Chassis Systems
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CHAPTER 4 Chassis systems 4.1 Suspension 4.1.1 Overview of suspension The suspension system is the link between the vehicle body and the wheels ( Fig. 4.1 ). Its purpose is to: ● locate the wheels while allowing them to move up and down, and steer ● maintain the wheels in contact with the road and minimize road noise ● distribute the weight of the vehicle to the wheels ● reduce vehicle weight as much as possible, in particular the unsprung mass ● resist the effects of steering, braking and acceleration ● work in conjunction with the tyres and seat springs to give acceptable ride comfort. This list is diffi cult to achieve completely, so some sort of compromise has to be reached. Because of this, many different methods have been tried, and many are still in use. Keeping these requirements in mind will help you to understand why some systems are constructed in different ways. Unsprung mass (sometimes described as unsprung weight) is usually the mass of the suspension components, the wheels and the springs. However, Figure 4.1 Suspension plays a key role Automobile Mechanical and Electrical Systems. © 2011 Tom Denton. Published by Elsevier Ltd. All rights reserved. CH004.indd 347 3/30/2011 5:37:22 PM 348 4 Automobile mechanical and electrical systems Figure 4.2 Spring just before a bump Figure 4.3 Spring hitting a bump Defi nition only 50% of the spring mass and the moving suspension arms is included. This Unsprung mass is because they form part of the link between the sprung and unsprung masses. The mass of the suspension It is benefi cial to have the unsprung mass as small as possible in comparison components, wheels and springs. with the sprung mass (main vehicle mass). This is so that when the vehicle hits a bump the movement of the suspension will have only a small effect on the main part of the vehicle. The overall result is therefore improved ride comfort. A vehicle needs a suspension system to cushion and damp out road shocks ( Figs 4.2 and 4.3 ). This provides comfort to the passengers and prevents damage Defi nition to the load and vehicle components. A spring between the wheel and the vehicle body allows the wheel to follow the road surface. The tyre plays an important Damper role in absorbing small road shocks. It is often described as the primary form A device that reduces vibration. of suspension. The vehicle body is supported by springs located between the body and the wheel axles. Together with the damper (also described as a shock absorber), these components are referred to as the suspension system. As a wheel hits a bump in the road, it is moved upwards with quite some force. An unsprung wheel is affected only by gravity, which will try to return the wheel to the road surface. However, most of the energy will be transferred to the body. When a spring is used between the wheel and the vehicle body, most of the energy in the bouncing wheel is stored in the spring and not passed to the vehicle body. The vehicle body only moves upwards through a very small distance compared to the movement of the wheel. The springs in the suspension system take up the movement or shock from the road. The energy of the movement is stored in the spring. The actual spring can be in many different forms, ranging from a steel coil to a pressurized chamber of nitrogen. Soft springs provide the best comfort, but stiff springs can be better for high performance. Vehicle springs and suspension therefore are made to provide a compromise between good handling and comfort. Modern vehicles use a number of different types of spring medium, but the most popular is the coil (or helical) spring. Coil or helical springs used in vehicle Key fact suspension systems are made from round spring-steel bars. The heated bar is Modern cars usually use a coil (or wound on a special former and then heat-treated to obtain the correct elasticity helical) spring. (springiness). The coil spring can withstand any compression load, but not side thrust. It is also diffi cult for a coil spring to resist braking or driving thrust. Suspension arms are used to resist these loads. Coil springs are generally used with independent suspension systems; the springs are usually fi tted on each side of the vehicle, between the stub axle assembly and the body ( Figs 4.4 and 4.5 ). The spring remains in the correct position because recesses are made in both the stub axle assembly and body. CH004.indd 348 3/30/2011 5:37:25 PM Chassis systems 4 349 Figure 4.4 Figure 4.5 Coil spring in position Details of a coil spring Figure 4.6 Laminated springs The spring is always under compression owing to the weight of the vehicle and hence holds itself in place. The leaf spring can provide all the control for the wheels during acceleration, braking, cornering and general movement caused by the road surface. Leaf Defi nition springs are used with fi xed axles and usually on larger vehicles. They can be Laminated described as: Consisting of many thin layers. ● laminated or multileaf springs ● single-leaf or monoleaf springs. The multileaf spring was widely used at the rear of cars and light vehicles, and is still used in commercial vehicle suspension systems ( Fig. 4.7 ). It consists of a number of steel strips or leaves placed on top of each other and then clamped together. The length, cross-section and number of leaves are determined by the loads carried. The top leaf is called the main leaf and each end of this leaf is rolled to form an eye ( Fig. 4.8 ). This is for attachment to the vehicle chassis or body. The leaves of the spring are clamped together by a bolt or pin known as the centre bolt. CH004.indd 349 3/30/2011 5:37:28 PM 350 4 Automobile mechanical and electrical systems Figure 4.7 Commercial vehicle leaf spring 1 2 4 3 Figure 4.8 Details of a leaf spring: 1, fi xed shackle; 2, swinging shackle; 3, U-bolts; 4, damper Key fact The curve of leaf springs straightens The spring eye allows movement about a shackle and pin at the rear, allowing out when a load is applied to it, and the spring to fl ex. The vehicle is pushed along by the rear axle through the front its length changes. section of the spring, which is anchored, fi rmly to the fi xed shackle on the vehicle chassis or body. The curve of leaf springs straightens out when a load is applied to it, and its length changes. Because of the change in length as the spring moves, the rear end of a leaf spring is fi xed by a shackle bolt to a swinging shackle. As the road wheel passes over a bump, the spring is compressed and the leaves slide over each other. As it returns to its original shape, the spring forces the wheel back in contact with the road. The leaf spring is usually secured to the axle by means of U-bolts. As the leaves of the spring move, they rub together. This produces interleaf friction, which has a damping effect. A single-leaf spring, as the name implies, consists of one uniformly stressed leaf ( Fig. 4.9 ). The spring varies in thickness from a maximum at the centre to a minimum at the spring eyes. This type of leaf spring is made to work in the same way as a multileaf spring. Advantages of this type of spring are: ● simplifi ed construction ● constant performance over a period, because interleaf friction is eliminated ● Figure 4.9 Tapered single leaf reduction in unsprung mass. CH004.indd 350 3/30/2011 5:37:30 PM Chassis systems 4 351 Figure 4.10 Torsion bar in a guide tube Figure 4.11 Torsion bar spring Torsion bar suspension uses a metal bar, which provides the springing effect as it is twisted ( Fig. 4.10 ). It has the advantage that the components do not take up Key fact too much room. The torsion bar can be round or square section, solid or hollow. A torsion bar can be round or square The surface must be fi nished accurately to eliminate pressure points, which may section, solid or hollow. cause cracking and fatigue failure. They can be fi tted longitudinally or laterally. Torsion bars are maintenance free but can sometimes be adjusted. They transmit longitudinal and lateral forces and have low mass. However, they have limited self-damping. Their spring rate is linear and their life may be limited by fatigue ( Fig. 4.11 ). Steel springs must be stiff enough to carry a vehicle’s maximum load. However, this can result in the springs being too stiff to provide consistent ride control and comfort when the vehicle is empty. Pneumatic suspension can be made self- compensating. It is fi tted to many heavy goods vehicles and buses, but is also becoming popular on some off-road light vehicles. The pneumatic or air spring is a reinforced rubber bellow fi tted between the axle and the chassis or vehicle body ( Figs 4.12 and 4.13 ). An air compressor is used to increase or decrease the pressure depending on the load in the vehicle. This Key fact is done automatically, but some manual control can be retained for adjusting the Air springs are like a (strong) balloon height of the vehicle or stiffness of the suspension.