Vehicle Ride
The answer to Question 1:
Strut in a vehicle is not installed at the top of the wheel but it is located a little sideways and due to this, the wheel rate is not equal to the spring rate.
So, Wheel rate = Spring rate + (Motion Ratio)^2
motion Ratio arrises due to this sideways assembly of the strut from the wheel.
Motion ratio is less than or equal to 1.
In the case of Mcpherson Strut, Motion ratio is 1 and Wheel Rate = Spring Rate
The answer to Question 2:
Ride Rate is the vertical movement of tire contact patch with respect to the chassis or sprung mass whereas the Wheel Rate is the vertical movement of the wheel center with respect to the sprung mass.
The answer to Question 3:
(i) No, for low and high-frequency regions we will have to use the different type of dampings. For a low-frequency region, we have to use high damping whereas for high-frequency regions low damping is preferred. The idea here is to keep the transmissibility ratio low.
From the graph, we can see that the transmissibility ratio for low damping at high-frequency regions is low whereas, in the case of low-frequency regions, a higher amount of damping will give lower transmissibility ratio.
(ii) For compression in the spring, the energy is going into the spring and because of that, the spring is restricting the further compression of it so in this case, we will need a lower amount of damping whereas, in the case of rebound the spring is releasing its energy and so for that we will need a high amount of damping to restrict its further expansion.
The answer to Question 4:
When a vehicle Encounters a bump, the front wheel hits it at first and the rear wheel hits it after some time. This time difference causes pitching in the vehicle if the two ride frequencies are same.
Consider the given graph below,
it can be clearly seen in the case of the two ride frequencies being equal or near to the equal, the pitching is more pronounced whereas in the case when the rear ride frequency is higher than that of the front right frequency, the rear ride phase catches up with the front ride phase and the ride becomes flat which is preferred because of in phase flat ride.
The answer to Question 5:
(i) A lower unsprung mass will provide better vibration isolation in the wheel hop region.
Lower the unsprung mass more will be its natural frequency.
More the natural frequency of the unsprung mass more will be the requirement of the vibration frequency by the external force to get into resonance with the unsprung mass natural frequency.
hence, for a lower unsprung mass, we get good vibration isolation.
(ii) More damping decreases the natural frequency of the unsung mass.
Hence, less damping is required to achieve a high natural frequency of the unsprung mass so that for resonance to occur at wheel hop region than it could happen at a high-frequency point.
hence, lesser damping is preferred for good vibration isolation.
The answer to Question 6:
An Air Suspension is a type of suspension where we have an airbag in place of the strut. These airbags are further connected to a compressor which pumps in or out air as required.
The advantage with Air Suspension is that we can change the stiffness of it by pumping in or out some gas.
Another advantage is that we can change the ride height too.
Now in the case of wheel hop region, the pressure inside this suspension increases which increases the natural frequency of the wheel and resonance does not happen.
Therefore a flat ride is insured.
The answer to Question 7:
Given, Total Sprung Mass = 2000 kg
unsprung mass, Mus = 50 kg (each corner)
Sprung mass in quarter car model, Ms = kg = 500 kg
spring stiffness, Ks = 25 N/mm = 25 kN/m
tyre stiffness, Kus = 250 N/mm = 250 kN/m
The answer to Question 8:
1.Front ride frequency of 1.2Hz, Rear ride frequency of 0.8Hz
Wheelbase = 98.42 inches = 2.5 m
speed = 37.42 m/h = 16.72 m/s
Time lag due to wheelbase, t = 2.5/16.72 s = 0.15 s
Front Ride Time period, Tf = 1/1.2 = 0.83 s
rear ride time period, Tr = 1/0.8 = 1.25 s
Clearly, Tf < Tr + t
0.83 < 1.25 + 0.15
In this case flat ride is not possible.
2.Front ride frequency of 1.2Hz, Rear ride frequency of 1.45Hz
Front Ride Time period, Tf = 1/1.2 = 0.83 s
rear ride time period, Tr = 1/1.45 = 0.68 s
Clearly, Tf = Tr + t
0.83 = 0.68 + 0.15
In this case flat ride is possible.