Unsprung weight

Ignoring the flexing of other components, a car can be modeled as the sprung weight, carried by the springs, carried by the unsprung weight, carried by the tires, carried by the road. Unsprung weight is more properly regarded as a mass which has its own inherent inertia separate from the rest of the vehicle. When a wheel is pushed upwards by a bump in the road, the inertia of the wheel will cause it to be carried further upward above the height of the bump. If the force of the push is sufficiently large, the inertia of the wheel will cause the tire to completely lift off the road surface resulting in a loss of traction and control. Similarly when crossing into a sudden ground depression, the inertia of the wheel slows the rate at which it descends. If the wheel inertia is large enough, the wheel may be temporarily separated from the road surface before it has descended back into contact with the road surface.

This unsprung weight is cushioned from uneven road surfaces only by the compressive resilience of the tire (and wire wheels if fitted), and which aids the wheel in remaining in contact with the road surface when the wheel inertia prevents close-following of the ground surface. However, the compressive resilience of the tire results in rolling resistancewhich requires additional kinetic energy to overcome, and the rolling resistance is expended in the tire as heat due to the flexing of the rubber and steel bands in the sidewalls of the tires. To reduce rolling resistance for improved fuel economy and to avoid overheating and failure of tires at high speed, tires are designed to have limited internal damping.

So the "wheel bounce" due to wheel inertia, or resonant motion of the unsprung weight moving up and down on the springiness of the tire, is only poorly damped, mainly by the dampers or shock absorbers of the suspension. For these reasons, high unsprung weight reduces road holding and increases unpredictable changes in direction on rough surfaces (as well as degrading ride comfort and increasing mechanical loads).

This unsprung weight includes the wheels and tires, usually the brakes, plus some percentage of the suspension, depending on how much of the suspension moves with the body and how much with the wheels; for instance a solid axle is completely unsprung. The main factors that improve unsprung weight are a sprung differential (as opposed to live axle) and inboard brakes. (The De Dion tube suspension operates much as a live axle does, but represents an improvement because the diff is mounted to the body, thereby reducing the unsprung weight.) Aluminum wheels also help. Magnesium alloy wheels are even lighter but corrode easily.

Since only the brakes on the driving wheels can easily be inboard, the Citroën 2CV had inertial dampers on its rear wheel hubs to damp only wheel bounce.

torsion bar suspension

A torsion bar suspension, also known as a torsion spring suspension or torsion beam suspension, is a general term for any vehicle suspension that uses a torsion bar as its main weight bearing spring. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end terminates in a lever, the torsion key, mounted perpendicular to the bar, that is attached to a suspension arm, a spindle, or the axle. Vertical motion of the wheel causes the bar to twist around its axis and is resisted by the bar' storsion resistance. The effective spring rate of the bar is determined by its length, cross section, shape, material, and manufacturing process.

CAR NEW MODELS: control arm or wishbon

CAR NEW MODELS: control arm or wishbon: In automotive suspension, an automobile's  control arm  or wishbone (aka. A-arm or A-frame) is a nearly flat and roughly triangular su...

Advantages and disadvantages of MacPherson strut

Although it is a popular choice, due to its simplicity and low manufacturing cost, the design has a few disadvantages in the quality of ride and the handling of the car. Geometric analysis shows it cannot allow vertical movement of the wheel without some degree of either camber angle change, sideways movement, or both. It is not generally considered to give as good handling as a double wishbone or multi-link suspension, because it allows the engineers less freedom to choose camber change and roll center.

Another drawback is that it tends to transmit noise and vibration from the road directly into the body shell, giving higher noise levels and a "harsh" feeling to the ride compared with double wishbones, requiring manufacturers to add extra noise reduction or cancellation and isolation mechanisms.

Despite these drawbacks, the MacPherson strut setup is still used on high performance cars such as the Porsche 911, several Mercedes-Benz models and lower BMWs models (including the new Mini but excluding the 2007 X5, 2009 7-series, 2011 5-series and 5-series GT).

The Porsche 911 up until the 1989 model year (964) use MacPherson strut designs that do not have coil springs, using a torsion bar suspension instead.

control arm or wishbon

In automotive suspension, an automobile's control arm or wishbone (aka. A-arm or A-frame) is a nearly flat and roughly triangular suspension member (or sub-frame), that pivots in two places. The base of the triangle attaches at the frame and pivots on a bushing. The narrow end attaches to the steering knuckle and pivots on a ball joint.

The upper control arm can clearly be seen at the top portion of the suspension components in the attached photo, where it is the silver part horizontally attached to the frame inside the red body portion and connecting to the steering knuckle near the side of the tire's wheel rim. Note the roughly A-shaped design with the top of the A near the tire and the bottom two points connected to the frame inside the body's space. In the photo, the A-shape is reinforced with a solid triangular plate near the top of the A.

Two such devices per wheel make up a double wishbone suspension, while one control arm per wheel makes up a part, usually the lower part, of a MacPherson strut suspension or of various other configurations.

Intermittent wipers

The inventor of intermittent wipers might have been Raymond Anderson who, in 1923, proposed an electro-mechanical design. (US Patent 1,588,399). In 1958 Oishei et at. filed a patent application describing electro-mechanical, thermal and hydraulic designs. (US Patent 2,987,747). Then in 1961 John Amos, an engineer for the UK automotive engineering company Lucas Industries, filed in the UK the first patent application for a solid-state electronic design. (See US patent 3,262,042).

In 1963, another form of intermittent wiper was invented by Robert Kearns, an engineering professor at Wayne State University in Detroit, Michigan. The road to his intermittent wipers began earlier, on his wedding night in 1953, when an errant champagne cork shot into Kearn's left eye, which eventually went almost completely blind. Nearly a decade later, Kearns was driving his Ford Galaxie through a light rain, and the constant movement of the wiper blades irritated his already troubled vision. He got to thinking about the human eye, which has its own kind of wiper, the eyelid, that automatically closes and opens every few seconds. Finally in 1963, Kearns put his idea into action, building his first intermittent wiper system using off-the-shelf electronic components. Kearns showed it to the Ford Motor Company, and proposed manufacturing the design.

In the Kearns design, the interval between wipes was determined by the rate of current flow into a capacitor. When the charge in the capacitor reached a certain voltage, the capacitor was discharged, activating the wiper motor for one cycle. After extensive testing, Ford executives decided to offer a design similar to Kearns’ intermittent wipers as an option on the company's Mercury line, beginning with the 1969 models. Kearns and Ford became involved in a multi-year patent dispute that eventually had to be resolved in court. A fictionalized version of the Kearns invention and patent lawsuit was used for the 2009 film Flash of Genius, which is billed as "based on the true story", but does not claim to be historically accurate in all respects.

In March 1970, Citroën introduced rain-sensitive intermittent windscreen wipers on their SM model. When the intermittent function was selected, the wiper would make one swipe. If the windscreen was relatively dry, the wiper motor drew high current, which set the control circuit timer to delay the next wipe longest. If the motor drew little current, it indicated that the glass was wet, setting the timer to minimize the delay.

Rain-sensing Wipers

In the past, automakers have tried to either eliminate the wipers or to control their speed automatically. Some of the schemes involved detecting the vibrations caused by individual raindrops hitting the windshield, applying special coatings that did not allow drops to form, or even ultrasonically vibrating the windshield to break up the droplets so they don't need to be wiped at all. But these systems were plagued by problems and either never made it to production or were quickly axed because they annoyed more drivers than they pleased.

However, a new type of wiper system is starting to appear on cars that actually does a good job of detecting the amount of water on the windshield and controlling the wipers. One such system is made by TRW Inc., here is a PDF describing their rain sensor system. TRW Inc. uses optical sensors to detect the moisture. The sensor is mounted in contact with the inside of the windshield, near the rear view mirror.

The sensor projects infrared light into the windshield at a 45-degree angle. If the glass is dry, most of this light is reflected back into the sensor by the front of the windshield. If water droplets are on the glass, they reflect the light in different directions -- the wetter the glass, the less light makes it back into the sensor.

The electronics and software in the sensor turn on the wipers when the amount of light reflected onto the sensor decreases to a preset level. The software sets the speed of the wipers based on how fast the moisture builds up between wipes. It can operate the wipers at any speed. The system adjusts the speed as often as necessary to match with the rate of moisture accumulation.

The TRW system, which is found on many General Motors cars, including all Cadillac models, can also be overridden or turned off so the car can be washed.

For more information on windshield wipers and related topics, check out the links on the next page!