Cant deficiency

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The term "cant deficiency" is defined in the context of travel of a rail vehicle at constant speed on a constant radius curve. Cant itself is a British synonym for the superelevation of the curve, that is, the elevation of the outside rail minus the elevation of the inside rail. Cant deficiency is present when a vehicle's speed on a curve is greater than the speed at which the components of wheel to rail force normal to the plane of the track would be the same in aggregate for the outside rails as for the inside rails.

The forces that bear on the vehicle in this context are illustrated in the following figure.

A vehicle's motion at speed v along a circular path embodies centripetal acceleration of magnitude v^2 / R toward the center of the circle, the curvature of that path being 1 / R where R is the radius of the circle. 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 M * v^2 / R where M is the mass of the vehicle.

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.

The component normal to the track acts together with the much larger component of gravitational force normal to the track and is generally neglected. It can slightly increase the vertical load seen by the vehicle suspension but it does not create lateral acceleration as perceived by passengers or and does not cause lateral deflection of the vehicle suspension.

The track is superelevated so that the component of the acceleration of gravity in the plane of the track will provide some fraction of the horizontal acceleration in the plane of the track due to the circular motion. 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 balance equation,

      ( V^2  / R )  cos( bank_angle )   =   g * sin( bank_angle )   ,

is satisfied. For a given curve radius and bank angle (i.e., superelevation) the speed V that satisfies the balance equation is called the balancing speed and is given by

       Vbal   =    ( R * g * tan( bank_angle ) )^1/2.

For reasons that will be mentioned below, passenger vehicles usually traverse a curve at a speed higher than the balance speed. The amount by which the actual speed exceeds the balance speed is conveniently expressed via the so-called cant deficiency, i.e., by the amount by which the superelevation would need to be increased to raise the balance speed to the speed at which the vehicles actually travel. Letting gauge_se denote the rail gauge from low rail gauge side corner to high rail field side corner, letting super_el 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

       CD  =  gauge_se  / ( 1 + R^2 * g^2 / Vact^4)1/2   -  super_el.

Taking an example, a curve with curvature 1.0 degree per 100 ft chord (radius 5,729.65 ft = 1,746.40 m), gauge_se = 59.5 inches (1511.3 mm), and superelevation 6.0 inches (152.4 mm) will have

  Vbal   =    (1746.4 * 9.80665 * tan( asin( 152.4 /1511.3 ) ) )1/2   
         =   41.6638  meters/s  =   149.99 km/h  =   93.20  miles/h.

If a vehicle traverses that curve at a speed of 125 mph = 201.17 km/h = 55.880 meters/s, then the cant deficiency will be

     CD  =  4.67 inches  =  118.7  mm.

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. This is to lessen the tendency of heavy wheel loads to crush the head of either rail.

For passenger traffic superelevations and authorized speeds can be set so that trains run with as much cant deficiency as is allowed based on safety, on relevant regulations, and on passenger comfort. As of 2007 the FRA regulations limit CD to 7.0 inches for tilting passenger vehicles, but work is underway to see if this limit can be safely increased to 9.0 inches. (In England, where axle loads are typically lower than those in the USA, tilting trains are allowed to operate with 12.0 inches CD in some cases. Russia, as an example, could allow greater CD in light of its wider track gauge.) Allowed CD could be set below the value that would be allowed based on safety in order to reduce wheel and rail wear and to reduce the rate of degradation of geometry of ballasted track. Choice of design CD will be less constrained by passenger comfort in the case of vehicles that have so-called tilting capability. One historical approach to determining safe cant deficiency was the requirement that the projection to the plane of the track of the resultant of the centrifugal and gravitational forces acting on a vehicle fall within the middle third of the track gauge. Contemporary engineering studies would likely use vehicle motion simulation including cross wind conditions to determine margins relative to derailment and rollover.

If the superelevation determined for a dedicated passenger route curve on regulatory and safety bases is below 6.0 inches (152.4 mm) it may be desirable to increase the superelevation and reduced the cant deficiency. However, if on such a curve some trains regularly travel at low speeds, then raising the superelevation may be inadvisable for passenger comfort reasons.

On a mixed traffic route owned by a freight rail company, freight considerations are likely to prevail. On a mixed traffic route owned by a passenger rail company some kind of compromise may be needed.

Cant deficiency is generally looked at with respect to ideal track geometry. As geometry of real track is never perfect it may be desirable to supplement the static considerations laid out above with simulations of vehicle motion over measured geometries of actual tracks. Simulations are also desirable for understanding vehicle behaviour traversing spirals, turnouts, and other track segments where curvature changes with distance by design. Where simulations or measurements show non-ideal behaviour traversing traditional linear spirals, results can be improved by using advanced spirals. Good track geometry including advanced spirals is likely to foster passenger acceptance of higher CD values.