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Fishtailing (also called "tank slapping") is a car handling problem which occurs in rear wheel drive vehicles when too much power is applied for the driving conditions. This is similar to power over-steer but applies with power for a longer period or when the vehicle is essentially travelling in a straight line rather than in a curve. Typically this applies to low friction roads due to debris (eg. sand, gravel), or bad weather conditions (rain, snow, or ice). Cars with sufficiently great power to lose traction can be subject to fishtailing on any surface. During fishtailing, the rear end of the car skids to one side, which must be offset by the driver by turning the front wheels in the same direction, steer into the skid, (e.g. left if the rear swings left) and reducing engine power. Over-correction will result in a skid in the opposite direction, - hence the name. Without a proper driver's reaction, the fishtailing car will spin; due to eventual loss of grip from the front wheels as well. The instinctive response (to steer right if the rear swings left or vice versa) will worsen the spin. Most modern rear wheel drive cars solve this problem by using a form of traction control which limits engine power when fishtailing occurs. The ability of the rear suspension to keep tyres in contact with, and perpendicular to the road is also a key factor in the amount of grip available through the rear axle. For example a live beam axle suspension will have far less grip on a bumpy road than an independent rear suspension, due to its far greater unsprung weight, and forces from one wheel being transmitted through the axle to the other wheel, leading to the tire being out of contact with the road surface more of the time.
Similar behaviour is evident during heavy braking in all types of road vehicles. During braking weight transfer shifts to the front, and without the proper distribution of braking force the rear wheels are subject to locking. Most modern cars use anti-lock brakes (ABS) which prevents this problem. Older cars use solely hydraulic pressure limiting valves or axle load sensing valves to ameliorate this problem.
Fishtailing is also a term for the police pursuit technique[citation needed] of deliberately inducing rear instability in a suspect vehicle (see PIT maneuver).
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