A smash number is the load, in pounds, that a coil-over spring is holding when it's compressed to a specific shock center-to-center length — for example, 606 pounds at 18.250 inches on a Late Model.
You measure a smash number on a spring smasher — a bench fixture that compresses the coil-over to a target shock length and reads load off a gauge. Pick the center-to-center height you actually run at that corner, load the coil-over (spring and shock), compress, and read. BSB Manufacturing’s Late Model example, “606 @ 18.250,” is exactly that: 606 pounds of load at 18.250 inches. The payoff comes when you want a different spring rate without losing your baseline — smash the new spring to the same load and the corner stays where it was. Scale the car first, though. A load number describes one corner; it can’t see a cross-weight problem.
Get it right and you can swap springs at the track without re-scaling the whole car; get it wrong and every spring change is a fresh guess.