Four-wheel alignment is the positioning of four tires, and the four-wheel alignment angle is the relative angle between the suspension system and the moving parts. Keeping the correct four-wheel alignment angle can ensure the straight-line driving and controllability of the vehicle, improve the steering performance of the vehicle, ensure the automatic righting performance of the steering system, and avoid premature damage and loss of accuracy of each axle due to improper bearing capacity. It can also ensure that each tire maintains a correct and tight joint with the ground, and reduce improper tire wear (eccentric wear or tire gnawing, etc.). ), and ensure the maneuverability and driving stability of the vehicle when turning. Therefore, the accuracy of the four tire positioning angles will directly affect the tire wear and the steering maneuverability and stability of the car when driving.

First, the content and significance of four-wheel alignment

The main contents of automobile four-wheel alignment include camber (front and rear wheels), camber (kingpin) (K.P.I), kingpin (kingpin), toe angle (front and rear wheels), and now it also includes toe-in when turning. Its specific content and significance are as follows:

1. arc

Camber is the angle formed by the tire centerline and the vertical line when viewed from the front of the vehicle, which is positive outward and negative inward. Different angles can change the contact point and stress point between the tire and the ground, which directly affects the grip and wear of the tire. The stress distribution of the load on the axle is changed, and the abnormal wear of each wheel bearing is avoided. In addition, the existence of camber can be used to offset the angle change caused by the deformation of suspension system parts and the gap between movable surfaces after the car body is loaded. The existence of camber will also affect the direction of the vehicle, just as a motorcycle can turn by tilting its body. Therefore, the camber angles of the left and right wheels must be equal, which will not affect the straight-line running of the vehicle under the balance of force. Combined with toe angle, the stability of straight-line running can be improved and uneven tire wear can be avoided. Increasing negative camber needs to be combined with increasing toe; ; In order to increase positive camber, it is necessary to increase toe-in.

2. Angle of inclination (K.P.I)

The tilt angle is the angle between the center line of the steering shaft (or kingpin) and the vertical line. With the inclination angle, the weight of the car can be evenly distributed on the bearing, which can protect the bearing from damage, make the steering force uniform and turn easily. On the other hand, if the camber angle is 0, the reaction force of the vehicle weight and the ground will produce great transverse shear stress on the axle, which will easily damage the axle and make the steering heavy. In addition, camber angle is also the source of righting force after front wheel steering. The camber angle has been set at the beginning of vehicle suspension design, and usually only the steering wheel of the steering shaft has it and cannot be adjusted.

Step 3: Spice bottle

The caster angle of the kingpin is the angle between the center line of the steering shaft (or kingpin) and the vertical line when viewed from the vehicle side, which is negative forward and positive backward. The existence of caster angle of kingpin can make the intersection of steering shaft and road surface in front of the contact point of tire, and can make use of the resistance of road surface to tire to keep the car running in a straight line. The principle is the same as that the front wheel of the shopping cart will automatically turn to the direction of your force and keep moving in a straight line. The greater the caster angle of the kingpin, the better the straight-line driving performance of the car and the better the recovery performance of the steering wheel after steering, but it will make the steering heavy. The caster angle of an ordinary car is about 1~2 degrees.

4. toe angle (toe)

Toe angle (sometimes called toe-in) and toe angle are the included angle formed by the center lines of the left and right tires when viewed from above, with toe-in inward and toe-out outward (toe-in is the difference between the rear dimension of the center of the left and right tires minus the front dimension at the tire height 1/2). The function of toe angle is to compensate the tendency of tire rolling inward or outward due to camber angle and road resistance, and to ensure the straight running of the car. Toe-in will lead to understeer, while toe-in will increase the tendency of oversteer.

5. Turn your toes outward.

The forward extension during steering is the difference between the steering angles of the two front wheels during steering. When turning, the turning angle of the inner wheel is usually larger than that of the outer wheel, with a difference of about 2 degrees. Its purpose is to make the vehicle turn smoothly around the instantaneous center of the extension line of the rear axle when turning. In addition, when the turning angle of the inner wheel is large, the resistance is also large, and the difference in resistance can make the vehicle lean to the side with large resistance, making it easy to turn (imagine the turning mode of the tanker).

offset

Offset refers to the distance between the rim mounting surface and the rim center, with positive offset towards the outside and negative offset towards the inside of the rim. Changing the offset of the rim will change the wheel track of the car, which refers to the distance between the center lines of the tires, so if we simply increase the rims and tires without changing the offset, it will not affect the wheel track.

Influence of changing offset

If the rim with smaller positive offset value is used, the track width will be widened, which can reduce the deviation of the center of gravity of the car body during cornering and improve the cornering speed limit of the vehicle. However, the distance between the center of the steering shaft and the center of the tire is relatively large, which makes steering difficult, increases the load of the steering mechanism, and causes the deformation of the steering linkage to increase, which needs to be corrected by appropriately increasing the toe-in. But these are abnormal ways, so the front wheel offset should be as close as possible to the original offset value. For the rear wheel, if it is replaced with a larger rim, if the bias is not changed, the inner side of the tire will often encounter the problem of hitting the suspension mechanism, so it is beneficial to use a rim with a smaller positive bias value without polishing the wheel arch. However, it should be noted that for cars with independent rear wheels, such changes will increase the changes in the toe-in of the rear wheels during acceleration and braking, which has no effect on ordinary street cars, but it is a big problem for racing cars. Let's take BMW's Series 5 (E34) as an example to see how the offset should change when the rim is enlarged. Initially, the iron rim provided by the original factory was 15*7J with an offset of 47, and the aluminum rim was 15*6J with an offset of 36; . When using 17 inch aluminum rim, the original factory provides 17*7.5J with an offset of 35, while Racing-Dydamic provides 17*8.5J with an offset of 18 front wheel and an offset of 17*9J. Changing the offset will also affect the bearing load. Generally speaking, the offset design of vehicles aims at the lowest bearing load when going straight. Although using a rim with a small positive offset will slightly increase the load when going straight (you don't have to worry too much about the load when the offset changes within 50mm), it can reduce the load when cornering.

Second, how to choose a four-wheel positioning store?

With the evolution of suspension system, from the most basic McPherson, trailing arm and double A-arm to three-link, four-link, five-link and compound link; The more connecting rods are, the more complicated the structure is, and the higher the requirements for four-wheel positioning angles are. So there will be a four-wheel alignment instrument designated by a certain model. The four-wheel locator is not used to adjust or change the positioning angle, but only to measure the positioning angle for technical reference. Technicians compare the angle measured by the instrument with the angle set by the original factory, and if it exceeds the design allowable range, adjust or replace some parts to restore the original design angle. So when you choose a four-wheel positioning store, you must remember that the advantages and disadvantages of positioning instruments are important, but the "person" who adjusts the positioning angle is more important. Experienced and skilled technicians and advanced instruments are the best choice.

Third, common positioning problems

In daily driving, how to judge the abnormality of chassis and suspension, and judge its causes, I think everyone wants to know most. Here are some typical questions for your reference. Poor linearity: When driving left or right, or the direction is not skewed, but the steering wheel is not straight, which is usually a typical positioning problem, but uneven tire wear or different types of tires used on the left and right wheels will also affect the linearity of the car. One of the most annoying problems with poor linearity is that sometimes the steering wheel is in line with the road surface, and sometimes it will have a small angle deviation when going straight, so the steering wheel can never be' stable'. The most likely reason is that the caster angle of the left and right wheels is deviated, resulting in different restoring forces of the left and right wheels, which will naturally be affected by the road surface when the two forces are unbalanced. Steering wheel jitter: Steering wheel jitter is mainly caused by problems such as tires, except for the wear of the transmission shaft (FF car). Excessive tire pressure or deformation will cause the whole vehicle to shake, and poor tire roundness and inaccurate balance are the main reasons for steering wheel shake. In addition, the uneven brake disc causes the jitter when braking, and the uneven braking force of the left and right wheels causes the deflection of the driving direction when braking, which are not solved by four-wheel positioning for you.

Fourth, four-wheel positioning q &；; A

1. How often does four-wheel alignment need to be done?

According to the use situation, it is best to check the chassis and positioning once every six months. If any angle exceeds the allowable value of the benchmark, some parts should be adjusted or replaced in time to restore the correct angle.

When do you need four-wheel alignment? Four-wheel alignment is not necessarily fixed. Generally, four-wheel alignment is required in one of the following six situations.

① Hold the steering wheel tightly when going straight, otherwise the car will run off (i.e. drive sideways).

There are many reasons for driving deviation: a, the height of the car body is not equal to the left and right, and the air pressure of the left and right tires is not equal; B, the left and right tires are different in size or pattern, and the tires are deformed or defective; C. The caster angle of the kingpin is not equal left and right, and the caster angle of the kingpin is not equal left and right; D, the thrust angle is too large; E, steering system card position, brake pad card position, etc.

② Abnormal tire wear, such as uneven, unilateral tire wear or uneven, feather tire wear.

A. Tire block wear: The general reasons are static imbalance of wheels and poor toe-in of rear wheels.

B. One-sided tire wear: The general reason is poor camber.

C. Tire feather wear: The reason is generally poor toe-in.

D. Concave-convex wear: Generally, the reason is uneven wheel movement and poor toe-in of the rear wheel.

③ The steering wheel is too heavy and too light when turning, and the steering wheel shakes when driving fast.

(4) After the suspension system accessories such as tires, steering knuckles and shock absorbers are replaced.

⑤. After vehicle collision.

⑥ When the vehicle travels 3000km or1000km.

2. What are the common positioning problems?

The most common positioning problems are the error of camber and toe angle caused by constant time vibration, and the change of caster angle caused by convex roads and holes.

3. How to detect the abnormality of positioning angle?

Generally speaking, about 60% of car owners who seek help because of abnormal positioning are due to poor straight-line driving performance, steering wheel angle leaning to one side, followed by steering wheel jitter, and uneven tire wear after driving for a period of time.

4. Should the positioning angle be modified when the size (upward inch) and offset change?

When increasing the size, try to use the same offset as the original size to ensure that the chassis can maintain its original performance. At present, the products on the market are limited by the limited product form and aesthetic considerations provided by the manufacturer, so when you change them, the other party will suggest that you change them to a positive offset value, as long as the difference is not big and the wheel arch will not be worn, just slightly increase the toe-in.

5. When the short spring is used to lower the car body, will it change the positioning angle and how to correct it?

When the short spring is used, the car body is lowered. For suspension system parts, its geometric change is just like the change of body lowering after the body is loaded. Therefore, unless the reduction amount is large, if the spring for the car body is reduced by 3~4 cm, there is no need to correct the positioning angle.

In a word, the accuracy of the four-wheel alignment angle will directly affect the tire wear and the steering maneuverability and stability of the car when driving.