The car drives perfectly in town, but get onto a main road and reach a hundred and a shake appears in the wheel, or an even hum that was not there before. It is one of the commonest complaints, and behind it lie two different groups of causes: vibration nearly always comes from the wheels and their mounting, hum from bearings or tyres. The speed of 100 km/h is no accident: that is where wheel oscillation falls into resonance with the suspension. Here is how to work out in one drive which group you are dealing with.
Why around 100 km/h specifically
A typical passenger car wheel turns 12–14 times a second at 100 km/h. If there are an extra 20–30 grams on one side of the rim — a balance weight that fell off, caked dirt, uneven wear — every revolution creates a centrifugal force of several kilograms. At lower speeds the suspension damps it, at higher speeds the frequency leaves the resonant zone, but in the band of roughly 90–120 km/h the oscillation coincides with the natural frequency of the suspension and body — and is amplified. So classic imbalance vibration lives in a window: it appears at a hundred and often fades by 130.
A hum works differently: it rises smoothly with speed, without a window, and at a hundred it simply becomes loud enough to notice — the general breakdown of that class of sound is on the symptom page humming while driving.
If it is vibration: the wheels and their mounting
Common causes of shake, in descending order of likelihood:
- Lost balance. A weight came off on a pothole or at a pressure wash, or the balancing was done carelessly in the first place.
- Something stuck to the rim. Snow, ice or dirt inside the rim — free imbalance that disappears once the rim is washed.
- A bent rim or a bulge in a tyre. After hitting a pothole the wheel’s geometry is out; it shows up on a balancing machine.
- A wheel fitted off-centre. If after a wheel change the rim sat on the hub skewed or without spigot rings, the vibration appears immediately after the visit — that scenario is covered in detail in a knock after a tyre change.
- Worn suspension. Hammered-out bushes and tired dampers do not create vibration themselves, but they stop damping it — shake from a small imbalance becomes noticeable, signs of worn shock absorbers.
The location guide is simple: the wheel shakes — look at the front wheels; the seat and floor shake while the wheel is calm — the rear.
If it is a hum: bearing or tyres
An even low hum that rises with speed has two main candidates. A wheel bearing — the unit the wheel turns on: its hum is compared to a plane taking off, it does not depend on the quality of the tarmac and it changes in gentle bends at speed (first one side is loaded, then the other). All the stages and tests are in signs of a worn wheel bearing and on the symptom page wheel bearing hum.
Tyres hum because of a noisy tread pattern, a hard compound or uneven sawtooth wear (steps on the tread blocks — usually from dead dampers or an alignment out of specification). That hum changes noticeably with the road surface and barely reacts to cornering. A detailed comparison of the two versions with tests is in humming: wheel bearing or tyres.
Telling them apart in one drive
| Sign | Wheel imbalance | Bearing | Tyres |
|---|---|---|---|
| Character | Shake in wheel or body | Even low hum | Hum or rustle |
| Link to speed | A window around 90–120 km/h | Rises smoothly, no window | Rises smoothly |
| Reaction to surface | None | None | Changes on different tarmac |
| Reaction to a gentle bend | None | Quieter or louder | Almost none |
| Coasting in neutral | Persists | Persists | Persists |
| What to do | Balancing | Hub diagnosis | Inspect the tread, rotate wheels |
Coasting in neutral rules nothing out within that trio — all three sources rotate with the wheels — but it does exclude the engine and gearbox: if the sound goes in neutral, look under the bonnet rather than at the wheels. And one more clue: swap the front and rear wheels. The hum moved — it is the tyres; it stayed put — the bearing or the suspension at that corner.
How the repair is decided
It is sensible to start with the cheapest step — balancing: it either solves the problem or honestly reveals a bent rim or a bulge. A hum with the signs of a bearing should not be left in the long grass: a disintegrating bearing eventually wears its seat in the hub carrier, and what could have been a bearing becomes a complete hub assembly.
Two things worth asking. Whether the wheels were fitted with the correct spigot rings and torqued to specification with a wrench rather than a rattle gun set by feel — off-centre or over-torqued wheels produce exactly this symptom and cost nothing to put right. And, if a bearing is diagnosed, whether the design allows the bearing alone to be pressed in or whether the whole hub unit is replaced — that single answer changes the scale of the job entirely.
If after the drive it is still unclear whether it is vibration, a hum or both, record the sound in the Pro-Stuk app out on the road. The app matches the recording with your answers — at what speed it appears, whether it changes in bends, what was recently done to the wheels — and shows the likely causes with a level of urgency. With that recording you will not have to reproduce the problem on the spot at the workshop.
