There is a positive relationship between the roll stiffness of the suspension and the vertical stiffness of the suspension, and the stiffness of the roll is reduced along with the reduction of the vertical stiffness. When the car is subject to lateral force and the body roll occurs, if the roll angle is too large, passengers will feel uncomfortable and reduce driving safety; if the roll angle is too small, the body will be subject to greater lateral shocks, poorer ride, and passengers. Will feel uncomfortable and the driver’s road feels bad. The vehicle roll stiffness should meet the following requirements: When the vehicle body is subject to lateral acceleration of 0.4g, its roll angle is in the range of 2.5° to 4°. The stiffness of the front suspension roll angle should be greater than the stiffness of the rear suspension roll angle, and the vehicle has a certain lack of steering. characteristic. Generally, the stiffness ratio between the front suspension roll stiffness and the rear suspension roll angle should be within the range of 1.4 to 2.6. If the front and rear suspensions cannot meet the above requirements or improve the ride comfort, the stabilizer bars can be installed in the front and rear suspensions. , Improve the car's handling stability without increasing the vertical stiffness.
Suspension's static deflection suspension's static deflection directly affects the body's vibration bias frequency (the front and rear part of the car's natural frequency), in selecting the front and rear suspension static deflection values, it should be close to, and hope the rear suspension The static deflection fc2 of the frame is smaller than the static deflection fc1 of the front suspension, which helps to prevent the vehicle body from generating a large longitudinal angular vibration. It is recommended that fc2 = (0.8 0.9 0.9) fc1 be taken. However, due to the short wheelbase of the mini-car, the rear seats are close to the rear wheels. In order to improve the comfort of the rear passengers, the rear suspension design is often softer. For each type of car, the frequency deviation value and the general value range of the static and dynamic deflection values ​​are taken as the lower limit value for the frequency deviation value of the car with high comfort requirements, and for the truck to take into account the difference in front and rear axle load and to avoid driver fatigue, before and after The ratio of static deflection is greater.
Another suspension indicator that is unsprung mass affects vehicle ride comfort is unsprung mass. The total mass of the car can be divided into two parts: sprung mass and unsprung mass. Parts of the mass carried by the elastic element, such as the body, frame and all other components and loads above the spring, are of sprung mass. Wheels, steering knuckles, and axles of non-independent suspensions are unsprung masses. If the unsprung mass is reduced, the vibration frequency of the body can be reduced, and the wheel vibration frequency is increased, which is advantageous for reducing the resonance and improving the smoothness of the automobile. The influence of unsprung mass on ride comfort is often evaluated by the ratio of unsprung mass to sprung mass m/M. The smaller the ratio, the better. The ratios of unsprung mass and sprung mass of several typical suspensions are listed.
The ratio between the unsprung mass and the sprung mass of several typical suspensions varies with the loading quality of the vehicle. Especially for the rear suspension of trucks, the axle load at full load is far from the maximum. Suspension with a constant stiffness within the expected range of load variation, ie, the elasticity of the suspension is linear, it may meet the frequency deviation requirement at full load and reduce the ambassador smoothness at the time of no load, or be satisfied at no load. When the frequency deviation is required and the full load is not enough, the dynamic deflection is too small, so that the limit block is frequently hit during running. In order to solve this contradiction, the suspension of the bus and the rear suspension of the truck often use the method of air springs and variable stiffness leaf springs to make it have non-linear elastic characteristics, so that the natural frequencies of no-load and full-load can be kept constant or Little change. The theoretically ideal elastic characteristics are shown. With this kind of suspension, the natural vibration frequency of the system remains unchanged under any load condition, ie: P0 - design load, N; CS0 - design load Suspension stiffness, N/mm; f - deflection of the suspension, mm.
Another important indicator of the relative damping ratio that affects vehicle ride comfort is the relative damping ratio. When the vehicle suspension has only elastic elements without friction or vibration damping devices, the vibration of the suspension mass of the vehicle will continue for a long time, causing the passengers to feel uncomfortable. Therefore, there must be vibration damping force in the suspension. For the selected suspension stiffness, only properly selecting the damping force can give full play to the damping effect of the suspension.
For a suspension system with a linear damper, the relative damping ratio can be used to evaluate the degree of damping or the speed of vibration attenuation. It is expressed as: The resistance coefficient of the k-suspension damper element shows that the damping effect of the shock absorber is related to the suspension stiffness and the sprung mass, in addition to its damping coefficient k. Different damping systems with different stiffnesses and different masses produce different damping effects. In order to obtain good ride comfort, typical relative damping is shown.
The bias frequency and relative damping ratio of the vehicle suspension require different damping forces for different road conditions. When driving on a flat, well-planned road, the suspension is required to have small damping, and when it is running on a bad road, it requires a large amount of damping. In addition, the damper coefficient of the shock absorbers in the modern car is not the same in the compression stroke and the extension stroke. In order to make full use of the spring in the compression stroke, the damping ratio of the compression stroke is often designed to be smaller than that of the extension stroke, which is approximately the extension stroke. 1/2 or so.
Due to the rational design of the valve system, the hydraulic damper can conveniently realize the nonlinear damping characteristics. The damping force of the hydraulic damper can change with the relative movement of the piston and the steel cylinder in terms of the direction, speed, and relative position between the two. An additional channel for the absorber fluid is added near the design load position so that the damping coefficient can be made smaller to obtain good ride comfort while running on a good road surface, and at the same time, sufficient damping force can be provided in a large stroke.
Concluding remarks By analyzing the block diagram of the “road-automobile-human†system, the main parameters and evaluation indexes of the suspension that affect the ride comfort of the vehicle are analyzed. When the suspension system parameters are designed, in addition to considering the smoothness of the car, it is also necessary to comprehensively consider The impact of the vehicle's steering stability and longitudinal stability, and take measures to improve ride comfort on the premise of ensuring vehicle safety.
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