Turning motion into heat

Your brakes do not stop your car. Your tyres stop your car. The brakes' entire job is to turn motion into heat fast enough that the tyres get a chance — and then to survive the heat they just made.

Hold onto that division of labour, while remembering that tyres, hydraulics, controls and mechanical parts can also cause braking faults that are not primarily thermal.

How much heat, exactly

A moving car is holding kinetic energy. When it stops, all of that kinetic energy is converted, but not all of it necessarily enters the friction brakes: aerodynamic drag, tyre deformation, driveline losses, road gradient and regenerative braking can take shares.

E = ½mv²

Car mass m = 1500 kg

At 100 km/h: v = 100 ÷ 3.6 = 27.78 m/s, so v² = 771.6 m²/s²

E = 0.5 × 1500 × 771.6 = 578,700 J ≈ 580 kJ

At 50 km/h: v = 13.89 m/s, so v² = 192.9 m²/s²

E = 0.5 × 1500 × 192.9 = 144,700 J ≈ 145 kJ

580 ÷ 145 = 4.00. Half the speed, a quarter of the energy. Exactly, not approximately — because the only thing that changed was v, and v is squared.

Five hundred and eighty kilojoules is an abstraction until you put it next to something domestic. It is enough to take 1.7 litres of water from tap temperature to boiling: 580,000 ÷ (4186 × 80) = 1.73 kg. One stop. One set of lights.

Predict, then run

Same car: how much kinetic energy does it carry at 100 km/h compared with 50 km/h?

About the same — you are still going from moving to stopped

Twice as much

Three times as much

Four times as much

Commit

You learn more from being wrong on purpose than right by accident.

Where 580 kilojoules actually ends up

Into the discs, mostly, and mostly into the front ones. Braking pitches the car forward onto its nose, which loads the front tyres and unloads the rears, so the front brakes are built to take the larger share — typically 60 to 80 per cent of the work in a hard stop.

Assume 70% of the stop goes to the front axle: 0.70 × 580 kJ = 406 kJ

Assume two grey cast-iron discs at 7 kg each: m = 14 kg

Specific heat of grey cast iron: c ≈ 460 J/(kg·K)

ΔT = E ÷ (m × c) = 406,000 ÷ (14 × 460) = 406,000 ÷ 6,440 = 63 K

About sixty-three degrees for this deliberately simplified bulk-temperature model. During a real stop the friction surfaces heat faster than the disc core and transient thermal stress can contribute to cracking, but the depth, gradient and result depend on disc geometry, material, cooling and braking history.

Your brakes are the most powerful thing on your car

Energy is only half the story. The other half is how fast you have to move it, and that is power.

Take a firm stop at 0.8 g: a = 0.8 × 9.81 = 7.85 m/s²

Time to stop from 27.78 m/s: t = v ÷ a = 27.78 ÷ 7.85 = 3.54 s

Average power: P = E ÷ t = 578,700 ÷ 3.54 = 164 kW

Peak power, at the instant you first hit the pedal, where the speed is still highest:

Force F = ma = 1500 × 7.85 = 11,775 N

P = Fv = 11,775 × 27.78 = 327,000 W ≈ 327 kW

Three hundred and twenty-seven kilowatts is roughly 440 horsepower as an instantaneous energy-conversion rate, from a family car whose engine may make much less. It is not a component nameplate rating, and its duration depends on the stop.

Which is the entire design problem. Everything about a brake is a compromise between absorbing an enormous burst and then getting rid of it before the next one.

Fade is two different failures wearing one name

When a driver says the brakes faded, pad friction and fluid boiling are two important possibilities. Fluid aeration, hose expansion, adjustment and mechanical faults can produce related symptoms, so pedal feel guides diagnosis but does not prove it.

Pad fade. The friction material has a temperature range it was designed for. Past it, the resins and binders holding the material together start to break down and gas off, and the coefficient of friction falls away. The symptom: the pedal still feels firm, and the car will not slow. You are pushing just as hard and getting less back.

Fluid fade, or vapour lock. Heat conducts out of the disc, through the pad, into the caliper piston, and reaches the brake fluid sitting behind it. Boil that fluid and you have vapour in a system designed around a liquid — and liquid does not compress, while vapour does. The symptom: the pedal goes long and soft. Your foot travels further and the pads move less.

The descent, which is where this really bites

A single stop is a burst. A long descent is a tap left running, and the arithmetic is unkind.

Gravitational potential energy: E = mgh

E = 1500 × 9.81 × 500 = 7,357,500 J ≈ 7.36 MJ

Against one stop from 100 km/h at 580 kJ:

7,360,000 ÷ 578,700 = 12.7

Nearly thirteen full-speed stops, delivered back to back with no gap in which to cool. Thirteen stops' worth of heat is roughly 800 K of temperature rise on those two front discs if none of it escapes — and while plenty does escape to the air, the balance runs one way.

The fluid can boil locally near a hot caliper before the whole system reaches the fluid's measured boiling point. The actual threshold depends on the fluid condition, pressure and temperature distribution; 155 °C is the DOT 4 minimum wet ERBP test value, not a universal in-service caliper trigger.

Select a suitable lower gear before a long descent and follow the vehicle handbook so engine braking shares the continuous load. Do not exceed a safe engine speed. If braking performance changes, stop in a safe place and seek assistance; do not continue to test a soft or fading pedal.

Two levers, and only one of them is linear

Look at ½mv² as a design equation and it tells you what to do about brakes.

• Mass, m, is linear in the energy equation. Add 50% to mass and the kinetic energy rises 50% at the same speed. Brake sizing is not directly proportional because tyres, axle loads, cooling, duty cycle and regulations also matter.
• Speed, v, is squared. Add 50% to the speed and you add 125% to the energy.

That is why a 2200 kg electric SUV needs more brake than a 1500 kg hatchback — 0.5 × 2200 × 771.6 = 849 kJ from 100 km/h, about 47% more — and why the same hatchback needs more brake on a country road than it does in the suburbs, by a much larger factor.

What the electric version changes, and what it does not

Regenerative braking uses the traction motor as a generator, so some kinetic energy returns to the pack instead of leaving as heat. How much is available varies with the vehicle, battery state and temperature, speed, grip and requested deceleration.

It is a partial counterpart, not a replacement. Regenerative capacity is limited and can reduce with low speed, a cold or nearly full battery, grip conditions or system constraints. Friction brakes remain necessary for the balance and for reliable stopping when regeneration is unavailable.

So electric cars keep their discs, and two honest consequences follow:

• Higher vehicle mass increases kinetic energy at a given speed.
• Reduced friction-brake use can allow disc corrosion in some climates and duty cycles, so inspection remains important.

The one everyone gets wrong

If a pedal pulses under braking, disc thickness variation and lateral run-out are common causes, but they are not the only ones. Measurement with the correct instruments and comparison with manufacturer limits should come before machining or replacement.

The distinction is worth money because the fix differs. If the pedal has gone long, do not drive the vehicle; arrange safe recovery and inspection. For a pulsation in Melbourne's west, we will measure before quoting and tell you when the disc remains within specification.

What you now know

Where the energy went, how much of it there was, why the number goes up with the square, what heat does to the friction material and to the fluid, and why the answer on a long descent is a lower gear rather than a firmer foot.

It is the thermal foundation for understanding many brake behaviours, alongside hydraulic, mechanical, tyre and control-system diagnosis.