What Causes the Car Body to Sway Left and Right When Passing Over Speed Bumps?
4 Answers
The reasons for the car body swaying left and right when passing over speed bumps are as follows: 1. Unbalanced Wheels: During manufacturing, it's impossible to ensure uniform quality for both tires and steel rims, so wheels must undergo dynamic balancing after assembly. As tires wear over time, periodic dynamic balancing is required at certain mileage intervals. 2. Loose Suspension Framework: The chassis suspension consists of various links and support arms connected through ball joints and rubber bushings. Increased wear of these components leads to looseness in the suspension, causing the car body to sway. 3. Excessive Steering Gear Clearance: The vehicle's steering system converts the rotational motion of the steering wheel into wheel oscillation via a rack and pinion mechanism. Over time, wear between the gear and rack creates play, allowing slight left and right wheel oscillation even without steering input, which can make the car body feel unstable.
Recently, my car has also been experiencing this issue when going over speed bumps. Upon closer inspection, I found it’s mainly a problem with the suspension system. Worn-out shock absorbers lose their support, causing the car body to sway from side to side like a boat. Another common cause is cracked bushings on the lower control arm—once those rubber pieces crack, the wheel alignment goes off. I recommend checking the tires, as uneven tire pressure on both sides can also make the car bounce unevenly over bumps. Loose stabilizer links are another frequent issue; if these small components are loose, the car body becomes particularly prone to swaying. To prolong your suspension’s lifespan, try to slow down to under 20 km/h when crossing speed bumps.
I've been driving this car for eight years, and it always feels like it's swaying from side to side when going over speed bumps. The mechanic said this is a typical symptom of aging lower control arms, where the rubber bushings crack and lose their cushioning effect. During the chassis inspection, it was also found that the shock absorbers are leaking oil, which is very dangerous. Once the oil is completely gone, the car body relies solely on the springs, causing particularly large swings when driving over bumpy roads. Additionally, wear on the ball joints of the triangular arms increases the gap, leading to unsynchronized movements of the left and right wheels. As for the tires, if the tread depth difference exceeds 1.5 millimeters, the car will tilt and sway when going over bumps. It's recommended to check the suspension bushings annually; these parts are inexpensive but crucial.
Last time after modifying the suspension, a similar situation occurred where the rear of the car swayed left and right when going over speed bumps. Upon investigation, it was found that the damping coefficient of the original shock absorbers did not match that of the modified springs, resulting in excessive after-shocks. Loose stabilizer bar links are also a hidden culprit; a gap exceeding 2mm in that metal rod can cause lateral sway. Additionally, deviations in wheel alignment data can lead to abnormal wheel camber angles, directly causing instability when going over bumps. The most easily overlooked issue is slight wheel rim deformation—an out-of-roundness exceeding 1.5mm can cause periodic swaying. Professional matching and tuning are essential when modifying the suspension.
