Chinese and English catalogue
The driving stability of a car refers to the ability of the car to keep the normal driving state and direction under the action of external factors during driving, so as not to lose control and cause slipping and overturning.
The main factors that affect the driving stability of the car are the structural parameters of the car itself, the driver's operation technology and external factors such as roads and environment.
First, the longitudinal stability of automobile driving
Fig. 2- 10 is the stress diagram when the car goes uphill at a constant speed. Inertia resistance is zero, so air resistance and rolling resistance can be omitted because of low speed. In the figure, G is the total gravity of the vehicle, α is the inclination angle, hg is the height of the center of gravity, Z 1 and Z2 are the normal reaction forces acting on the front and rear wheels, X 1 and X2 are the tangential reaction forces acting on the front and rear wheels, L is the wheelbase of the vehicle, l 1 and l2 are the distances from the center of gravity of the vehicle to the front and rear axles, and point O is the center of gravity of the vehicle, O65.
1. Longitudinal overturning
The critical state of longitudinal overturning is that the normal reaction force Z 1 of the front wheel is zero. At this time, the car may roll over around 02 o'clock. Let the moment of O2 point be Z 1=0 to obtain
Gl2cosα0- Ghgsinα0=0
Where: α 0-z1is zero, and the ultimate slope inclination;
I0-z 1 is the longitudinal slope of the road.
When the slope angle α≥α0 (or the longitudinal slope i≥i0), the car may overturn longitudinally. According to Formula (2-30), the longitudinal overturning stability is mainly related to the distance l2 from the center of gravity to the rear axle and the height hg of the center of gravity. The greater l2, the lower hg and the better longitudinal stability.
2. Longitudinal sliding
For rear-wheel drive vehicles, according to the attachment conditions, the critical state that the driving wheel does not slip is
Gsinαj=jGk
Because of sinαj? tgαj？ So, Ij
ij = tgαj =
Where: α j refers to the slope inclination angle when the critical state of longitudinal slip occurs;
Ij —— the longitudinal slope of the road when the critical state of longitudinal slip occurs. Other symbols have the same meanings as before.
When the slope angle α≥αj (or longitudinal slope i≥ij), the car may slip longitudinally. Ij mainly depends on the ratio of the driving wheel load Gk to the total vehicle gravity G and the value of the adhesion coefficient J, as shown in Formula (2- 15) and Table 2-5.
3. Guarantee of longitudinal stability
Analytical formulas (2-30) and (2-3 1) are generally close to 1, but far less than 1, so
Me? & lti0
That is to say, when the car is driving on the ramp, the longitudinal sliding phenomenon occurs first before the longitudinal overturning occurs. In order to ensure the longitudinal stability of the car, the road design should meet the conditions of no longitudinal sliding and avoid the car from overturning vertically. Therefore, the conditions of longitudinal stability when the car is running are as follows
As long as the designed road longitudinal gradient I meets the above conditions, the stability of the longitudinal driving of the car when it is fully loaded can generally be guaranteed. However, when the load in transportation is too high, the longitudinal stability condition will be destroyed due to the increase of the height of the center of gravity hg, so the loading height of the car should be limited.
Second, the lateral stability of automobile driving
1. Force balance when the car is driving on a horizontal curve
When a car runs on a horizontal curve, it will produce centrifugal force, which acts on the center of gravity of the car and deviates from the center of the circle horizontally. The centrifugal force of a car with a certain mass is directly proportional to the square of the driving speed and inversely proportional to the radius of the horizontal curve. The calculation formula is as follows
Where: f-centrifugal force (n);
R—— radius of plane curve (m);
V- vehicle speed (m/s).
Centrifugal force has a great influence on the stability of the car driving on a horizontal curve, which may lead to the car slipping or overturning to the outside. In order to reduce the effect of centrifugal force and ensure the car to run smoothly on the horizontal curve, it is necessary to make the pavement on the horizontal curve into a one-way transverse slope with high outside and low inside, which is called transverse superelevation. As shown in Figure 2- 1 1, when a car runs on an ultra-high horizontal curve, the horizontal component of its weight can offset part of the centrifugal force, and the rest is balanced by the lateral friction between the car tire and the road surface.
Centrifugal force f and automobile gravity g are decomposed into transverse force x parallel to the road surface and vertical force y perpendicular to the road surface, namely
Because the lateral inclination angle α of the pavement is generally very small, then sinα≈tgα=ih, cosα≈ 1, where ih is called lateral superelevation slope (abbreviated as superelevation rate), so
The lateral force X is the unstable factor of automobile running, and the vertical force is the stable factor. As far as lateral force is concerned, it is impossible to reflect the stability of cars with different weights only from its numerical value. For example, a lateral force of 5kN acting on a car may cause the danger of lateral overturning, but it may be safe to act on a heavy truck. Therefore, the lateral force coefficient is used to measure the stability, that is, the lateral force per unit vehicle weight, that is,
Convert the vehicle speed V (m/s) to V (km/h), and then
Where: r-plane curve radius (m);
F—— transverse force coefficient;
V—— driving speed (km/h);
Ih- lateral superelevation slope.
Equation (2-33) shows the relationship between lateral force coefficient and vehicle speed, horizontal curve radius and superelevation. The greater the μ value, the worse the stability of the vehicle on the horizontal curve. This formula is of great significance for determining the radius of horizontal curve, superelevation rate and evaluating the safety and comfort of automobile driving on horizontal curve.
2. Analysis of lateral overturning conditions
When a car is driving on an ultra-high plane curve, it may be in danger of overturning sideways around the contact point of the outer wheel due to the lateral force. In order to prevent the car from overturning, the overturning moment must be less than or equal to the stable moment. that is
Because Fih is much smaller than G and can be ignored, then
Where: b—— car wheel track (m);
Hg—— the height of the center of gravity of the vehicle (m).
Replace formula (2-34) with formula (2-33) and sort, and get
This formula can be used to calculate the minimum curve radius r or the maximum allowable driving speed v without rollover when the car is driving on a horizontal curve.
3. Analysis of lateral slip conditions
When the car is driving on a horizontal curve, the existence of lateral force may cause the car to slip sideways in the direction of lateral force. In order to prevent the automobile from skidding sideways, the lateral force must be less than or equal to the lateral adhesion between the tire and the road surface, that is,
Where: transverse adhesion coefficient, generally = (0.6 ~ 0.7), and its value is shown in Table 2-5.
Replace formula (2-36) with formula (2-33) and sort, and get
This formula can be used to calculate the minimum curve radius r or the maximum allowable driving speed v without sideslip when the car is driving on a plane curve.
4. lateral stability guarantee
It can be seen from equations (2-34) and (2-36) that the lateral stability of a car driving on a horizontal curve mainly depends on the value of the lateral force coefficient μ. Modern cars have a low center of gravity when they are designed and manufactured. That is to say, when the car is driving on a flat curve, it will slip before overturning. Therefore, the stability against overturning should be ensured in road design. As long as the μ value used in the design meets the conditions of Formula (2-36), the stability of lateral driving can be guaranteed when the vehicle is fully loaded. However, when the load is too high, overturning may occur.
Thirdly, the vertical and horizontal combination stability of automobile driving.
When the car is driving on a plane curve with a certain small radius, it increases a curve resistance relative to the straight line. The power consumed by the above-mentioned cars increases, which reduces the running speed. For downhill cars, it is possible to tilt, slip and overload along the composite slope direction of vertical and horizontal combination, which is dangerous for car driving. Therefore, the maximum value of compound gradient should be limited to facilitate the stability of driving.
As shown in fig. 2- 12, the car is driving on the downhill section of longitudinal slope I(tgα) and ultra-high cross slope ih(tgβ), and the load W 1 acting on the front axle is
Centrifugal force F distributes load W2 to the front axle as follows
As the sum of inclination angles is very small, the total load ∑W of the front axle is
In a straight section, the load w' acting on the front axle is
On a ramp with a flat curve, the ratio of the front axle load increment to W is
For trucks, generally hg/l2≈ 1, then
On a straight ramp, if ih≈0, I = I That is, when the car goes downhill on a straight ramp, the ratio of the front axle load increment to the front axle load on a straight section is equal to the longitudinal slope of the section. If the maximum longitudinal slope imax of the same size on the straight line is also used as the control on the curve, the following equation holds.
Convert v (m/s) into v (km/h) and arrange it to get.
This formula is not only the stability condition of the automobile along the vertical and horizontal combination direction, but also the reduction condition of the maximum longitudinal slope on the horizontal curve.