With flat-bottomed vehicles [like an armored Hummer], the blast effect can be amplified if an explosion occurs directly beneath. A blast wave striking the underside will be ‘reflected’ back at the ground. Before dissipating, that energy will be redirected at the vehicle (if the BOP [blast over pressure] from the initial explosion is strong enough to lift the vehicle, an assymetric, reflected blast wave may well topple the vehicle onto its side). In most cases, the weight and ground clearance of a LAV [light armored vehicle] means that the vehicle will not be ‘lifted’ by blast.
*The undersides of a LAV may be flat but it is also relatively smooth and free from projections. This is in direct contrast with a ‘soft-skinned’ vehicle like a [Humvee] with its add-on armour. Like a liquid wave, blast will curve around a corner – finding its way into seams and openings. Irregular shapes found in conventional frames and firewalls become ‘gas traps’ focusing blast energy. These nooks and crannies are exactly what the designers of mine- resistant vehicles tried to avoid. Combining gas traps with light weight is a recipe for disaster. *
[The so-called "Mine Resistant Ambush Protected" vehicles], which were designed to thwart the effects of landmines, have hulls care- fully shaped to deflect blasts from below. Usually, this means a ‘V’-shaped bottom with a minimum of angles or excrescences, allowing the blast to ‘flow’ unimpeded past the structure. Parts of the vehicle likely to be in the path of the blast are designed to be sacrificed – wheels and axles are easily blown off but such running gear parts can be just as readily re-attached.
An excellent primer in the Canadian-American Strategic Review explains:
