In modern vehicles, a smooth comfortable ride is taken for granted. A great deal of technology is involved. When the ride becomes rough, there are reasons and this article may help find them.
The compromise of the ride
Achieving a good ride in a vehicle involves compromises. Each vehicle has desirable characteristics, and passenger comfort is an important one. For instance, a truck is expected to carry a load. A heavy suspension can achieve this, but ride quality will suffer. A lighter suspension improves the ride but at the expense of the load.
Technology can solve the dilemma, for example, by using adjustable air springs. Such devices solve the ride and load problems but add heavily to both the initial and maintenance cost to the vehicle. For instance, the air bags go bad and have to be replaced. Air pumps fail and cost quite a bit to replace. Production vehicles do a very good job of hitting a compromise between these three points, including cost.
The earliest vehicles had different needs and expectations. A straight front axle, with a transverse spring were often used. These were incredibly durable, inexpensive and gave ground clearance needed for the bad roads. Ride quality was of little concern. Maintenance cost was also extremely low, consisting of occasionally adding lubricant to the oil caps on the king pins.
Years later, independent suspension was added. This improved passenger comfort and now each wheel could travel over bumps with little disturbance to the others. Heavy springs supported the load and shock absorbers dampened the jounce and rebound caused by a bump. As long as weight was of little concern, this system gave good service. The ride was good, and it was very durable.
Jounce and rebound
When a tire strikes a bump, the wheel moves up and the spring compresses to compensate. The force of the bump is absorbed by the spring. This is called jounce. The energy temporarily stored in the spring is then released. On the rebound, the body of the vehicle is forced up. Jounce and rebound cause the vehicle body to slam up and down, decreasing passenger comfort.
Among other things, shock absorbers resist jounce and rebound and improve the perception of ride quality. On jounce, the shock absorber slows the upward thrust, preventing inertia from slamming the vehicle down. Shock absorbers also resist rebound, slowing the release of energy. The shock absorber limits motion and eliminates the annoying oscillation that would otherwise occur. The green line in the illustration above shows vehicle-body motions with good shock absorbers. Worn shocks are shown by the red line.
The drastic weight reduction of later vehicles made the older independent suspension obsolete. McPherson strut suspension greatly reduces weight, improves ride and lowers the cost. Drawbacks included a reduction in durability and load capacity. This is why McPherson struts are rarely used in trucks.
Struts and shocks
The McPherson strut replaces the upper control arm and shock absorber, in earlier independent suspension systems. Initially, there was a clear distinction between the newer struts and the older shocks. Struts normally support weight and shocks merely dampen motion. Actually, every McPherson strut contains a shock absorber.
Modern designs blur the distinction. Today, shock absorbers often have springs and support weight. Struts sometimes only dampen motion. The two terms have become largely interchangeable today.
Rough ride causes
With use, shock absorbers wear and may fail to suppress jounce and rebound. This allows inertia to increase motion within the vehicle body and diminish ride quality. A shock absorber may also seize or a valve can stick causing a very harsh ride. The old bounce test on each corner of the vehicle will reveal a seized shock. If the sidewall of the tire flexes, before the suspension moves, the shock absorber may be bad.
Testing resistance to jounce and rebound is more difficult. A better test for wear is measuring the temperature of the shock or strut and comparing it to the ambient temperature. An increased temperature, on the outside surface of the shock, means it is working. A good shock absorber will feel warm, after being driven on a bumpy road. Excess bounce, dipping and suspension-travel are other indicators of worn shocks and struts.
Cheap, out of round tires cause a poor ride. Tires that are not round will damage shocks and struts as they attempt to dampen the motion of the tire. Often shocks and struts are blamed for chopped wear on tires. In reality, out of round tires damaged the shock, and the tire wear is the result.
Tires also come in many types for different applications. For example, trucks may have load-range C, which is a six-ply, load-range E, ten-ply and so on. Heavier tires require more air pressure and increase load capacity. They also drastically decrease ride quality on a vehicle not designed for them. A half-ton pickup, with ten-ply tires, will give a very harsh ride.
Other causes of a poor ride
When suspension components attach to the vehicle body, rubber mounts are used to isolated noise and vibration. As these mounts wear, the bumps and holes in the road transfer noise to the passengers.
A common source of poor ride complaints is the upper strut mount. This rubber mount attaches the strut to the body and absorbs a good deal of bumps. When the mount wears out, the strut shaft can contact the vehicle body. This transfers road noise and vibration to passengers, decreasing ride quality.
Stabilizer bar bushings and end-links also wear with use. Worn bushings can give a loud knock on bumps and make the ride feel harsher than it is. Broken motor and transmission mounts produce the same effect. If the weight of the engine and transmission are not restrained, they slam into their mounts on bumps, making the ride very bad.
Body mount cushions, between the frame and passenger compartment can deteriorate. These cushions absorb most road noise and when they fail the passengers begin to feel every bump.
Improper ride height reduces suspension travel and ride quality
Lowering or raising the normal height of a vehicle will change the geometry of the suspension. This will result in a degradation of ride quality. Worn or broken springs, air springs that leak and air-pumps that fail will also change ride height and passenger comfort. Diagnosing air suspension is covered in another Detailed Topic.
A ride that is rough is the result of seized shocks, worn bushings, improper ride height or a tire issue. It is not only uncomfortable it signals a problem, which will only get worse. If your vehicle does not ride properly the experts at AGCO can set you straight.