A tool to compute the Static Stability Factor (SSF) — the single geometric number that sets the lateral acceleration at which a vehicle begins to tip over in a corner. Adjust the track width, CG height, speed and turn radius to see the cornering lateral acceleration, the critical rollover speed and the safety margin in real time, and grasp why tall, narrow vehicles are so prone to rollover.
Parameters
Track width t
m
Spacing of the left and right tyre contact points. Wider resists rollover
CG height h
m
Height of the vehicle's centre of gravity above the road. Lower resists rollover
Travel speed v
km/h
Vehicle speed through the corner
Turn radius R
m
Radius of the corner. A smaller radius means higher lateral acceleration
Results
—
Static Stability Factor SSF
—
Cornering lateral accel. (g)
—
Critical rollover speed (this R) (km/h)
—
Safety margin (×)
—
Rollover verdict
—
Vehicle-class rating
—
Vehicle in a turn (rear view) — rollover moments
A rear view of the vehicle. Gravity acts straight down through the CG and the lateral (centripetal) force acts sideways, producing rollover moments about the outer tyre contact point. Colour shows the margin against rollover (green = stable / red = at risk).
Static Stability Factor SSF (t: track width, h: CG height) and the cornering lateral acceleration a_lat (v: speed in m/s, R: turn radius, g: gravity). The vehicle begins to tip over when the lateral acceleration, in g, reaches the SSF.
Critical rollover speed v_crit (the speed at which, on this turn radius, the lateral acceleration reaches the SSF) and the safety margin — how far below the rollover limit the current cornering is.
What is the rollover threshold?
🙋
Student: The "rollover threshold" is the line between a car tipping over in a corner or not, right? Is it set just by speed?
🎓
Professor: Good question. To a first approximation, whether a car tips over in a corner is governed by a single number called the Static Stability Factor, the SSF. It is half of the track width — the spacing between the left and right tyres — divided by the height of the centre of gravity. With a 1.5 m track and a 0.55 m CG height, SSF = 0.75/0.55 ≈ 1.36. That number is the lateral acceleration, in units of g, at which the car begins to tip.
🙋
Student: Wait — so it tips over once the lateral acceleration matches the SSF? Why does it work out that way?
🎓
Professor: Picture the car from behind. In a corner, the lateral (centripetal) inertial force acts outward at the CG and tries to roll the body about the line of the outer tyres. Gravity, acting straight down through the CG, tries to keep it planted. Those two moments balance exactly when the lateral acceleration in g equals the SSF. So the SSF is the rollover threshold of the rigid vehicle.
🙋
Student: I see! So the reason tall SUVs are said to roll over easily — can that be explained from the same formula?
🎓
Professor: Exactly. The denominator of SSF = (t/2)/h is the CG height h, so a tall, narrow vehicle has a large h and a small t and therefore a small SSF. A typical SUV or light truck has an SSF of only about 1.0-1.2. A low, wide sports car reaches 1.4-1.6 and barely rolls on a flat road. Move the slider on the "SSF vs CG height" chart below and you will see the SSF drop steeply as the CG rises.
🙋
Student: So as long as the SSF is high, the car is completely safe?
🎓
Professor: Two caveats there. First, a real vehicle has soft suspension and tyres, so the body rolls in a turn and the CG shifts outward — which means a real car starts to tip a little sooner than the rigid SSF predicts. Second, and more important: most real-world rollovers are "tripped" — the tyres catch a kerb, soft shoulder or guardrail. A tripped rollover can flip even a stable car below its SSF. So a high SSF is no licence to be careless if there is a kerb where you slide.
🙋
Student: The tool also gives a critical rollover speed — how should I use that number?
🎓
Professor: The critical rollover speed is the speed at which, for this corner radius, the lateral acceleration reaches the SSF. It is v_crit = √(SSF·g·R). The tighter the corner — the smaller the radius — the lower that speed. In practice it is used in accident reconstruction and road design to gauge "would taking this corner at this speed put you in the rollover region". Make the turn radius smaller on the left and watch the critical speed fall.
Frequently Asked Questions
The Static Stability Factor SSF is the dimensionless ratio SSF = (t/2)/h — half of the track width t divided by the centre-of-gravity height h. For a vehicle modelled as a rigid body, this number equals the lateral acceleration, in units of g, at which the inside wheels lift and rollover begins. A larger SSF means a vehicle is harder to tip: a typical SUV or light truck has an SSF of about 1.0-1.2, while a low, wide sports car reaches 1.4-1.6.
When cornering on a turn radius R, the lateral acceleration in g is a = v²/(R·g). The speed at which this equals the SSF is the critical rollover speed, v_crit = √(SSF·g·R) (multiply by 3.6 for km/h). The smaller the radius and the smaller the SSF, the lower the critical rollover speed. This tool shows the critical speed for the turn radius you enter.
Not precisely. SSF is an ideal value that treats the body as rigid. A real vehicle has compliant suspension and tyres, so the body leans (rolls) in a turn and the centre of gravity shifts outward. Because of this, a real vehicle starts to roll at a slightly lower lateral acceleration than the rigid SSF predicts. Moreover, most real-world rollovers are "tripped" — the tyres strike a kerb, soft soil or a guardrail — which can flip even a stable vehicle below its SSF.
Because the denominator of SSF = (t/2)/h is the CG height h. A tall, narrow vehicle has a large h and a small t, so its SSF is low and it rolls at a lower lateral acceleration. A low, wide vehicle has a high SSF and is almost impossible to tip on a flat road. SUVs, vans and trucks raise their CG further when loaded with cargo, passengers or a roof rack, so the rollover risk when laden deserves special attention.
Real-World Applications
New-vehicle safety ratings: The US NHTSA uses the SSF as the primary metric for the rollover-risk part of its star ratings. Vehicles with a low SSF receive fewer rollover stars, letting consumers compare before buying. Because the SSF needs only two dimensions — track width and CG height — it gives a highly practical estimate of rollover tendency without ever having to tip a real vehicle.
Designing SUVs, vans and trucks: Tall body styles inherently tend toward a low SSF, so manufacturers widen the track, place heavy components (battery, fuel tank) low in the floor, and raise the suspension roll stiffness to curb the rollover tendency. Electric vehicles resist rollover well because the heavy battery pack sits in the floor, dropping the CG dramatically and raising the SSF.
Road and corner design: Road designers estimate the cornering lateral acceleration from the corner radius and design speed, then add superelevation (banking / cross-slope) so that neither rollover nor side-slip occurs. Tighter corners have a lower critical rollover speed, so small-radius ramps — such as motorway on/off ramps — are given speed limits and sharp-curve warning signs. The critical speed in this tool serves as a first-pass estimate for such studies.
Accident reconstruction and forensic analysis: In rollover-crash analysis, the speed at the time of the crash is back-calculated from tyre marks and corner radius at the site. Because the cause and speed estimate differ depending on whether the rollover was a pure cornering (SSF-type) event or a tripped one (mounting a kerb), the SSF and critical speed are the starting point for that distinction. They are also used as a reference when evaluating how electronic stability control (ESC) or rollover-detection systems perform.
Common Misconceptions and Pitfalls
The biggest misconception is believing the SSF directly tells you the rollover speed. The SSF is a limit on lateral acceleration (in g), not a speed. The same vehicle can corner fast on a wide, large-radius bend without rolling because the lateral acceleration stays small, yet on a tight small-radius corner the acceleration reaches the SSF even at low speed. The critical rollover speed is always a quantity that comes "paired with a radius". Change the turn radius in this tool and you will see the critical speed of the same vehicle move dramatically.
Next, overlooking that the SSF is a rigid-body ideal. A real vehicle has suspension and tyres that compress like springs, so the body rolls outward during a turn. The CG then shifts outward and the rollover moment arm lengthens, so a real vehicle's inside wheels start to lift at a lower lateral acceleration than the SSF predicts. Cargo, passengers and a roof load also push the CG height h up, directly lowering the SSF. Understand that the catalogue SSF is for the unladen, standard condition, and assume even less margin when laden.
Finally, the complacency of thinking "a high SSF means rollover cannot happen". Statistically, the overwhelming majority of real rollover crashes are not pure cornering (on-track) events but "tripped" rollovers — the vehicle side-slips and the tyres catch a kerb, soft shoulder, guardrail or ditch, receiving a sudden rotational moment. In a tripped rollover, even a passenger car with a high SSF can roll over. Always keep in mind that the SSF is a metric for cornering rollover on a flat, smooth road — it is not a number that prevents side-slip or run-off-road events themselves.
How to Use
Enter track width in mm (e.g., 1520 mm for standard passenger vehicle) using the trackNum and trackRange inputs
Input center of gravity height in mm (e.g., 680 mm for sedan, 950 mm for SUV) via cgNum and cgRange
Set cornering speed in km/h and turn radius in meters to compute lateral acceleration and compare against critical rollover threshold
Review the Static Stability Factor (SSF) output—values above 1.4 indicate low rollover risk; below 0.9 indicates high risk
Check the Critical Rollover Speed result to see maximum safe speed for the given turn radius
Worked Example
A mid-size sedan: track width 1480 mm, CG height 560 mm, cornering at 80 km/h through a 100 m radius curve. Lateral acceleration = (80/3.6)² / 100 = 0.49 g. SSF = 1480 / (2 × 560) = 1.32. Critical rollover speed at 100 m radius = √(1.32 × 9.81 × 100) = 114 km/h. Safety margin = 114 / 80 = 1.43×. Verdict: Safe cornering within normal driving envelope.
Practical Notes
SUVs and pickup trucks (CG height 900–1100 mm) typically achieve SSF 0.85–1.1; narrower track widths amplify rollover risk on identical maneuvers compared to sedans
Load condition matters: placing cargo on roof racks raises CG by 80–150 mm and reduces SSF by 10–20%, significantly lowering critical speed thresholds
Adjust speedNum incrementally when exploring margin sensitivity; a 10% speed increase can reduce safety margin by 15–20% due to quadratic acceleration relationship
Track width increases (lowered suspension, wider wheels) improve SSF more effectively than modest CG lowering for production vehicles