You’ve got a vehicle in the shop with a customer complaint that points likely in the direction of a steering or suspension issue. You’ve made an initial assessment of the problem, and it’s time to identify the suspension prior to going on a test drive, so you know the type of handling characteristics to expect.
Independent suspension allows each wheel on the same axle (in this case “axle” referring to the front or rear location of the wheels) to move up and down independently of the other. On an axle- or beam-style suspension, both wheels are linked together by a solid axle or beam and the movement of one side effects the movement of the other. Independent suspension has enormous advantages related to handling and comfort, most notably in the front. Independent front suspension (IFS) was one of the first major advancements of automotive technology early in the 20th century, and there was no looking back.
Independent rear suspension (IRS) offers the same advantages, but due to the complexity and expense of IRS, and the fact that its advantages aren’t outwardly evident for most drivers, IRS is primarily found on high-end luxury, performance and sports cars. The most common arrangement for many years has been IFS combined with axle or beam rear suspension, and this is still utilized on heavy trucks due to the durability of a rear axle suspension. However, IRS is becoming much more common with the popularity of all-wheel drive and advanced driver assistance systems (ADAS) that rely on refined suspension technology to provide the highest level of safety available.
A short/long arm (SLA) suspension, also referred to as a double wishbone or double “A” arm suspension, is a form of independent suspension, and is one of the oldest types of modern suspension. It has been used as one of the primary front suspensions since the 1930s, and while that may not strike you as modern, it’s survived the test of time due to many desirable characteristics. The upper control arm is shorter than the lower, which causes geometry changes during suspension movement.
These geometry changes allow this type of suspension to offer better load handling capabilities, higher comfort, and finely tunable handling characteristics. For this reason, SLA suspension continues to be used on heavy-duty applications, as well as performance and racing cars. SLA suspensions primarily utilize coil springs or torsion bars to support the weight of the vehicle.
The MacPherson strut style of suspension, which is undoubtedly the most common today, houses the spring, shock absorber and steering pivot in one unit. The major advantage to strut suspension is simplicity and low cost. There are different variations of strut suspension, however the most common is a single wishbone independent design, which features the strut, a steering knuckle and lower control arm. Struts are also used in rear axle beam -type (non-independent) suspensions.
In general, a strut suspension doesn’t handle as well as an SLA suspension because of the limits of geometry changes during suspension travel, but it is still used on some performance cars because they have limited suspension travel and don’t need to rely on this particular aspect of SLA suspension.
A multi-link suspension, sometimes called a 4-link suspension, replaces the control arms with individual front and rear links. They are complex and the most expensive type of suspension, but with the aid of computer design, they take the advantages of SLA geometry and handling characteristics to the next level. This type of suspension is often found on high-end luxury performance vehicles.
Sway bars, also known as stabilizer or anti-roll bars, connect one side of the suspension to the other. They can be located in the front, rear or both. When any suspension movement occurs, that movement is transferred through the sway bar to the suspension on the other side. This balances the compression of the suspension on both sides, eliminating body roll, balancing the weight distribution of the vehicle, and providing optimal traction and handling.
A beam or solid-axle suspension can be divided into two primary types. A live axle is one that not only spans between the suspension springs on each side, but it transfers power through it to the wheels. This was the most common arrangement on cars for many years, is still the most common setup on trucks and is still used on the front of some heavier-duty four-wheel-drive trucks.
An axle beam spans between the suspension on both sides, but does not transfer power through it, and is sometimes called a dead axle. This is the most common rear suspension on front-wheel-drive vehicles.
There are many different varieties of solid-axle suspensions. For example, they may utilize leaf springs, which position the axle, or they may utilize trailing arms and struts, or even coil springs in conjunction with a multi-link attachment system. Solid axle suspensions also utilize sway bars to improve handling, and some also use Panhard bars or a Watts linkage to control the sideways motion of the axle.
In the field, you will likely encounter a few different types of suspension that seem like an awkward combination of them all, but with this core knowledge, you’ll be able to quickly identify the important operational aspects of any of them.
Steering systems can primarily be broken down into the type of steering gear and the type of linkage. The steering gear transfers the rotational motion of the steering wheel into the reciprocating or linear motion required to move the steering linkage. A traditional steering gearbox utilizing a recirculating ball and worm system transfers rotational motion into reciprocating motion on the end of a pitman arm. The pitman arm connects to the steering linkage. The most common form of linkage for these systems is the parallelogram linkage, named for its relative geometric configuration.
These are the most complex steering systems, and the linkages include an idler arm, center link and tie rods. Drag link steering utilizes the same traditional pitman arm gear box, however the drag link connects directly to a steering knuckle, and one single tie rod travels from that knuckle, or, in some cases, from the drag link to the other steering knuckle.
A rack and pinion steering gear transfers rotational motion into linear motion. This is a much simpler system, and initially became popular in small, front-wheel-drive vehicles due to space savings and lower cost. The only steering linkage required by rack and pinion steering is a tie rod on each side to connect to each steering knuckle. The benefits of rack and pinion steering make it the most popular system on vehicles today, including full-size vehicles, trucks and vans.
Coming up next: Understanding alignment angles and their effect on steering.
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