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Pair of railroad wheels fixed onto an axle
A
wheelset
is a pair of
railroad vehicle
wheels
mounted rigidly on an
axle
allowing both wheels to rotate together. Wheelsets are often mounted in a
bogie
("
truck
" in
North America
) ? a pivoted frame assembly holding at least two wheelsets ? at each end of the vehicle. Most modern
freight cars
and
passenger cars
have bogies each with two wheelsets, but three wheelsets (or more) are used in bogies of freight cars that carry heavy loads, and three-wheelset bogies are under some passenger cars. Four-wheeled
goods wagons
that were once near-universal in
Europe
and
Great Britain
and their colonies have only two wheelsets; in recent decades such vehicles have become less common as trainloads have become heavier.
Conical wheel-tread
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Most
train wheels
have a
conical
taper of about 1 in 20 to enable the wheelset to follow
curves
with less chance of the wheel
flanges
coming in contact with the rail sides, and to reduce
curve resistance
. The rails generally slant inwards at 1 in 40, a lesser angle than the wheel
cone
. Without the conical shape, a wheel would tend to continue in a straight path due to the
inertia
of the rail vehicle, causing the wheelset to move towards the outer rail on the curve. The cone increases the effective diameter of the wheel as it moves towards the outer rail, and since the wheels are mounted rigidly on the axle, the outer wheels travel slightly farther, causing the wheelsets to more efficiently follow the curve. Abnormal wear at the
wheel?rail interface
is thus avoided,
[1]
along with the loud, piercing, very high-pitched squeal which usually results from it ? especially evident on curves in tunnels, stations and elevated track, due to flat surfaces slipping and flanges grinding along the rail. However, if the degree of conicality is inappropriate for the suspension and track, an unpleasant
oscillation
can occur at high speeds. Recent research is also showing that marginal changes to wheel and rail profiles can improve performance further.
[2]
Not all railroads have employed conical-tread wheels. The
Bay Area Rapid Transit
(BART) system in
San Francisco
, built with cylindrical wheels and flat-topped rails, started to re-profile the wheels in 2016 with conical treads after years of complaints about the
squeal
by its passengers.
[3]
Australia's
Queensland Railways
used cylindrical wheels and vertical rails until the mid-1980s, when considerably higher train loads made the practice untenable.
[4]
[5]
[6]
Specialised wheelsets
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Some
rubber-tyred metros
feature special wheelsets with
rubber
tyres
outside of deep-flanged steel wheels, which guide the bogie through standard
railroad switches
and keep the train from
derailing
if a tyre deflates. The
system
was originally conceived by
Michelin
for the
Paris Metro
; the
first line
opened in 1956.
Gallery
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See also
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References
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Further reading
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External links
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