Ground clearance

Also called running ground clearance, belly clearance, diff clearance

Ground clearance is the distance from the ground to the lowest fixed point under a vehicle, which means it is set by whatever hangs lowest rather than by how tall the tires are or how much lift is fitted.

What is ground clearance?

The gap between the ground and the lowest thing bolted to the vehicle. On a solid-axle truck that is almost always the differential housing, sitting a few inches off-centre on the axle tube. On an independent-front-suspension truck it is the front differential, the lower control arms, or a skid plate under one of them. On a lot of modern trucks it is a plastic air dam nobody thinks about until it is torn off.

It is the single number people quote when comparing vehicles, and it is the one that misleads most, because it describes one point and off-road you get stopped by the whole underside.

The thing lift does not do

A suspension lift raises the body and the frame away from the axle. On a solid-axle vehicle it does not raise the differential at all, because the differential is part of the axle and the axle is still sitting on the tires. You get more clearance for the frame, the exhaust and the fuel tank, and exactly zero more under the diff.

Bigger tires do raise it, and this is the part worth committing to memory: going up an inch in tire diameter raises the axle centreline by half an inch, because the increase is split between the top and the bottom of the tire. Thirty-three inch tires in place of thirty-ones is one inch of extra diameter and half an inch of extra clearance under the diff.

United States federal law splits it a different way and the second half is the one that matters off-road. Running clearance means the distance from the surface on which an automobile is standing to the lowest point on the automobile, excluding unsprung weight. Axle clearance means the vertical distance from that surface to the lowest point on the axle differential. The first is the belly. The second is the pumpkin, and it is the number a suspension lift does not change.

Those definitions come with thresholds. To be classified as an off-road vehicle in the EU, a car or light commercial vehicle needs at least 180 millimetres under the front axle, 180 under the rear, and 200 between them, together with the approach, departure and ramp angles, four-wheel drive, a differential lock or equivalent, and the ability to climb a 25 percent gradient. Five of those six geometric criteria have to be met, not all six, which is the regulation quietly admitting that every vehicle is a compromise.

Clearance is also only a third of the geometry. A vehicle with enormous belly clearance and a long wheelbase still high-centres on a ridge, and one with a low air dam still stops at the bottom of a climb. Approach, departure and breakover angles are the other three quarters of the picture.