Gravel vs Tarmac: The Science of Suspension Setup

Surface friction coefficient (μ) sets the upper limit of grip. Dry tarmac μ ≈ 0.8–1.0 versus gravel μ ≈ 0.4–0.6. On tarmac, the suspension's primary role is suppressing cornering roll and pressing tyres to the road at maximum camber — high spring rates and low rebound damping are the baseline. On gravel, surface irregularities send wheels airborne; the longer a wheel is off the surface the more grip is lost. Soft springs (for compliance over bumps) and strong rebound damping (to return the wheel quickly) prevent this. Japanese dirt trial gravel courses are typically wider and more uniformly surfaced than WRC gravel stages, so somewhat higher spring rates (20–26kg/mm) are viable.

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