There is currently an attempt to re-industrialize South Africa, in light of this the company has decided to locally manufacture a low cost hybrid car. This car is to be aimed at the South African market so it must suit the South African consumer.
1.2 Problem Statement
A suspension system must be chosen to be used for the low cost, locally built, hybrid car.
This report aims to compare the feasibility of Macpherson strut suspension systems and double wishbone suspension systems in order to make an informed choice of which suspension system is to be used in the hybrid car. The feasibility will based on a number of different factors concerning the manufacturing and functionality of the suspension system. These factors will be the costs, functionality and design limitations.
1.4 Overview of Study
This report will going forward discuss the feasibility of the system to be implemented in the low cost hybrid car. It will do this by going through several relevant pieces of literature and looking at the car system as a whole. The literature serves to bring forth information needed to make the best choice of system which must meet the system considerations. The link between the two topics will be made clear in the discussion before closing off with a final decision in the conclusion.
Suspension systems are used in cars to help ensure a smoother ride and help the driver with control of the vehicle. The control aspect is often called handling. These systems are essential because even the smoothest roads will always have flaws and, although small, their impact would be felt greatly by the people in the car were it not for suspension systems. Here a comparison will be made between different aspects of three suspension system types.
Figure 1. How car suspension works (Harris, 2005)
The two types to be reviewed are the Macpherson strut and double wishbone suspension systems. Both systems make use of a wishbone shaped strut with the main difference being that the Macpherson strut uses a single wishbone while a double wishbone suspension system makes use of two. These “wishbones” are mounted to the chassis of the car. (Harris, 2005) (Linde, 2011)
Figure 2. Honda Coupe, example of double wishbone suspension (Harris, 2005)
They are independent suspension designs so they are placed separately on the required wheels. The double wishbone suspension system and MacPherson struts have been chosen for this literature review as both are generally the most popular suspension systems used in passenger cars. (Harris, 2005)
Macpherson struts can be used in either rear wheel suspension or in front wheel suspension. This is because of its more flexible design which allows for it to take up less space while still absorbing vibrations straight before they reach the passengers. They have a simpler design, with a single wishbone that only needs to be mounted once, which makes them the preferred option. (Mathis, 1995)
Figure 3. Example of a Macpherson strut (Gilles, 2005).
Unlike MacPherson struts, double wishbones are a lot more complicated in their design. As the name states they make use of two wishbone structures instead of the single one, used in Macpherson struts. The system must then be mounted twice. These “more complex” aspects of the parts allow for more flexibility in the use of this system.
The aspects allows for the system to be placed at different angles which influence certain specific factors based on the characteristics required. Double wishbone suspension systems are more suited to rear wheel suspension because of the space that they take up in the car. They are usually more situated to the rear where they can take up some of the boot space at the back instead of space for essential parts in front. (Gilles, 2005)
Due to the complexity of the system design; double wishbone suspension systems are quite expensive to manufacture. The assembly of the pieces means that when a part of the system breaks the whole system must be replaced. This means that the cost of repairs is higher and so is the cost of maintenance as again the entire system must be removed to make any and all adjustments.
On the other hand MacPherson struts have a much simpler design making the manufacturing process easier and hence, saving costs. Repairs and maintenance on the system would also be much simpler than if a double wishbone suspension system was used. (Gilles, 2005) (Mathis, 1995)
Since the aim of a suspension system is to make sure that the ride of the car is not only more comfortable but also to improve the handling of the car; in accordance with other specifications it is important to analyze these the most significant factors in suspension systems.
Both suspension systems are typically used for passenger cars like the one to eventually be chosen will be used in. Macpherson struts will give the riders a more comfortable, smoother ride and increased acceleration due to this fact. The drawbacks to this suspension system are that because of its design the car is lowered which means that at times when driving collisions may occur between higher parts of the ground and the actual structure of the car.
Also, the car’s handling ability is reduced making driving harder for the driver. This is because the car tires make less contact with the ground.
The double wishbone counters this as it creates more stability by making the outside of the tires make more contact with the surface of the road when cornering. I gives the driver better control increasing the handling of the car. This more rigid system does not give as smooth a ride as the Macpherson strut but the driver will appreciate having control. (Gilles, 2005) (Mathis, 1995)
3 System considerations
This section will clarify where and how the selected piece fits into the general system being the car. It looks at how the suspension system will interact with other parts of the car and the environment.
3.1 Rear wheel or Front wheel drive
Double wishbone suspension must be rear wheel drive because otherwise there is a limitation of space for other essential parts of the car. This is an important feature to consider in a hybrid car because there is more than one engine; a factor which already limit the amount of space available for parts to fit. At the back end of the car the space that gets used up is the space that would generally belong to the boot or the trunk of the car making it be able to carry slightly less.
The Macpherson strut can be used either in the front end or back end of the car depending on where it is needed most. Hence, whether the car is rear wheel drive or front wheel drive.
3.2 Off road or on road
The car in question is to be a low cost passenger car. It does not need to have off road capabilities as it is most likely a small sedan. This means no special adjustments or alterations must be made in that accord.
3.3 Elemental/climatic factors
As it is not meant for off-road navigation not much provision needs to be made for factors like excess mud or water like is prevalent in the wilder parts of the country. Our country is not constantly bombarded by things such as sand storms so excess dust will not be a factor in the wear of the parts.
From the observations made and all the data and information collected it can be seen that double wishbone suspension systems create a safer driving environment yet to do so they must compromise on comfort. The Macpherson strut suspension system has optimum comfort yet compromises the safety of the drive.
The hybrid car to be made must be cost efficient to appeal to the market. Looking at costs the Macpherson strut suspension system is not only cheaper to manufacture for the company but it is also is also cheaper than the double wishbone system to repair and maintain.
From an engineering standpoint more can be done with the Macpherson Strut system to modify it to suit the constraints than can be done with the double wishbone system. However, the Macpherson strut would not require those modifications as its simple mechanism allows for it to work well.
It is clear from the information stated above that the Macpherson strut suspension system is best suited for use in the low cost hybrid car. It has far more advantages than the double wishbone suspension system for the purpose of our application.
The system chosen is best for the applications but it still has a few major drawbacks. In future modifications should be made to the system to raise it higher off the ground to avoid constant collisions. There is also the safety concern of handling around corners. Ways must be found to make the system more rigid. A more stable system will make better contact with the ground reducing any risks that may ensue.
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