The situation when a car begins to make extraneous sounds precisely at the moment of releasing the gas pedal often confuses the driver. It would seem that the load on the engine and transmission is minimal, the car rolls by inertia, but a dull or loud knock continues to disturb the ear. Coasting - this is a specific operating mode of the chassis, in which the vectors of forces acting on the suspension and drive shafts change.
Unlike engine acceleration or braking, coasting creates conditions when the suspension elements are in a “floating” state, and any play becomes audible. This symptom cannot be ignored, as it may indicate critical wear of parts that can lead to loss of controllability. In this article we will analyze in detail the mechanics of the occurrence of such sounds and methods for eliminating them.
Physics of the process: why it knocks without gas
To understand the nature of sound, it is necessary to consider the change in torque on the wheels. When overclocking drive shafts are in tension, transferring energy from the engine to the wheels, which often masks small backlashes in the connections. As soon as you release the accelerator, the torque disappears or becomes negative (under engine braking), and the parts are free to move within their clearances.
It was at this moment inertia the vehicle causes the wheels to roll forward, while the suspension elements may lag slightly or move in the opposite direction due to rolling resistance. If the system has worn out hinges, they begin to hit their seats. This phenomenon is often referred to as “bouncing” of parts.
It is important to note that the nature of the sound may vary depending on the road surface. On smooth asphalt, the knocking sound can be rare and dull, while on small gravel or “comb” it turns into a rapid fraction. This is due to the fact that micro-irregularities in the road force the wheels to constantly make vertical movements, provoking impacts in weakened units.
Technical information
Thrust and backlash vector: When coasting, the thrust vector changes direction. If during acceleration the splined joints and hinges are pressed against each other by the force of the engine, then they are opened by coasting. Any gap, even a few millimeters, when encountering an unevenness turns into an impact load, which is perceived by our ears as a knock.
Diagnostics of external and internal CV joints
One of the most common causes of knocking noise when coasting is wear and tear. constant velocity joints (CV joints). Drivers are often accustomed to the fact that a crunching sound when the wheels are turned out is a sign of a “grenade,” but when coasting, the symptoms may be different. The internal CV joint that connects the drive to the gearbox most often manifests itself as vibration, but when it is heavily worn, it begins to knock.
The outer joint is usually silent when moving in a straight line, but if a groove has been developed in its cage or chips have appeared, then when the load changes (gas roll-up), the ball can roll with an impact. This is especially noticeable on front-wheel drive cars with high mileage, where the resource drive shafts is already coming to an end.
To diagnose the outer CV joint, you can use the rocking method. Raise the car on a lift, grab the wheel with your hands at the 12 and 6 o'clock position and try to loosen it. If you hear a clicking sound when you pull it hard, the problem may be in the hinge. However, a more accurate result is obtained by visual inspection of the anthers: the presence of grease around the hinge indicates its destruction.
Checking the CV boot: Even if a knock has not yet appeared, but the boot is torn, the lubricant has leaked, and dust has gotten inside - the joint is doomed. Replace the boot and lubricant immediately, this will extend the life of the unit. If sand crunches inside, replacing the unit is inevitable.
Silent blocks of levers and subframe
Rubber-metal joints, or silent blocks, are the main element that dampens vibrations. When coasting, when the traction force disappears, the suspension arms may rotate slightly in their seats. If the rubber bushing has dried out, cracked, or completely peeled off from the metal cage, a characteristic dull knock occurs.
Particular attention should be paid to the rear silent blocks of the front levers and fastenings subframe. It is these points that experience maximum loads when the motion vector changes. In this case, the knock is often described as “wooden” or “iron”, especially on cold asphalt when the rubber becomes dull.
You can diagnose a faulty silent block using a mounting spatula. You need to rest your spatula on the lever and try to rotate it in the hinge. Lack of elasticity, visible tears in the rubber, or free movement of the metal bushing inside the rubber indicate the need for replacement. Sometimes it is enough to replace the rubber bushing itself, but more often the lever assembly is replaced.
Tie rods and rods
Although the steering is more associated with cornering, worn tie rod ends and the rods can also knock when coasting in a straight line. The mechanism is simple: the wheel, encountering an unevenness, receives an impulse, which is transmitted through the ball joint of the tip. If there is play there, metal hits metal.
You can distinguish the knocking of steering rods from the suspension by the nature of the sound. Tips often produce a louder, “clunking” sound, especially over small bumps. The rods connecting the rack to the tips knock more dullly, and this sound is often transmitted to the steering wheel in the form of light blows.
The check is carried out as follows: lift the front of the car, grab the steering rod closer to the tip and shake it up and down. There shouldn't be any play. You can also ask an assistant to sharply turn the steering wheel left and right while you keep your hand on the tip - vibration and gaps will immediately become noticeable.
The table below shows the main symptoms of malfunctions of various elements of the front suspension:
| element | Character of the knock | When it appears | Test method |
|---|---|---|---|
| Steering end | Voiced, frequent | Small irregularities, rolling | Rocking the wheel, checking the play by hand |
| Lever silent block | Deaf, solitary | Swing, start of movement | Visual inspection, spatula test |
| Inner CV joint | Vibration, hum | Acceleration, coasting under load | Rocking the shaft with your hand |
| Stabilizer link | Rumble, rattle | Any irregularities | Rocking the stand with your hand |
Support bearings and shock absorber struts
Support bearing serves to connect the shock absorber to the car body and ensures that the strut rotates along with the wheel when the steering wheel is turned. When coasting, especially over uneven surfaces, a faulty bearing can make crunching or creaking sounds, which drivers often confuse with a knock.
If we are talking specifically about knocking, then most often it is the fault shock absorber strut. If the internal valves become ineffective or destroyed, the shock rod can move freely in the cylinder, creating rebound shock. The upper support may also knock if the seat for the rod has worn out.
Diagnostics of the support bearing is carried out with the vehicle stationary. Open the hood, place your hand on the shock absorber spring and have a helper rock the steering wheel from side to side. Vibration and crunching under the palm will indicate a problem. To check the shock absorber, you need to sharply press on the corner of the car and release: the body should return to its original position and stop. If the “swing” begins, the shock absorber is faulty.
☑️ Front suspension diagnostics
⚠️ Attention: Operating a vehicle with a faulty support bearing or a damaged shock absorber strut can result in the steering locking or the wheel lifting off the road during a sudden maneuver. Don't put off repairs.
Transmission problems and engine
The power plant itself should not be discounted. The engine and gearbox are attached to the body through supports (pillows). If the rubber part of the support is destroyed, then when coasting, when the engine tends to turn or move due to inertia, it may hit the metal parts against the subframe or body.
The sound in this case will be low and bassy, reminiscent of the blows of a large lightning bolt. This is especially noticeable when releasing the gas, when the torque drops sharply and the engine nods. It could also be due to wear and tear damper disk clutch, which begins to rattle at low speeds when coasting.
Use a jack or lift to check the engine mounts. Carefully lift the engine (observing safety precautions!) and inspect the cushions from below. Cracks, tears and complete absence of rubber indicate the need for replacement. Sometimes the engine may touch the body due to misalignment, which also causes a knocking noise.
The secret to quiet operation
Many drivers forget about the “guitar” (lower engine mount). It is she who often knocks when starting and coasting on front-wheel drive cars. Check it first if the upper supports are intact.
Methods of elimination and prevention
Eliminating knocking noise when coasting begins with an accurate diagnosis. Don't rush to change all the details in a row. Use the method of elimination: check the simplest and cheapest elements (rods, tips) before climbing into transmission or engine.
When replacing silent blocks, be sure to lubricate them with a soap solution when pressing them in, but under no circumstances use oils or grease, which can destroy the rubber structure. After replacing any suspension elements, it is necessary to check and, if necessary, adjust wheel alignment.
Prevention consists of regular visual inspection of the engine compartment and the suspension on the lift. Timely replacement of torn CV joint boots and shock absorbers prolongs the life of adjacent components. Careful driving, especially on bad roads, also reduces the load on the chassis components.
The main rule: A coasting knock is always a backlash. No play - no knock. Your task is to find exactly where the parts gain extra freedom of movement when the thrust vector changes.
⚠️ Attention: After any intervention in the steering elements or replacement of suspension arms, adjustment of the wheel alignment angles is mandatory. Ignoring this requirement will lead to rapid wear of the tires and the vehicle pulling to the side.
Frequently asked questions (FAQ)
Can the suspension only knock when it's cold?
Yes, this is possible. Rubber elements (silent blocks, stabilizer bushings) harden in the cold and lose their elasticity. The gaps in them become more noticeable, and the sound becomes louder. After warming up, the rubber softens and may temporarily stop knocking, although wear will not go away.
Is CV joint knocking dangerous when driving in a straight line?
If the CV joint begins to knock even when coasting in a straight line, this is a sign of severe wear. Operating such a car is dangerous: the joint may jam or fall apart while moving, which will lead to loss of drive and a possible accident.
Why does the knocking sound only appear at a certain speed?
This may be due to resonance. At certain speeds, the frequency of vibration of the wheel coincides with the natural frequency of vibration of the worn suspension element, increasing the amplitude of the knock. It may also indicate wheel imbalance or runout of brake discs.
How to distinguish engine knock from suspension knock?
Suspension knock directly depends on the quality of the road surface. The knocking sound of the engine (mounts) depends on the load (acceleration) and speed. If you are driving on a perfectly smooth road and there is a knocking noise only when you release the gas, most likely it is the engine or transmission mounts.