Center of gravity
Also called centre of gravity, CoG, center of mass, CG height
A vehicle's center of gravity is the single point its whole weight acts through, and how high that point sits above the ground is the largest factor in whether the vehicle leans, tips or rolls.
What is center of gravity, and why does height matter?
Every part of a vehicle has weight, and the center of gravity is the one point you could balance all of it on. It sits somewhere around seat height in a stock truck. What matters off-road is not where it is exactly but how high it is, because height is the lever arm that turns a lateral force into a roll.
NHTSA reduces this to one number for its rollover ratings, the Static Stability Factor. In the agency's own words, the SSF for a vehicle is calculated as a ratio of one half its track width divided by the height of its center of gravity above ground. Widen the track and the vehicle gets harder to roll. Raise the center of gravity and it gets easier, and the relationship is direct: the number sits in the denominator.
NHTSA draws the obvious conclusion in the same paragraph. Trucks and sport utility vehicles generally have lower SSF values than passenger cars because of their higher centre of gravity. That is where a build starts, before anything has been bolted to the roof.
NHTSA has also measured what moving that number is worth. In its 2017 study of rollover reduction, each increase of 0.01 in SSF cut the odds of a first-event rollover in a single-vehicle crash by about 3.4 percent across all passenger cars and about 5.8 percent across light trucks and vans. A lift kit moves SSF the other way.
Why roof weight is the expensive kind
Because it has the longest lever. A hundred pounds in the bed sits below the existing center of gravity and pulls it down. The same hundred pounds on the roof sits four or five feet above it and pulls it up, and it does that whether you are using the thing or not.
This is the real argument behind roof rack weight limits, and it is a different argument from the one about the rack failing. The rack will usually hold. The vehicle is the part whose behaviour changes, and it changes on the road as well as off it, on every emergency swerve and every off-camber sidehill for the life of the build.
None of this makes a roof tent wrong. It makes it a choice with a price, and the price is paid in stability, not just in payload.
See also
Seen in
- Where should the weight go on an adventure motorcycle?
- Should you put your truck camper on a flatbed?
- What is GVWR, and how do you know if you are over it?
- How much weight can your roof carry?
- Higher, further back, and strapped loose: The Rolling Hobo's KTM 690 Enduro R
- Basil: KTM put the fuel low, Poskitt hung the luggage on it
- Too high and top-heavy: Chris Scott's Desert Riders XR650L, and what he would change
- Three 4x4 Sprinters: the drivetrain was the easy part
- The truck that got weighed, twice
- Buying 500 pounds and losing 30
- The cab they made smaller
Sources
- New Car Assessment Program (NCAP); Removal of Dynamic Rollover Resistance Test and Side Air Bag Out-of-Position Testing; Updates to NCAP's 10-Year Roadmap, Federal Register, 17 August 2026, NHTSA docket 2026-16735, read 2026-08-27States the Static Stability Factor formula in NHTSA's own words. SSF entered the program in 2001 (66 FR 3388, https://www.federalregister.gov/documents/2001/01/12/01-973/consumer-information-regulations-federal-motor-vehicle-safety-standards-rollover-resistance). Note that the rule this notice announces removes the dynamic rollover test from NCAP, which leaves SSF as the surviving rollover measure. We looked for a codified definition first: 49 CFR 575.303 is cited for this all over the web, and no section in that range exists in the current CFR. SSF is a New Car Assessment Program parameter, not codified anywhere.
- Trends and Rollover-Reduction Effectiveness of Static Stability Factor in Passenger Vehicles, DOT HS 812 444, National Highway Traffic Safety Administration, August 2017, read 2026-08-27Gives the formula as track width divided by twice the centre of gravity height, and the effect size per 0.01 of SSF.