What inputs do momentum analysis and crush analysis use to estimate impact speed?
Momentum analysis uses vehicle masses including occupants and cargo, the directions of travel entering and leaving contact, and the post-impact paths and distances that establish separation velocities, while crush analysis uses the depth and distribution of permanent deformation, applied to the struck structure’s stiffness properties. Momentum analysis conserves the system’s total momentum across the impact. Crush analysis works from energy: the permanent deformation, applied to the struck structure’s stiffness properties, yields the energy absorbed and the closing speed that energy implies. The two methods therefore estimate impact speed from almost entirely separate inputs.
The only meaningful overlap between the momentum analysis inputs and the crush analysis inputs is mass. Everything else — geometry and rest positions on the momentum side, deformation and stiffness on the crush side — is independent.
What does momentum analysis need to estimate impact speed reliably?
Momentum analysis needs impact and departure headings taken from scene geometry rather than assumed, a defensible point of impact, and drag factors for the surfaces each vehicle crossed on its way to rest. The point of impact is usually located from gouges, fluid deposits and debris distribution. The drag factors depend on the surface, on whether the wheels were rolling, locked or damaged, and on whether the vehicle rotated as it slid.
The angles are the sensitive part of momentum analysis. Small heading errors propagate strongly, which is why total-station or laser-scanned scene documentation is worth more to a momentum analysis than a scaled sketch.
What does crush analysis need to estimate impact speed reliably?
Crush analysis needs a measured crush profile and stiffness coefficients appropriate to that vehicle and that damaged face. A measured crush profile is a set of depths taken along the damaged face at recorded intervals from a defined reference, on both vehicles where possible. SAE J224 provides the collision deformation classification used to describe the damage consistently, with SAE J2433 serving the heavy-vehicle equivalent, so that a crush profile means the same thing to a second engineer.
In crush analysis, the measurement discipline matters as much as the arithmetic. Crush measured from photographs, or from a vehicle already partially disassembled or moved by recovery equipment, carries an error band that should be stated rather than absorbed.
Where do crush stiffness coefficients come from, and what are their limits?
Crush stiffness coefficients derive largely from instrumented barrier crash testing, much of it conducted or published under NHTSA’s compliance and consumer-information programs. That origin defines the domain of the stiffness coefficients: frontal impacts into a flat rigid barrier, at test speeds and directions chosen for a regulatory purpose.
Real collisions frequently sit outside that barrier-test domain — narrow-object and pole impacts, underride, override, oblique and corner engagements, and impacts into structures that deform themselves. Applying a frontal stiffness coefficient set to a side or rear structure, or to a vehicle represented only by a class average, is a modeling choice that belongs on the page.
What collision energy does crush analysis not capture?
Crush analysis captures only the energy that stayed in permanent deformation. Crush does not capture the elastic energy returned as the structures rebound, which restitution accounts for, and it does not capture energy dissipated in tire scrub, in ground engagement, in rotation, or in bodies the vehicle struck that absorbed energy of their own.
That uncaptured energy is one structural reason a crush estimate can sit low relative to a momentum estimate, and it is why the coefficient of restitution assumed at low closing speeds is rarely negligible.
Why does sensitivity analysis matter more than a single impact-speed number?
Sensitivity analysis matters more because a single impact-speed number states less than the range it sits in, so the sensitivity is the real deliverable. Competent reconstruction work runs each input across its plausible interval — mass, drag factor, departure angle, crush depth, stiffness set — and reports which inputs actually move the answer and by how much.
The result of a sensitivity analysis is usually that two or three inputs dominate and the rest are noise. Knowing which inputs are which tells everyone where the remaining investigative effort should go, and where argument is not worth having.
What does it mean when momentum analysis and crush analysis give different impact speeds?
When momentum analysis and crush analysis give different impact speeds, the divergence has recognizable causes, and it usually points at an assumption that does not fit the collision. A crush figure well below the momentum figure may indicate an unrecorded secondary impact, an override or underride geometry in which one structure passed through rather than into the other, energy absorbed by roadside furniture, or an inapplicable stiffness set.
A momentum figure that looks low may indicate a misplaced impact point, a drag factor set for a surface the vehicle did not actually cross, or a rotation that was not accounted for. In each case, the reconciliation of the momentum and crush figures, not their average, is the analysis.
How does scene evidence constrain impact speed alongside momentum and crush analysis?
Scene evidence constrains speed through the marks and positions left at the scene: skid marks bound speed through a drag factor, critical-speed yaw marks bound speed through the arc radius the vehicle carved, and rest positions and debris distribution bound the departure conditions that momentum analysis depends on. Tire marks constrain impact speed independently of both momentum analysis and crush analysis, which makes scene evidence a third constraint alongside the two methods. Scuff and gouge marks locate the collision sequence in space.
Dash-camera, surveillance or bystander video adds timing, and photogrammetry can recover geometry from scene photographs after the tire marks themselves have weathered away — which they often do within days.
What does a defensible impact-speed reconstruction look like?
A defensible impact-speed reconstruction gives a stated range rather than a point value; presents momentum analysis and crush analysis each with its own inputs and uncertainty; shows the sensitivity of the conclusion to the inputs that dominate; and, where the methods disagree, explains the disagreement rather than quietly preferring the more convenient number.
That kind of disclosure is also what makes an opposing analysis testable. Two reconstructions that disclose their inputs can be compared; two that report only conclusions can only be asserted at each other.
This article is general technical orientation on momentum and crush-energy analysis, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.