A Flat spot, or Wheel flat, is a fault in railroad wheel shape. A flat spot occurs when a rail vehicle's

Rolling stock

Rolling stock comprises all the vehicles that move on a railway. It usually includes both powered and unpowered vehicles, for example locomotives, railroad cars, coaches and wagons...

 wheelset

Wheelset (railroad)

A wheelset is the wheel-axle assembly of a railroad car. The frame assembly beneath each end of a car, railcar or locomotive that holds the wheelsets is called the bogie...

 is dragged along the rail after the wheel/axle has stopped rotating. Flat spots are usually caused by faulty brakes or wheelset bearings. It can also be caused by use of the emergency brake

Emergency brake

On trains, the expression emergency brake has several meanings:* The maximum brake force available to the driver/engineer from his conventional braking system, usually operated by taking the brake handle to its furthest postion, through a gate mechanism, or by pushing a separate plunger in the cab*...

, or slip and slide

Slippery rail

Slippery rail is a condition of railways caused by fallen moist leaves that lie on and cling to the top surface of the rails of railway tracks. The condition results in significant loss of friction between train wheels and rails, and in extreme cases can render the track temporarily unusable...

 conditions that causes wheels to lock up while the train is still moving.

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

Alloy

An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...

 structure.

If the flat spot is very large (as in the picture), strands of molten metal may have got 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, a wheel with an untreated flat spot can damage the track

Rail tracks

The track on a railway or railroad, also known as the permanent way, is the structure consisting of the rails, fasteners, sleepers and ballast , plus the underlying subgrade...

 and cause a derailment

Derailment

A derailment is an accident on a railway or tramway in which a rail vehicle, or part or all of a train, leaves the tracks on which it is travelling, with consequent damage and in many cases injury and/or death....

.

Automobile use

In automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 parlance, a flat spot occurs when the driver presses the accelerator pedal

Throttle

A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...

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