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 using wheeled vehicles running in railway track, tracks, which usually consist of two parallel steel railway track, rails. Rail transport is one of the two primary means of ...

wheel shape. A flat spot occurs when a rail vehicle's wheelset stops rotating while the train is still in motion, causing part of the wheel to ablate against the hard steel of the rails. Flat spots are usually caused by use of the emergency brake, or slippery (low-adhesion) conditions that cause 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.

Consequences

NORAC guidelines state that the train may continue at normal speed if the flat spot is less than 2.5 inches long, or, in the case of multiple adjacent flat spots, each is shorter than 2 inches. 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 in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...

structure. If the flat spot is sufficiently large, 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 A railway brake is a type of brake used on the cars of railway trains to enable deceleration, control acceleration (downhill) or to keep them immobile when parked. While the basic principle is similar to that on road vehicle usage, operational ...

. 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 rail transport, a derailment is a type of train wreck that 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 sys ...

Automobile use

Engine response

automobile A car, or an automobile, is a motor vehicle with wheels. Most definitions of cars state that they run primarily on roads, Car seat, seat one to eight people, have four wheels, and mainly transport private transport#Personal transport, peopl ...

parlance, a flat spot occurs when the driver presses the

gas pedal "Gas Pedal" is a song by American rapper Sage the Gemini, featuring guest vocals from fellow Bay Area rapper Iamsu!. It was released by Republic Records on March 21, 2013 as the second single from the former's debut studio album, '' Remember Me ...

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

electronic fuel injection Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of a fuel injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines. All co ...

. 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 la ...

, 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 sports, 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 g ...

is locking the wheels during heavy braking.

References

Train wheels {{rail-transport-stub