Key Factors to Consider When Choosing Side Skirt Materials

Side skirts do more than just visually lower your Mustang’s stance. They redirect airflow along the rocker panels, reduce lift, and protect the lower body from gravel, salt, and debris. Choosing the right material directly affects how well the skirts perform these duties and how long they last. Before you settle on a material, evaluate these five critical factors:

  • Durability against road debris and weather: Daily drivers face stone chips, road salt, and temperature swings. Track cars may encounter curb contact. The material must resist cracking, warping, and UV degradation.
  • Weight and impact on vehicle dynamics: Heavy skirts add unsprung or aerodynamic load. While most side skirts are light enough not to hurt handling, carbon fiber offers the best strength-to-weight ratio and can improve throttle response by reducing overall mass.
  • Ease of installation and maintenance: Some materials require drilling and bonding, others use factory mounting points. Repairability also varies – a polyurethane skirt can often be plastic-welded, while fiberglass cracks may require resin and cloth.
  • Style and aesthetic compatibility: Your Mustang’s generation (S550, S197, SN95) and existing body kit influence which shape and finish look cohesive. Carbon fiber’s exposed weave suits aggressive builds, while painted polyurethane matches OEM-style lines.
  • Cost versus longevity: Cheap ABS skirts may deform in heat. Carbon fiber costs 3–5× more than polyurethane but lasts indefinitely with proper care. Calculate total cost of ownership including paint, hardware, and potential replacements.

Top Materials for Side Skirts

Polyurethane (PU)

Polyurethane is the go-to material for drivers who want a balance of flexibility, impact resistance, and affordability. OEM-grade PU skirts for the Mustang are usually injection-molded, which gives even thickness and consistent fitment. This material can flex under stress – if you scrape a parking stop or curb, PU tends to bounce back rather than crack. It also absorbs vibrations and road noise better than rigid materials.

Advantages: Resists cracking in cold climates; easy to paint with flexible additive; relatively lightweight (1–3 lbs per side). Many aftermarket brands like American Muscle’s polyurethane skirts use factory-grade polyurethane for bolt-on installation.

Disadvantages: Can show minor warpage if stored improperly; not as stiff as carbon fiber for aerodynamic downforce; may require sanding and flexible primer before paint.

Best for: Daily drivers, street performance builds, and anyone who wants a durable skirt that won’t shatter on rough pavement.

Fiberglass

Fiberglass has been a staple in aftermarket bodywork for decades. Hand-laid fiberglass allows manufacturers to create complex curves and aggressive duct work that injection molding cannot replicate. For Mustang owners building a widebody or custom show car, fiberglass side skirts can be shaped to match unique flares or splitters.

Advantages: Highly moldable; relatively cheap material cost; can be repaired with common fiberglass kit; accepts paint well after proper gel coat sanding.

Disadvantages: Brittle under point impact – a rock strike or jack placement can cause spider cracks; absorbs moisture if the gel coat is breached; heavier than polyurethane or carbon fiber (typically 4–6 lbs per side).

Maintenance: Inspect for micro-cracks every season. Seal edges with clear coat to prevent water ingress. Brands like CJ Pony Parts offer fiberglass rocker panels that require careful prep but deliver a show-quality finish.

Carbon Fiber

For enthusiasts chasing every ounce of weight savings and a race-bred appearance, carbon fiber sits at the top. Pre‑preg carbon fiber (cured under heat and pressure) is used by premium manufacturers to achieve the signature weave while maintaining high stiffness. On a Mustang, carbon side skirts can save up to 60% in weight compared to fiberglass, reducing unsprung mass and improving suspension response.

Advantages: Extremely light (as little as 0.8 lbs per side); superior tensile strength; UV-resistant with proper clear coat; resists impact better than fiberglass in many cases (layers can delaminate rather than shatter).

Disadvantages: High cost ($400–$1,000+ a pair); requires careful drilling to avoid splintering; exposed weave demands clear coat to prevent yellowing; non-repairable if structural damage occurs – replacement is usually the only option.

Best for: Track days, serious road course builds, or show cars where the woven look is a statement. Check with Roush Performance’s carbon packages for an OEM‑inspired option with warranty support.

ABS Plastic (Acrylonitrile Butadiene Styrene)

ABS is common in factory bumpers and some budget side skirts. It is rigid at room temperature but can become brittle in extreme cold. While cheaper than polyurethane, ABS lacks the same flexibility and impact memory. Many entry‑level Mustang skirts use ABS because it molds easily and holds a crisp edge.

Advantages: Low cost; good paint adhesion; lightweight (2–4 lbs per side).

Disadvantages: Prone to cracking in cold climates; may warp when parked in direct sun; can shatter on high‑speed debris impacts. Not recommended for cars driven in northern winters or on gravel roads.

Polypropylene (PP)

Polypropylene is another thermoplastic used in some OEM underbody panels. It is lightweight and has good impact resistance, but it is difficult to paint effectively without special adhesion promoters. Most aftermarket side skirts for Mustangs do not use pure PP, but it appears in some molded rocker guards and lower splitters.

Advantages: Very low cost; excellent chemical resistance; can withstand moderate impacts without cracking.

Disadvantages: Difficult to achieve a high‑gloss paint finish; may need vinyl wrap instead of paint; limited aesthetic options compared to PU or fiberglass.

Installation Considerations

Proper installation determines both the look and longevity of your side skirts. Fitment check: Always dry‑fit the skirts with painter’s tape before drilling or bonding. Ensure the contour matches your Mustang’s rocker panel profile – S550 models have subtle body lines that some universal skirts miss. Use a flexible gap filler (like 3M panel bond) for glass‑smooth seams. Hardware: Stainless steel screws with rubber washers prevent corrosion and vibration loosening. For PU and ABS, self‑tapping screws into the rocker flange work well. For carbon fiber, drill with a carbide bit and use nylon inserts to avoid cracking. Preparation: Clean surfaces with isopropyl alcohol, scuff paint with 400‑grit sandpaper, and apply adhesive promoter if recommended. If you are unsure about drilling, consider professional installation at a shop like those recommended by Ford Performance certified installers.

Maintenance and Longevity

Side skirts live in the blast zone of road debris, so regular cleaning and inspection extend their life. Washing: Use a low‑pressure rinse and a microfiber mitt to avoid scratching painted surfaces. Carbon fiber clear coat should be waxed with a ceramic sealant to prevent UV yellowing. Repair: Polyurethane cracks can be plastic‑welded with a filler rod; fiberglass chips require resin and cloth repairs; carbon fiber damage usually means replacement. Protection: Apply a Paint Protection Film (PPF) to the leading edges – even on unpainted carbon, a clear film stops rock chips. Seasonal care: In salty winter regions, remove skirts if possible (or apply an undercoat spray to the rocker panel behind them) to prevent trapped moisture from rusting the body.

Performance Impact: Aerodynamics and Weight

Side skirts work with the front splitter and rear diffuser to create a low‑pressure zone under the car, reducing lift. A properly designed skirt – with a straight bottom edge that nearly touches the ground – can decrease rear lift by up to 10–15% at highway speeds. However, the material’s stiffness matters: a flexible polyurethane skirt may flutter above 100 mph, reducing its aerodynamic effect. Carbon fiber maintains its shape, making it the choice for serious track cars. Weight savings also add up: swapping factory plastic rockers for carbon fiber skirts can save 4–6 lbs overall. While that alone won’t win races, every pound of unsprung weight reduction improves suspension compliance and reduces rotational inertia.

How to Choose Based on Your Mustang Build

  • Daily driver / street cruiser: Polyurethane is the smart choice. It handles the abuse of potholes and parking curbs without cracking, and it accepts paint flawlessly. Budget 2–3 hours for installation with basic hand tools.
  • Show car / weekend cruiser: Fiberglass or carbon fiber. Fiberglass allows custom sculpting and paint matching; carbon fiber gives a premium, motorsport look. Both require more careful installation and maintenance.
  • Track / autocross car: Carbon fiber for minimal weight and maximum stiffness. Look for skirts with integrated ducting for brake cooling if your Mustang sees heavy track use.
  • Bargain build / test mule: ABS plastic skirts can be painted and replaced cheaply. They will show wear faster, but they allow you to experiment with different styles.

Final Thoughts

Your Ford Mustang’s side skirts are an investment in both appearance and function. Polyurethane remains the best all‑around material for most owners, offering a blend of flexibility, repairability, and cost. Fiberglass satisfies those who want custom shapes and are willing to maintain them. Carbon fiber delivers the ultimate in weight reduction and visual impact for dedicated enthusiasts or track‑focused builds. No matter which material you choose, invest in quality hardware, proper prep work, and regular maintenance – your Mustang will thank you with years of clean looks and improved aerodynamics.