What Is a Unibody Pull?by John Willis
In response to the gas crisis of the 1970s, automakers sought lightweight manufacturing solutions. Unibody construction was one of the major resulting innovations. Rather than heavy I-beam frame rails and individual sheet-metal panels, bolted together, the unibody is all one piece. Unibodies required new repair techniques. When a unibody car is damaged in an accident and the damage actually changes the frame dimensions, auto repair shops "pull" the frame back to factory dimensions. The process is referred to as a "unibody pull."
Because the unibody is lighter, it is also designed to disperse the energy of accidents more widely. Accident damage, either minor or severe, is less likely to be localized to the point of impact in a unibody frame. The whole frame is design to absorb energy. While that is an excellent safety feature, it can result in mild damage spreading through the entire frame, even if it's not obvious damage.
A key part of repairing or "pulling" a unibody frame is the ability to accurately measure it in three dimensions. Auto repair shops employ sophisticated equipment so they can measure to the millimeter to determine the integrity of the frame's dimensions.
Unibodies are pulled on heavy-duty steel racks. These racks give repair technicians a variety of anchor points to use to pull bends and deformations out of the damaged car. Other specialized tools are used in conjunction with these racks to get exactly the right angles. The greatest force will be applied in exactly the opposite direction from which the impact came. Damage can be pulled out until the dimensions of the car's form once again matches factory specifications.
Occasionally the term "unibody pull" references misalignment in a previously wrecked unibody car which has not been repaired correctly. The "pull" will feel and behave much like wheel misalignment. In fact, the wheels can be misaligned, but not because of the relative position of the wheels changed, but because the relative position of the body to which the suspension is mounted has changed. Car shoppers looking at used unibody cars are wise to make sure the vehicle's frame is undamaged.