STEERING SYSTEM

In the Oxford dictionary, a steering wheel is defined as an apparatus for steering a vehicle. A steering mechanism in an automobile, therefore, converts the driver rotational input at the steering wheel into a change in the steering angle of the road wheels. More often than not a steering wheel it an automobile is the quickest and easiest object in The single most important factor that governs the relationship between the driver and the steering wheel is the irreversibility factor. In layman's terms steering is said to be irreversible if road shocks are not transferred to the driver. So you would think that every vehicle should have a steering box that is as irreversible as possible. Well, not quite. If the steering box won't transfer shocks then, as expected, there won't be much feel either. Also the efficiency will, understandably, be low and consequently require d heavier steering action. So ideally, every steering should be irreversible enough not to transfer any shocks, but reversible enough to give enough leverage for easy operation and good efficiency. Apart from this a steering mechanism should not have any play in the dead, straight position and should also be easy to adjust. Most passenger cars usually employ one of two widely used steering, mechanisms - a rack and-pinion system or recalculating ball steering system The steering system, no doubt, forms an important part of vehicle handling dynamics. It is the most important mechanism where direction changes of a vehicle at different speeds are concerned. However, there is yet another factor that forms an important part of the steering and handling dynamics. It is called the slip angle and it is dependent on the weight, construction, tyres, performance and drive configuration of the vehicle (whether a front wheel drive or rear wheel drive). etc. Slip angle is a term used to describe a particular kind of flex in tyres. Even though the term uses the word 'slip this characteristic has nothing to do with slipping of skidding .Slip angle is a measurement of how much the tyre's contact patch has twisted (steered) in relation to the wheel. A good way to understand this characteristic is to stand beside your car whilst the steering wheel is being turned. If you observe the left front wheel, you will see that it steers a few degrees before the tyre's contact patch starts to turn. It is not uncommon for this slip angle to be as large as six to ten degree. on a race track. 'This characteristic is important because, as long as the tyres have traction, the car tries to go where the tyres are pointed. So, even though the wheels may not be steered, the tyres themselves could be steering due to the slip angle. On a small oval track the tyres only need to steer slightly over three degrees for the car to travel through the turns. On large race tracks the tyres may need to steer two degrees or less. However, steering can also occur at the rear tyres due to their slip angles. Very often, in a corner, you will turn The steering wheel a certain number of degrees initially, and you then need to reduce the stee.ing angle before you reach the apex of the turn. This is because the rear tyres are generating a slip angle and some of the steering is taking place at the rear tyres. It is even possible to travel through the turn with the steering wheel in the straight-forward position because the rear tyres are doing all of the steering. Automotive engineers call this condition 'critical speed . On big race tracks, these turn in and turn-back characteristics are less pronounced than on shorter tracks, but when they do occur, you'd better be alert as you are nearing the limit. Increasing the slip angles at the rear of the car will make it over steer (the front end tends to turn into a corner more than expected). Likewise, reducing the silp angles at the rear will make the car understeer (the front end tends to turn in later than expected). Weight transfer from the inside tyres to the outside tyres affects the slip a angles. When a race car is turning, equally loaded tyres will run at smaller slip angles. Conversely, a large difference between the inside tyres load and outside tyre's load will cause the slip angle to increase. Springs , spring location. Roll centers , etc affect weight transfer and, in turn, trigger the differences in slip angles, in addition to the factors mentioned above. If you haven't understood any of that technical mumbo -jumbo, the following will surely help you understand. You may have often heard a Formula 1 commentator shouting himself hoarse about a car starting to understeer when it loses front end grip as the tyres wear out. Now, this may lead you to believe that understeer is a bad thing. Hang on a minute and think about the time you sneaked out in your father's Amby and went for a joyride. Surely you still remember how you cornered too fast and the car fishtailed and spun out. Still gives you goose bumps doesn't it? Well, if you still don't know what hit you let me welcome you to the world of over steer. Simply put, it is when the car will turn more than you expect it to. 'Over-steer' is difficult to control and requires the application of opposite lock, which in itself an art learned over time. Also, you need to do a tap dance on the accelerator and brakes. But, try your childhood antics in your new front wheel-driven car, and it just won't fishtail. What it will do, though. is corner less than you expect it to and this is what understeer is. So, to regain control of your car all you need to do is get off the throttle and the car will fall back into line. Now you tell me. Which would you rather have ? if you still say over steer you are our kind of fun-loving guy. Remember, changes that increase the weight transfer are going to increase the slip angle. Changes that reduce the weight transfer will reduce the slip angle. lncreasing slip angles at the rear makes your car looser, and reducing the slip angles (at the rear) will make your car lighter in the corners. Also, keep in mind that these causes and effects are true as long as the tyres have traction. If you drive on oil/water. Or for some other reason your tyres are already sliding, then you can pretty much forget what has been explained above. The purpose of the steering system is to turn the front wheels. In some cases, it also turns the rear wheels. The wheels constantly change direction, while switching lanes, rounding sharp turns, and when avoiding roadway obstacles.

MANUAL-STEERING SYSTEMS

The steering system is composed of three major subsystems: the steering linkage, steering gear, and steering column and wheel. As the steering wheel is turned by the driver, the steering gear transfers this motion to the steering linkage. The steering linkage turns the wheels to control the vehicle’s direction . Although there are many variations to this system, these three major assemblies are in all steering systems.

Steering Linkage

The term steering linkage is applied to the system of pivots and connecting parts placed between the steering gear and the steering arms that are attached to the front or rear wheels that control the direction of vehicle travel. The steering linkage transfers the motion of the steering gear output shaft to the steering arms, turning the wheels to maneuver the vehicle. The type of front-wheel suspension (independent wheel suspension as compared with a solid front axle) greatly influences steering geometry. Most passenger cars and many light truck and recreational vehicles have independent front-wheel suspension systems, Therefore, a steering linkage arrangement that tolerates relatively large wheel movement must be used.

Parallelogram Steering Linkage

A parallelogram type of steering linkage arrangement was at one time the most common type used on passenger cars. Now it is found mostly on larger cars, pickups, and larger SUVs. It is used with the recirculating ball steering gear and can be classified into two distinct configurations: parallelogram steering linkage placed behind the front-wheel suspension and parallelogram steering linkage placed ahead of the front-wheel suspension. This type of steering linkage is most often used where motor and chassis components would interfere with normal operation of the steering linkage. These designs are the basic steering systems used in conjunction with independent front-wheel suspensions. This type of linkage also provides good steering and suspension geometry. Road vibrations and impact forces are transmitted to the linkage from the tires, causing wear and looseness in the system, which permits intermittent changes in the toe setting of the front wheels, allowing further tire wear. In a parallelogram steering linkage, the tie-rods have ball socket assemblies at each end. One end is attached to the wheel’s steering arm and the other end to the center link. The components in a parallelogram steering linkage arrangement are the pitman arm, idler arm, links, and tie-rods.

PITMAN ARM

The pitman arm connects the linkage to the steering column through a steering gear located at the base of the column. It transmits the motion it receives from the gear to the linkage, causing the linkage to move left or right to turn the wheels in the appropriate direction. It also serves to maintain the height of the center link. This ensures that the tie-rods are able to be parallel to the control arm movement and avoid unsteady toe settings or bump steer. Toe, a critical alignment factor, is a term that defines how well the tires point to the direction of the vehicle.

There are two basic types of pitman arms: wear and nonwear. Service needs different 4depending on which 6pe of arm is used. Nonwear arms have tapered holes at their center link ends and normally need to be replaced only if they have been damaged in an accident or have been mounted with excessive tolerance. Wear arms have studs at the center link end and are subject to deterioration from normal operation. These arms must be inspected periodically to determine whether or not they are still serviceable.

IDLER ARM

The idler arm or idler arm assembly is normally attached, on the opposite side of the center link, from the pitman arm and to the car frame, supporting the center link at the correct height. A pivot built into the arm or assembly permits sideways movement of the linkage. On some linkages, such as those on a few light-duty trucks, two idler arms are used. Idler arms normally wear more than pitman arms because of this pivot function, with wear usually showing up at the swivel point of the arm or assembly. Worn bushings or stud assemblies on idler arms permit excessive vertical movement in the idler arms.

LINKS

Links, depending on the design application, can be referred to as center, drag or steering links. Their purpose is to control sideways linkage movement, which changes the wheel directions. Because they usually are also mounting locations for tie-rods, they are very important for maintaining correct toe settings. If they are not mounted at the correct height, toe is unstable and a condition known as the toe change or bump steer is produced. Center links and drag links can be used either alone or in conjunction with each other, depending on the particular steering design. There are several common designs of center links. Like pitman arms, they can be broadly characterized as either wear or nonwear. Center links with stud or bushing ends are likely to become unserviceable from the effects of normal operation and should be inspected periodically. Links with open tapers are nonwear and usually need to be replaced only if they have been damaged in an accident or through excessive tolerance at the mounting position of the idler or pitman arms.

TIE-RODS

Tie- rods are actually assemblies that make the final connections between the steering linkage and steering knuckles. They consist of inner tie-rod ends, which are connected to the opposite sides of the center link: outer tie-rod ends, which connect to the steering knuckles; and adjusting sleeves or bolts, which join the inner and outer tie-rod ends, permitting the tie-rod length to be adjusted for correct toe settings Tie-rods are subject to wear and damage, particularly if the rubber or plastic dust boots covering the ball stud have been damaged or arc missing. Contaminants such as dirt and moisture can enter and cause rapid part failure. A special bonded ball stud, in which no boot is used, is available for use on certain light-duty two-wheel-drive and four-wheel-drive trucks. An elastomer bushing bonded to the stud ball provides strong shock absorption and steering return in downsized vehicles.

Rack and Pinion Steering linkage

Rack and pinion is lighter in weight and has fewer components than parallelogram steering .Tie-rods are used in the same fashion on both systems, but the resemblance stops there. Steering input is received from a pinion gear attached to the steering column. This gear moves a toothed rack that is attached to the tie-rods. In the rack and pinion steering arrangement, there is no pitman arm, idler arm assembly, or center link. The rack performs the task of the center link. Its movement pushes and pulls the tie-rods to change the wheel’s direction. The tie-rods are the only steering linkage parts used in a rack and pinion system. Most rack and pinion constructions are composed of a tube in which the steering rack can slide. The rack is a rod with gear teeth cut along one end-spur and helical. The other end is fitted with two balls to which the ends of the divided track rods are attached. The rack meshes with the teeth of a small pinion at the end of the steering column. The two inner tie-rod ends, which are attached to the rack, are covered by rubber bellows boots that protect the rack from contamination. The inner tie-rods connect to outer tie-rod ends, which connect to the steering arms. The rack and pinion housing is fastened to the vehicle at two or three points. In some cases, the rack and pinion steering gear on unibody can is bolted directly to a body panel, like a cowl. When this is done, the body panel must hold the steering gear in its correct location. The unibody structure must maintain the proper relationship of the steering and suspension parts to each other. Along with other advantages, the rack and pinion steering system combined with the MacPherson strut suspension system is found in most front-wheel-drive unibody vehicles because of their weight and space-saving feature. The driver gets a greater feeling of the road with rack and pinion because there are less friction points. This means a higher probability of car owners with steering complaints. Fewer friction points can reduce the systems total ability to isolate and dampen vibrations.

RACK

The rack is a toothed bar contained in a metal housing. The rack maintains the correct height of the steering components so that the tie-rod movement is able to parallel control arm movement. The rack is similar to the parallelogram center link in that its sideways movement in the housing is what pulls or pushes the tie-rods to change wheel directions.

PINION

The pinion is a toothed or worm gear mounted at the base of the steering column assembly where it is moved by the steering wheel. The pinion gear meshes with the teeth in the rack so that the rack is propelled side-ways in response to the turning of the pinion.

YOKE ADJUSTMENT

The rack-to-pinion lash, or pre-load, affects steering harshness, feedback, and noise. It is set according to the manufacturer’s specifications. An adjustment screw plug, or shim pack are located on the outside of the housing at the junction of the pinion and rack to correct or set the yoke

TIE-RODS

Tie -rods are very similar to those used on parallelogram systems. They consist of inner and outer ends and adjusting sleeves or bolts. The inner tie-rod ends on rack and pinion units are usually spring-loaded ball sockets that screw onto the rack ends They are preloaded and protected against contaminant entry by rubber bellows or boots.