Static Stability Factor (SSF) Calculator — Vehicle Rollover Threshold

How it works

SSF = T ÷ (2 × H) · T = track width, H = centre-of-gravity height · rollover threshold (rigid vehicle) = SSF in g · quasi-static rollover speed on radius r: v = √(SSF × g × r)

The static stability factor is the simplest measure of how top-heavy a vehicle is: it is the sideways acceleration, in g, at which a rigid vehicle would start to tip on a flat road. NHTSA uses SSF, together with a dynamic manoeuvre test, to give vehicles their rollover star rating. Cars typically have SSFs of about 1.3–1.5, SUVs and pickups about 1.0–1.3. Real vehicles roll at somewhat lower forces because suspension and tyres let the body lean, and most real rollovers are "tripped" by a curb, soft ground or a ditch.

Worked example

A crossover with a 1.55 m track and its centre of gravity 0.58 m above the ground has SSF = 1.55 ÷ (2 × 0.58) = 1.34 — in the crossover or minivan range. In theory it would tip at 1.34 g, more than tyres can give on dry asphalt, and on a 50 m curve that would need about 92 km/h.

Sources: NHTSA — Car safety ratings (rollover resistance) · Vehicle rollover — static stability factor

Frequently asked questions