of the driven wheels often look similar to those of undriven ones,
but there are some differences in the way they are constructed because the
driven hubs have to incorporate
In a front-wheel-drive car the hub has two jobs: to drive the wheels, and to
swivel so that the car can be steered. To make this possible, the hub rotates
in or next to a large casting called a hub carrier. The carrier itself is
mounted to the suspension with
that allow it to swivel and move up
In the case of cars with MacPherson
, the hub carrier has a ball joint
at the bottom and is either bolted to or is part of the strut at the top. The
upper swivel is then formed by the strut's top mount where it joins the inner
The amount the hub carrier can swivel is controlled by the
which it is attached by a
with another joint.
Driving the hub
The hub is driven by a shaft running from the final drive. The shaft has a
flexible joint (usually a constant velocity joint) at each end to allow for
steering swivel and suspension movement.
The end of the drive shaft runs through the centre of the hub carrier and
engages with the rotating part of the hub. When the drive shaft turns, the
rotating part of the hub turns with it.
Just as with an undriven wheel, there must be a pair of bearings - an inner
and an outer - to hold the hub steady and allow it to rotate smoothly. But
for undriven wheels
inside the hub and over the stub
, the bearings for driven wheels usually fit inside the hub carrier and
over the hub. The drive shaft then runs through the centre of the hub.
Some cars, notably older BL models, effectively have a rotating stub axle
instead - the hub does not extend into the hub carrier. The drive shaft fits
tightly inside the bearings that are inside the hub carrier. The hub bolts
directly to the end of the drive shaft which sticks through the hub carrier.
With this type of design, the shaft takes the load of the wheel as well as
With both types of design, it is the nut on the end of the drive shaft that
holds the hub on to the car. The hub fits from the outside of the hub carrier
and the drive shaft runs from the inside to pass through the middle of the hub.
on the drive shaft rests up against the back of the inner track of the
inner bearing to stop the shaft pulling right through. The nut is always done
up tightly and is staked or held by a split pin.
Driven hubs usually have
rather than the
found in undriven hubs. In most designs the bearings are quite separate and are
into the hub carrier, although they can be held in by a
threaded retaining ring.
Where the bearings are separate, a
or tube is fitted between the two
inner races to save them from excessive side loads when the central hub nut is
are always included to keep dirt out.
Driven rear hubs
Rear-wheel-drive cars with
usually have rear hubs
similar to those on the front of front-wheel-drive cars, except the hub does
not have to steer.
Some manufacturers have used unusual features on their driven rear hubs -
for instance, the Triumph Herald used a key running in a slot to transmit the
drive between the drive shaft and hub, rather than the usual splines.
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