A beam axle, rigid axle or solid axle is a
dependent suspension design in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once commonly used at the rear wheels of a vehicle, but historically they have also been used as front axles in four-wheel-drive vehicles. In most automobiles, beam axles have been replaced with front and rear
independent suspensions.
Implementation
With a beam axle the
camber angle
Camber angle is one of the angles made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of a wheel and the vertical axis of the vehicle when viewed from the front or rear. It is used in the design of steering a ...
between the wheels is the same no matter where it is in the travel of the suspension.
A beam axle's fore & aft location is constrained by either:
trailing arm
A (semi) trailing-arm suspension, sometimes referred to as (semi) trailing-link is a vehicle axle or wheel suspension design in which one or more horizontal arms (or "links"), perpendicular to and forward of the axle, are connecting the axle or ...
s, semi-trailing arms,
radius rod
A radius rod (also called a radius arm, torque arm, torque spring, and torsion bar) is a suspension link intended to control wheel motion in the longitudinal (fore-aft) direction. The link is connected (with a rubber or solid bushing) on one en ...
s, or
leaf spring
A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Originally called a ''laminated'' or ''carriage spring'', and sometimes referred to as a semi-elliptical spring, elliptical spring, or cart spring, it ...
s. The lateral location can be constrained by a
Panhard rod
A Panhard rod (also called Panhard bar, track bar, or track rod) is a suspension link that provides lateral location of the axle. Originally invented by the Panhard automobile company of France in the early twentieth century, this device has been ...
, a
Scott Russell linkage
A Scott Russell linkage is a linkage which translates linear motion through a right angle.
The linkage is named after John Scott Russell (1808β1882), although watchmaker William Freemantle had already patented it in 1803.
A different form o ...
or a
Watt's linkage
In kinematics, Watt's linkage (also known as the parallel linkage) is a type of mechanical linkage invented by James Watt in which the central moving point of the linkage is constrained to travel on a nearly straight line. It was described i ...
, or some other arrangement, most commonly by the leaf springs.
Shock absorbers and either leaf springs,
coil spring
A selection of conical coil springs
The most common type of spring is the coil spring, which is made out of a long piece of metal that is wound around itself.
Coil springs were in use in Roman times, evidence of this can be found in bronze Fib ...
s, or
air bags
An airbag is a vehicle occupant-restraint system using a bag designed to inflate extremely quickly, then quickly deflate during a collision. It consists of the airbag cushion, a flexible fabric bag, an inflation module, and an impact sensor. Th ...
are used to control vertical movement.
The
Twist-beam rear suspension
The twist-beam rear suspension (also torsion-beam axle, deformable torsion beam or compound crank) is a type of automobile suspension based on a large H or C-shaped member. The front of the H attaches to the body via rubber bushings, and the ...
is a similar suspension design, however its beam axle is able to twist, thereby functioning as an
anti-roll bar
An anti-roll bar (roll bar, anti-sway bar, sway bar, stabilizer bar) is a part of many automobile suspensions that helps reduce the body roll of a vehicle during fast cornering or over road irregularities. It connects opposite (left/right) whee ...
to control the roll motion of the body, and is considered to be a semi-independent suspension design.
Live axle vs dead axle
A live axle is a type of beam axle in which the shaft (or, commonly, ''shafts'' connected to move as a single unit) also transmits power to the wheels; a beam axle that does not also transmit power is sometimes called a dead axle. While typically used in vehicles with
Hotchkiss drive, this suspension system can also be used with other types of power transmission.
Advantages
*A beam axle is typically simple in design, rugged, and inexpensive to manufacture.
**Only one
universal joint
A universal joint (also called a universal coupling or U-joint) is a joint or coupling connecting rigid shafts whose axes are inclined to each other. It is commonly used in shafts that transmit rotary motion. It consists of a pair of hinges ...
or
constant-velocity joint
Constant-velocity joints (also known as homokinetic or CV joints) are mechanical joints which allow a drive shaft to transmit power through a variable angle, at constant rotational speed, without an appreciable increase in friction or play. The ...
is needed at each steered and driven wheel and none are needed at non-steered wheels; this reduces maintenance requirements and manufacturing costs compared to independent suspensions, which typically require two such joints at each driven wheel.
*A beam axle is space-efficient, an important advantage for
off-road applications, as it provides better vehicle
articulation and durability in a high load environment.
*
Camber angle
Camber angle is one of the angles made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of a wheel and the vertical axis of the vehicle when viewed from the front or rear. It is used in the design of steering a ...
is rigidly fixed by axle geometry; for a live axle,
toe
Toes are the digits (fingers) of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being ''digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being ''plan ...
is typically fixed as well.
**As the vehicle's body rolls during hard cornering, the unchanging camber yields predictable
handlingβat least on smooth surfaces.
**
Wheel alignment
Wheel alignment sometimes referred to as breaking, or tracking is part of standard automobile maintenance that consists of adjusting the angles of wheels to the car manufacturer specifications. The purpose of these adjustments is to reduce tire ...
is simplified.
**Traction, braking and tire wear characteristics do not change as the suspension is compressed. These are great benefits in a vehicle that carries heavy loads, and together with the beam axle's characteristic strength, this has resulted in front and rear beam axles being nearly universal in
buses and
heavy-duty trucks. Most light and medium duty
pickup truck
A pickup truck or pickup is a light-duty truck that has an enclosed cabin, and a back end made up of a cargo bed that is enclosed by three low walls with no roof (this cargo bed back end sometimes consists of a tailgate and removable covering) ...
s,
SUV
A sport utility vehicle (SUV) is a car classification that combines elements of road-going passenger cars with features from off-road vehicles, such as raised ground clearance and four-wheel drive.
There is no commonly agreed-upon definiti ...
s, and
vans also use a beam axle, at least in the rear.
Disadvantages
*A beam axle does not allow each wheel to move independently in response to uneven surfaces, and handling is typically worse than more sophisticated suspension designs.
**In turns, the outside wheel is often subjected to adverse camber angles when the inside wheel hits a bump, which can suddenly reduce cornering grip and destabilize the vehicle.
**Camber angle cannot change during body roll, and the geometric
roll center of the suspension is always fixed at the physical axle midpoint, limiting suspension tuning options. Furthermore, the roll center moves in reaction to road irregularities
**Toe is typically fixed at zero for a live axle, and dynamic toe control is difficult to implement.
*The mass of the beam is part of the
unsprung weight
The unsprung mass (colloquially unsprung weight) of a vehicle is the mass of the suspension, wheels or tracks (as applicable), and other components directly connected to them. This contrasts with the sprung mass (or weight) supported by the ...
of the vehicle, hurting
ride quality
Ride quality refers to a vehicle's effectiveness in insulating the occupants from undulations in the road surface (e.g., bumps or corrugations).
A vehicle with good ride quality provides a comfort for the driver and passengers.
Importance
Good r ...
.
**The need for lateral location devices such as a Panhard rod or Watt's linkage adds more unsprung weight and partially offsets the beam axle's advantages in simplicity, space efficiency and cost.
*In a vehicle with conventional
Hotchkiss drive, the entire axle may twist in its mounts in reaction to torque loads; during hard acceleration, this may reduce traction and induce wheel hop or sudden adverse toe changes.
*The bulky
differential housing of a Hotchkiss live axle reduces ground clearance, hindering the vehicle's ability to ford deep mud, clear obstacles and negotiate deeply rutted roads.
**The differential housing can only be raised by using larger wheels and tires, typically with a penalty in unsprung weight, braking effectiveness and acquisition costs; furthermore, vehicle modifications may be necessary for adequate
fender or
frame
A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent.
Frame and FRAME may also refer to:
Physical objects
In building construction
*Framing (con ...
clearance.
*Front beam axle suspension is unusually sensitive to any lack of concentricity in the hub and wheel assembly which can cause a side-to-side oscillation ("shimmy") of the steering at certain speeds (typically ), commonly referred to as "death wobble" within the 4x4 community.
This is addressed on some vehicles with steering dampers although removal and careful refitting of the front wheels often cures the problem.
See also
*
Axle
An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, bearing ...
*
Twist-beam rear suspension
The twist-beam rear suspension (also torsion-beam axle, deformable torsion beam or compound crank) is a type of automobile suspension based on a large H or C-shaped member. The front of the H attaches to the body via rubber bushings, and the ...
*
Wheelset (rail transport)
A wheelset is a pair of railroad vehicle wheels mounted rigidly on an axle such that both wheels rotate in unison. Wheelsets are often mounted in a bogie ("truck" in North America) β a pivoted frame assembly holding ...
Notes
{{DEFAULTSORT:Beam Axle
Automotive suspension technologies