Tyre Contact Area

Load divided by pressure gives area. That part is trivial. What is not trivial is the pressure you divide by, and this is one of the most reliably misstated numbers in pavement engineering.

Contact pressure is usually LOWER than inflation pressure, not higher. Generations of textbooks repeat a rule attributed to Yoder: take the contact pressure as the inflation pressure plus about ten per cent, on the reasoning that the tyre wall is in tension and so must add to the air's push. Van Vuuren tested that directly and reported the results in Transportation Research Record 523 (TRB, 1974, pp. 76-87). He measured contact pressures running as much as 30% below inflation pressure and described the assumption as completely unacceptable.

The mechanism is straightforward once you stop thinking of a tyre as a balloon. A pneumatic tyre is a structure: its carcass plies, its belts and above all its sidewalls carry a real share of the vertical load in bending and tension. The air is not doing all the work, so the pavement does not feel the full inflation pressure over the footprint. There is a transition — load a tyre heavily enough relative to its inflation and contact pressure climbs above inflation pressure — but where that transition sits depends on the tyre's construction, and bias-ply and radial tyres cross it at different points. Which is exactly why this page takes contact pressure as an input rather than deriving it from a rule.

If all you have is a sidewall pressure, say so to yourself and treat the resulting area as an estimate with real uncertainty attached, not a measurement. If you can get a footprint, do: ink and paper under a known load on a level floor still works, and pressure-sensitive film works better. Then run the equation the other way and let the pavement tell you what pressure it is feeling.

The patch is also not uniform, and the equation gives an average. Real pressure distributions across a truck tyre footprint are lumpy — higher under the belt edges on a radial, higher through the middle on an over-inflated bias-ply — and they change with speed, camber and cornering. What this calculation produces is the mean over the gross contact area, which is what layered-elastic pavement models want as an input and is not what any particular square centimetre of road actually experiences. The peaks matter for surface distress; the average matters for what happens deeper down.

Finally, the scale gap that hangs over all of this. The AASHO Road Test ran bias-ply tyres at 75 to 80 psi. Modern highway trucks run radials at 85 to 145 psi. Higher pressure over a smaller patch concentrates the load nearer the surface, which is where thin asphalt layers crack, and the axle-load equivalency factors inherited from the Road Test cannot see any of it — they know the axle weight and nothing else. The same blindness applies to wide-base single tyres replacing duals: same axle weight, same equivalency factor, more concentrated contact, and a real argument in the literature about whether the pavement notices. It does.