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.