Monthly Archives: January 2011

How a Car’s Steering Works – PART ONE

The steering mechanism in a car seems like a fairly simple mechanism: you turn the steering wheel, the car’s wheel turns, and the car goes in a different direction. Right? Actually, a car’s steering mechanism, while simpler than, say, the transmission, is still a fairly complex piece of machinery. Today, I’ll discuss how basic steering systems function.

Steering basics

When you round a corner, not all of your car’s wheels turn at the same angle. Think about it. Say you are turning right. The right front wheel, or inside wheel, will need to turn around a smaller circle than the outer wheel, which means it will need to be positioned at a different angle (see figure one).

 

Figure 1: turning angles

 

The image above provides an illustration of this phenomenon. Before the car enters the turn, both front wheels are straight. As the car goes into the turn, however, the two front wheels must both be placed at an angle; the angle is much steeper for the inside wheel. Notice that if you draw lines perpendicular to the center-line of each wheel, these lines will intersect at the center of the turn.

The way in which the wheels and steering wheel are linked together allows this to happen smoothly when you turn the wheel (See figure two).

 

Figure 2: Steering linkage

 

The steering wheel itself is attached to the track rod, which shifts from left to right when you turn the wheel. Ball bearings connect the track rod to two tie rods, which are then connected, also by ball bearings, to the steering arms. These are connected to the wheels. Taken together, these parts form a sort of parallelogram. As a result, when you turn the wheel, the inside wheel is turned at a steeper angle than the outside wheel (See figures three and four).

 

Figure 3: Before turning

 

Figure 4: After turning

 

The Rack and Pinion Gearset

In most modern cars, the connection between steering wheel and track rod is made by a rack-and-pinion gearset, a fairly simple arrangement. A notched rod, called the rack, extends out of the track rod. The steering shaft itself ends in a round gear called a pinion. As you turn the wheel, the pinion rolls through the notches on the rack, pushing the track rod to the left or right (see figure five).

 

Figure 5: Rack and pinion gear

 

This system does two things. First, it turns the rotational motion of the steering wheel into the linear motion needed to turn the wheels. Second, it makes it easier for you to turn the wheels by providing a gear reduction. As in the transmission, the locking together of gears of different diameters means that you don’t have to turn the wheel as far (or as hard) in order to get a response.

A car’s steering ratio indicates how many degrees you have to turn the steering wheel in order to turn the wheels by a certain amount. For example, if a car has a 20:1 steering ratio, then you need to turn the wheel 20 degrees for every 1 degree that the wheels turn. The steering ratio is determined by the spacing of the teeth on the rack and the size of the pinion gear.

If a car has a low steering ratio, it means that the car will respond quickly to input from the steering wheel but will be difficult to steer. Cars with higher steering ratios are easier to steer but less responsive, i.e. you will have to turn the wheel further in order to produce a response. Most cars have steering ratios between 12:1 and 20:1. However, the steering ratio in a racing car will be closer to 1:1, so that the car is more responsive; since these cars are generally very light, difficulty in turning isn’t an issue.

Some cars have something called variable ratio steering. In this system, the teeth at the center of the rack are placed close together, so that the car responds quickly when you enter a turn. However, as you get closer to the edges of the rack, the teeth are spaced further apart, so that the wheels don’t become difficult to move as you approach the limits of the turn.

Rack-and-pinion is the most common design in use on the road today. However, there are a number of other steering mechanisms in use. Re-circulating ball steering is used on many heavier vehicles, such as trucks and SUVs. In this system, the rack and pinion gear is replaced by a worm gear, which is filled with re-circulating ball bearings that help to keep the teeth in this gear in contact with one another. Since this system isn’t as common, I won’t go into its workings detail. Basically, this system gives a greater mechanical advantage, i.e., like the brake system, it converts the input force into a greater output. This is why it is often used on heavier vehicles. However, since power steering is now common on most vehicles, this type of steering arrangement is no longer used as frequently. I’ll discuss power steering in my next entry.

To read more on a broad range of subjects from “How To Change A Tire” to “How To Jumpstart Your Car”, visit DefensiveDriving.com’s Safe Driver Resources website!

Check out these sites for more information about defensive driving and business driver safety.

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How A Car’s Suspension Works

Good handling on the road depends on more than a car’s steering system. The steering works hand in hand with the suspension and tires to create a smooth ride and reliable steering. The suspension acts to improve a car’s ride and handling.

Even the smoothest roads have bumps. A car’s ride is its ability to absorb these bumps while keeping the car body fairly steady. Without a suspension, every little nick in the road would produce a corresponding jolt in the body of the car. The suspension also improves a car’s handling, i.e. the vehicle’s ability to turn, stop, and accelerate safely.

In order to accomplish these goals, a suspension faces several challenges. To create a smooth ride, the suspension has to absorb energy from bumps in the road and distribute this energy throughout the car frame. The suspension also works to keep the tires in contact with the road. Remember that when a car brakes, weight shifts from the rear to the front; the reverse occurs during acceleration. Weight also shifts when a car makes a turn. This weight transfer can weaken tires’ grip on the road; to combat this, the suspension minimizes the transfer of weight from back to front and vice versa. It also helps to transfer weight from the “high” side of the car to the “low” side when the car turns a corner.

The suspension has several components that work together to accomplish these goals: frame, springs, and damper (shock absorber).

The frame is the rigid structure that supports the main weight of the car. This part of the car is referred to as the sprung mass because it rests on springs; these springs absorb the increased vertical velocity of the wheels as they travel over bumps. The unsprung mass is the weight of the car below the frame: wheels, tires, axles, etc.

Over the years, several different types of springs have been used in car suspensions. Leaf springs are curved metal bars composed of several layers of metal that bend to absorb energy; they were common on older cars and are still used on most trucks. Torsion bars, which twist to absorb energy, were used by European carmakers in the 50s and 60s. Air springs were also used in some older models. Today, coil springs, like those found in a mattress, are most common in passenger cars.

The stiffness of the springs affects the performance of the vehicle. If a car is loosely sprung, it will easily absorb bumps in the road, providing a very smooth ride. However, the handling of the car won’t be as good, as the vehicle body will be prone to moving forward, backward, and side to side. Tightly sprung cars, while offering bumpy rides, maneuver more effectively. Car manufacturers aim to find a balance between these qualities.

Springs absorb energy easily; however, they don’t dissipate it. As soon as you release a compressed spring, it snaps back in the reverse direction and continues to oscillate until all the energy has been used up. If suspensions relied entirely on springs, you would have a very bumpy and uncontrollable ride.

To account for this, springs are usually paired with dampers, or shock absorbers. These devices use hydraulics to turn kinetic energy (motion) into thermal energy (heat.) This way, the energy stored in the spring dissipates quickly, without causing unnecessary motion in the body of the car.

A typical shock absorber is, in essence, a piston inside two oil filled tubes. The piston is attached to a casing, which is in turn attached to the spring. As the spring moves, it pushes the piston up or down, compressing the oil inside the pressure tube.  Tiny perforations in the pressure tube allow the oil to slowly escape into the reserve cylinder. The system is designed to provide enough resistance to absorb all of the energy from the spring without moving too much. (See figure 1)

 

Figure 1: a damper or shock absorber

The shock absorber has two cycles: the extension cycle, when the piston moves “down,” and the compression cycle, when the piston moves back up. The compression cycle controls the unsprung mass of the car, as the wheels compress the spring when they move upwards. The extension cycle controls the sprung mass of the car, which is effected by the “release” of the coiled springs. As the system provides greater resistance during the extension cycle, the shock absorber is very effective in keeping the body of the car fairly still. Modern shock absorbers are also velocity sensitive, so that the system provides more resistance as the car moves faster. When a shock absorber is combined with a coil spring, it becomes a strut, which, in addition to absorbing energy from the wheels, helps to provide structural support.

Another component of the suspension is the anti-sway bar. This is a solid metal bar that spans the axle of the car, joining one side of the car to another. Also called an anti-roll bar, it helps to prevent too much lateral motion in the car body.

These are the basic elements of a suspension, which can be arranged in different ways on different types of cars, depending on the arrangement of the wheels. If the wheels are dependent, i.e. linked by a solid axle, then a combination of leaf springs and shock absorbers is used. While still common on trucks, the dependent front and rear suspensions are no longer common on passenger cars.

Instead, independent suspensions, in which the wheels are each allowed to move on their own, are used. If both the front and the back wheels use an independent suspension, then a car can be said to have four-wheel independent suspension, a phrase you may encounter in car advertisements. One of the most common designs used on the front suspension is the McPherson strut, which is named after its inventor, Earle S. McPherson of General Motors. Invented in 1947, this design is still common today. (See figure 2)

 

Figure 2: McPherson Strut

Another common design for front suspensions is the double wishbone or double A arm suspension. In this design, two wishbone shaped supports are attached to each wheel, joining the wheel at one point and the frame at two points. Each of these arms carries a shock absorber and coil spring. Similar systems are used in the rear suspension in most cars. As the rear suspension doesn’t have to accommodate a steering element, these designs are usually a bit simpler.

Each of these basic designs has been modified in a number of different ways to produce a range of suspension options. All of these designs, however, employ the same basic principles to produce a safe and comfortable ride. 

To read more on a broad range of subjects from “How To Change A Tire” to “How To Jumpstart Your Car”, visit DefensiveDriving.com’s Safe Driver Resources website!

Check out these sites for more information about defensive driving and business driver safety.

Drunk Driving

Drunk driving, driving under the influence, driving while intoxicated, drink-driving, impaired driving, operating under the influence: no matter what you call it (and no matter where you are), drunk driving is one of the most reckless activities you can participate in. Not only is it illegal in all US states and most countries around the world, but most repeat drunk drivers end up irrevocably altering (or ending) many lives: their own and those of strangers, friends, loved ones, and children.

Although specific drunk driving regulations may differ from state to state, the penalties are high. Those who kill or injure another while driving under the influence can face heavy fines and civil suits as well as jail time. Most states have adopted strict sentencing laws that ensure those convicted will serve the full time they are sentenced to.

Since 2004, the most common legal limit for drunk driving is .08% Blood Alcohol Content (BAC). Different people will reach this BAC at different points; on average, consuming 3-5 drinks in an hour will bring your BAC close to this limit. However, it’s important to note that your BAC keeps going up for three hours after you’ve finished drinking and that alcohol affects different people in different ways. Many jurisdictions also have open container laws which prohibit the presence of open containers of alcohol within a motor vehicle, even if the driver is not consuming alcohol. In other states, it is illegal to even sleep in the driver’s seat while drunk. 

Drivers can experience impaired faculties at BAC levels as low as .02%. At .05%, drivers suffer from a reduced ability to track moving objects, to respond to emergencies, and to steer effectively. At .08%, drivers cannot control speed effectively, suffer from impaired concentration and memory, and have a harder time processing information, such as the appearance of new obstacles or changing traffic conditions.

Knowing this, it’s not surprising that roughly 10% of traffic-related fatalities are caused by drunk driving. In the United States, on average one person dies from an alcohol-related crash every 40-45 minutes. According to the National Highway Traffic Safety Association (NHTSA), these accidents result in $50 billion in costs each year. Young drivers are particularly at risk. Drivers in the 21-24 year old bracket are the most likely to drive drunk. Alcohol is also a factor in 31% of fatal crashes in the 15-20 year old bracket.

While for some drivers drunk driving may be a rare poor choice, many are repeat offenders. On average, a driver arrested for the first time for drunk driving has driven drunk 87 times before! Mothers Against Drunk Driving (MADD), an organization which has radically changed the way Americans view drunk driving, estimates that over 2 million drunk drivers with three or more prior convictions are on the road on any given day.

However, these drivers face stern consequences. In many places, drivers arrested with BACs of over .2 or .15 suffer from additional penalties, including the installation of ignition interlock devices (an in-car breathalyzer which drivers must blow into before the car will start), larger fines, and longer DUI programs. In many states, physicians are allowed to violate doctor-patient confidentiality in order to report drunk driving. In other states, such as Ohio and Minnesota, repeat offenders are issued special license plates. In South Africa, a country which has notoriously high rates of drunk driving accidents, local newspapers have begun to print “lists of shame” on Monday morning which make public the names of drivers convicted of drunk driving over the weekend.

Although rates of drunk driving arrests and convictions have declined in recent years, this is clearly still a serious issue. You can help to prevent drunk driving by making sure that you and your friends make safe choices. If you know you will be drinking, make arrangements to take a taxi, use public transportation, or call a safe ride service. You can also take turns serving as designated driver or choose bars and restaurants within walking distance. If worse comes to worst, call a family member or (very good) friend. While they may be annoyed at having to fetch you late at night, they’ll be glad that you’re safe in the morning.

If you host a party, make provisions for your guests. Either make sure that each group has a designated driver or provide space for guests to stay over. There are lots of delicious recipes for non-alcoholic cocktails and punches out there, so think about providing an attractive alternative for your non-drinking guests.

If you see a driver on the road who looks like he or she may be driving drunk, don’t hesitate to alert your local police. A report like yours will be enough cause for an officer to pull over the driver in question. Symptoms of drunk driving include rapid and erratic acceleration and deceleration, stopping suddenly and/or in an inappropriate place, driving in the center of two lanes, driving very slowly (i.e. under 10mph) and other similar odd behaviors.

Educate your friends and family as well. While 80% of drivers in the US know the term “BAC,” many don’t know or understand the legal limits for their states. Don’t let friends or acquaintances get behind the wheel after drinking and make sure they know what the consequences can be if they do. 

If you’d like to get more involved in the fight against drunk driving, there are a number of non-profit organizations dedicated to doing so. The most famous of these is Mothers Against Drunk driving (www.madd.org). Among their current initiatives is a lobby to have ignition interlock devices installed in the cars of all convicted drunk drivers, so that they cannot easily become repeat offenders. The Century Council (www.centurycouncil.org) is another leader in the fight against drunk driving.

Check out these sites for more information about defensive driving and business driver safety.

Airbag Safety: Making Your Airbag Work For You

Airbags are built to save lives, and they do. The National Highway Traffic Safety Administration (NHTSA) estimates that airbags saved 13,967 lives between 1987 and 2003. Since 1998, all new cars have been required to have front air bags; side airbags are now also increasingly common. These devices can reduce the risk of accident fatality by 11-13%.

However, airbags can also cause injury. As crashes happen very quickly, airbags deploy at around 200 mph. For those who are positioned incorrectly, as well as for children 12 and younger, this can pose a serious hazard. Each year sees a number of airbag-related injuries and fatalities, particularly among children. In order for airbags to do their job effectively, you, as the driver, have to do yours by making sure that everyone in the car is seated appropriately.

For Children

Even in cars without airbags, children 12 and under are safest in the back seat. Always try to plan ahead so that you won’t have to transport children in the front seat. Because children are smaller and more fragile than adults, airbags do not impact them appropriately. As a result, children sitting in the front seat can suffer serious injury from airbags if involved in a crash.

Infants under one year should be seated in rear-facing seats, as their head and neck muscles aren’t yet strong enough to resist the force of a crash. These seats must ALWAYS be placed in the back seat. Children older than one year can be placed in forward-facing convertible safety seats, which should also be placed in the back seat. Booster seats should then be used until seat belts fit the child appropriately.

If there is a compelling reasons for a child to be seated in the front seat, such as a medical condition that requires constant supervision, then the child should be properly restrained and the seat pushed back as far as possible. Installing an on/off switch for the airbag may also be a good idea in such a situation. However, you’ll need to be diligent about turning off the airbags when children are in the front seat and turning them on for adult passengers.

For Adults

Driver and passengers should always be sure to sit at least ten inches away from the airbags. It’s not a good idea for passengers to put their feet up on the dash or lean too close to the airbag. While short stature drivers may be inclined to sit close to the wheel, it is much safer to sit farther away; pedal extenders or other modifications may help to make this positioning more comfortable for short stature adults.

Figure 1:  Safe Distance from Airbag

 

While you were probably taught the “10 and 2” hand position, this is no longer the best alternative in the days of near-universal airbags. If your hands are too close to the top of the wheel when the airbags deploy, you’ll end up punching yourself in the face.

Instead, driving instructors—including those who teach drivers of emergency vehicles—recommend placing hands between “8 and 4” and “9 and 3.” In addition to being safer, this is also a more natural position that causes less stress to the arms.

 

Figure 2: “9 and 3” hand position

Instructors also recommend using modified hand-to-hand steering when turning. Instead of bringing your hands together at the top of the wheel, push up with one hand; then, before that hand reaches the top of the wheel, slide the other hand up and pull down. You will be feed the wheel through your hands without actually bringing either hand to the top of the wheel.

The future of airbags

Today, many car manufacturers are developing smart air bag technology that is making airbags safer to use. Since 2006, all car manufacturers have been required to implement sensors that can detect when a child or improperly positioned adult is in the front seat and modify (or stop) the deployment of airbags.

“Smart” airbags are also designed to deploy in two stages: a smaller deployment for minor fender benders and a full deployment for more serious accidents. In addition to being safer, these designs can also save money, as most airbags need to be replaced after deployment. Even if you have smart airbags in your car, however, you should still seat children in the back seat.

To read more on a broad range of subjects from “How To Change A Tire” to “How To Jumpstart Your Car”, visit DefensiveDriving.com’s Safe Driver Resources website!

Check out these sites for more information about defensive driving and business driver safety.

Work-related Car Accidents: Who Is Responsible?

If your employee has an accident while driving his or her own car to work, are you as the employer responsible? What about an employee who uses a personal car to take a prospective client to dinner and hits a pedestrian? What if a different employee has an accident while using a company car on personal business?

Whether you are responsible for a corporate fleet or simply have one or two employees who use their cars to run work-related errands, these are questions you should be able to answer. However, the answers can be a bit more complicated than you might think.

In general, employers have vicarious responsibility for the actions of their employees. Under the principle “respondeat superior,” an employer is legally responsible for any actions undertaken by employees during the course of employment. Determining what exactly the “course of employment” is can be a bit tricky.

Basically, if an action is undertaken to accommodate the needs of an employer and/or benefits the employer, then it is considered to be in the “course of employment.” For example, say an employer asks his employee to fetch work-related materials on his or her way home. The employer would then be liable for any accidents or damage incurred by the employee while driving home from work. This holds true whether or not the employee is driving a company car or a personal car and even if the driver only uses the vehicle for work-related purposes sporadically.

For example, in a recent case in California, Lobo v. Tamco (2010), the court ruled that an employer was responsible for the actions of an employee while travelling to and from work, as the employer required that the employee bring his car to work in order to use it for company errands. While the employee had only been required to use his car for work-related errands on 12 occasions during 16 years of employment, the employer was nonetheless held responsible.

Courts do often distinguish between “detours” and “frolics.” If an employee goes on a detour while carrying out his or her duties, then the employer will still be responsible. For example, if an employee stops for food while on the way to a sales call and an accident occurs at the drive-through, the employer will likely be held responsible. However, should an employee use a personal OR company vehicle for reasons of personal pleasure, this is defined as a “frolic,” and the employer is not held responsible. For example, if an employee has an accident with a company car while taking his family out to dinner, the employer won’t be held responsible. However, this may not necessarily be true. In some states, owner liability laws will ensure that the employer is responsible for any accidents that involve company cars, even if they occur while personal tasks are being carried out.

Employers can also be held responsible for negligent hiring. Employers need to conduct the necessary background checks to ensure that employees are suitable for the tasks that will be required of them. For example, an employer who hired a convicted child molester to drive an ice cream truck would be guilty of negligent hiring. This holds true for positions which require driving. If you are hiring an employee who will be required to carry out ANY kind of work-related driving, you, as an employer, will be responsible for checking that the prospective employee has a clear driving record, any necessary licensing, and the knowledge and skills necessary to drive safely. Employers can also be found guilty of negligent retention. If a driver accumulates repeated fines and offenses, his or her employer must take steps to prevent further problems, including suspension, firing, or the completion of a defensive driving course.

One area of employer liability that has recently come under intense scrutiny is the rise in distracted driving accidents. Employers can be held liable for cell-phone related accidents, particularly if the employer has created a work environment in which employees are pressured to be in constant communication, even while driving. If an employer issues cellphones to employees, the employer can also be held liable for phone related accidents.

As an employer, you can take several steps to minimize your liability for cell-phone related accidents, including:

  • Creating a clear written policy on cell phone usage and safe driving that is signed by all employees. For tips on how to do this, see my previous entry on how to create a safe driving policy.
  • Provide printed information on state laws regarding driving and cell-phone usage.
  • Have all employees sign an indemnity statement that absolves the employer of responsibility should they violate the company’s cell-phone usage policy.

Communication of expectations and regular safe driving education can also help to reduce the number of accidents and promote safe driving habits within the work place. Enrolling your workforce in a defensive driving course is a great way to educate your employees and prevent accidents. Find out more at http://www.businessdefensivedriving.com/