Lap shear strength is also measured by pounds per square inch until failure. To test lap shear strength, objects are pulled in a diagonal fashion. If you were to pull the same coffee cup off the table diagonally, the strength needed to separate it from the table would be lap shear strength. Automotive urethane has more than 500psi of lap shear strength. This force adds to the structural strength of the vehicle body and is especially important in collisions when a rollover occurs.
Flexibility is equally important.
Urethane must have flexibility to allow for vehicle body movement. While a car bends and flexes with the roadway, the glass is rigid. So if the adhesive is too inflexible, the glass would break over a rough surface or other expected road conditions. Urethane must also adhere to different types of surfaces. Glass is very smooth, yet the metal frame of the vehicle is rough.