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1. |
How do antilock brakes work? |
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2. |
Are all antilock brake systems
the same? |
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3. |
Why don't antilocks reduce
stopping distances as much on dry surfaces as wet ones? |
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How do antilock brakes work? |
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Antilock brakes are designed to help drivers
avoid crashes. When a driver hits regular brakes hard, the wheels may lock and
the vehicle may skid. Wheel lockup can result in longer stopping distances,
loss of steering control and, when road friction is uneven, loss of stability
if the vehicle begins to spin. The main advantage of antilocks is that they can
reduce these problems on wet and slippery roads. Antilocks work with your car's
normal service brakes to decrease stopping distance and increase the control
and stability of the vehicle during hard braking. Vehicles equipped with
antilocks have speed sensors mounted at each wheel and a secondary
electro-hydraulic braking circuit. The principle behind antilocks is that a
skidding wheel provides less stopping force and control than a wheel that is
rotating. Antilocks prevent wheels from skidding by monitoring the speed of
each wheel and automatically pulsing the brake pressure on any wheels where
skidding is detected. Antilocks shouldn't make much difference in stopping
distances on dry roads, although they can enhance vehicle stability and allow
drivers to maintain steering control during emergency stops when conventional
brakes might allow wheel lockup and skidding. |
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Are all antilock brake systems the
same? |
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Antilocks differ among vehicles, but there
are some basic similarities. Each system has sensors that monitor the
rotational speeds of selected wheels when brakes are applied. When one of these
wheels approaches lockup, a control unit reduces brake pressure to that wheel
(or set of wheels) just enough to allow rotation again. This typically happens
many times per second, resulting in improved control and, on many wet and
slippery surfaces, shorter stopping distances. Differences among antilock brake
systems include the following :
• Cars and many SUVs have four-wheel systems with wheel-speed sensors on each
wheel. In one type of system, the antilocks reduce brake pressure to both rear
wheels whenever one approaches lockup. Brake pressure to the front wheels of
four-wheel systems is controlled independently to maximize stopping power,
which is concentrated in the front. In four-wheel independent systems, each
wheel is controlled individually, so when any one approaches lockup, the
antilocks reduce brake pressure to that wheel.
• Some pickups and cargo vans have rear-wheel only antilock systems to address
different braking needs when vehicles are loaded versus unloaded. The antilocks
monitor the rotational speeds of rear wheels only and release pressure to both
when either is about to lock.
• Tractor-trailers have separate antilock systems for the tractors and the
trailers. Ideally, both the tractor and trailer of a combination rig should
have antilock brakes, but putting antilocks on either component should produce
improvement compared with conventional brakes. With antilocks on the tractor
only, a driver can maintain better steering control even if trailer wheels lock
and the trailer swings. If only the trailer has antilocks, trailer swing can be
reduced even if steering control is lost. |
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Why don't antilocks reduce
stopping distances as much on dry surfaces as wet ones? |
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Adequate braking is easy to achieve on dry
roads with or without antilock brakes. Even if wheels lock, the coefficient of
friction between tires and road surface is still relatively high, so a vehicle
stops relatively quickly. It is even possible on some surfaces to stop sooner
without antilocks than with them, although such instances are rare. They occur,
for example, when loosely packed snow or gravel creates a "dam" effect in front
of locked wheels, shortening the stopping distance more than antilocks could. |
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