LAYOUT OF CHASSIS & TYPES OF DRIVES OF AUTOMOBILES

LAYOUT OF CHASSIS

The main parts of and automobile are mounted on the chassis. The layout of these components on the chassis are different in different types of vehicles, i.e., cars, jeeps, trucks, buses, etc. The main difference in the layout of this chassis is the position of the engine.

The engine is located at the front of the vehicle, followed by a clutch, gear box, propeller shaft, universals joints, differential, rear axle, etc. The radiator is located in front of the engine. Various other parts of the vehicle not shown in the layout  are dynamo, horn, steering box, fan, timing gear, carburetor, air filter, gear  control, steering wheel, cylinder, petrol tank, rear axle, front axle. The drive from the gear  box is conveyed through a short shaft to the front universal joint of the propeller shaft. From the propeller shaft it is conveyed to the rear wheel through a sliding splined type of joint. The bevel gear of the short shaft is driven by rear universal joint. This bevel gear meshes with a larger bevel gear which drives the two rear axle shafts through a differential gear.

1.8. TYPES OF DRIVES OF AUTOMOBILES

Some of the important drives of automobiles may be classified as follows:

            1.         Front engine - Rear wheel drive

            2.         Rear engine - Rear wheel drive

            3.         Front engine - Front wheel drive

            4.         Four wheel drive

1.         Front Engine - Rear Wheel Drive

In this layout a front mounted engine-clutch-gear box unit drives a beam type rear axle suspended on leaf sprints through a propeller shaft with two universal joints. With the help of coil sprints, the front wheels are independently sprung. As shown in Fig. 1.4 this layout is one of the oldest layout which remained unchanged for many years. some of the advantages provided by this system are :

            (a)        Balanced weight distribution between the front and the rear wheels.

            (b)        Easy front wheel steering.

            (c)        Behind the rear seats, large luggage space is available.

            (d)       Accessibility to various components like engine, gearbox and rear axle is better in  comparison to other layouts. The control linkages-accelerator, choke, clutch and gearbox are short and simple.

            (e)        Full benefits of the natural air stream created by vehicle’s movement is taken by the forward radiator resulting in reduced power losses from a large fan.

            (f)        Small length of the propeller shaft permits the angularity of the universal joints to be small and easily provided by simple types.

            By mounting the rear wheel drive assembly on the body unit and using universally jointed shafts to independently steer rear wheels as shown in Fig. 1.5, the layout design can be modified and improved. It provides number of benefits like improved handling, comfort and rear wheel grip as well as reduced unspring weight.

2.         Rear engine-Rear wheel drive

            This arrangement eliminates the necessity for a propeller shaft when the engine is mounted adjacent to the driven wheels. The engine-clutch-gear box-final drive form a single unit in this layout. As shown in Fig. 1.6, to reduce the ‘overhang’ distance between the wheel centres and the front of the engine, the final drive is generally placed between the clutch and the gear box. In comparison to front wheel drive it has a simpler drive shaft layout. Further, the weight of rear engine on the driving wheels provides excellent tranction and grip especially on steep hills as well as when accelerating. Inspire of the low proportion of the vehicle weight transferring to the front wheels, very effective rear wheel braking is possible. Due to the absence of the propeller shaft the obstructed floor space is reduced. The front of the vehicle can, therefore, be designed for good visibility and smooth air flow. the exhaust gases, fumes, engine heat and noises are also carried away from the passengers. It results in compact layout and short car.

            The layout also has got certain disadvantages like restricted luggage space due to narrow front compartment which houses the fuel tank also. Natural air cooling is not possible, it requires a powerful fan. The floor is further obstructed due to long linkage required for the engine, clutch and the gear box controls. The rearward concentration of weight causes the vehicle to be more affected by side winds at high speeds. this makes the vehicle unstable resulting in over steering and turning very sharply into a curve. This necessitates the steering correction in the opposite direction.

3.         Front engine-front wheel drive.

            This layout provides optimum body-luggage space and a flat floor line resulting in a transverse longitudinal engine position. This drive pulling the car along provides good grip and good road holding on curves due to major weight at the front. The chances of skidding especially on slippery surfaces are very much reduced. Good road adhesion is provided by the large proportion of the vehicle weight acting on the driven wheels. when the vehicle is to be ‘steered in’ to the curve, it provides ‘understeer’ characteristics always preferred by drivers.

The combination of steered and driven wheels with short drive shafts provides the main disadvantage. This requires special universal joints and a more complicated assembly. to prevent the rear wheels from skidding under heavy braking, the ‘reduced’ weight at the rear usually necessitates special arrangement.

4.         Front wheel steering Rear wheel drive

            1.         Access to the engine is very easy.

            2.         Slowing down of the water circulation causing cooling troubles can be avoided and long hose connections can be saved due to situating of the radiator in the main air stream.

            3.         This arrangement helps minimize the linkage between the clutch, gear box and engine.

            4.         The angularity of the propeller shaft is kept to minimum and there is no need of joints due to the shaft length.

                                   

Rear Engine-Rear Wheel Drive

Advantages

1.  Better road adhesion preferably on steep hills and while accelerating with increased weight on the driving wheels.

2.         Generally a proportional part of weight of the car is transferred to the front wheels while braking. Therefore, due to the firm road surface contact maintained by rear engined car results in assistance to stopping of the vehicle.

3.         In this arrangement, front wheels are only for steering purposes.

4.         The necessaity of the propeller shaft is altogether eliminated due to the combination of engine, gear box and final drive. This also requires only one common oil sump.

5.         Good visibility and stream lining is provided by proper design of vehicle front.

6.         The passengers are kept away from inconveniences like noise, heat and fumes.

Disadvantages

 1.        At high speed, the increased weight at the rear end makes the vehicle unstable.

2.         To control the engine, clutch and gear box, long linkages are required.

3.         The width of the car at the front gets reduced for accommodating the movement of the steering wheels resulting in reduction of size of the luggage compartment for given length and with of the car.     

  4.       The wheels get turned too sharply into the curve due to tendency of over-steering.  This necessitates the turning of the steering wheels in the opposite direction to make correction by the driver.

5.         Efficient cooling becomes very difficult to obtain due to screening of the engine by the vehicle body.

Front Engine - Front Wheel Drive

Advantages

1.         As compared to rear wheel driven car, there is a faster and safer travelling due to good road holding on curves.

2.         Good road adhesion is obtained due to a large part of the vehicle’s weight being carried on the driving wheels under normal conditions.

3.         Under-steer conditions generally preferred by many drivers are promoted by this type of drive. The car comes back to closer radius if the throttle is released. This makes the steering wheel to run more in the direction of turn to make it a better condition.

4.         A lower flat floor lines is provided due to dispensing with the propeller shaft resulting in lowering of centre of gravity.

5.         The engine, clutch, gear box and final drive are combined similar to the rear engine car. This provides a more comfortable drive due to final drive spring. 

Disadvantages

1.         Due to the weight of the vehicle moving to the rear, the weight on the driving wheels is reduced on steep gradients as well as while accelerating.

2.         The tractive effort which is most needed on steep gradients and during accelerating is reduced.

3.         This disadvantage becomes more serious on slippery gradients.

4.         Under these conditions certain modifications in modern designs have been made to ensure provision of sufficient traction.

5.         Four-wheels drive

To increase maneuverability of the vehicle required to travel on rough unconstructed roads and tracks another arrangement known as four-wheel drive is provided. due to all the four wheels getting driven, whole of the weight of the vehicle is available for traction. But this advantage is not worth the additional cost on good road surfaces. The system is provided in jeeps which are known as 4 X 4 wheel drive vehicles.

6.         Left hand and Right and drives

In different countries, the automobiles are driven on different sides of the road, In United Kingdom and all the countries, which were once colonies of the British Rule. The vehicles are driven on the left hand side of the road. In all other countries of the world, normally vehicles are driven on the right hand side of the road. For better driving control, the vehicle drivers must be nearer to one another while passing or crossing. Similarly for safety consideration, the drivers must be in the centre of the road while driving. Therefore, two types of vehicles are manufactured.

            (a)        Left hand drive: The steering is fitted on the left hand side of the automobile and such vehicles are convenient to drive in countries following right hand drive rules, e.g. U.S.A., Russia, European countries.

            (b)        Right hand drive :  The steering is fitted on the right hand side of the automobile and such vehicles are convenient to drive in countries following left hand drive rules, e.g. U.K., India, Pakistan. However, though rare, left hand cars also driven in such countries.

References

1. Automobile Engineering (Vol. 1 & 2)            -           K.M.Guptha
2. Automotive Mechanics                                   -           Joseph Heitner
3. Automobile Engineering                                 -           Harbans Singh Reyd
4. Automotive Mechanics                                   -           William H. Course