Vehicle Ride
1.Why is the wheel rate usually different from the spring rate?
It is because of the mounting location of the spring is different from the wheel center.
Wheel rate = Spring rate X (Motion ratio)^2
If the springs are mounted exactly at the wheel center with an angle 90 degree then the wheel rate and the spring rate will be same but practically it is not possible so wheel rate is different from spring rate.
2.Why is the ride rate different from the wheel rate?
Riderate = (Ks)(Ktr)/(Ks+Ktr)
Ride rate is calculated by considering the tire rate and spring rate connected in series.Vertical force per unit displacement of the tire ground contact point w.r.t tire stiffness and spring stiffness.
But in case of wheel rate it only depends on spring stiffness and motion ratio without considering the tire stiffness.ie, Wheel rate = Spring rate X (Motion ratio)^2
3.Can you use the same amount of damping for
I.Low and high frequency regions
No. For low frequency high damping is prefered because to lower the transmissibility ratio.But for mid and high frequency range low damping is prefered to lower the transmissibility ratio.
II.Compression and rebound
No, Because during compression damping can be low as compared to rebound because spring is also absorbing energy during compression. So about 2 to 3 times low damping compare to rebound.
During rebound spring will release the energy so damper needs to control the energy realease also , so high damping is prefered compare to compression.
4.Why does the rear ride frequency need to be higher than the front?
When car hitting bump, because of wheel base reartire will hit the bump after some time delay than front. So If front and rear frequency is same then pitching will happen because of time time delay. So to compensate this time delay rear frequency needs to be higher than the front to match the frequency to avoid pitching effect of the vehicle.
5.For improved vibration isolation in the wheel hop region, would you prefer:
I.A higher or lower unsprung mass?
A lower unprung mass is prefered in wheel hop frequency (High frequency) to lower the transmissibility ratio
II.Higher or lower damping?
At wheel hop frequency range (High frequency) low damping is prefered to lower the trasmissibility ration for vibration isolation.
6.From a ride frequency standpoint, why are air suspensions beneficial, especially in heavy vehicles (do some research to answer this)
Main advantage in Air suspension is spring stiffness can be automatically adjustable for weight that is added to the truck.Level sensors can automatically adjust the height to meet required ride height. So it can able to maitain almost constant required ride frequency so ride and handling will be achieved compare to leaf spring suspension.
7.Calculate sprung mass and unsprung mass natural frequencies for a vehicle with a 2000 kg sprung mass, 50/50 weight distribution, 50 kg of unsprung mass in each corner, spring stiffness of 25 N/mm and tire stiffness of 250 N/mm. Assume gravity to be 10 m/sec^2 to simplify calculations. Report your answer in hertz
Riderate = (Ks)(Ktr)/(Ks+Ktr)
=(25e3)(250e3)/(25+250)e3
=22X10^3 N/m
Body bounce frequency =1/2*pi(√ (kr/Ms)
=1/2*3.14(√ (22X10^3/500)
= 1.055Hz
Unsprung mass nat.Freq=1/2*pi(√ ((Ks+Ktr)/Mus)
=1/2*3.14(√ ((275X10^3)/50)
= 11.8Hz
8.Calculate the time lag between the front and rear suspensions for:
Time lag = wheel base/Vehicle speed
= 2.499/16.728 (sec)
= 0.149 sec
CASE1: Front ride frequency of 1.2Hz, Rear ride frequency of 0.8Hz
Period of Front = 1/FR Frequency
=1/1.2
=0.833 Sec
Period of Rear =1/RR Frequency
=1/0.8
=1.25 Sec
Adding time delay with period of rear = 1.25 + 1.49
=2.74 sec
So period in FR(0.833 sec)< RR(2.74 Sec), Because of the varation in periods of front and rear frequency pitching will happen.
CASE2: Front ride frequency of 1.2Hz, Rear ride frequency of 1.45Hz
Period of Front = 1/FR Frequency
=1/1.2
=0.833 Sec
Period of Rear =1/RR Frequency
=1/1.45
=0.689 Sec
Adding time delay with period of rear = 0.689 + 1.49
=0.838 sec
So period in FR(0.833 sec) = RR(0.838 Sec), Because the periods of front and rear frequency is almost same pitching will not happen. So case 2 is prefered