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I am trying to understand why a bigger swept area on disc brakes provides higher braking force for the same force being exerted on the brake lever/pedal by the rider.

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If we use the same exact brake pad material, same thickness, same disc material/surface treatment etc. and the only variable we change is the width of the brake pads and therefore increase the swept area, would this result in higher braking force for the same lever/pedal input force? If so, please explain why.

Larger swept area means better cooling of the pads during braking but that shouldn't make a difference when the brakes are still cold. Larger swept area also means the pad material can be softer since the brake pads are larger so won't wear as quickly as the smaller variety, is this a significant reason why larger swept are provides better braking performance?

Would increasing the length of the pads (thus not increasing swept area) also increase braking performance (braking force higher for same lever/pedal force)? If so, would it have the same effect as making the pads wider?

source: Hydraulic Brakes - Why motorcycle brakes more powerful than bicycle brakes? (check first answer)

Thank you for any input, it is greatly appreciated!

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  • $\begingroup$ How does increasing the length of the pad not increase surface area - perhaps you should do a drawing about what you said. $\endgroup$
    – Solar Mike
    Sep 6, 2020 at 17:54
  • $\begingroup$ @solar Mike increasing length of pad does indeed increase pad surface are but if I understand the definition of swept area correctly it should not change with length of pads. My understanding of the definition: the total area of the disc that the brake pads operate on; area of the disc between smallest diameter where pad touches rotor and biggest diameter where pad touches rotor. Will add a drawing $\endgroup$
    – Maarten -Monica for president
    Sep 6, 2020 at 18:13
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    $\begingroup$ Do you think the outer surface of the drum will also conduct / radiate heat? If you want to check just do some serious braking then touch the outside of the drum... Also, some drums were cast with fins... $\endgroup$
    – Solar Mike
    Sep 6, 2020 at 19:11
  • $\begingroup$ Next question, are you assuming solid disks or vented? $\endgroup$
    – Solar Mike
    Sep 6, 2020 at 21:17
  • $\begingroup$ @SolarMike I was assuming vented discs $\endgroup$
    – Maarten -Monica for president
    Sep 6, 2020 at 21:18

2 Answers 2

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Brakes primarily convert kinetic energy to heat energy. So a large area can absorb more heat lowering the peak temperatures ;of course this is strongly affected by the thickness/mass of the discs and other factors. AND the larger area can get rid or more heat . High temperatures cause deterioration of pad materials ,so lower ( not as high) temperatures extend pad life. I anticipate brake usage and apply brakes "lightly" to increase stop time which reduces peak pad temperatures. As a result , when I last looked at my Murano pads at about 70,000 miles they showed very little wear. Years ago a friend did amateur road racing which required extreme braking and new pads and rotors every racing weekend.

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There are a lot of reasons, but we focus more on just heat transfer.

To bring a car to stop the brakes have to convert the kinetic energy of the moving car to heat on the disks and pads by friction and then transfer it to the air.

The disk heat is released into the ambient air by conduction, convection, radiation. In the case of disks, direct conduction is a major component.

The larger sweep area means the larger interface surface of the hot disk with the cool air, therefore the increased cooling rate of the disks, and increased performance of the brakes under demanding braking situations, before disks overheat and become warped and burn the pads.

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  • $\begingroup$ Am I correct in assuming this cooling effect doesn't play a significant role /shouldn't make a significant difference in terms of braking performance when the brakes are still cold? $\endgroup$
    – Maarten -Monica for president
    Sep 6, 2020 at 18:38
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    $\begingroup$ No. Unless we are talking about subzero temps, which can be dealt with by gently braking a few times to warm up the disks. $\endgroup$
    – kamran
    Sep 6, 2020 at 18:47

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