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Wet and slippery surfaces

Grip depends on the surface. On dry tarmac the friction coefficient is about 0.8, on wet tarmac often around 0.5, and on road markings, manhole covers, rails and leaves lower still. The first rain after a dry spell is extra slippery, because oil and dust float up. Slow down and increase your gap when it is wet.

s_{\text{brake}} = \dfrac{v^2}{2 \mu g}

the braking distance

Symbols

$v$	speed	m/s
$\mu$	friction coefficient
$g$	gravitational acceleration, 9.81	m/s²

Example

You brake from 60 km/h ( $v \approx 16.7$ m/s).

Dry: $s = \dfrac{16.7^2}{2 \cdot 0.8 \cdot 9.81} \approx 18$ m. Wet: $s = \dfrac{16.7^2}{2 \cdot 0.5 \cdot 9.81} \approx 28$ m.

Ride over road markings and manhole covers with the bike upright, without braking, accelerating or turning.

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Part of Motorcycle licence (A1, A2, A): Grip, road surface and weather.