Vehicle ride
1. Wheel rate is the vertical force acting on the tire per unit vertical displacement at the location along the spindle corresponding to the wheel centerline, measured relative to the chassis.
Wheel Rate= Spring Rate * (Motion Ratio)^2
Spring rate is a measurement of the amount of force needed to compress a spring at a particular distance.
2. Ride rate is vertical force per unit displacement of the tire\'s contact patch concerning chassis.
Wheel rate is the vertical force acting on the tire per unit vertical displacement at the location along the spindle corresponding to the wheel centerline, measured relative to the chassis.
3. No, we cannot use the same amount of damping in both the cases
i) High damping is preferred at the sprung mass natural frequency and low damping is preferred in the mid and high-frequency range.
ii) Compression damping is less than the rebound damping because during compression, the spring absorbs energy and hence the damping force needs to be lower.
4. The out of phase motion between the front and rear vertical motion, caused by the time delay between when the front wheel and rear wheel hit the bump is accentuated by the frequency difference. A result of the phase difference is pitching of the body. To reduce the pitch induced by hitting a bump, the rear needs to have a higher natural frequency to catch up with the front. This notion is called producing a flat ride, meaning that the induced body pitch from road bumps is minimized.
5. i) A higher unsprung mass is preferred for improved vibration isolation in the wheel hop region.
ii) A lower damping is preferred for improved vibration isolation in the wheel hop region.
6. The air spring system is well known for its low transmissibility coefficients and its ability to vary load capacities with only the change of the gas pressure within the springs. One of the advantages of air springs is that the energy-storage capacity of air is far greater per unit weight than that of mechanical spring material, such as steel. Because of the efficient potential energy storage of springs of this type, their use in a vibration-isolation system can result in a natural frequency for the system which is almost 10 times lower than for a system employing vibration isolators made from steel springs. This leads to applications where low-frequency vibration isolation is necessary. Air springs have not only lower resonance frequencies but also smaller over-all length than mechanical springs with equivalent characteristics. The ability to change the load-carrying capacity simply by changing the air pressure rather than changing out the air spring is a major advantage that air springs have over steel springs. The air spring is mainly used in commercial vehicles, but lately, it is also used in higher classes of passenger vehicles.
7. sprung mass= 2000kg
weight distribution = 50/50
sprung mass in quarter model= 2000/4 = 500kg
unsprung mass in quarter model= 50kg
Kr= Kt*Ks/(Kt+Ks) = (250*25)/275 =22.72N/mm
Fs=(1/2π)√(Kr/Ms) = (1/2π)√(22.72*1000/500) = 1.073Hz
Ke= Kt+Ks =250+25=275N/mm
Fus=(1/2π)√(275*1000/50) = 11.80Hz
8. Case:1
front ride frequency=1.2Hz
Rear ride frequency=0.8Hz
Tf=1/1.2= 0.83sec
Tr=1/0.8= 1.25sec
Tf=Tr+Δt
0.83=1.25+Δt
Δt=-0.42sec
Case:2
front ride frequency=1.2Hz
Rear ride frequency=1.45Hz
Tf=1/1.2=0.83sec
Tr=1/1.45=0.69sec
Tf=Tr+Δt
0.83=0.69+Δt
Δt=0.14sec
Wheelbase= 98.42inches= 2.5m
Speed=37.42mph=16.73m/s
time lag=wheelbase/speed = 2.5/16.73= 0.15 sec
Ride frequency in case 2 works best for a flat ride.