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 tire 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 this 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 let go steering wheel. Used in sports cars and supplied in standard equipment with most suspension designs with double wishbones.
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 arm was large positive, on the Niva VAZ-2121, thanks to a more compact brake mechanism with a floating caliper, it was reduced to almost zero (24 mm), and on the front-wheel drive LADA Samara family, the run-in arm became narrower 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.
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 run-in shoulder. Initially, the transverse angle of inclination of the axis of rotation was used 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 the angles of the 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 the steering wheel is 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.
Why do we need camber, toe and caster angles?
Pendant without corners
If no corners are made at all, the wheel will remain perpendicular to the road during compression and rebound, in a constant and reliable contact with her. 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 rear wheel drive car, for example "Zhiguli"), since both ball joints coupled with brake mechanism wheels do not fit inside. 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. As a result, the steering wheel will constantly tear out of your hands.
In the transverse plane, the position of the wheel is characterized by the angles α (camber) and β (tilt axis)
In addition, muscle strength will have to overcome this very considerable 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. It is important not to overdo it here, so that when going up, the wheel does not fall too much inward.
The rolling of an inclined wheel resembles the rolling of a cone
In practice, they do this: by slightly tilting the axis of rotation (β), the desired value is obtained by tilting the plane of rotation of the wheel (α). The angle of wasps is the collapse. At this angle, the wheel rests on the road. The tire in the contact zone is deformed.
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. 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.
With a positive run-in shoulder, turning the wheel is accompanied by lifting the front end of the body
The other two angles stabilize the steered wheels - in other words, make the car go straight with the steering wheel released. The angle of the transverse inclination of the axis of rotation (β) is responsible for the weight stabilization. It is easy to see that with this scheme (Fig.), 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.
Caster - pitch angle
Longitudinal angle tilt axis of rotation - caster - gives dynamic stabilization. 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 ...
This is how caster works
Now when cornering, the lateral reactions of the road applied behind... (thanks to the caster!) try to put the wheel back in 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.
Positive (a) and negative (b) run-in shoulders
IN front wheel drive car with the MacPherson suspension, the situation is completely different. This design allows you to get a zero and even negative (Fig. b) 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 have a camber of 0 ° ± 30 ", a toe-in 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 by 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 rolling shoulder - with an increase in the rolling resistance of the wheel, it automatically corrects the trajectory.
The angles for each car model are determined after a lot of testing, finishing work and re-testing. 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 waste of time. 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, then it is simply impossible to predict what will happen in the end.
Proper wheel alignment is one of the critical factors, providing normal controllability, stability and stability of the car in a straight line and when cornering. Optimal suspension geometry parameters for each model are set at the design stage. The specified wheel alignment values are subject to change and require periodic adjustment due to natural wear and tear components and elements of the running gear or after suspension repair.
Assignment of wheel alignment angles
Correctly tuned suspension geometry allows the car to more effectively perceive the forces and moments that occur in the contact patch of the wheel with the road surface during different modes movement. This ensures predictable behavior of the car, namely: stability in a straight line, stability in turns, stabilization during acceleration and braking. Also, due to the absence of excessive rolling resistance of the wheels, more uniform tire wear occurs, which allows to increase their service life.
The wheel alignment values specified by the manufacturer are optimal for specific vehicle and correspond to its purpose and suspension tuning features. However, if necessary, the possibility of their change or adjustment is structurally provided. The number of parameters that can be adjusted for each car is individual.
Types of basic car wheel alignment angles
| Parameter | vehicle axle | Adjustable parameter | What does it affect |
|---|---|---|---|
| Camber (camber) | Front rear | Yes (depending on vehicle) | Driving stability in a turn Premature wear tires |
| Toe Angle (Toe) | Front rear | Yes | Straight-line stability Premature tire wear |
| Roll Pivot (KPI) | Front | No | |
| Pitch Angle (Caster) | Front | Yes (depending on vehicle) | Vehicle stabilization while driving |
| Breaking shoulder | Front | No | Vehicle stability under braking Vehicle stabilization while driving |
Camber
wheel camber (English) camber) is the angle formed by the median plane of the wheel and the vertical passing through the point of intersection of the median plane of the wheel and the supporting surface. Distinguish between positive and negative camber:
- positive (+) - when the top of the wheel is tilted outward (away from the car body);
- negative (-) - when the top of the wheel is tilted inward (toward the car body).
Positive and negative camber angles Structurally, the camber is formed by the position of the hub assembly and provides the maximum area of the tire contact patch with the road. In the case of a double lever independent suspension the position of the hub is determined by the upper and lower wishbones. In the formation of the camber angle is affected lower arm and shock absorber.
The deviation of the camber angle from the norm affects the car as follows.
- good stability car in corners;
- the grip of the wheels deteriorates during rectilinear movement;
- increased wear inside tires.
- good grip wheels with the road;
- stability in turns worsens;
- increased wear on the outer side of the tire.
wheel alignment
wheel alignment (English) toe) - the angle between the longitudinal axis of the car and the plane of rotation of the wheel. It can also be defined as the difference in distance between the front and rear sides of the wheel rims (in the figure this is the value A minus B). Thus, convergence can be measured in degrees or millimeters.
Car wheel alignment Distinguish between total and individual convergence. Individual convergence is calculated separately for each wheel. This is the deviation of the plane of its rotation from the longitudinal axis of symmetry of the car. The total toe-in is calculated as the sum of the individual toe-in angles of the left and right wheels of the same axle. Similarly, the total convergence in millimeters is determined. With a positive convergence (eng. toe-in) the wheels are mutually turned inward in the direction of travel, with a negative value (eng. toe-out) out.
Positive and negative wheel alignment The deviation of the values of the angle of convergence from the norm affect the car as follows.
Too big negative angle:
- increased tire wear on the inside;
- sharp reaction of the car to the steering.
Too large positive angle:
- maintaining the trajectory of movement worsens;
- increased tire wear on the outside.
The transverse angle of inclination of the axis of rotation of the wheel The transverse angle of inclination of the axis of rotation (eng. KPI) is the angle between the axis of rotation of the wheel and the perpendicular to the supporting surface. Thanks to this parameter, when the steered wheels are turned, the car body rises, as a result of which forces arise,
seeking to return the wheel to a straight position. Thus, KPI has a significant impact on the stability and stability of the vehicle in a straight line. The difference in the values of the angles of the transverse inclination of the right and left axles can lead to the withdrawal of the vehicle to the side with a large inclination. This effect can also manifest itself if the other wheel alignment angles correspond to the normal values.
Pitch Angle
Longitudinal angle of inclination of the axis of rotation Longitudinal angle of inclination of the axis of rotation (eng. caster - the angle between the axis of rotation of the wheel and the perpendicular to the supporting surface in the longitudinal plane of the vehicle. Distinguish between positive and negative angles of longitudinal inclination of the axis of rotation of the wheel.
A positive caster contributes to the emergence of additional dynamic stabilization of the car when driving at medium and high speed. As a result, the turning ability deteriorates. low speed.
Breaking shoulder
In addition to the above parameters, another characteristic is of great importance for the front axle - the running-in shoulder. This is the distance between the point formed by the intersection of the axis of symmetry of the wheel and the ground, and the point of intersection of the line of the transverse inclination of the axis of rotation and the ground. The run-in shoulder is positive if the point of intersection of the surface and the axis of rotation of the wheel lie to the right of the axis of symmetry of the wheel (zero shoulder), and negative if it is located to the left of it. If these points coincide, then the running-in shoulder is zero.
Breaking lever value This parameter affects the stability and steering of the wheel. The optimal value for modern cars is zero or positive run-in shoulder. The sign of the running-in shoulder is determined by the camber, the transverse inclination of the axis of rotation of the wheel and the offset of the rim.
Automakers do not recommend installing wheel disks with a non-standard departure, because this may result in changing the set running-in shoulder to a negative value. This can seriously affect the stability and handling of the vehicle.
Changing the values of the angles of installation of the wheels and their adjustment
Wheel alignment angles are subject to change due to natural wear of parts, as well as after replacement with new ones. Without exception, all steering rods and tips have threaded connection, which allows you to increase or decrease their length to adjust the values of the angles of convergence of the wheels. Convergence rear wheels, as well as the front ones, is adjustable on all types of suspensions, with the exception of the rear dependent beam or axle.
