Skid plate

Also called bash plate, sump guard, underbody armor, skid plates

A skid plate is a panel of steel, aluminium or plastic bolted under a vehicle to protect the engine, transmission, transfer case or fuel tank from rocks, and to let the vehicle slide over an obstacle instead of catching on it.

What is a skid plate?

A flat or gently curved plate covering something vulnerable underneath the vehicle. Most factory four-wheel drives come with at least a thin one under the engine. A full aftermarket set covers the engine, the transmission, the transfer case, the fuel tank and sometimes the rear differential, in overlapping sections that bolt to the frame.

It has two jobs that people conflate. The first is impact: stopping a rock from putting a hole in an oil pan or a fuel tank, which turns a bad day into a recovery and a tow. The second is sliding: presenting a smooth continuous surface so the vehicle rides over an obstacle rather than snagging on a bracket, a crossmember or a hose.

The materials, and what each is bad at

Steel is cheap, very strong, and heavy. It dents rather than cracking, which means it can be beaten back into shape, and it rusts where it has been scraped to bare metal. A full steel set on a mid-size truck is commonly 130 to 200 pounds.

Aluminium is roughly half the weight for a given thickness at similar strength, does not rust, and gouges rather than sliding as well as steel does. It is also several times the price.

UHMWPE plastic is light and slides better than either metal, which makes it good for the sliding job and poor for the impact one. It is what a lot of factory plates are, and it is a reasonable choice for terrain that is more about scraping than about spearing.

What armouring the underside costs

Weight is the obvious one, and it is payload spent before you have packed anything. Access is the next: an oil change now starts with removing a plate, which is why good sets have drain access holes and bad ones do not.

The one that catches people out is heat. Sealing the underside of an engine bay restricts the airflow that was leaving through it. Patrick's Tacoma is the ordinary version: after fitting skid plates he fitted hood louvers, and reported engine bay temperatures dropping twenty to thirty degrees as a result. Plates can also trap mud and hold water against components that were designed to dry out.