny mass penalty.

Fitment and Ground Clearance

Side skirts that extend too low will scrape on driveways, speed bumps, and curbs. This can damage the skirts and the underlying chassis. It also creates a safety risk if they detach. Strike a balance: the lower lip of the skirt should be as low as possible without hitting normal road obstacles. Many aftermarket skirts are designed to fit factory pinch welds and offer a 10–20 mm drop from the rocker panel height.

Pairing with Suspension Modifications

To fully realize the CG reduction, lowering springs or coilovers are necessary. Side skirts alone on a stock-height car will have little effect on CG. The combination of a 25–40 mm drop with properly designed side skirts yields the best results. Consider adjustable coilovers to fine-tune ride height while keeping the skirts off the ground.

Side Skirts vs. Other Aerodynamic Mods

Side skirts don’t work in isolation. They are most effective when paired with complementary aero parts:

  • Front Splitter: Directs air around and under the car. Side skirts connect the splitter’s effect to the rear, preventing air from spilling out the sides.
  • Rear Diffuser: Expands the low-pressure area under the car. Side skirts seal the sides so the diffuser can work efficiently.
  • Canards and Vortex Generators: Manage flow at the front and over the body, respectively. Side skirts close the side flow, enhancing the effect of these devices.

Without side skirts, a front splitter or rear diffuser will lose effectiveness because high-pressure air can re-enter under the car from the sides. This is why almost all race cars and high-performance road cars come factory-equipped with side skirts or side sills.

Real-World Performance Gains

Drivers who install side skirts along with a mild lowering often report noticeable improvements in:

  • Cornering stability: Reduced body roll and better steering feel during fast corners.
  • High-speed confidence: The car feels more planted and less twitchy on the highway, especially in crosswinds.
  • Braking stability: Less lift under heavy braking keeps the rear tires in better contact with the road.
  • Fuel economy: Reduced drag can improve highway mpg by 1–3%, though gains vary by vehicle.

These benefits are most noticeable on track or during spirited driving. For daily commuting, the aesthetic upgrade is the primary reason, but the aerodynamic bonus is always present.

Potential Drawbacks

No modification is without compromises. Side skirts come with some downsides:

  • Reduced ground clearance: Scraping on driveways and speed bumps is common. Some owners choose to remove them for winter or install removable versions.
  • Increased weight: Heavy side skirts add unsprung or at least low-mounted weight, which can negate some CG benefits if combined with a very high mounting point. Choose materials wisely.
  • Installation complexity: Proper fitting may require drilling or riveting into body panels. Some kits use double-sided tape, but long-term durability is questionable.
  • Repair costs: Damage to side skirts is common from road debris, and replacing them can be expensive, especially carbon fiber.
  • Water and snow accumulation: In wet conditions, skirts can trap moisture against the rocker panels, promoting rust if not properly sealed or drained.

Before installing, consider your driving environment. If you regularly encounter steep ramps, snow, or unpaved roads, side skirts may not be practical.

Conclusion

Side skirts are a deceptively simple upgrade with a meaningful effect on your car’s center of gravity. By enabling a lower ride height, reducing aerodynamic lift, and adding low-mounted mass, they work together with suspension and other aero parts to improve handling, stability, and safety. While not a replacement for proper coilovers or weight reduction, side skirts are an effective tool for any driver looking to sharpen their car’s dynamic behavior.

When shopping for side skirts, focus on material quality, fitment precision, and your intended use. Pair them with a moderate lowering and complementary aerodynamic mods for the best results. With the right setup, you’ll feel the difference every time you take a corner or hit the highway.

For further reading on vehicle dynamics, check out Car and Driver’s guide to center of gravity. To learn more about aftermarket side skirts, this MotorTrend article covers selection and installation. For a deeper dive into automotive aerodynamics, the AutoSpeed series on underbody flow is a great resource.