Cant deficiency

In railway engineering, cant deficiency is defined in the context of travel of a rail vehicle at constant speed on a constant-radius curve.

The amount of cant deficiency is expressed in terms of required superelevation to be added in order to bring the resultant force into balance between the two rails.

This centripetal acceleration is produced by horizontal forces applied by the rails to the wheels of the vehicle, directed toward the center, and having sum equal to ⁠

The net horizontal force producing the centripetal acceleration is generally separated into components that are respectively in the plane of the superelevated (i.e., banked) track and normal thereto.

Referring to the figure above, it can be seen that the components of gravitational and centripetal acceleration in the plane of the track will be equal when the following balance equation is satisfied, where α is the bank angle.

Letting GE denote the rail gauge from low rail gauge side corner to high rail field side corner, letting SE denote the actual superelevation, and letting Vact denote the actual speed, it follows from the definition that the cant deficiency, CD, is given by the formula Taking an example, a curve with curvature 1.0 degree per 100 ft chord (radius 1,746.40 m = 5,729.65 ft), GE = 1511.3 mm (59.5 inches), and SE = 152.4 mm (6.0 inches) will have If a vehicle traverses that curve at a speed of 55.880 m/s (= 201.17 km/h = 125 mph), then the cant deficiency will be On routes that carry freight traffic in cars with the maximum allowed axle loads it will be desirable to set superelevations so that the balancing speed of each curve is close to the speed at which most such traffic runs.

Contemporary engineering studies would likely use vehicle motion simulation including cross wind conditions to determine margins relative to derailment and rollover.

However, if on such a curve some trains regularly travel at low speeds, then raising the superelevation may be inadvisable for passenger comfort reasons.

Simulations are also desirable for understanding vehicle behaviour traversing spirals, turnouts, and other track segments where curvature changes with distance by design.

Approval is governed by conditions outlined in CFR chapter 49 section 213.329 part (d)[3] and based on the idea that the car cannot unload the inside wheel on a curve by more than 60% of static loading.