The 16-Inch Center Stud Problem:
If we have a 48-inch bracket (96" divided by 2) the 48-inch won't cause any problems.
However, if we cut a 30-inch bracket out of a 48", we will not hit 3 studs (16+16 = 32 center). On the other hand, should we cut it at (for the sake of this example) 32", now we're left with an extra 16-inch bracket... and we have the same issue as with the 32-inch bracket. So what gives?
They call them "two by four" studs but in actuality they are 3 and 1/2" by 1 and 3/4". Herein lies the problem:
A 16” bracket is not long enough to safely hit two studs dead centre. Using an 18” bracket will allow the bracket to stick out past the stud by 1/8”, and enable you to centre your screws in your studs.
Likewise, on a 34” bracket, you will also extend past the stud by 1/8”, whereas a 32” bracket will not.
As you can see, the screw head is already 6/16" (3/8) big and we only have 7/16" of room left in the stud to drill into. The screw itself is only 3/16" thick though, and if you drill to the right, you will miss the stud. Even if you're perfectly straight on, you are only in the stud by 1/4 of an inch. If you drill to the right you are fine, but then the pan head screw does not sit flush on the bracket anymore. And we must also remember that no stud is perfect - they are warped, not exact at 16" center, etc.