Automobile Systems Design

If you ever thought of making your own car, you might have come across a point where you thought of designing your car. Basically one can be divided into two different segments

 

  1. Exterior body designing for looks

vartan-980x650

Image credit: Scouladesign

2.  Automotive components and system design

chas5

This blog is about the later one: Automotive components and system design, which includes design of 5 basic systems of an automobile, viz.

  1. Chassis/Body
  2. Transmission
  3. Suspension
  4. Brakes
  5. Steering

For a better understanding, I would recommend you to refer a good book on automobile engineering

Automobile Engineering by R.B. Gupta for initial level

Theory of Ground Vehicles by J.Y. Young for deep understanding


Chassis/Body

Chassis is the base frame of an automobile. It is responsible for holding all the other components safely. Conventionally, chassis is a tubular structure upon which the components are mounted.

Read more about what is chassis

The chassis is designed to withstand the internal load by the components as well as external loads ranging from road bumps to severe accidents. Chassis design is started by fixing some basic quantities like Wheel base, Track width and approx dimensions of the vehicle. With these basic quantities fixed, a rough layout of chassis is made, something like below.chassis-frame1_thumb

This is the raw design of chassis with no consideration of its structural safety, ergonomics or manufacturability. taking this raw design as a base, designers design the chassis of the vehicle by considering above factors. Engineers perform various tests on the designed model by using softwares, which mainly includes strength check for front impact, rear impact, side impact, roll over and bump. Other than these Noise, Vibration and Harshness (NVH) test is done for vibrating components attached to chassis.

For the body works, the major concern is of the aerodynamics of vehicle. The measure for aerodynamics is determined by determining the value of Drag Coefficient, Cd by using Computational Fluid Dynamics (CFD).

Image credit: Google Search


Transmission

Transmission is responsible for motion of the vehicle, and it includes Engine, Clutch, Gearbox and wheels and shafts. Designing of these components is a subject in itself. I can’t cover their designing procedure in this blog. However, the specifications of these components are also decided by using mathematical calculations and applied physics.

For transmission design, again some quantities are fixed, like power, top speed and acceleration. By fixing the power, engine specification is already fixed and other parts are designed to achieve our desired performance. Clutch is always designed to transmit 1.5 times the power of engine. To better understand the working of clutch, you can go through the books refered above.

Gearbox and Wheels will be the final governing parts for the vehicles speed and acceleration. Out of these, Wheel size is generally fixed based on the requirement according to feasibility, traction and availability of standard parts. The final thing remaining is Gearbox.

Gearbox is one of the most important part of vehicles’ transmission system. A good gearbox is one which transmits maximum power to the wheel and at best proportion of RPM and torque. One must understand that high-speed and high torque cannot be achieved at same time and that is why we need gearbox. The gearbox is then designed for the required number of shifts and required gear ratio based on the engine RPM range and Torque range.

Since, transmission system has the maximum number of rotating parts, thus it goes through a large number of vibration analyses. NVH simulation is done mainly for the transmission system


Suspension

Suspension design is one of the most difficult step in automobile design. For a common person, suspension just includes shock absorbers which will prevent bumps from road to transmit to the passengers. But for engineers, there are a lot of parameters to be controlled in order to make a good suspension system.

There are total 8 types of suspension geometry, Namely

  • Swing axle.
  • Sliding pillar.
  • MacPherson strut/Chapman strut.
  • Upper and lower A-arm (double wishbone)
  • Multi-link suspension.
  • Semi-trailing arm suspension.
  • Swinging arm.
  • Leaf springs.

Each geometry has its own pros and cons and thus are selected for the vehicle based on their properties. For eg. Leaf spring is used for Heavy vehicles, where comfort is not the main priority. MacPherson strut, double wishbone and Multi-link suspension are used in cars. The type of geometry selected decides the ride comfort.

Suspension has many physical quantities to take care of such as roll centre height, motion ratio and frequency. Controlling these quantities is again a subject in itself.

After creating the geometry, it can be tested in simulation softwares like Adams or Lotus

Image source: Official Websites

These softwares gives a detailed result of ride comfort, ride frequency, camber range, Motion Ratio, wheel rate etc.


Brakes

Basically, automobile brakes work on pascal’s law (Principle of Transmission of Fluid Pressure). A very large force is required between brake pads and Disc/Drum to be able to stop the vehicle. This large force is achieved by a combined leverage of brake paddle and hydraulic force achieved by Pascal’s law. Thus we are able to stop our vehicle easily with a comparably very small force.

brake_system

Brakes are designed by keeping in mind various factors like desired deceleration, paddle force and cost. Cost mainly decides the use of either drum brakes or disc brakes. The braking system is considered good when it can stop the vehicle running in good speed, in minimum distance. Human comfort is also need to be taken care of while designing braking system. The paddle force and paddle travel should also be less.


Steering

The steering system of the vehicle is designed such that it provides maximum control over the vehicle, along with easy operation and ergonomic characteristics. There are two steering geometries available:-

  1. Ackerman Geometry
  2. Davis Geometry

Steering design starts with the selection of steering geometry and no on goes with Davis.

A good steering geometry is determined by the Ackerman percentage. Basically Ackerman percentage decide how much your inner tire turns compared to outer tire. 100 % Ackerman condition is when the turning circles of both inner and outer tire are concentric while 0 % means both circles are the same i.e inner tire turns the same angle that of outer tire.

Ackermann

While Ackerman percentage decides steering ability, but there is no such ideal Ackerman percentage. Low or High Ackerman percentage leads to under-steering or over-steering respectively. So an approximate value of Ackerman Percentage is fixed for steering design, based on the requirement.

Now the components of steering system are designed to fulfill the above conditions. The main components of steering are:

  • Rack and Pinion
  • Tie Rod
  • Knuckle
  • Steering Column
  • Steering Wheel

Rack and pinion is a gear mechanism which converts Circular motion into linear motion.

what-happens-when-rack-and-pinion-goes-out

It is designed such that it minimises the steering force with optimum lock to lock steering wheel turns. That means The Gear ratio of Rack and Pinion is decided purely for driver’s ergonomics. Tie Rod and Knuckle are designed to withstand forces acting on them during turn and bumps. Their design is also dependent on suspension. The complete geometry is designed and tested in Adams to check whether it is following the designed path.


This was the basic overview of Automobile System Design. There is a lot to learn in design of each system and their components. I will be writing about them soon. I hope you liked this post. Please like and share if you found it helpful. Comment for suggestion or clarifications

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