A flat spot, or wheel flat, also called spalling or shelling, is a fault in

railroad Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in tracks. In contrast to road transport, where the vehicles run on a prep ...

wheel shape. A flat spot occurs when a rail vehicle's wheelset is dragged along the rail after the wheel/axle has stopped rotating. Flat spots are usually caused by use of the emergency brake, or slip and slide conditions that causes wheels to lock up while the train is still moving. Flat spots are more common in the autumn and winter when the rails are slippery. Flat spots can also be caused by faulty brakes or wheelset bearings.

Consequences

If the flat spot is very small, the rail vehicle will be able to continue being used. The fault is removed later in the wheelset turning process, using a wheel lathe. However, because of the heat suffered while being dragged along the rail and the impacts suffered afterward, these wheels are more likely to break due to changes in the

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductili ...

structure. If the flat spot is very large (as in the picture), strands of molten metal may have stuck on one side of the flat spot, making it impossible for the wheel to turn due to insufficient clearance between the rolling surface and the brake block. In this case, the wheelset must be replaced immediately. In extreme cases such as the

1971 Salem, Illinois derailment The 1971 Salem, Illinois derailment occurred on June 10, 1971, when Amtrak’s ''City of New Orleans'' passenger train derailed near Salem, Illinois. It is sometimes referred to as the Tonti derailment, after the unincorporated community of T ...

a wheel with an untreated flat spot can damage the

track Track or Tracks may refer to: Routes or imprints * Ancient trackway, any track or trail whose origin is lost in antiquity * Animal track, imprints left on surfaces that an animal walks across * Desire path, a line worn by people taking the shorte ...

and cause a

derailment In rail transport, a derailment occurs when a rail vehicle such as a train comes off its rails. Although many derailments are minor, all result in temporary disruption of the proper operation of the railway system and they are a potentially ...

Automobile use

Engine response

automobile A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people instead of goods. The year 1886 is regarded ...

parlance, a flat spot occurs when the driver presses the

accelerator pedal Car controls are the components in automobiles and other powered road vehicles, such as trucks and buses, used for driving and parking. While controls like steering wheels and pedals have existed since the invention of cars, other controls have ...

and there is a delay in the engine's response. This fault was more common before cars had

electronic fuel injection Manifold injection is a mixture formation system for internal combustion engines with external mixture formation. It is commonly used in engines with spark ignition that use petrol as fuel, such as the Otto engine, and the Wankel engine. In a man ...

. Many engines, especially modified ones, still have flat spots in the torque curve due to resonances in the intake system, although manufacturers try to eliminate these by use of a

plenum chamber A plenum chamber is a pressurised housing containing a fluid (typically air) at positive pressure. One of its functions is to equalise pressure for more even distribution, compensating for irregular supply or demand. It is typically relatively l ...

, careful design and testing.

Car tires

A literal flat spot can occur on car tires if the vehicle is parked without moving for some time (generally longer than a week), and the tire deformation at the bottom of the wheel becomes semi-permanent. The flat spot gradually relieves itself when the car is driven but can temporarily give similar symptoms to an unbalanced wheel. Cars being laid up for extended periods, or intermittently-used caravans and trailers, should be kept on axle stands (tyres not in contact with the ground) or have the tires over-inflated to eliminate or reduce this problem. "Tire savers", curved wheel stands, are also available for use during storage. These reduce or avoid the problem by cradling the lower part of the tire tread and preventing the usual deformation where it rests on the ground. Another cause seen frequently in

racing In sport, racing is a competition of speed, in which competitors try to complete a given task in the shortest amount of time. Typically this involves traversing some distance, but it can be any other task involving speed to reach a specific go ...

is locking the wheels during heavy braking.

References

Train wheels {{rail-transport-stub