A disc brake presses stationary friction pads against a rotating disc. In the single-piston floating caliper shown here, hydraulic pressure moves one pad and the caliper’s sliding body applies the opposite pad. Friction on both faces creates torque opposing rotation.
Disc Brake Interactive Calculator
Calculate the ideal torque from a single-piston floating caliper. The face view shows the rotor and stationary caliper; the axial cutaway shows how one piston and the sliding body press both pads. Adjust pressure, piston diameter, friction and effective radius.
Equation Used
- One hydraulic piston in a floating caliper with freely sliding guides.
- Both pads contact opposite disc faces with equal normal force.
- Constant friction coefficient and a specified effective friction radius.
- No pressure loss, caliper deformation, fade or pad wear in the calculation.
- Illustrated rotor outer radius equals effective radius plus 35 mm; this is not a rotor selection rule.
One piston acts on one pad, while the floating body supplies the opposing pad reaction. Both friction faces contribute torque. This ideal model does not calculate stopping distance, heat, tyre grip or component ratings.
One piston, two loaded faces
The first view shows a rotor turning beneath a stationary caliper. The second view cuts through the assembly along its axis, exposing the hydraulic chamber, piston, pads and sliding guide arrangement.
The piston moves one pad toward the disc. Reaction moves the caliper body in the other direction until its outer finger applies the second pad. With both faces in contact, each carries the same ideal normal force. Released clearances are enlarged to make the motion visible.
Floating and fixed arrangements
The calculator specifically models one piston in a floating caliper. Fixed calipers instead use opposing pistons and a body fixed to its support. Their area accounting must use the appropriate total piston area on each side.
Hydraulic force and friction torque
The piston area is A = πd²/4. Pressure P acting on that area produces normal force N = PA at each face in this ideal floating arrangement. Each face contributes μN tangential force, so the total is F = 2μN.
Multiplying by the effective friction radius gives T = 2μPAr. Convert megapascals to pascals by multiplying by one million and millimetres to metres by dividing by 1000. The displayed force per face is N, not the sum of the two axial force magnitudes.
Doubling pressure, friction or effective radius doubles torque. Doubling piston diameter quadruples piston area and torque. These relationships provide direct checks on the calculator.
54 mm piston at 7 MPa
A 54 mm piston has approximately 22.90 cm² of area. At 7 MPa, each pad has approximately 16.03 kN of normal force. With friction coefficient 0.40, the two faces provide approximately 12.83 kN of total tangential force.
At 130 mm effective radius, that gives approximately 1667 N·m of ideal torque at one disc. Reducing pressure to 3.5 MPa halves that result. Setting pressure to zero gives zero normal force, friction force and torque while retaining the piston area.
Torque is not stopping performance
This calculation does not include wheel radius, vehicle mass, rotating inertia, tyre grip, brake balance or thermal capacity. It therefore cannot establish stopping time or distance. Constant animation speed represents an imposed drive, not an assertion that braking has no effect on a freely rotating wheel.
Friction is entered as a model parameter and held constant. Temperature-dependent friction, pressure losses, deformation and uneven contact require additional data. The results are not component pressure ratings or vehicle brake recommendations.
Disc brake calculation questions
Why multiply by two?
There are two friction faces, each carrying normal force PA in the modeled arrangement.
Does “force per face” mean total clamp force?
It is the normal force on one face. The two opposing axial forces have equal magnitude; they should not be confused with the sum of the tangential friction forces.
Is effective radius the outside rotor radius?
No. It is the moment arm representing the distributed friction force over the pad contact area.
Why does the rotor keep turning?
The animation shows contact and load transmission with imposed rotation. It has no inertia or stopping-time model.
Can piston diameter be changed?
Yes. The cutaway diameter and calculated area respond to that input. Torque follows the square of diameter.
Reference
Brembo Parts — Brake caliper technology and operation, description of floating and fixed calipers, sliding guides and piston action. The calculator equations follow ideal hydraulic area force and friction torque.
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