Thecant of arailway track orcamber of a road (also referred to assuperelevation,cross slope orcross fall) is the rate of change in elevation (height) between the two rails or edges of the road. This is normally greater where the railway or road is curved; raising the outer rail or the outer edge of the road creates abanked turn, thus allowing vehicles to travel round the curve at greater speeds than would be possible if the surface were level.

Importance of superelevation

In curved railway tracks, the outer rail is elevated, providing a banked turn. This allows trains to navigate curves at higher speeds and reduces the pressure of the wheel flanges against the rails, minimizing friction and wear. The difference in elevation between the outer and inner rails is referred to as cant in most countries.

How superelevation works

The main functions of cant are the following:

• Improve distribution of the load across both rails
• Reduce wear on rails and wheels
• Neutralize the effect of lateral forces
• Improve passenger comfort

On horizontal curves, curvature causes acentrifugal force acting outward on the outer wheel. The smaller theradius of curvature, the greater the centrifugal force. Superelevation means that the outer edge of the track is raised relative to the inner edge. This results in a gravitational force acting in the opposite direction to the centrifugal force. This improves the distribution of the load across both rails, ensuring stability and safety for trains navigating the curve and improving passenger comfort.

This stability prevents the wheel flanges from touching the rails, minimizing friction, wear andrail squeal.

The necessary cant in a curve depends on the expected speed of the trains and theradius of curvature: the higher the speed, the greater the centrifugal force. However, the curve may use a compromise value, for example if slow-moving trains may occasionally use tracks intended forhigh-speed trains.

Generally the aim is for trains to run withoutflange contact, which also depends on thetire profile of the wheels. Allowance has to be made for the different speeds of trains. Slower trains will tend to make flange contact with the inner rail on curves, while faster trains will tend to ride outwards and make contact with the outer rail. Either contact causeswear and tear and may lead toderailment. Many high-speed lines do not permit slower freight trains, particularly with heavieraxle loads. In some cases, the impact is reduced by the use of flangelubrication.

Ideally, the track should have sleepers (railroad ties) at a closer spacing and a greater depth ofballast to accommodate the increased forces exerted in the curve.

At the ends of a curve, where the rails straighten out, the amount of cant cannot change from zero to its maximum immediately. It must change (ramp) gradually in atrack transition curve. The length of the transition depends on the maximum allowable speed; the higher the speed, the greater length is required.

For the United States, with a standard maximum unbalanced superelevation of 75 mm (3 in), the formula is this:

${\displaystyle v_{max}=\sqrt{\frac{E_a+3}{0.00066d}}}$

where${\displaystyle E_a}$ is the superelevation in inches,${\displaystyle d}$ is the curvature of the track in degrees per 100 feet, and${\displaystyle v_{max}}$ the maximum speed in miles per hour.

The maximum value of cant (the height of the outer rail above the inner rail) for astandard gauge railway is approximately 150 mm (6 in).^{[citation needed]} For high-speed railways in Europe, maximum cant is 180 mm (7 in) when slow freight trains are not allowed.^[1]

Track unbalanced superelevation (cant deficiency) in the United States is restricted to 75 mm (3 in), though 102 mm (4.0 in) is permissible by waiver. The maximum value for European railways varies by country, some of which have curves with over 280 mm (11 in) of unbalanced superelevation to permit high-speed transportation. The highest values are only fortilting trains, because it would be too uncomfortable for passengers in conventional train cars.^[2]

Ideally, the amount of cant${\displaystyle E_a}$, given the speed${\displaystyle v}$ of a train, the radius of curvature${\displaystyle r}$ and the gauge${\displaystyle w}$ of the track, the relation

${\displaystyle v^2=\frac{E_{a}rg}{\sqrt{w^2-E_a^2}}\approx \frac{E_{a}rg}{w}}$

must be fulfilled, with${\displaystyle g}$ the gravitational acceleration. This follows simply from a balance between weight, centrifugal force, and normal force (the horizontal component of the tilted gravitational force). In the approximation it is assumed that the cant is small compared to the gauge of the track. It is often convenient to define theunbalanced cant${\displaystyle E_u}$ as the maximum allowed additional amount of cant that would be required by a train moving faster than the speed for which the cant was designed, setting the maximum allowed speed${\displaystyle v_{max}}$. In a formula this becomes

${\displaystyle v_{max}\approx \sqrt{\frac{(E_a+E_u)rg}{w}}=\sqrt{\frac{(E_a+E_u)g}{dw}}}$

with${\displaystyle d=1/r}$ the curvature of the track, which is also the turn in radians per unit length of track.

In the United States, maximum speed is subject to specific rules. When filling in${\displaystyle g=32.17\mathrm{f}\mathrm{t}/{\mathrm{s}}^2}$,${\displaystyle w=56.5\mathrm{i}\mathrm{n}}$ and the conversion factors for US customary units, the maximum speed of a train on curved track for a given cant deficiency or unbalanced superelevation is determined by the following formula:

${\displaystyle v_{max}\approx \frac{3600}{63360}\sqrt{\frac{32.17\cdot 12(E_a+E_u)}{56.5\cdot d\frac{\pi }{1200\cdot 180}}}\approx \sqrt{\frac{E_a+E_u}{0.00066d}}}$

with${\displaystyle E_a}$ and${\displaystyle E_u}$ in inches,${\displaystyle d}$ thedegree of curvature in degrees per 100 feet and${\displaystyle v_{max}}$ in miles per hour.

In Australia, theAustralian Rail Track Corporation is increasing speed around curves sharper than an 800-metre (2,625 ft) radius by replacing wooden sleepers with concrete ones so that the cant can be increased.^[3]

The rails themselves are now usually canted inwards by about 5 to 10 percent.

In 1925 about 15 of 36 major American railways had adopted this practice.^[4]

Incivil engineering, cant is often referred to ascross slope or camber. It helps rainwater drain from the road surface. Along straight or gently curved sections, the middle of the road is normally higher than the edges. This is called "normal crown" and helps shedrainwater off the sides of the road. During road works that involve lengths of temporary carriageway, the slope may be the opposite to normal – for example, with the outer edge higher – which causes vehicles to lean towards oncoming traffic. In the UK, this is indicated on warning signs as "adverse camber".

On more severe bends, the outside edge of the curve is raised, orsuperelevated, to help vehicles around the curve. The amount of superelevation increases with itsdesign speed and with curve sharpness.

Anoff-camber corner is described as the opposite of abanked turn, or a negative-bank turn, which is lower on the outside of a turn than on the inside.^[5][6] Off-camber corners are both feared and celebrated by skilled drivers.^[7][8] Handling them is a major factor in skilled vehicle control, both single-track and automotive; both engine-powered and human-powered vehicles; both on and off closed courses; and both on and off paved surfaces.^{[citation needed]}

On race courses, they are one of a handful of engineering factors at the disposal of a course designer in order to challenge and test drivers' skills.^[9] Off-camber corners were described by a training guide for prospective racers as "the hardest corners you will encounter" on the track.^[10] Many notable courses such asRiverside International Raceway combine off-camber corners with elevation and link corners for extra driver challenge.^[11]

On the street, they are a feature of some of the world's most celebrated paved roads, such as The"Dragon" (US 129) through Deals Gap^[12] and the"Diamondback" (NC 226A) in North Carolina,^[13]Route 78 in Ohio,^[14]Route 125 in Pennsylvania,^[15]Route 33 in California,^[16] and Betws-y-Coed Triangle inSnowdonia National Park in Wales.^[17]

To mountain bikers and motorcyclists on trails and dirt tracks, off-camber corners are also challenging, and can be either an engineered course feature, or a natural feature of single-track trails.^{[18][19][20][21]} Incyclocross, off-camber sections are very common as the courses snake around ridges, adding difficulty.

Camber in virtual race circuits is carefully controlled byvideo gamerace simulators to achieve the designer's desired level of difficulty.^[9]

• Banked turn
• Camber angle
• Profilograph
• Tilting train

• Bosworth, Brian; Sanders, Michael (2003).Destination Highways: Washington. Vancouver, B.C.: Twisted Edge Publishing.ISBN 978-0968432815. — includes camber in evaluating engineering of roads, one of six numerical factors modeled to determine desirability formotorcycle touring

• Track geometry
• Road infrastructure

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