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Vehicle Safety
Vehicle Roll Stability SSF (NHTSA) Simulator
Compute the NHTSA Static Stability Factor SSF = (Track Width / 2) / CG Height and compare it against the lateral acceleration produced by the current cornering condition. Adjust vehicle class and loading to see the 5-star rating, maximum safe cornering speed, and tip-first vs slide-first failure mode update in real time.
Parameters
Vehicle class
Preset reference. Sliders override individual values
Track width T
mm
CG height H_CG
mm
Higher CG (SUV / truck) → easier to roll
Wheelbase
mm
Vehicle mass
kg
Cornering speed
km/h
Turn radius R
m
Loading condition
Roof load raises CG significantly
Results
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SSF (Static Stability Factor)
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NHTSA rating
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Lateral acceleration (g)
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Safety margin (g)
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Max safe speed (km/h)
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Failure mode
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Front view — roll moment visualisation
Shows track width T, CG height H_CG, the centrifugal force vector during cornering, and the roll moment pivoting about the outer wheel. Low SSF (high CG) means a small lateral-g already topples the vehicle.
a_lat: current lateral acceleration in g. Positive margin = stable; negative = rollover. The dynamic threshold including suspension and tyre compliance is about SSF × 0.85.
Vehicle Roll Stability SSF — Static Stability Factor NHTSA
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Why do SUVs roll over so much more often than regular sedans?
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One-word answer: they sit high, so the centre of gravity (CG) is high. An SUV or pickup has 200+ mm of ground clearance and a CG that is 600-800 mm above the road. A sedan is closer to 500 mm. Even with the same track width, that CG difference feeds straight into a number called the Static Stability Factor. NHTSA defines SSF = (half the track width) / (CG height), and that number is also the minimum lateral acceleration, in g, needed to start a rollover. So SSF=1.0 means about 1g of lateral force (roughly the size of gravity itself) is enough to lift the outer wheels.
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OK. So how high does SSF need to be for "safe"? The tool shows a star rating.
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NHTSA's 5-star scheme is: SSF ≥ 1.45 → 5 stars, 1.35-1.44 → 4 stars, 1.25-1.34 → 3 stars, 1.15-1.24 → 2 stars, less than 1.15 → 1 star. Low sports cars sit above 1.5. The first-generation SUVs (the 1980s Suzuki Samurai or the early-1990s Ford Bronco II) were only 1.05-1.10, and that became a social-cost issue with thousands of fatal rollovers per year in the US. Even today over 10,000 US road deaths a year involve rollover, and SSF is the single most practical predictor.
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The "Failure mode" stat says "Tip-first" or "Slide-first". What does that mean?
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It is the question of "which limit comes first". Tyre friction μ is about 0.8 on dry pavement, so the highest lateral acceleration tyres can give you is around 0.8g. If SSF > 0.8, the tyres lose grip first and the car slides out (slide-first). That is controllable with ESC and the crash outcome is usually milder. If SSF < 0.8 (the old high-CG SUVs), the tyres are still gripping when the vehicle starts to roll — tip-first. That is the lethal mode. Modern passenger cars are deliberately designed to be slide-first.
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When I switch "Loading condition" to "With roof load", SSF drops a lot. Does a roof box really hit it that hard?
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It hits surprisingly hard. A roof rack is at the very top of the vehicle, so even 50 kg up there shifts the overall CG up by 30-80 mm. This tool applies a 0.80 multiplier as a rule of thumb, but real-vehicle tests show 15-25% drops in effective SSF. An SUV with SSF=1.21 (2 stars) and a permanent roof tent drops to effective SSF ≈ 0.97 — 1-star territory, where a single steering input can roll the car. North American camping accessory makers are required to ship warning labels for that reason. Switch the class to "Pickup" and add roof load in the sliders, and you will see the bars move into the danger zone.
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What can the vehicle side actually do about it?
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Three big levers. (1) Lower the CG — EVs that pack the battery into the floor get high SSF for free (Tesla Model X is about 1.4). (2) Widen the track — wide-body kits or flared fenders increase T. (3) Active control — ESC (Electronic Stability Control) is mandatory on US new cars since 2012 and cut rollover fatalities by about 50%, and Mercedes ABC / Active Roll Control reduces body roll dynamically. But unless you raise SSF itself, the underlying limit does not move, so the most effective decision is at the buying and loading stage.
Frequently Asked Questions
SSF = (Track Width / 2) / CG Height. Enter track width T and centre-of-gravity height H_CG in mm and the dimensionless index represents the minimum lateral acceleration, in g, at which the outer wheels start to lift. For T=1600mm and H_CG=660mm, SSF=1.21, meaning roughly 1.21g of lateral acceleration is needed to begin tip-up. SUVs and pickups have a high CG, so their SSF is lower and their rollover risk is higher than sedans.
NHTSA's Rollover Resistance Rating combines SSF with a dynamic fish-hook test. This tool uses SSF only: SSF ≥ 1.45 → 5 stars, 1.35-1.44 → 4 stars, 1.25-1.34 → 3 stars, 1.15-1.24 → 2 stars, <1.15 → 1 star. A 1-star vehicle has about 4× the rollover rate of a 5-star vehicle in real-world crashes, and the rating is widely used by US buyers when choosing SUVs.
Heavy loads on a roof rack raise the centre of gravity sharply and effectively lower SSF by 15-25%. This tool applies a 0.80 multiplier in the 'roof-loaded' mode, so an SUV with SSF=1.21 drops to an effective 0.97 — a 1-star equivalent and a zone where evasive steering can easily cause rollover. Always check the SSF impact of long-term loads such as ski boxes, roof tents and cargo carriers.
When cornering past the limit, a vehicle with SSF < μ (tyre friction, about 0.8 on dry pavement) tips first — the outer wheels lift and rollover begins. A vehicle with SSF > μ slides first — the tyres lose grip before rollover. Sliding can be controlled by ESC, but rollover is hard to recover. Since the 1990s passenger cars are designed for SSF ≥ 1.3 so that they slide before they tip, and this tool reports the failure mode directly.
Real-World Applications
SUV and pickup safety design: Since the Ford Explorer / Firestone rollover crisis (2000 recall, 271 deaths) SSF has been a buying-side safety metric. New vehicle programmes target SSF ≥ 1.30 internally; when the architecture cannot reach it, designers lower the battery, drop suspension mounts and widen the track to compensate. Tesla Model X distributes its battery across the floor and reaches SSF ≈ 1.4 — exceptional for an SUV-class body.
Commercial van and RV upfit planning: Adding roof tents, ladders or large rooftop AC units to a base van requires recalculating CG height. In North America, fleet operators increasingly require upfitters to submit an SSF estimate before delivery, and tools like this one are used for the go/no-go check. An empty SSF=1.25 commercial van plus 100 kg on the roof can fall below SSF=1.0.
Bus and tanker truck rollover countermeasures: Transit buses and hazmat tankers sit at SSF=0.6-0.9 and tip easily on sharp ramps. Over 70% of tanker accidents are single-vehicle rollovers, and liquid sloshing further lowers the dynamic SSF. Baffle plates inside the tank and stronger stabiliser bars are common fixes, and modern fleets use Roll Stability Control (RSC) where the ECU continuously estimates SSF and actively reduces speed.
Race cars and motorsport: F1, GT and rally cars sit at SSF=1.8-2.5 and routinely sustain 5g of cornering without rolling. That comes from flat floors, very wide aero packages and ultra-low engine packaging. Conversely, the truck class in rally-raid events (e.g., Dakar) lives near SSF=1.1, where in-race rollovers are common and the roll cage is the last line of defence.
Common Misconceptions and Pitfalls
First, "high SSF means safe in every situation" is false. SSF is a static (stationary) metric; in real crashes suspension travel, tyre slip and roll-bar response push the effective dynamic threshold down to about SSF × 0.80-0.90. This tool already includes that factor, but on snow or gravel tyre grip drops before body roll grows, and rollover can begin at lower g than expected. SSF=1.3 does not authorise hard steering plus braking on a snowy road.
Second, "all SUVs are dangerous" is also wrong. 2020s SUVs with optimised suspension and standard ESC reach safety levels comparable to sedans. Latest NHTSA statistics show post-2015 SUV rollover fatality rates at one-third or less of 1990s SUVs. Look at SSF together with ESC, curtain airbags and roof strength when judging total safety. When buying a used SUV, check both SSF and the presence of ESC.
Finally, "tip-first does not equal instant rollover". Real-world rollover triggers are mostly (1) curb contact, (2) road-surface irregularities, and (3) simultaneous braking and steering — all of which transiently push beyond the SSF limit. Rollover from a clean 60 km/h cornering on flat tarmac is rare; most cases are "trip-overs" from striking a curb or being pulled by gravel. Even when the SSF margin is positive, staying inside the lane and away from curbs is just as important as raising SSF itself.
How to Use
Enter track width (distance between tire centerlines left-to-right) in mm—typical sedans 1500–1600 mm, SUVs 1650–1800 mm
Input CG height (vertical distance from ground to vehicle center of gravity) in mm—sedans typically 450–550 mm, SUVs 650–800 mm
Enter wheelbase and vehicle mass; simulator calculates SSF = (Track Width / 2) / CG Height and compares against NHTSA critical threshold of 1.4
Review lateral acceleration limit, safety margin, and maximum safe speed before rollover risk increases
Worked Example
Mid-size SUV with track width 1720 mm, CG height 720 mm, wheelbase 2850 mm, mass 1800 kg: SSF = (1720 / 2) / 720 = 860 / 720 = 1.194. This falls below NHTSA's 1.4 minimum, indicating elevated rollover risk. Lateral acceleration threshold ~0.62 g. Maximum safe speed approximately 68 km/h in emergency evasive maneuvers before tipping. Compared to a sedan (track 1550 mm, CG 500 mm): SSF = 1.55, safer lateral acceleration limit ~0.78 g, max speed ~85 km/h.
Practical Notes
Higher SSF (above 1.4) indicates greater resistance to rollover; vehicles below 1.35 require driver awareness on banked curves and lane-change avoidance
CG height increases with roof racks, cargo loads, or aftermarket suspension lifts—recalculate SSF when modifying vehicle configuration
Lateral acceleration varies with tire grip, road surface, and suspension stiffness; NHTSA threshold assumes standard highway tires on dry asphalt
Failure mode typically progresses: load transfer → wheel lift → rollover; safety margin shows cushion above critical lateral acceleration