Flat spot

A flat spot, or wheel flat, is a fault in railroad wheel shape. A flat spot occurs when a 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.

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 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 such as the 1971 Salem, Illinois derailment a wheel with an untreated flat spot can damage the track and cause a derailment.

In automobile parlance, a flat spot occurs when the driver presses the accelerator pedal and there is a delay in the engine's response. This fault was more common before cars had electronic fuel injection. 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, careful design and testing.

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Wikipedia