A brake is a device that converts a vehicle's kinetic energy into frictional heat. Adjust the vehicle mass, the speed before braking, the number of discs and the material to see in real time how hot one disc gets in a single stop, and how close repeated braking comes to brake fade.
Parameters
Vehicle mass m
kg
Speed before braking v
km/h
Number of brake discs n
discs
Discs that share the heat (usually 4 on a 4-wheel car)
Mass of one disc m_disc
kg
Heat fraction into the disc f_disc
The rest escapes to the pads, tyres and air
Disc material
Sets the specific heat c and fade threshold
Results
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Braking energy E (kJ)
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Heat absorbed per disc (kJ)
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Temperature rise ΔT (K)
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Final disc temperature (°C)
—
Energy density (kJ/kg)
—
Fade verdict
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Brake disc heating animation
A caliper grips the rotating disc and kinetic energy turns into frictional heat. The disc's glow colour tracks the final temperature (dull → orange → red-hot).
Temperature rise ΔT vs braking speed
Disc temperature over consecutive stops
Theory & Key Formulas
$$E=\tfrac12 m v^{2},\qquad \Delta T=\frac{f_{disc}\,E}{n\,m_{disc}\,c}$$
Braking energy E (m: vehicle mass, v: speed before braking) and the temperature rise of one disc ΔT (f_disc: heat fraction into the disc, n: number of discs, m_disc: mass of one disc, c: specific heat).
$$T_{final}=T_{amb}+\Delta T$$
Final disc temperature T_final (T_amb: ambient temperature, 25°C). v must be converted to m/s, and c is the specific heat of the disc material.
What is the Disc Brake Thermal Simulator?
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A disc brake is the device that stops a moving car, right? Where does the energy go when it stops?
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Good question — energy can't just vanish. A moving car carries kinetic energy E = ½mv². The brake turns that energy into heat through friction between the pads and the disc, and throws it away. So the disc gets hot every time you stop. Just stopping a 1.5-tonne car from 100 km/h releases about 580 kJ of heat in an instant — about as much heat as running a household hair dryer for ten minutes straight.
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580 kJ! Does all that heat really go into that thin metal disc?
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Not all of it, but most of it. This tool assumes 90% flows into the discs; the rest escapes to the pads, tyres and air. The problem is that the heat is concentrated in discs weighing only a few kilograms. The temperature rise equals absorbed heat ÷ (mass × specific heat). Raise the "speed before braking" on the left — because it acts as the square of speed, going to 150 km/h makes the energy 2.25× larger and the temperature rise jumps by the same 2.25×.
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If one stop raises it that much, what happens if I brake many times on a long downhill?
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That is exactly the dangerous pattern. If the next braking comes before the disc has cooled down, heat keeps stacking up. Look at the "disc temperature over consecutive stops" chart below. When more heat is added than cooling removes, the temperature climbs step by step. Keep that up and, past a certain temperature, the friction coefficient of the pad suddenly drops — that is "brake fade". The brakes going soft on a long mountain descent is caused by exactly this.
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I've heard of fade! How do you prevent it?
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Basically you either "make it tougher against heat" or "stop heat from building up". Make the disc bigger and thicker and its thermal capacity rises, so the same heat gives a smaller temperature rise. A ventilated disc with internal cooling passages lets the airflow carry heat away. To go further, switch the material to carbon-ceramic — high specific heat and a fade threshold up at 900°C. Switch the material on the left and watch how the final temperature and fade verdict change. Sports cars and race cars run large discs precisely for this heat management.
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On a downhill they always say "use engine braking" — is that also about heat?
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Exactly. Using engine braking or downshifting lets the engine absorb part of the kinetic energy, so less heat flows into the discs. Relying only on the foot brake keeps this tool's f_disc high — that is, all of the heat is concentrated in the disc. "Don't ride the brake on a long descent" is a driving technique that gives the disc time to cool and keeps it short of the fade threshold.
Frequently Asked Questions
A brake turns the vehicle's kinetic energy E = ½mv² into heat by friction. A fraction f_disc of that heat flows into the discs while the rest escapes to the pads, tyres and air. The heat absorbed by one disc is E_perDisc = f_disc·E / n, where n is the number of discs. The temperature rise is ΔT = E_perDisc / (m_disc·c), where m_disc is the mass of one disc and c is the material's specific heat. The final temperature is the ambient temperature of 25°C plus ΔT.
Brake fade is the loss of braking power that occurs when the discs and pads get so hot that the friction coefficient drops, so pressing the pedal no longer slows the car as much. On a long downhill, repeated braking gives the disc no time to cool, the temperature builds up, and once it crosses a threshold (about 650°C for cast iron) the brakes suddenly become weak. This tool compares the final temperature with the material's fade threshold and grades it as comfortable margin, running hot, or fade risk.
Kinetic energy scales with the square of speed, so even one hard stop from 100 km/h in a 1.5-tonne car releases about 580 kJ of heat in an instant. Most of that heat (90% in this tool) is dumped into discs that weigh only a few kilograms, so a single stop already produces a temperature rise of tens of degrees. Double the speed and the energy — and the temperature rise — quadruple. That is why braking from high speed is especially demanding.
There are three measures. (1) Use a ventilated disc with internal cooling passages to improve heat rejection. (2) Make the disc larger and thicker to increase its thermal capacity m_disc·c. (3) Switch to a carbon-ceramic material with a higher specific heat and heat resistance. In this tool, selecting carbon-ceramic raises the specific heat c to about 1.7 times that of cast iron, so the same heat produces a smaller temperature rise, and the fade threshold rises to 900°C.
Real-World Applications
Brake design for passenger and commercial vehicles: Carmakers size the discs and choose the material around the most severe use cases — a long mountain descent, or towing a fully loaded trailer. Like this tool, they estimate the energy of a single stop and the temperature rise under repeated braking, and secure a margin against the fade threshold. Heavier and faster vehicles need larger discs precisely to gain thermal capacity.
Motorsport: Race cars repeat hard braking many times per lap, and disc temperatures reach several hundred degrees. In Formula 1, carbon brake discs glow red-hot in use, and combining high heat resistance with low weight can decide the race. Selecting carbon-ceramic in this tool lets you feel how much higher the specific heat and the fade threshold become.
Heavy vehicles and rail: Trucks, buses and rail vehicles have large mass and an enormous amount of kinetic energy. The foot brake alone cannot handle the heat, so designers add engine brakes, exhaust brakes and retarders (hydraulic or electromagnetic auxiliary brakes) to lower the heat fraction f_disc that flows into the discs in the first place. The "use engine braking" signs on long descents are this heat management put into practice.
Pre-study for thermal CAE analysis: Before running a detailed transient heat-conduction FEM or a thermal-stress analysis, a lumped thermal-capacity model like this tool (treating the whole disc as one temperature) gives a first read. Once the estimate fixes the order of magnitude of the final temperature, the disc dimensions can be revised before investing in mesh and convection boundary conditions. Conversely, if the FEM result differs sharply from this estimate, it is a sanity check that points to a mistake in the heat input or boundary conditions.
Common Misconceptions and Pitfalls
The most important caveat is that this tool is a lumped thermal-capacity model and cannot capture the temperature distribution inside the disc or local hot spots. It assumes the whole disc heats up uniformly and reports an average temperature. In reality the friction surface (the rotor face) where the pad contacts gets far hotter than the interior, and in the instant of a hard stop the surface alone can be several hundred degrees hotter. A steep temperature gradient causes thermal stress that can warp the disc or open heat cracks. Treat this tool's final temperature as a guide to the average; the surface peak temperature is higher.
Next, assuming you are safe as long as you stay below the fade threshold. The fade threshold is only the point at which the friction coefficient begins to drop sharply; braking performance degrades gradually well before that. High temperature also accelerates pad wear and can boil the brake fluid, creating bubbles — "vapour lock" (the pedal sinking to the floor). When this tool gives a "running hot" verdict at 0.6 times the threshold, the margin is already not generous. For sustained use, driving and designing to keep temperatures low is the safer side.
Finally, heavier discs are not always better. It is true that increasing the disc mass m_disc raises the thermal capacity and reduces the temperature rise, but the disc is unsprung mass (mass below the suspension). Making it heavier hurts ride comfort and handling stability and is bad for acceleration and fuel economy. That is why high-performance cars choose carbon-ceramic — light and heat-resistant — or ventilated structures that increase heat rejection. A disc is designed by balancing thermal capacity, heat rejection and light weight.
How to Use
Enter vehicle mass (kg) using massNum or massRange slider; typical range 800–2500 kg for passenger cars.
Set initial velocity (km/h) via vNum or vRange; higher speeds generate exponentially greater braking energy.
Specify number of brake discs (ndNum or ndRange); dual-disc setups distribute heat load more evenly than single discs.
Select disc material (dmNum or dmRange): cast iron (ρ≈7200 kg/m³, c≈460 J/kg·K), carbon-ceramic (ρ≈1600 kg/m³, c≈800 J/kg·K), or sintered metal.
Click Calculate to output braking energy E (kJ), heat per disc, temperature rise ΔT, and fade verdict.
Worked Example
A 1500 kg sedan braking from 100 km/h with two cast-iron discs (diameter 330 mm, thickness 28 mm each, mass ~3.2 kg per disc): Braking energy E = 0.5×1500×(27.78)² = 579 kJ. Heat per disc = 289.5 kJ. ΔT = 289.5/(3.2×460) ≈ 198 K. If initial disc temperature is 85°C, final temperature reaches 283°C, well below fade threshold (350°C for cast iron). Energy density = 289.5/3.2 = 90.5 kJ/kg. Verdict: No fade risk.
Practical Notes
Carbon-ceramic discs tolerate 900°C before fade; cast iron fades severely above 350°C. Mountain descents with repeated hard braking raise temperatures 150–200 K per stop.
Four-disc systems (sport vehicles, large SUVs) split energy into four loads: each disc absorbs 25% less heat, reducing fade probability for aggressive driving or towing scenarios.
Ambient temperature affects final disc temperature directly; a 40°C summer day requires ~30 K margin reduction versus winter baseline.
Ventilated discs dissipate ~15–20% more heat than solid discs; simulate by reducing effective ΔT by 0.15 multiplier.