A Tonneau ( or ) in automotive terminology is an area of a car or truck open at the top. It can be either a people or cargo space.
A tonneau cover describes a hard or soft cover used to protect unoccupied passenger seats in a convertible or roadster, or the cargo bed in a pickup truck. Hard tonneau covers open by a hinging or folding mechanism while soft covers open by rolling up.
The tonneau cover also conceals and/or covers cargo. When the cover is pulled out, it keeps items out of the sun and provides extra security by keeping personal items out of sight.
A tonneau was originally an open rear passenger compartment, rounded like a barrel, on an automobile and, by extension, a body style incorporating such a compartment. The word is French, meaning 'cask'.
Early tonneaus normally had a rear-facing hinged door, but single and dual side doors were soon introduced. The first US side-door tonneau was made by Peerless, and others quickly followed.
When the street was muddy or dirty the car could be backed up to the curb so tonneau passengers could exit directly onto the sidewalk.
Early open-bodied touring-type automobiles used tonneau covers to protect unoccupied rear seats. As early as the 1930s, lakes racers, searching for that extra competitive edge, pulled a page directly from early aircraft construction and skinned the cockpits of their roadsters and streamliners with removable canvas. The skins covered gaping cockpits that would otherwise disturb airflow and create undue drag; as a result, tonneau-equipped cars ultimately went faster with a given amount of power.
Tonneau covers are available for open sports cars such as the Porsche Boxster, MG, Triumph, and Austin-Healey. These covers, often made of natural or artificial leather, cover the entire passenger compartment, and are zippered so the driver's seat can be uncovered while the rest of the interior remaines covered. Tonneau covers may be used in lieu of hard or soft convertible tops.
As air courses over the windshield at speed, it creates turbulence as it cascades into and bounces out of the cockpit. This condition is called buffeting, and can be annoying. A tonneau cover reduces buffeting at all speeds. Heat produced by a heater leaves as that turbulence exchanges the cockpit's warmer air for cooler ambient air. A tonneau cover is capable of both retaining heat and shielding a driver from UV exposure.
Tonneau covers are used in utility vehicles and pickup trucks to cover and secure the truck bed and come in a variety of styles.
The most common style is the roll-up tonneau made from cloth or vinyl, which uses a rib-like structure to support the fabric and keep it taut. A snap-based system is also used, but has become less common due to truck owners not wanting to install the snaps on their vehicle as they typically require drilling or permanent adhesive. Roll-up Tonneaus are opened by rolling the cover up toward the cab of the truck. Hard roll-up tonneau covers are more firm than regular roll-up tonneau covers. They are made of a wall of individual aluminum slats, covered with soft vinyl. In the unrolled position, these aluminum slats form a hard aluminum shell, which not only covers the cargo inside the bed, but also may support loads up 400 pounds on top of it.
Another style of truck bed tonneau cover is a retractable unit, which is mounted at the front and sides of the bed and rolls up or retracts from the tailgate towards the cab. The retractable tonneau is typically made of vinyl, plastic or aluminum. Retractable tonneaus are more secure than soft tonneau covers, since they usually lock and are made from a harder composition, but they take more time to install and are designed for semi-permanent installation.
Fiberglass, hard plastic or aluminum tonneau covers are also common. Some may be painted to match the truck, are solid in construction, and can be locked. These covers are usually heavy and require gas struts to assist in opening and closing. They operate much like a vehicle's hood, typically opening from the tailgate end of the bed (back to front). Some have multiple compartments that open front to back, back to front, side to side, or even rise vertically. Fiberglass, hard plastic or aluminum tonneau covers are sometimes installed as a factory option on new vehicles.
Many sellers claim that tonneau covers improve gas mileage because they make the truck more streamlined. However, air currents create a wake inside the pickup bed, which helps the aerodynamics. A tonneau cover interferes with this wake, and scientific tests have shown little to no improvement in mileage by using a tonneau cover traveling at less than 70 mph (110 km/h). A similar effect is seen when the tailgate is down and the mileage goes down.
"Tonneau case" is used to describe a type of watch case, with rounded, bulging sides resembling a barrel (or cask).