A familiar situation: alone, the car runs quietly along a road you know, but seat three passengers or load the boot and an impact appears on every bump. The sound has nothing to do with the engine and does not change with speed. That is a fairly precise clue: it is almost always the suspension, which under weight has started working in a different part of its travel. Here is what happens down there and where to look first.
What changes when the car is loaded
The spring compresses under weight, the body sits lower, and the suspension moves into a different working range. It has a reserve of upward travel — up to the point where the strut meets the bump stop, the rubber or polyurethane travel limiter. On an empty car that reserve is enough for any town pothole. With four passengers and luggage the reserve shrinks, and the same pothole drives the suspension all the way to the stop — hence the dry impact.
The second effect is the redistribution of clearances. A worn bush or a hammered-out bushing sits in a middle position on an empty car, but under load it is pressed through and starts knocking clearly. In other words, loading does not create the fault, it reveals the one that was already there. That is exactly why such a knock is a good diagnostic sign rather than a reason to “just drive more gently”.
What most often knocks under load
| Source | How it shows itself |
|---|---|
| Destroyed bump stops | Sharp dry impact on medium potholes, only with a load |
| Sagged springs | The body noticeably lower at the rear, a knock on any undulation |
| Anti-roll bar bushes and links | Rattle on fine washboard, audible without a load too |
| Arm and beam bushes | Dull impact, the car wanders in ruts |
| Dampers | Body float after a speed hump joins the knock |
| Luggage in the boot | Jack, spare wheel, tools — the most common false alarm |
Do not skip the last line: before driving to a workshop it is worth taking everything out of the boot and checking that the spare is secured. That is a free check which closes a surprising number of cases.
How to check it yourself
Start by looking at how the car sits. Measure the distance from the arch to the centre of the wheel on the left and right, empty and loaded: a noticeable difference between sides points at a sagged spring — sagging springs symptoms and signs of worn suspension springs. Then bounce it: press down on a corner of the body and let go. A healthy suspension stops almost at once, a worn one keeps oscillating, as in checking shock absorbers with the bounce test.
Comparing types of bump is useful too. Fine washboard and expansion joints more often give away the anti-roll bar, deep potholes the bump stops and bushes — signs of worn bump stops and signs of worn anti-roll bar bushes. If the knock is heard differently inside the cabin and outside, the trick from a knock you only hear with the window open helps. The general catalogue of similar sounds is on the symptom page knocking over bumps.
How the repair is decided
The most frequent outcome with this symptom is an inexpensive one: bump stops, bushes, fasteners. The expensive items surface when the knock was ignored for a year and the arm bushes have been destroyed along with the dampers. So the decision usually turns on one question — is the travel gone because the consumables have crumbled, or because the springs have sagged and the dampers no longer control the movement.
Two checks separate those cases. The ride height measured left to right and front to rear, compared against the figure in the manual: sagged springs show up as centimetres, not impressions. And the state of the bump stops and boots with the wheel off — a crumbled stop is visible immediately and explains the impact without any further theory.
Three things worth knowing before the work. Springs and dampers are replaced in pairs across an axle, otherwise the car pulls under braking and over bumps. Bump stops and boots are cheap, but they are only reached with the strut apart, so they go in together with a damper — coming back for them later means paying the same labour twice. And bushes are finally torqued with the car’s weight on its wheels: tightened with the suspension hanging, the rubber lives twisted and tears within months.
If you want to understand what is sounding before the workshop visit, try recording the sound in the Pro-Stuk app, driving the same stretch twice — empty and loaded. The difference between the recordings is informative in itself, and the app matches it with your answers and shows the likely causes with percentages.
