Why did added armour fail to protect Humvees from the most lethal roadside bombs?

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armoured vehicle A British Warrior mechanized combat vehicle serving in NATO's Stabilization Force in Bosnia and Herzegovina, 1997.

The Humvee (High Mobility, Multipurpose Wheeled Vehicle) was originally designed for mobility rather than as a heavily armoured combat vehicle. When the U.S. military attempted to address the vulnerability of these vehicles to improvised explosive devices (IEDs) during the Iraq War and Afghanistan War by adding extra armour, these modifications proved insufficient against the most lethal threats.1

The primary reason for this failure was that the added armour could not adequately protect the vehicle's underside. Many of the most dangerous IEDs were buried in roadways, specifically designed to target the bottom of a vehicle where the armour was weakest.1

In contrast, the Mine-Resistant, Ambush-Protected (MRAP) vehicles were engineered with a V-shaped hull. This design is fundamentally more effective because it deflects the force of an explosion upward and away from the troop compartment, rather than absorbing the blast through flat-bottomed plating.1