From correct adjustment wheels depend on many factors: handling, tire life, fuel consumption. Let's look at them - what they influence and why they are needed.
What are they for?
The recommendations of manufacturers for the installation of wheels should be taken with full responsibility. For each model, the recommendations are different. These angles provide best performance stability and handling, as well as minimal tire wear.Periodically, during the operation of a car (after 30,000 km of run), it is useful to control them, and if the car has been replaced individual elements suspension, and even more so after serious blows, this must be done immediately. It should be remembered that the adjustment of the angles of the steered wheels is the final suspension repair operation, running gear and steering parts.
Max turning angle
characterizes maximum angle, at which the wheel of the car will turn with the steering wheel completely turned out. The smaller it is, the greater the accuracy and smoothness of control. After all, to turn even a small angle, only a small movement of the steering wheel is required.Do not forget that the smaller the maximum turning angle, the smaller the turning radius of the car. Those. it will be difficult to deploy in a limited space. Manufacturers have to look for golden mean, maneuvering between large radius rotation and precision control.
Run-in shoulder
This is the shortest distance between the center of the tire and the pivot point of the wheel. If the axis of rotation and the middle of the wheel coincide, then the value is considered zero. With a negative value - the axis of rotation is shifted outward of the wheel, and with a positive value - inward. For vehicles with rear wheel drive a run-in shoulder with a zero or negative value is recommended. In practice, due to the design of the machine, this is difficult to do, because. the mechanism does not fit inside the wheel. The result is a car with a positive rolling shoulder, which behaves unpredictably: when driving over bumps, the steering wheel can be pulled out of your hands, when cornering, a noticeable moment is created that prevents uniform movement.
To combat the positive rolling shoulder, the specialists tilted the steering axis in the transverse direction and made a positive camber. Although this reduced the rolling shoulder, it had a bad effect on driving in a turn.
Caster Angle
Responsible for the dynamic stabilization of the steered wheels. If it's simple, then it makes the car go straight with the steering wheel released. Those. if you take your hands off the steering wheel, then the car should ideally go straight and not deviate anywhere. If a lateral force acts on the car (for example, wind), then the caster must provide smooth turn vehicle in the direction of the force when the steering wheel is released. In addition, the caster prevents the car from tipping over. The main function of the caster is to tilt the wheels in the direction the steering wheel is turned. The tilt of the wheel affects the traction, and hence the handling. If the car is moving straight, the wheels have the most traction, which provides the driver with a quick start and late braking.
When the wheel is turned, the tire is deformed under the action of lateral forces. To maintain maximum contact patch with the road, the wheel also leans in the direction of the turn. But you need to know the measure, because with a large caster, the wheel will tilt a lot, and then lose traction.
Roll axis
Responsible for the weight stabilization of the steered wheels. The bottom line is that at the moment the wheel deviates from “neutral”, the front end begins to rise. And since it weighs a lot, then when the steering wheel is released under the action of gravity, the system tends to take its original position, corresponding to movement in a straight line. True, in order for this stabilization to work, it is necessary to maintain a (albeit small, but undesirable) positive rolling shoulder. Initially, transverse angle The tilt of the axis of rotation was applied by engineers to eliminate the shortcomings of the car's suspension. He got rid of such "illnesses" as positive camber and rolling shoulder.
Many vehicles use MacPherson strut suspension. It makes it possible to obtain a negative or zero run-in shoulder. After all, the axis of rotation consists of a support of a single lever, which can be placed inside the wheel. This suspension is not perfect, because it is almost impossible to make the angle of inclination of the axle small. In a turn, it leans the outside wheel at an unfavorable angle (like positive camber), while the inside wheel leans in the opposite direction at the same time.
As a result, the contact patch at the outer wheel is greatly reduced. Because the outer wheel in a turn bears the main load, the entire axle loses a lot of traction. This, of course, can be partially offset by caster and camber. Then the grip of the outer wheel will be good, while the inner one will practically disappear.
wheel alignment
There are two types of convergence: positive and negative. It is easy to determine: you need to draw two straight lines along the wheels of the car. If these lines intersect in front of the car, then the convergence is positive, and if behind - negative. If the convergence is positive, then the car enters the turn easier, and also acquires additional understeer, it will be more stable when driving in a straight line. If the convergence is negative, then the car drives inadequately, scours from side to side. But it should be remembered that an excessive deviation of the convergence from zero will increase the rolling resistance in a straight line, in turns it will be less noticeable.
Camber
It happens negative and positive. When viewed from the front of the car, and the wheels will lean inward, this is negative camber. If they deviate outward - positive. The camber is necessary to maintain the grip of the wheel with the roadway. On serial machines make zero or slightly positive camber. If needed good handling- make it negative.
Rear wheel adjustment
Many machines do not adjust angles rear wheels. For example, on front wheel drive cars VAZ, where a rigid beam is installed at the back. Violations can only occur in case of a serious accident, when the rear beam. Also not regulated back corners on SUVs with a rigid axle. On many foreign cars is multi-link suspension behind. This means that you can adjust the toe and camber of the rear wheels.This must be done after hitting a curb or an accident. Because any car is very sensitive to changes in the angle of convergence of the rear wheels. If it is negative, then the car will constantly skid when cornering. If positive - also bad, the car will show understeer. When cornering, the car will tend to go straight.
What to do first?
First, the angles of the rear wheels are adjusted (it is possible), and only then the front ones. First, the caster is set, then the collapse and the last (required) is the convergence. You also need to make sure that steering wheel stood straight. For this use special devices to fix it.Also note that the use of sports settings will adversely affect comfort. If you make the caster too big or too much negative camber, the force on the steering wheel will increase. But this The best way change the behavior of the car to a more sporty one.
CAR CLUB
/WANT TO KNOW EVERYTHING
ANGULAR SUSPENSION
A LITERAL DRIVER WILL USE THE BASICS OF GEOMETRY
TEXT / EVGENY BORISENKOV
The simplest and seemingly obvious solution is to not make any corners at all. In this case, the wheel during compression-rebound remains perpendicular to the road, in a constant and reliable contact with her (Fig. 1). True, it is structurally quite difficult to combine the central plane of rotation of the wheel and the axis of its rotation (hereinafter, we are talking about the classic two-lever suspension of rear-wheel drive Zhiguli), since both ball joints Together with the brake mechanism, they do not fit inside the wheel. And if so, then the plane and the axis “diverge” by a distance A, called the rolling shoulder (when turning, the wheel rolls around the axis ab). In motion, the rolling resistance force of the non-driving wheel creates a tangible moment on this shoulder, which changes abruptly when driving through bumps. Few people enjoy driving with the steering wheel constantly torn from their hands!
In addition, you will have to sweat a lot, overcoming this very moment in the turn. Therefore, positive (in this case) it is desirable to reduce the rolling shoulder, or even completely reduce it to zero. To do this, you can tilt the axis of rotation ab (Fig. 2). It is important not to overdo it here, so that when going up, the wheel does not fall too much inward. In practice, they do this: by slightly tilting the axis of rotation (b), the desired value is obtained by tilting the plane of rotation of the wheel (a). Angle a is the collapse. At this angle, the wheel rests on the road. The tire in the contact zone is deformed (Fig. 3).
It turns out that the car moves as if on two cones, tending to roll to the sides. To compensate for this trouble, the planes of rotation of the wheels must be brought together. The process is called convergence adjustment. As you may have guessed, both parameters are tightly coupled. That is, if the camber angle is zero, there should be no convergence, negative - a divergence is required, otherwise the tires will “burn”. If the camber is set differently on the car, it will be pulled towards the wheel with a large slope.
The other two angles stabilize the steered wheels - in other words, make the car go straight with the steering wheel released. The first, already familiar to us, the angle of the transverse inclination of the axis of rotation (b) is responsible for weight stabilization. It is easy to see that with this scheme (Fig. 4), at the moment the wheel deviates from the “neutral”, the front end begins to rise. And since it weighs a lot, when the steering wheel is released under the influence of gravity, the system tends to take its original position, corresponding to movement in a straight line. True, for this it is necessary to maintain the same, albeit small, but undesirable positive rolling shoulder.
The longitudinal angle of inclination of the axis of rotation - caster - gives dynamic stabilization (Fig. 5). Its principle is clear from the behavior of the piano wheel - in motion, it tends to be behind the leg, that is, to take the most stable position. To get the same effect in a car, the point of intersection of the pivot point with the road surface (c) must be ahead of the center of the wheel-road contact patch (d). To do this, the axis of rotation and tilt along. Now, when turning, the lateral reactions of the road applied behind... (thanks to the caster!) (fig. 6) try to return the wheel to its place.
Moreover, if the car is subjected to a lateral force that is not related to the turn (for example, you are driving on a slope or with a side wind), then the caster ensures that the car turns smoothly “downhill” or “downwind” when the steering wheel is accidentally released and does not allow it to tip over.
IN front wheel drive car with the MacPherson suspension, the situation is completely different. This design makes it possible to obtain a zero and even negative (Fig. 7b) rolling shoulder - after all, only the support of a single lever needs to be “pushed” inside the wheel. The angle of collapse (and, accordingly, convergence) is easy to minimize. So it is: the VAZs of the “eighth” family, familiar to everyone, have a camber of 0 ° ± 30 ", a convergence of 0 ± 1 mm. Since the front wheels are now pulling the car, dynamic stabilization during acceleration is not required - the wheel no longer rolls behind the leg, but pulls it along. A small (1°30") caster angle is retained for braking stability. A significant contribution to the "correct" behavior of the car is made by the negative shoulder of the run-in - with an increase in the rolling resistance of the wheel, it automatically corrects the trajectory.
As you can see, it's hard to overestimate the impact of suspension geometry on handling and stability. Naturally, the designers pay close attention to it. The angles for each car model are determined after a great many tests, finishing work and more tests! But only ... based on a serviceable car. On an old, worn-out car, the elastic deformations of the suspension (primarily, rubber elements) are much greater than on a new one - the wheels diverge noticeably from much smaller forces. But it is worth stopping, as in statics all corners are again in their place. So adjusting a loose suspension is a monkey job! First you need to repair it.
You can nullify all the efforts of developers in other ways. For example, take a good bite back car. You look - the caster changed the sign and from dynamic stabilization memories remain. And if during acceleration the “sportsman” can still cope with the situation, then with emergency braking- hardly. And if you add non-standard tires and wheels with a different offset, who will undertake to predict what will happen in the end? ahead of time worn rubber and "killed" bearings are not so bad. It could be worse...
Rice. 1. "Suspension without corners."
Rice. 2. In the transverse plane, the position of the wheel is characterized by angles a (camber) and b (tilt).
Rice. 3. The rolling of an inclined wheel resembles the rolling of a cone.
Rice. 4. When positive leverage running in, turning the wheel is accompanied by lifting the front end of the body.
Rice. 5. Caster - the angle of the longitudinal inclination of the axis of rotation.
Rice. 6. This is how the caster works.
Rice. 7. Positive (a) and negative (b) run-in shoulders.
The driver is driving a car. There is an obstacle ahead. It slows down, but the brakes "take" a little differently. In most cases, this difference is practically negligible. But at very hard braking(Fig. 1) the car throws to the side, maybe only half a meter, or skids and ... an accident. It often also occurs due to the fact that during braking the wheels of one side of the car were on ice, mud or water.
What do these cases have in common? The common thing is that the wheels of the right and left sides got into different conditions on the forces of resistance to movement. And, of course, these different conditions “provoked” a skid or a spontaneous turn of the car, which the driver did not always have time to correct in time.
"Self-defense" against skidding
All modern models necessarily have two independent circuits in the hydraulic brake drive (see). In order to guarantee that braking efficiency and, therefore, safety is maintained, it is necessary that at least one front wheel is braked in case of any malfunction. For this reason, the cheapest and simplest of the double-circuit - the diagonal scheme of the separate hydraulic drive brakes. But the transition to it forced the designers to put "self-defense measures" into the geometric ratios of the parameters of the front suspension and steering gear. This measure is the negative running-in shoulder.
A few words about the term itself. The running-in shoulder (Fig. 2) is the distance between point G of the tire’s contact with the road and point B. It denotes the intersection with the road of the continuation of an imaginary axis passing through the centers of the upper and lower ball joints of the double-lever front suspension. If the GV segment is located inside the vehicle track (Fig. 2a), it is considered positive. If, due to a certain combination of sizes of parts in the front suspension, the GV segment is outside the track, then the running-in shoulder r is considered negative (Fig. 2b).
Now let's see what happens when a car with a diagonal separate hydraulic brake circuit is braked. Suppose one of the contours (say, servicing the brakes front right and rear left wheels) is out of order. Pressing the pedal brakes the front left and rear right wheel(Fig. 3). At the points of their contact with the road, braking forces arise, Ftp and Ftz, respectively.
The moment from the force of inertia Fн, applied in the center of gravity of the car's CG on a shoulder equal to half the track, will turn the car around the front left wheel. It will only be neutralized to a small extent by the moment from the force Fтз, turning the car in the opposite direction around the braked rear right wheel. Let us separately consider the force Fтп. It is much larger than Ftz (due to the redistribution grip weight when braking), therefore, to simplify the scheme of action of forces, we will conditionally assume that only one front wheel, and the force of inertia turns the car around it. But approximately the same situation occurs in any scheme, and even if the drive is fully operational, but the wheels of one side of the car fall on a surface with a low coefficient of adhesion (icy, snowy, wet) during braking or in the event of a tire rupture on the move of one of the front wheels. Save while given direction very difficult and sometimes impossible. In addition, here the steered wheels tend to turn in the direction where the braking force can be realized through a higher friction coefficient, sharply increasing the turn of the car.
Let's turn to Fig. 4. When braking, the steered wheel rotates relative to the “pivot”, the imaginary axis AB, under the action of the braking force Ftp.
Effort on the steering wheel is reduced to almost zero
With a traditional, positive running-in arm (section GV in Fig. 4a), a moment Mt arises, acting in the same direction as the moment Mi, formed by the inertia force Fn on a shoulder equal to half the track.
If, however, the suspension of the front wheels is designed so that the running-in shoulder turns out to be negative (segment VG in Fig. 4b), then the product of this shoulder and the force Ftp applied at the point of contact Г of the wheel with the road will give the moment Mt, acting in the opposite direction to the moment Mi and will neutralize it.
During comparative tests of vehicles with negative and positive break-in shoulders, braking was carried out from an initial speed of 80 km/h in the absence of wheel lock and the steering wheel was released. One of the circuits of the diagonal drive circuit was artificially turned off. For the model with a positive running shoulder, the turning angle relative to the initial direction of motion was 140-160° with a significant lateral displacement. And the model with a negative running shoulder built into the design had a turn angle in the range of 15-17 °, that is, it practically did not deviate from the original trajectory. This is a clear evidence of the undoubted advantage of the negative break-in shoulder during asymmetric braking of the car.
Particularly interesting in this regard are the data obtained during tests on the amount of force or torque that the driver must apply to the steering wheel in order to keep the car on the desired trajectory when braking. The moment on the steering wheel required for this with a positive break-in shoulder reaches approximately 130 kgf * cm, that is, with a steering wheel radius of 20-25 cm, the driver must apply a force of more than 5-6 kgf. On a car with a negative break-in arm, the torque on the steering wheel under the same conditions is negligible and fluctuates around zero. At the same time, the adjustment of the trajectory of the steering wheel does not cause any difficulties for the driver.
Skid during braking - 10 times less
This is the positive effect of the negative break-in arm, which improves safety by maintaining a straight line trajectory when braking or when the wheels of one side hit the slippery area roads.
And how big can the negative running-in shoulder be? Too large its value can lead to a deterioration in the stabilizing properties of the steering, which will have to be compensated for by an increase in the longitudinal inclination of the kingpin. But such "compensation", in turn, will increase the force on the steering wheel, which is undesirable. Therefore, for most cars, the value of the negative running-in shoulder ranges from 2 to 10 mm, reaching 18 mm in extreme cases (as done on the Audi-80). The other extreme is models with a running shoulder equal to zero ("Mercedes-Benz").
When you are “tinkering” with repairs, experimenting with wheel sizes, or setting up a newly installed suspension, an embarrassment can happen that you may not have even heard of - it is likely that the break-in shoulder radius will change. This "thing" can have a serious impact on the handling of your car.
Without a clear and full understanding With all the factors that affect suspension performance, wheel placement and geometry, it's easy to make a tuning mistake that will end up making your car feel worse than it was before. At the same time, it is quite difficult to catch the moment when an unfortunate oversight was made.
IN in general terms shoulder radius is an elusive, almost mythical setting that sits somewhere on the edge of key adjustments such as camber, offset and wheel size. Essentially, it is defined by the location of the point in space where an imaginary line through the center of the suspension intersects a vertical line through the center of the wheel, the two lines will meet somewhere. It is important that this angle is calculated on the car without load. For calculations carried out by engineers, this is extremely important.
Note the larger suspension angle relative to the wheel
In general, there are three main shoulder radius options:
If the two lines intersect exactly at the tire-to-road contact patch, the vehicle does not have a break-in shoulder radius.
If the lines intersect below the contact patch, theoretically underground, then this is called a positive break-in shoulder radius.
When both lines converge over the contact patch, this is the negative running shoulder.
Depending on these settings, they can seriously affect how the vehicle handles, accelerates and stops. Different axle load ratings and drive configurations require different settings, which will be calculated long before the engineers start realizing the desired handling characteristics. Yes, automakers have a lot of hard work, and this stage is just one of them. Change just one parameter in the suspension and you will initiate a chain reaction that can ultimately nullify your main goal.
Break-in shoulder radius refers to the relative angle between the suspension and the wheel axle
At zero radius, the common perception is that this setting can make the car feel slightly wobbly in the front when cornering and under heavy braking.
On the other hand, in a stationary state, when turning the steering wheel, it is necessary to turn the contact patch, which is maximally spread over the road surface, which requires more effort and wears out the tire more. These days, such a setup (with zero leverage) on cars is extremely rare. A little more or a little less, but not zero.
You can, of course, change the zero setting. For example, "push out" the wheels with shims or install fully adjustable coilovers and the radius can become positive. This will cause the tire to "scrape" on the ground when cornering, adding uneven wear and reducing its service life. A car with a positive break-in arm can behave unpredictably on the road: the steering wheel can be pulled out of your hands when driving through bumps, and when cornering, a “perceptible moment that prevents uniform movement” is created.
The positive moment of such a setting exists for rear wheel drive vehicles. To them, such a setting is useful in order to help keep the front wheels in forward direction even when you release the steering wheel. Used in sports cars and supplied in standard equipment with most double wishbone suspension designs.
Front axle Volkswagen Scirocco
The positive radius of the shoulder does not contribute to braking if, for any reason, between the sides vehicle there is a different force. Let's say if the left wheels have less grip and ABS system does not allow you to develop maximum effort on them. In this case, the car will try to turn towards the wheels with more grip.
The extreme positive shoulder radius can be very heavy, so much so that it was only really viable on older cars with very thin tires.
Most of us have negative shoulder radius on our cars because it tends to go hand in hand with MacPherson strut strut settings. This helps the steerable front wheels feel more stable on the road, which is good for cornering and overall vehicle handling if, say, you suddenly have one of your front tires flat. Another handy "side effect" is that if you wheel into the water on one side of the car, the negative radius will work against the car's natural displacement, mitigating the effects of passing through the hazard.
Negative shoulder radius is safer for hydroplaning
Adjust the suspension in the negative shoulder - the most safe option do it. It (setting) allows you to generate certain forces that will reduce any unintentional tendency to change direction by the driver, which in the case of a positive setting can take place.
In the original version of such a suspension, developed by MacPherson himself, the ball joint was located on the continuation of the axis shock absorber strut- thus, the axis of the shock absorber was also the axis of rotation of the wheel. Later, for example, on the Audi 80 and Volkswagen Passat of the first generations, the ball joint began to be shifted outward to the wheel, which made it possible to obtain smaller, and even negative values of the running-in shoulder.
Thus, run-in shoulder (Scrub Radius) is the distance in a straight line between the point at which the axis of rotation of the wheel intersects with the roadway and the center of the contact patch between the wheel and the road (when the vehicle is not loaded). When turning, the wheel "rolls" around the axis of its turn along this radius.
It can be zero, positive or negative (all three cases are shown in the illustration).
For decades, most vehicles have used relatively large positive roll-over leverage. This made it possible to reduce the effort on the steering wheel when parking compared to the zero shoulder of the break-in (because the wheel rolls when the steering wheel is turned, and not just turns on the spot) and frees up space in engine compartment due to the removal of the wheels "out".
However, over time, it became clear that the positive roll-over shoulder can be dangerous - for example, when the wheels of one side hit a curb section that has a different friction coefficient from the main road, the brakes on one side fail, one of the tires is punctured, or the steering wheel is out of adjustment out of hand." The same effect is observed with a large positive run-in shoulder and when driving through any bumps on the road, but the shoulder was still made small enough so that it remained unobtrusive during normal driving.
Starting from the seventies and eighties, as the speed of vehicles increased, and in particular with the spread of the MacPherson type suspension, which easily allows this with technical side, cars began to appear en masse with a zero or even a negative run-in shoulder. This allows you to minimize the dangerous effects described above.
For example, on the "classic" VAZ models, the run-in shoulder was large positive, on the "Niva" VAZ-2121 due to a more compact brake mechanism with a floating bracket, it was reduced almost to zero (24 mm), and on the front-wheel drive LADA Samara family, the roll-in shoulder became already negative. Mercedes-Benz generally preferred to have a zero break-in shoulder on their rear wheel drive models.
The rolling shoulder is determined not only by the suspension design, but also by the parameters of the wheels. Therefore, when selecting non-factory "disks" (according to the terminology adopted in the technical literature, this part is called "wheel" and consists of the central part - disk and the outer, on which the tire sits - rims) for the vehicle, the manufacturer's specifications must be observed. valid parameters, especially the offset, since when installing wheels with an incorrectly selected offset, the rolling shoulder can change dramatically, which has a very significant effect on the vehicle's handling and safety, as well as on the durability of its parts.
For example, when installing wheels with zero or negative offset with a positive (for example, too wide) offset provided from the factory, the plane of rotation of the wheel shifts outward from the axis of rotation of the wheel that does not change at the same time, and the run-in shoulder may acquire an unnecessarily large positive value - the steering wheel begins to “tear out of hand” on every bump in the road, the effort on it when parking exceeds all permissible values (due to an increase in the lever arm compared to the standard departure), and wear wheel bearings and other suspension components increases significantly.
