A situation when the car begins to spontaneously change the trajectory of movement, prowl from side to side or, as drivers say, “loop”, can scare even an experienced driver. This is especially dangerous at high speeds or when maneuvering in a dense flow, where every centimeter of the trajectory matters. The driver is forced to drive constantly, straining physically and psychologically to keep the car in its lane.
Most often, complaints about the fact that the car is driven from the bottom come when describing the behavior of the rear part of the body or the general instability of the rear axle. This condition does not occur out of the blue - it is always a consequence of wear of suspension parts, violation of geometry or problems with the chassis. Ignoring such symptoms can lead to loss of control at a critical time when a sharp braking or detour of an obstacle is required.
In this article, we will discuss in detail the mechanics of the process, explain why the rear axle begins to tear down, and which design elements require priority attention. You will learn how to independently conduct primary diagnosis and distinguish the banal under-pumped air in tires from serious breakage of the levers.
Effect of tyre condition and pressure on course stability
The first thing to start looking for the cause of the unstable behavior of the car is to check the wheels. Often drivers look for complex breakdowns in the suspension, forgetting about the most elementary. Different pressures. In the left and right side tires, there is a significant difference in the contact spot with the road. The car begins to tilt and pull towards the wheels with lower pressure, creating a yaw feeling.
The second critical factor is the degree of wear of the tread and its uniformity. If the rubber is “bald” or has uneven wear (for example, “eaten” the inside), the grip on the road becomes unpredictable. On wet asphalt or in the track, such a defect manifests itself especially clearly: the car begins to wander along the lane, reacting to the slightest irregularities.
- 🚗 Check the pressure in all four wheels with the pressure gauge, checking the indicators with the manufacturer's recommendations (usually indicated on the door rack).
- 🛞 Examine the tread for “hernia”, bloating or uneven erasure of the pattern, which indicates problems with the collapse.
- 🌡️ Pay attention to the seasonality of rubber: suffocated winter tire in summer or "bald" summer tires in the rain guarantee loss of exchange rate stability.
It is also worth mentioning such a phenomenon as the “cone” of the tire. This is a manufacturing defect or the result of improper storage when the tire profile is not perfectly round. When rotating, such a wheel works like an eccentric, constantly taking the car aside. This can be diagnosed only on a specialized stand or by eliminating the wheels.
Diagnosis of the rear suspension: Silentblocks and levers
If everything is in order with the wheels, then with a high degree of probability the reason lies in the elements of the rear suspension mounting. The main enemy of stability here is the backlash. When plumblock (resinometallic hinges) levers wear out, free passage appears in the connections. The lever begins to “walk” at its landing point, allowing the wheel to shift relative to the body.
Especially critical is the wear of the Silentblocks of the front of the lower lever or the so-called “crab” (crossbar) in multi-link suspensions. When braking or accelerating, the wheel under the action of inertia moves forward or backward, changing the angle of installation. For the driver, this is expressed in a sharp jerk or pullback of the car when pressing the gas or brake pedal.
⚠️ Note: When replacing silent blocks, it is extremely important to use the correct pressing technology. Heating with open fire or hammering often destroys the internal structure of new rubber, and after a couple of thousand kilometers backlash will appear again.
In addition, it is worth examining the levers themselves for deformation. If the car fell into a deep hole at speed, the metal could get residual deformation. Even a slight curvature of the lever changes the geometry of the suspension, making it impossible to properly adjust the angles of the wheel installation. Visual inspection often does not give a result, a check at the stand of the collapse-descendence is required.
When buying used Silentblocks, pay attention to the date of rubber production. Old, "oak" rubber even without mileage can quickly crack and fail.
The role of shock absorbers and springs in body stabilization
Shock absorbers are responsible not only for comfort, but also for the constant contact of the wheel with the road. If shock absorber "flow" or lost its properties, the wheel after the passage of irregularity begins to jump uncontrollably. At this point, the grip on the road is lost, and with a side wind or a tilt of the road, the car can sharply lead to the side.
Often the cause of withdrawal is a broken turn of the spring. Visually, this may not be noticeable, especially if the spring is covered with dirt. But even the loss of part of the turn changes the height of the clearance on one side and the angle of inclination of the body. This leads to the fact that the axis is skewered, and the machine begins to pull towards the lower part.
| Symptoms. | Probable cause | Method of verification |
|---|---|---|
| Body swing after the pit | Wear of shock absorber | Press on the corner of the body: if swinged more than 1 time - replacement |
| Knocking at the passage of irregularities | Broken support sleeve | Visual inspection and swaying of the installation |
| Oil leaks on the rod | Violation of the sealing of the oal | Inspection of the suspension on the lift |
| Different clearances on the left and right | Settled or broken spring | Measurement of height from the center of the wheel to the arch |
It is also important to check the upper supports of shock absorbers. They often have bearings (especially in McPherson racks), which, when destroyed, can emit crunch and create backlash that affects the accuracy of the suspension. While this is more commonly the case with the front axle, in some rear suspension schemes elements also require attention.
☑️ Checking of shock absorbers
Problems with the collapse and convergence of wheels
The geometry of wheels is the foundation of controllability. If the wheels are broken, the car will constantly tend to move away. Convergence (the angle between the wheel plane and the car's longitudinal axis) is the most sensitive parameter. Even a small deviation of 2-3 mm can cause a noticeable yaw and rapid wear of rubber "saw".
The shattering (wheel angle to vertical) also affects stability. With a negative collapse, the car becomes more stable in corners, but may be worse to keep straight on the track. Positive collapse, on the contrary, contributes to the withdrawal of the car in the direction of the wheel tilt. On modern cars with multi-link suspension, the angles of collapse are often unregulated without replacing parts, so their violation indicates deformation.
Often drivers forget that the adjustment procedure is carried out only on a serviceable suspension. It is pointless to twist the bolts of collapse if there are backlashes in the levers. First, all faults are eliminated, worn-out parts are replaced, and only then the car is sent to the stand.
Why is the collapse going down?
Most often this occurs after falling into a deep hatch, hitting a curb or an accident. Also, the corners can “swim away” after replacing the suspension parts, if the master has not adjusted again. Sometimes the cause is the natural wear of the hinges, which gradually changes the geometry.
Steering and its effect on the rear axle
While the phrase “loops at the rear” usually indicates rear suspension problems, the impact of steering cannot be ruled out. If there is a significant backlash in the steering rack or tips, the driver has to constantly “catch” the car. Micromotion steering is transmitted to the wheels, and the car begins to rush along the lane.
Particular attention should be paid to the steering shaft cardinal and crosses. Wear at these nodes creates a delay in steering wheel reaction. You turn the steering wheel, but the wheels react late. Trying to compensate for this, the driver twists the steering wheel, causing a sharp jerk of the car to the side. It feels like the back is “wobbling”, although the problem is the delay in the front reaction.
- 🔧 Check the back of the steering wheel: it should not exceed 10-15 degrees (about 5-7 cm in the outer diameter of the steering wheel).
- 🛠️ Swing the steering rods and tips: there should be no free ride or knocking when the wheel is swaying sharply with your hands.
- 💧 Inspect the anthers of the slat: the presence of oil or lubricant outside indicates depressurization and possible wear of internal friction pairs.
It is also worth checking the tightening of the bolts of the subframe attachment (if any). The weakening of these mounts allows the entire front suspension to shift relative to the body, which radically changes the behavior of the car on the road and can simulate problems with the rear axle.
Aerodynamics and external factors
The reason for the swelling is not always a breakdown. Sometimes external factors are to blame. Side wind - frequent companion of instability on the road, especially for cars with high sailability (minivans, SUVs, vans). A gust of wind can dramatically shift a lightweight car off the trajectory, and if the suspension is not ideal, this effect is amplified.
The road gauge is another enemy. If the asphalt has longitudinal indentations, the wheels tend to fall into them. To get out of the rut, it takes a steering effort. When letting go of the steering wheel, the car sharply twitches towards the track. In combination with the worn suspension, this creates the effect of constant looping.
⚠️ Warning: Installing non-standard aerodynamic body kits, roof trunks or boxes can disrupt the aerodynamics of the car. At high speeds, this creates lift or lateral sailing, making the machine unstable.
The car’s load also plays a role. If the trunk is overloaded and the front seats are empty, the weight shifts back. The front wheels lose downforce and steering efficiency drops. The car becomes "empty" and sensitive to any irregularities of the road.
Comprehensive diagnosis of the chassis is mandatory: you can not treat only the symptom, you need to find the root cause, whether it is a backlash in the lever or pressure in the tires.
Frequently Asked Questions (FAQ)
Can the car be “swiped” because of the brakes?
Yeah, maybe. If the caliper jams on one of the rear wheels, it begins to slow down. When accelerating or coasting, the car will lead towards the “braked” wheel. Also, uneven wear of brake pads or discs can cause beats and withdrawal.
Is it dangerous to drive if the car is a little sideways?
It is dangerous and economically unprofitable. In addition to the risk of accidents, constant steering increases the fatigue of the driver. In addition, the broken geometry leads to accelerated “eating” of rubber, and new tires can become unusable in one season.
Why does the car start driving only at speeds above 90 km/h?
At high speeds, aerodynamic forces and inertia come into force. The slightest imbalance of wheels (balancing is required), the beating of brake discs or backlashes in the suspension, imperceptible in the city, on the track are amplified and cause a resonance that feels like yaw.
Do I need to make a break-down after each replacement?
Yes, if this part affects the geometry of the suspension (levers, Silent blocks, steering tips, shock absorbers). Even if you replace the part with a similar one, the angles of the wheel installation will almost certainly get lost, and without adjustment, the car will behave unstable.