Reasons for grade compensation

Grade compensation is the reduction of the gradient on a sharp horizontal curve. The reasons are:

1. Curve resistance: on a curve the vehicle meets extra tractive resistance due to lateral friction, tyre slip and steering. If the full ruling gradient also exists there, the total resistance may be more than the engine can overcome, causing loss of speed or stalling.
2. Equal total resistance: the gradient is reduced by an amount equal to the curve resistance, so that the total tractive effort needed on the curve is the same as on the straight with the ruling gradient.
3. Safety and uniform speed: vehicles keep a uniform speed, driving is easier and the accident risk is reduced, especially for heavy trucks.
4. Hill roads: where hairpin bends and sharp curves lie on long steep grades, heavy vehicles cannot climb without compensation.

IRC rule: compensation $= \dfrac{30+R}{R}\%$, not exceeding $\dfrac{75}{R}\%$ (R in m); it is not needed for gradients flatter than 4%.