11 N M

where ^ = coefficient of friction r = brake efficiency F = friction force N = normal load.

Thus a braking efficiency of 100% is equal to a coefficient of friction of one.

11.1.3 Determination of brake stopping distance (Fig. 11.1)

A rough estimate of the performance of a vehicle's brakes can be made by applying one of the equations of motion assuming the brakes are 100%.

i.e. V2 = U2 + 2gs where U = initial braking speed (m/s) V = final speed (m/s) g = deceleration due to gravity

If the final speed of the vehicle is zero (i.e. V = 0)

Example Calculate the minimum stopping distance for a vehicle travelling at 60 km/h.

Stopping distance s = 0.004 U2(m)

11.1.4 Determination of brake efficiency (Fig. 11.1)

The brake efficiency can be derived from the kinetic energy equation and the work done in bringing the vehicle to a standstill.

M = coefficient of friction, W = vehicle weight (N), U = initial braking speed (m/s), m = vehicle mass (kg), s = stopping distance (m), V = brake efficiency.

Then equating work and kinetic energy,

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