The primary safety function of a nerf bar is to prevent tires from becoming entangled when two cars race side by side or make contact. On oval tracks where wheel-to-wheel contact is frequent, fast-spinning tires touching each other can cause one or both cars to lose control or become airborne, creating severe danger. By interposing a structural tube between the tire contact zone and a rival car's wheel, the nerf bar deflects the bump into a controlled slide rather than a tyre-lock.
Nerf bars are common on Modifieds such as those used in the NASCAR Whelen Modified Series. Sprint cars and midgets, which race on both dirt and asphalt ovals, use nerf bars as a standard fitment; on these cars the bars are typically lightweight steel or aluminium tubes bolted to the main frame or chassis rails at each side. The wheel pods fitted behind the rear wheels on the Dallara DW12 IndyCar chassis serve a closely analogous purpose and are frequently referred to informally as nerf bars by crews and commentators.
Racing nerf bars are built to be strong enough to absorb repeated low-speed contact without deforming into the wheel or tyre. They are generally fabricated from steel tubing and positioned so that, in a sideways contact, the bar meets the rival car's bodywork or bar rather than the tyre sidewall. On sprint cars they are sized and positioned to allow wheel-to-wheel racing without driver injury; a bar bent by contact is replaced between heats.
The term nerf bar is also applied to a tubular side step fitted to pickup trucks and sport utility vehicles, serving as a step for entry and exit and providing minor side protection against kerbs or rocks. These road-vehicle bars — sometimes called step bars, tube steps, or side rails — share only the name and basic tubular form with the racing component, and serve an entirely different function. Heavy-duty off-road versions that attach directly to a vehicle's frame rails and can support the full vehicle weight are typically called sliders or rock bars rather than nerf bars.
In dirt oval racing, contact between competitors is far more routine than in road-course or stadium racing, and the nerf bar is treated as consumable safety equipment. Drivers may deliberately use a nerf bar to "nerf" a rival — nudging them aside to complete a pass — a technique regarded as part of aggressive but accepted oval-racing tactics. Race officials distinguish between a nerf (acceptable side contact below a threshold of aggression) and a deliberate block or rough driving violation, though the line is often contested. Sprint car and midget drivers consider a bent or scratched nerf bar a badge of hard racing rather than an indication of damage requiring extended repair.