Which of the following should be checked first thing when doing a front-end alignment?

When performing a front-end alignment, curb height should be checked first as it establishes a baseline for the vehicle's overall dimensions relative to its suspension system. Curb height refers to the distance from the ground to a specified point on the vehicle's chassis when it is at rest and normally loaded. This measurement is crucial because it determines if the vehicle is within manufacturer specifications and can indicate if there are any underlying issues with the suspension components, such as sagging springs or worn-out parts.

If the curb height is not correct, it can lead to inaccurate alignment readings and potential handling problems. By ensuring that the curb height is within the correct specifications, a technician sets an accurate starting point for measuring other alignment angles, such as toe angle, camber, and caster. Thus, beginning with curb height allows for a more effective and accurate alignment process.