Sorry I misunderstood the question.
Fore and aft movement is a slightly different issue. Many rigs are designed to allow this. I once made new spreaders for my schooner that, unlike the originals, were rigid fore and aft. Fortunately, Nat Benjamin stopped me before final installation.
Masts do flex. Sometimes, hard core racers will induce some bend at times and take it out at other times. Sometimes it's just the stress of weather and is within designed levels. Either way, rigid spreaders will suffer fatigue at or near the base and sooner or later fail.
If you look at the geometry, you will see that in the fore and aft dimension, the spreader will always assume it's correct fore and aft alignment when under pressure. Draw a line from the chainplate straight to the stay attachment. It crosses the spreader. Any spreader that pushes the stay out (for better angle at the top) and has it's base past that straight line will always be forced to the stay's shortest path.
The real example of stay and spreader geometry is a bit more radical if you're looking at a dolphin striker to give a bobstay a better angle. Too many make a rigid base for the striker and it's subject to anchor line stresses from the side. And it's possible for the striker to be so short, to have a base so close to that direct line between the stay's ends, that it cannot assume the correct position by stress alone. Then, like a whisker pole set from the rail, it may need to be held in place.
Not a problem with spreaders. They will find the right place with tension.
G'luck
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