Vehicle Ride

Question 1.

To begin with, the distance from the wheel centre to the point of pivot on the link is different, i.e., usually greater than the distance from the spring to the point of pivot on the present link in the given suspension system. As the link goes in jounce and rebound, producing deflection as a result, there will be moments created by the wheel and the spring. Also, on the other hand, there will be forces acting on the spring and the wheel respectively. Now, wheel rate or the stiffness of the wheel, Kw is defined as force per unit deflection.

Since the distance from wheel centre to pivot point is greater than the distance from the spring to the pivot point, Force due to the wheel, known as wheel force is greater than the force due to the spring, known as spring force. Ultimately, the wheel rate will be greater, or different than the spring rate. It can also be seen from the equation below:

Kw = Ks*`(M.R.)^2`.

where, Kw = wheel rate, Ks = spring rate , M.R. = Motion Ratio.

Motion ratio is a geometric measure that relates the wheel centre rate to the spring rate and is a key factor in deteremining the difference between the two rates.

Question 2.

Wheel rate is the result of the vertical force on the whell subjected to unit deflection.If (delta-x) is the deflection of the wheel as a result of the jounce of the suspension link, and Fw is the vertical force acting on the wheel, then, the stiffness or the wheel rate is given by:

`Kw = (Fw)/(deltax)`.

The ride rate on the other hand, is a result of the combination of wheel rate and spring rate which are connected in series with the sprung mass of the vehicle. The ride rate is given by:

`Kr = (Ks*Kw)/(Ks+Kw)`. where Ks and Kw are spring rate and wheel rate of the vehicle repectively. This is turn is used to obtain the sprung mass frequency of the vehicle.

Ride rate is basically a function of both spring rate and wheel rate. Hence, as wheel rate changes the spring rate changes, and ultimately, the ride rate changes. Hence, the ride rate is usually different from the wheel rate of the vehicle.

Question 3.

(i) We cannot have the same amount of damping in low and high frequency regions. The main aim behind this is to keep the transmissibility ratio of the vehicle suspension as low as possible. Transmissilibility ratio is the ratio of sprung mass displacement to the road displacement. To keep this ratio as low as possible, we need to have a high amount of damping at the sprung mass natural frequency and a low amount of damping at mid to high range of frquencies.

(ii) No, we cannot have the same amount of damping at compression and rebound.

Compression damping should be lower than that of rebound damping, usually by a factor of 2. This is because, in compression, the springs absorb energy and hence damping force needs to be low.

On the other hand, in rebound, energy is released from the spring and the damper has to control this energy released and therefore rebound damping needs to be higher than that of compression damping.

Question 4.

For a flat ride, the rear ride frequency has to be higher than the front ride frequency since the rear supension tend to oscillate more than the front suspension because there is a time delay between the front and rear suspension encountering the bump or an obstacle on the road. If frequencies are same, the front and rear suspension are out of phase and it leads to pitching in the vehicle. The out of phase motion is caused because the rear hits the bump a little later than the front suspensipn due to the wheelbase seperation. Generally, the rear ride frequency is 1.2 times the front ride frequency which results in a In-pahse motion betwwen the front and rear suspension.

Question 5.

For improved vibration isolation, we must satisfy the following characterstics:

(i) We must have a lower unsprung mass at the wheel-hop region, since it provides better isolation at unsprung mass natural frequency and also keeps the transmissibility ratio low in the high frequency range. The main aim is to keep the transmissibility ratio of the suspension as low as possible and this supports the cause.

(ii) Similar to the above case, we must have low amount of damping in the wheel-hop region or in the mid to high frequency range to get a low transmissibility ratio and a desirable vibration isloation of the suspension system. For eg., if we have damping amounts of 0.5,0.07 and 0.01, damping amount of 0.01 is most preferred at the wheel-hop region and a damping amount of 0.5 is prefereed at low frequency range to provide better vibration isolation.

Question 6.

Air suspension is a type of vehicle suspension driven by an electric or an engine-driven pump or compressor. It provides more driver comfort due to reduction in noise, harshness and vibration on the road. It is mainly benefical in heavy vehicles in place of conventional steel springs, as it offers more consistency and it keeps all the wheels even. The air suspension system keeps heavy vehicles lke trucks level from side to side, especially when heavy cargo materilas are to be transported. This results in reduced body roll during cornering. It reduces the tendency of short wheelbase trucks to bounce over rougher roads and terrains when the vehicle is empty. It also improves the ride height based on load weight and vehicle's speed.

Question 7 and Question 8 are provided in the link below:

https://drive.google.com/file/d/1_Fv1rQR9xROMV5pqxHzVXLN0WtMSVb4f/view?usp=sharing