in an accident or have one or two of the front or rear wheels missing, or for pickup trucks and other vehicles that have steel bumpers. Cars equipped with all-wheel drive cannot be towed with a sling, as it can cause problems with the car's drivetrain. Wheel-lift (also called a "spectacle lift" or "underlift"): developed from the hook-and-chain technology to produce a large metal yoke that can be fitted under the front or rear wheels to cradle them, drawing the front or rear end of the vehicle clear of the ground by a pneumatic or hydraulic hoist so it can be towed. This apparatus generally picks up the drive wheels of the vehicle (i.e. the front wheels if it is front-wheel drive, the rear wheels if it is rear-wheel drive), touching only the tires. The wheel lift was designed by Arthur W. Nelson of Weld Built Body Co. in 1967. The name spectacle lift is common in Europe; the cradle resembles a pair of squared spectacles (eyeglasses). Medium and heavy trucks use a variation, the "underlift" or "chassis lift", which lifts the axle or frame instead of the wheels. Wheel-lift trucks can have adapters which can also lift the chassis. Integrated (also called a "self-loader", "snatcher", "quick pick" or "repo truck"): boom and wheel-lift integrated into one unit. Used in light-duty trucks to repossess vehicles or move illegally-parked vehicles. Most have controls for the apparatus inside the cab of the tow truck to make quick pickup possible without the inconvenience of exiting the truck to hook up the vehicle. Though similar to a wheel-lift truck, an integrated truck differs in that the end of its boom features movable arms that can more easily and quickly clamp onto the wheels of a vehicle, often controlled from the cabin of the truck. On a wheel-lift truck, the wheels of a vehicle must be manually secured to the yoke by the operator before being lifted. There are also heavy-duty trucks manufactured with integrated lifts. Flatbed (also called a "rollback", "slide" or "tilt tray"): the entire back of the truck is fitted with a bed that can be hydraulically inclined and slid back to ground level, allowing the vehicle to be placed on it under its own power or pulled on by a winch. Because they carry rather than tow the vehicle, they can be used on a completely immobile vehicle; in the US,