# Why Streets Don't Break Under Buses

How a heavy city bus's weight spreads through its tyres and the layers of a street, from the asphalt surface down to the soil\.

By bright\-koala\-6ec9696d

Topic: Engineering

[Open the interactive Jig](https://ask.jig.com/j/clU2zssvWnm3)

## Transcript

### 1. A bus weighs as much as three elephants

A full city bus weighs about 18 tonnes, roughly as much as three big elephants\. It rolls along the same street hundreds of times a week, yet the road stays in one piece\. Let's slice the street open and find out how\.

This model is built at 1:10 scale: the 12\-metre bus is 1\.2 metres long here\.

### 2. It all rests on six tyres

The bus touches the street only where its tyres meet it: two at the front and four at the back\. Each glowing patch is about the size of a large hand\. It presses down about as hard as the air inside the tyre, around 100 pounds on every square inch\.

The back axle carries about two\-thirds of the weight, so it has two tyres on each side\.

### 3. A street is a layer cake

Under the dark top is a stack of layers, each with a job\. A thin skin of asphalt, which is stones glued together with sticky tar, keeps out rain and gives grip\. Below it lie thick crushed stone, then gravel, then packed soil called the subgrade\.

On a typical city street the layers above the soil add up to about half a metre\.

### 4. Watch the push spread out

The colours show how hard the ground is being pushed\. Right under a tyre the push is strong but small\. Deeper down it spreads wider and gets weaker, like a torch beam spreading on a wall\. Drive the bus and watch the push travel with it\.

The lines mark a half, a quarter, a tenth and a fortieth of the tyre's push, from a simplified model\.

### 5. Gentle by the time it reaches the soil

Each layer hands the load to a wider area below it\. By the time it reaches the soil, half a metre down, the push is less than a tenth as strong\. The soil can carry that easily, so nothing sinks or cracks\.

Engineers choose each layer's thickness so that no layer is pushed harder than it can take\.

### 6. The street bends, just a little

A street is not perfectly stiff\. Under a bus wheel the surface dips by less than a millimetre, thinner than a coin, then springs back as the bus rolls on\. Bending a little instead of cracking is why asphalt streets are called flexible pavements\.

Measured dips under a heavy axle are around 0\.7 millimetres\.

### 7. Heavier axles wear roads much faster

One bus never breaks a street; millions of wheels over many years slowly wear it out\. Wear grows very fast with the weight on each axle: double the weight and the wear is about 16 times bigger\. That is why buses and trucks have axle weight limits\.

Engineers call this rule of thumb the fourth\-power law\. A car is so light that its wear hardly counts\.

### 8. Built to bend, not break

A street survives a bus by teamwork\. The tyres spread the weight, the asphalt spreads it more, and every layer below shares it until the soil feels only a gentle press\. Next time a bus rumbles by, picture the push spreading out beneath your feet\.

Water is a road's real enemy: cracks let rain soften the stone below, and that is how potholes start\.