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The Suspension System:
The Role Of Axles
A basic suspension system consists of springs, shock absorbers, arms, rods, ball joints and axles.
Solid, or beam, axles connect the wheels on each side of the vehicle. This means the movement of a wheel on one side of the vehicle is transferred to the wheel on the other side.
With independent suspension, the wheels can move independently of each other, which reduces body movement. This prevents the other wheel being affected by movement of the wheel on the opposite side, and this reduces body movement.
The solid, or beam, axle is used in the rear suspension of many front-engined, rear-wheel drive cars, and light commercial vehicles, and as the front suspension on many heavy commercial vehicles.
The solid or beam axle provides a simple means of locating and mounting the hub and wheel units. Together with leaf springs, it forms an effective, non-independent suspension system.
Similarly, with coil springs.
It is still used in the rear suspension of many front-engined, rear-wheel-drive cars, and light commercial vehicles, and as the front suspension on many heavy commercial vehicles.
On rear-wheel-drive vehicles, with leaf springs, the axle housing is held in place by the springs, and no other form of location is needed.
The drive is transmitted through the final drive unit and axles to the wheels, and therefore the axle is referred to as a live axle.
When a vehicle accelerates from rest, the resistance of its mass causes a torque reaction, producing a tendency for the axle housing to rotate in the direction that is opposite to wheel rotation.
A similar effect occurs during braking, but with the twisting effect in the direction of wheel rotation.
In both cases, this tendency can cause leaf spring wind-up, and the twisting action can interfere with suspension motion.
It is usually controlled by mounting the axle housing closer to the fixed shackle point, so that the spring’s front section is shorter than the rear section. The short, stiff front section resists twisting, and a bump-stop above the final drive housing can also be used to restrict upward motion during acceleration.
Beam axle
A beam axle is a suspension system, also called a solid axle, in which one set of wheels is connected laterally by a single beam or shaft. A live axle is a type of beam axle in which the shaft (or shafts, since live axles, while connected to move as a single unit, are seldom one piece) also transmits power to the wheels; a beam axle that does not also transmit power is sometimes called a dead axle. Beam axles are commonly used at the rear wheels of a vehicle.
Beam axles are typically suspended either by leaf springs or coil springs. In some cases, a Panhard rod or similar device may be used to control the lateral motion of the axle.
A similar suspension is the Twist-beam rear suspension, in which the beam axle also functions as an anti-roll bar to control the roll motion of the body.
The principal advantage of the beam axle is that it is simple and cheap to manufacture. It also engages little or no interior volume within the vehicle. Its drawbacks are that it does not allow each wheel to move independently in response to bumps, and the mass of the beam is part of the unsprung weight of the vehicle, which can further reduce ride quality.
Because beam axles do not ever exhibit any camber change as the suspension travels, they are ideal for carrying heavy or varying loads. Although this negatively impacts cornering compared to other suspension designs (because the wheels have zero camber gain during body roll), beam axles are nearly universally used in heavy-duty trucks. Most light and medium duty pickup trucks and vans also use a beam axle, at least in the rear.
Beam axles have an important advantage for off road applications, as it provides better vehicle articulation. They are also generally stronger and less prone to failure in a high load environement, such as for off-roading.
Dead Axle
A dead axle only supports the vehicle and doesn't transmit any drive. With a live axle, the drive is transmitted through the final drive unit and axles to the wheels.
On front-wheel-drive vehicles, a simple beam axle can be used on the rear, with coil spring suspension and control arms for location. This is called a dead axle, since it only supports the vehicle and doesn’t transmit any drive. It is also non-independent, as deflection of a wheel on one side of the vehicle will be transferred to the other wheel.
On some vehicles, this is reduced by using a U-shaped axle beam, with a torsion bar mounted inside it.
Trailing arms are welded to the beam, to locate the axle longitudinally.
A lateral rod prevents lateral movement when cornering, and coil springs provide for suspension. The torsion bar is connected between the left and right wheel units, and deflection of the wheel on one side causes the axle and its torsion bar to twist together.
Passenger cars no longer use beam axle front suspension, but it is still common on heavy commercial vehicles, and some 4-wheel-drives.
Trucks use an I-beam, in most cases located by leaf springs.
4-wheel-drives, with rigid axles, may use leaf springs for front and rear suspension.
Coil springs may also be used for front and rear, and as with other beam axle designs, control arms and a lateral rod must be used for location.
Plain or nonfloating axle
Early vehicles used a live axle known as the plain or nonfloating axle. This type of axle is not used on any modern wheeled vehicle. The axle resembled the semifloating axle on the outside. However, the axle shafts were supported by two roller bearings. One roller bearing was located just inside the outer end of the axle shaft housing. The other roller bearing was located at the center of the axle shaft and inside the axle shaft housing. The inner ends of the axles were connected to the differential side gears by keys and key ways.
These axle shafts had to carry the weight of the differential assembly on the inner ends. The vehicle wheels were attached to the outer ends of the axle shafts. Therefore, the outer ends of the axle shafts carried the
entire weight of the rear of the vehicle. End thrust on the axle shafts was absorbed, or taken up, by a ball-type bearing located on each side of the differential case and by a block between the inner ends of the axles.
Semi-floating axle
The semifloating axle is used on most passenger and light commercial vehicles. The difference between the semifloating and plain axle is the method of mounting the differential assembly and the support of the axle shafts.
On the semifloating axle assembly, the differential housing is supported in the axle housing on bearings rather than on the inner ends of the axle shafts. This design relieves the axles of the weight and some of the operating stresses of the differential assembly.
The inner ends of the axle shafts are splined to gears in the differential and have only to transmit turning effort. The outer ends of the axle shafts are mounted like the plain-type axle, with the shafts supporting vehicle weight and withstanding wheel side-thrust.
The outer end of the axle is supported on a tapered roller bearing. With this type of construction, the wheel may come off the vehicle if the shaft should break. This is also true of the plain axle above.
Three-quarter floating axle
The three-quarter floating axle was also used on some earlier commercial vehicles. The bearing supporting the outer end of the axle shaft is moved from inside the axle housing to the outside. This method of mounting the axles places most of the weight of the vehicle on the ends of the axle housing rather than the ends of the axle shafts. The wheel is solidly keyed to a taper on the end of the axle shaft, so side-thrust is still taken
by the axle shaft as the vehicle turns or skids.
Full-floating axle.
The fourth type of axle is known as the full-floating type axle. This type of axle is used on some off road or military tactical wheeled vehicles. The major difference between the three-quarter and full-floating axles is that two bearings are used to mount the wheel on the axle housing instead of one bearing. With this method of construction, the axle shaft may be removed without disturbing the wheel or differential assembly. The axle shaft on a full-floating axle has a flange at the outer end. The flange connects the axle shaft to the hub of the wheel. It is through the flange that the axle shaft drives the wheel. On rear axles, the flange is a part of the axle shaft. On front axles, the end of the shaft is splined and a splined flange slides over the end of the shaft and is bolted to the wheel hub. |
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Glossary
Article Pages
1. Suspension System
2. Principles Of Suspension Systems
4. Types of Suspension Systems
5. Construction Of Leaf Springs & Shackles
6. Coil Springs
7. Torsion Bar Suspension
8. Auxilary Spring Suspension
9. Variable-load Spring Arrangement
10. Bogie-Type Suspension
11. Shock Absorber Introduction
12. Hydraulic Shock Absorber Operation
13. Direct Acting Shock Absorber
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