# Tyres and Grip

> Every force that accelerates, brakes or steers a car passes through four tyre contact patches, each roughly the size of a postcard. Learn how grip is generated, why tyres must slip a little to work, and how pressure, tread and the friction circle set the limits.

- Level: Beginner · about 13 minutes · Wheels & Tyres
- Lesson page: https://maxrecursion.github.io/AutoVerse/topics/tyres-and-grip

## What you will learn

- Estimate the size of a tyre contact patch from load and pressure
- Explain the friction circle: braking and cornering share one grip budget
- Describe slip ratio and slip angle and why peak grip needs some slip
- Read a tyre size marking and explain the role of tread depth in the wet

## The idea, in plain language

However powerful the engine or big the brakes, a car can only accelerate, stop and turn as hard as its tyres allow. Each tyre touches the road over a patch roughly the size of a postcard or the palm of your hand, and all the car's control forces pass through those four small areas.

Grip comes from the rubber gripping tiny bumps in the road surface and briefly bonding to it. The amount of grip depends on the tyre, the road and the load pressing the tyre down. On a dry road a good tyre can produce a sideways or braking force almost equal to the weight it carries; on a wet or icy road far less.

Importantly, a tyre has one 'budget' of grip. If you use most of it to brake, there is little left for steering, and vice versa. That is why braking hard in the middle of a bend is risky and why ABS keeps the wheels turning so you can still steer. In the wet, the tread grooves pump water out from under the tyre; with too little tread, the tyre can ride up on a film of water and lose grip altogether.

## The engineering explanation

The contact patch area is approximately the wheel load divided by inflation pressure (the carcass carries some load, so this is an estimate): 4,000 N on a tyre at 2.3 bar gives about 174 cm². The maximum horizontal force is roughly F_max ≈ μ × F_z, with μ around 0.9–1.1 for road tyres on dry asphalt and considerably lower in the wet. μ itself falls slightly as load increases (load sensitivity), which is why load transfer reduces an axle's total grip and why weight distribution matters.

Tyres generate force through deformation, not pure sliding. Longitudinal force rises with slip ratio — the difference between wheel speed and road speed — and typically peaks around 10–15% slip before falling as the patch slides. Lateral force rises with slip angle, the angle between where the wheel points and where it travels, peaking at a few degrees for road tyres. ABS and traction control hold the tyre near these peaks. Combined demands obey the friction circle (strictly an ellipse): √(F_x² + F_y²) ≤ μ F_z, so braking force used leaves less available for cornering.

A marking such as 205/55 R16 91V means 205 mm section width, sidewall height 55% of width (about 113 mm), radial construction, 16-inch rim, load index 91 (615 kg) and speed rating V (240 km/h). Radial construction lets the sidewall flex while steel belts keep the tread flat. Tread grooves evacuate water; when the water cannot escape fast enough, a wedge lifts the tread and the tyre aquaplanes, with the onset speed falling as tread depth decreases and as pressure decreases. Many jurisdictions set a legal minimum tread depth (for example 1.6 mm in the UK and EU), but wet braking performance degrades well before that point. Under-inflation increases flexing, heat build-up and rolling resistance; over-inflation shrinks the patch and reduces grip.

## Step by step

1. **A postcard of rubber.** Each tyre touches the road over a small patch; all control forces pass through it. *In engineering terms:* A ≈ F_z / p_inflation. 4,000 N at 230 kPa gives ≈ 174 cm².
2. **Grip scales with load.** The harder a tyre is pressed onto the road, the more force it can produce — up to a limit set by rubber and surface. *In engineering terms:* F_max ≈ μ F_z with μ ≈ 0.9–1.1 dry; load sensitivity means μ falls as F_z rises.
3. **Grip needs a little slip.** Tyres produce the most grip while stretching and twisting slightly, just before they start to skid. *In engineering terms:* Longitudinal force peaks at roughly 10–15% slip ratio; lateral force peaks at a few degrees of slip angle.
4. **One budget for everything.** Braking, accelerating and turning all share the same grip; use more for one and less remains for the others. *In engineering terms:* √(F_x² + F_y²) ≤ μ F_z — the friction circle (ellipse in practice).
5. **Tread clears water.** Grooves pump water away so the rubber can touch the road; worn tread makes aquaplaning more likely. *In engineering terms:* Aquaplaning onset speed falls with lower tread depth, lower pressure and deeper standing water.

## Key concepts

- **Coefficient of friction**: Ratio of maximum horizontal force to vertical load at the tyre. (`F_max ≈ μ × F_z`)
- **Friction circle**: Combined braking/accelerating and cornering forces cannot exceed the tyre's total grip. (`√(F_x² + F_y²) ≤ μ F_z`)
- **Slip angle**: Angle between the direction a wheel points and the direction it actually travels; it creates cornering force.
- **Contact patch area**: Approximately the wheel load divided by inflation pressure. (`A ≈ F_z / p`)

## Common misconceptions

- **Myth:** Wider tyres always give more grip because there is more rubber on the road. **Reality:** For a given load and pressure the patch area is similar; width changes its shape. Wider tyres often help through compound, heat and load sensitivity, but can aquaplane sooner.
- **Myth:** A tyre at the legal minimum tread is as safe as a new one in the rain. **Reality:** Wet stopping distances lengthen considerably as tread wears, long before the legal minimum is reached.
- **Myth:** Maximum grip happens when the tyre is not slipping at all. **Reality:** Tyres generate force through small amounts of slip; peak grip occurs at a modest slip ratio or slip angle, and falls once the patch slides.

## In the real world

- From 100 km/h on dry tarmac with μ ≈ 0.9, the physics limit gives a braking distance of about 44 m — before adding reaction time.
- Tyre pressures should be set cold to the figures on the door pillar or fuel flap, which account for the car's weight and speed.
- On a wet roundabout, braking before the turn rather than during it leaves the tyres' full grip available for steering.

## Common questions

### How do car tyres grip the road?

The rubber in each contact patch deforms into the tiny bumps of the road surface and bonds to it on a microscopic level, and the tyre transmits force by stretching slightly, which is called slip. Grip depends on the load on the tyre, its rubber compound, its temperature and the surface.

### What is the tyre contact patch?

It is the area of tread touching the road at any moment, roughly the size of a postcard for a normal car tyre. Every force that accelerates, brakes or steers the car passes through these four small patches.

### Why do tyres have tread?

Grooves give water somewhere to go, so the rubber can stay in contact with the road in the wet. With too little tread depth, water builds up under the tyre faster than it can escape and the car can aquaplane.

### Does tyre pressure affect grip?

Yes. Under-inflation overheats the sidewalls and makes steering vague, while over-inflation shrinks the contact patch and reduces grip. Use the pressures on the vehicle's placard, checked when the tyres are cold.

## The story behind it

### John Boyd Dunlop (1887–1889)

Gave the bicycle an air-filled tyre. In October 1887 Dunlop, a veterinary surgeon with a large practice on May Street in Belfast, developed the first practical pneumatic tyre for his son's tricycle and tested it in his yard. Willie Hume won the tyres' first races in 1889, including all four events at Queen's College Sports in Belfast on 18 May. Among the beaten riders were sons of Harvey Du Cros, who then partnered with Dunlop.

*Worth knowing:* Robert William Thomson patented a pneumatic tyre in 1847, and Dunlop's own 1888 patent was later declared invalid, about two years after it was granted.

Sources: [Wikipedia: John Boyd Dunlop](https://en.wikipedia.org/wiki/John_Boyd_Dunlop)

### Marius Mignol and Michelin (1946)

Developed the first Michelin X radial car tyre. Michelin designed, developed, patented and commercialised the radial tyre, and Marius Mignol developed the first Michelin X car radial in 1946. Its cords run at 90 degrees to the direction of travel. Michelin owned Citroën, which fitted radials to the 2CV from its 1948 launch. In 1968 Consumer Reports credited radials with longer tread life, better steering and lower rolling resistance, and a Ford engineer, Jack Bajer, adapted a 1960s Falcon's steering and added bushings to suit the radial's different feel.

*Worth knowing:* Earlier radial patents were filed by G. H. Hamilton and T. Sloper (London, 1914) and Arthur W. Savage (US, 1916), but no products were made.

Sources: [Wikipedia: Radial tire](https://en.wikipedia.org/wiki/Radial_tire)

## Related lessons

- [How a Car Works: The Big Picture](https://maxrecursion.github.io/AutoVerse/topics/how-a-car-works)
- [Brake Hydraulics](https://maxrecursion.github.io/AutoVerse/topics/brake-hydraulics)
- [Steering Systems](https://maxrecursion.github.io/AutoVerse/topics/steering-systems)
- [The Differential](https://maxrecursion.github.io/AutoVerse/topics/differential)
- [Mechanical Sympathy](https://maxrecursion.github.io/AutoVerse/topics/mechanical-sympathy)

## Further reading

- [Avoiding tyre wear](https://www.howacarworks.com/wheels-and-tyres/avoiding-tyre-wear) — How a Car Works
- [Tyres](https://www.howacarworks.com/advanced-driving/tyres) — How a Car Works

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Original AutoVerse lesson (AutoVerse original content). Simulation values in AutoVerse labs are illustrative. Source page: https://maxrecursion.github.io/AutoVerse/topics/tyres-and-grip