Deadbolt Throw Length: How Much Engagement Depth a Longer Bolt Actually Buys Against Frame Splitting

A deadbolt is sold by its throw, the distance the bolt travels into the frame, yet what a household needs is resistance to the frame splitting around the strike. This study models that resistance from stated assumptions: a steel bolt of 11 mm by 20 mm section has a shear capacity near 61,600 N and never governs, while the timber frame around the strike pocket is modelled as failing at F = 700 + 330 e^0.8 newtons, where e is the engagement depth in millimetres after a 4 mm allowance for door gap and strike recess. A standard 25 mm throw then resists 4470 N and delays entry by about 3.99 minutes under the stated attack model, while the longest throw tabulated, 38 mm, resists 6242 N, a gain of 39.6 percent, for a delay of 5.26 minutes. Resistance reaches the 5,000 N level used here as a working target only at a throw of 28.8 mm. The practical rule is therefore to specify engagement depth, not catalogue throw, and to expect diminishing returns beyond about 32 mm because the exponent on engagement is well below one. SIMULATED, NOT MEASURED.