# Car Crash Design Lab by liwenting

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```									                 Car Crash Design Lab

This activity is designed to utilize your prior knowledge of physics and physical systems in order
select critical safety features of a car. In this lab you are asked to select components of a vehicle
that will enable a human passenger to survive a head on collision while travelling 45 mph.

In order to make educated selections for your car components, you must first become familiar
with the manner in which they interact.

Problem Statement:
You have been asked to select the final three design components that will complete the
construction of a car, but you must also make these choices without going over your allocated
budget. The three choices you have to make are as follows:

Bumper:
The bumper of the car is the first component to
come into play during a head on collision. You will
be asked to select a bumper based on a given
price and quality factor which will be explained
later during this introduction.
Hood:
The next component to consider is the hood of the car.
The hood acts as the second feature experiencing force
from the crash. You will be asked to select a bumper
based on a given price and quality factor which will be
explained later during this introduction.

Airbag:
The air bag is the final component that can help to
reduce the amount of force experienced by the driver.
Again, you will be asked to select a bumper based on a
given price and quality factor which will be explained
later during this introduction.
How they interact:

In order to understand how the different components work together we must first understand
how they work individually. For this lab, each component will be treated as either a spring or as
a system of springs. The three pieces together form a system of springs that absorb impact
from the crash and help reduce the amount of force experienced by the driver.

There are three different ways in which springs can be
represented. They can be either:

a) A single spring
b) Springs in series
c) Springs in parallel

As you have previously learned, a spring is associated with a
spring constant k, which in essence determines the strength
of the spring.

In this lab you will have to determine the overall spring
constant of the system by using the appropriate formulas as
well as the values associated with your component
selections. For each component of the car, you will be
making a decision that affects the overall strength of the
system.
The above table shows you how you are to calculate the overall spring constant for a given
system of springs. The following section of this introduction will show you what factors to base
your component selection process from and how to calculate each pieces contribution to the
system.

Bumper: Bumper Spring Constant k         B–   Number of Springs in Bumper NB

The bumper can be visualized as a component directly corresponding to the “In Parallel” spring
system. The factor you will be choosing is the number of internal springs contained within the
bumper. Regardless of how many springs you choose for your system to have, the spring
constant for each spring remains the same. This makes the overall spring constant for the
bumper:

Beq = NB * kB
Hood:       Hood Spring Constant kH – Number of Crush Points in Hood NH

The hood can be visualized as a series of springs due to the nature of the hood’s “crumple
zones.” These crumple zones, or crush points, act as individual springs along the length of the
hood. In order to help understand this effect, consider the following images:

You will be choosing the number of crumple zones
contained within the hood of the car. Regardless of how
many springs you choose for your system to have, the
spring constant for each spring remains the same. Each
zone acts as its own spring. Using this information you can
calculate the overall spring constant for the hood by:

1/Heq = Hn / kH

Airbag:      Airbag Spring Constant kA = Aeq

The airbag can be thought of as a simple 1 spring system that attempts to absorb any leftover
force from the crash and reduce the impact to the driver. The factor you will choose for the
airbag is the material it is made from which will affect the spring constant for the component.
You are essentially choosing the ka factor for your airbag.

Overall system: Overall Spring Constant - keq
Once you have made your selections you can now determine the overall spring constant for the
system. Here is the breakdown we have looked at so far.

1) The bumper is a spring system of springs in parallel
2) The hood is a system of springs in series
3) The airbag is a single spring

Taking these three components and viewing the overall system as one large series of springs,
you can refer once again to the formula for springs in series and calculate the total by:
Overall Spring constant for system:

1/keq = 1/ Beq + 1/ Heq + 1/ Aeq

Using this information:
Now that you know how to calculate the overall spring constant for your system, you will need
to know how to use this information to determine the level of safety in the vehicle you design.

Designing you car:

There are two critical elements to consider when thinking about the impact your car will have
during its crash:

1) The car’s mass  1360 kg
2) The velocity it is travelling 45 mph

Scientific research has shown that the amount of force a person experiences during a crash
relates in the following fashion:

Magnitude of Deceleration              Sustained Injuries            Acceptable Outcome
(measured in G’s)
0 ≤ F < 12                     Little to no injury                     Yes
12 ≤ F < 20                            Minor                            Yes
20 ≤ F < 35                      Serious Injuries                       No
35 ≤ F < 50                    Serious Injuries with                    No
possibility of Death

The potential energy residing in each spring prior to the crash is the key factor in limiting the
amount of force transferred to the driver. The potential energy present in each spring in
dependent on both the spring constant and its maximum displacement from equilibrium.

Process:

In order to ensure survival of the driver, it is necessary that they sustain less than 40 G’s of
force during the crash. In order to make this occur, the entire spring system must be capable of
mitigating much of the initial impact. The manufacturers of the car components provide you
with the corresponding spring constant for each piece. This information will be available within
the design simulation program. You are now ready to refer to the lab worksheet and begin