Brake Disc Size Question

Hello All :

For bicycles with disc brakes -

Why are bikes built with larger discs on the front wheel and small discs employed for the rear wheel ?

Thanks in advance.


Jai

Comments

  • webboo
    webboo Posts: 6,087
    Because the front brake is the one that stops you. The rear brake controls how you stop I.e in a straight line and not over the bars.
  • webboo said:

    Because the front brake is the one that stops you. The rear brake controls how you stop I.e in a straight line and not over the bars.

    Would that be because for a road bike the rider has more weight on the front than the rear ?

  • They’re not always different sizes. I’ve got a couple of bikes with 160mm front and rear rotors, some have 140mm front and back, and I know people that use 160mm front and 140mm rears. It’s personal preference a lot of the time, and given that you should be biasing your braking to the front, that makes a lot of sense.
  • They’re not always different sizes. I’ve got a couple of bikes with 160mm front and rear rotors, some have 140mm front and back, and I know people that use 160mm front and 140mm rears. It’s personal preference a lot of the time, and given that you should be biasing your braking to the front, that makes a lot of sense.

    So no real hard and fast rule.

    I get what you share - Thanks !
  • j_vora said:

    webboo said:

    Because the front brake is the one that stops you. The rear brake controls how you stop I.e in a straight line and not over the bars.

    Would that be because for a road bike the rider has more weight on the front than the rear ?

    Think of it like a car. You wouldn’t want to stop a car using mostly the handbrake, it wouldn’t be much fun. Same is true on a bike. The front brake takes the brunt of the braking because you want to be pushing back from the front, not dragging back from the rear.



  • The front brake takes the brunt of the braking because you want to be pushing back from the front, not dragging back from the rear.


    Makes sense - Got it !

    Thanks !
  • joe_totale-2
    joe_totale-2 Posts: 1,333
    edited September 2020
    Look at a motorbike, they have huge rotors on the front wheel and much smaller ones on the rear wheel.
  • pilot_pete
    pilot_pete Posts: 2,120
    Simply because about 70% of your stopping power should be coming from the front brake. The bigger the front rotor the better it will dissipate the heat generated under braking. I ride 140mm front and rear.

    If I were to go to the Alps for example, I would fit a 160mm front as I would be doing a lot more high speed and repeated hard braking efforts which will generate a lot of heat. At 82kg a 140mm on the front is fine on the relatively short descents I do in the Peak District.

    PP
  • It's not common for a road bike to have differing rotor sizes, much more common for MTB's. I think road bike manufacturers seemed to have settled on 160mm front and back, as far as I can see.
  • Mine is 160 at each end.
    Advocate of disc brakes.
  • andyh01
    andyh01 Posts: 599
    Doh, just wrote long post and not sent..
    Am.i right in thinking bigger the better for heat dissipation but weight penalty? Surely can't be that much difference in weight or is smaller more aero?
    I guess if holes in disc helps to keep cool but compromise on integrity
    Surely they don't get that hot with air flow to point that they would fail warp/buckle/melting point?

    Do the pads make any difference to heat build up?

    Also does the fluid help to keep disc cool or does it just stop the fluid from evaporating when using the higher boiling point fluid, which doesn't make contact with the disc and in sealed lines?
  • veronese68
    veronese68 Posts: 27,491
    I had a Parabox converter on my bike for a few years, the front had a bigger caliper and pads than the rear.
  • pilot_pete
    pilot_pete Posts: 2,120
    edited September 2020
    Some interesting questions in that response Andy.

    1. Yes, the bigger the disc the better heat dissipation (all else being equal), therefore the less likely to overheat. I have had a front 140mm disc overheat and warp under heavy breaking. It was no longer flat when laid on a bench - it had dished with the heat.

    2. I swapped that (and the rear) for Shimano Freeza rotors - these have aluminium sandwiched between the steel of the braking surface, which acts as a heat sink. Never experienced overheating with them as they operate at a lower temperature under the same braking forces due to dissipating the heat better. Still 140mm front and rear on two different bikes.

    3. Yes, there is a weight penalty for larger discs, but really that is irrelevant - the brakes are there to stop you, if a rotor overheats then it will fail to do so. So surely reliable stopping should be the number one concern? The weight difference is minimal. And as for aero losses - again negligible and irrelevant between 140 and 160mm discs, even less so if you are not racing against the clock.

    4. Holes in the disc do not compromise integrity. If you designed and sold a disc in that way you would end up liable for the inevitable accidents! The holes are there to help dissipate heat and water from the braking surface. They also help dissipate any small debris trapped between rotor and pad. Obviously on a bicycle they also help to keep the weight a bit lower too.

    5. Answered already - yes they can overheat. It depends on a number of factors such as rider and bike weight, speed at application and how hard you apply the brakes, cooling time between applications etc etc - so long, steep descents, with lots of hairpin turns and straights in between will mean repeated heavy braking from high speeds which will put more and more heat into a disc unless it can dissipate it at a sufficient rate.

    6. Pad material can affect heat transfer into a disc rotor.

    7. It’s not just rotors overheating though - you can get the caliper very hot too. Any small bubbles of air in the system can then expand due to the heat and the brake lever gets spongy, possibly even travelling to the handlebar without maximum braking force!

    8. The fluid doesn’t help keep the disc cool, that’s not it’s job. It is however designed with a boiling point that will have been tested to try to ensure it won’t boil until a temperature above what should be achieved under normal conditions. A higher boiling point fluid is just that - it’s not to cool a rotor, it’s to prevent it boiling until that higher temperature. The caliper and therefore fluid within getting hot is simply a byproduct of heat dissipation through the caliper, which is transferred from the pads. You will notice some pads have finned backing plates - again to improve airflow and increase surface area to assist cooling.

    The worst combination is a heavy rider, descending said steep hill with lots of heavy braking from high speed into repeated hairpin bends. A lighter rider, (assuming both use correct technique) may well be able to use smaller rotors and get adequate braking, when a heavier rider may need a larger disc to achieve the same, all other things being equal.

    Dragging the brakes downhill is the worst technique as heat will continually build...Good technique is to let the bike run, then brake hard to scrub the speed off before a bend, then let the brakes off and allow the bike to build speed again before the next application.

    PP
  • j_vora
    j_vora Posts: 63

    Simply because about 70% of your stopping power should be coming from the front brake. The bigger the front rotor the better it will dissipate the heat generated under braking. I ride 140mm front and rear.

    If I were to go to the Alps for example, I would fit a 160mm front as I would be doing a lot more high speed and repeated hard braking efforts which will generate a lot of heat. At 82kg a 140mm on the front is fine on the relatively short descents I do in the Peak District.

    PP

    Thanks pilot_pete and many who have taken time to answer my question - Much appreciated !


    Jai
  • andyh01
    andyh01 Posts: 599

    Some interesting questions in that response Andy.

    1. Yes, the bigger the disc the better heat dissipation (all else being equal), therefore the less likely to overheat. I have had a front 140mm disc overheat and warp under heavy breaking. It was no longer flat when laid on a bench - it had dished with the heat.

    2. I swapped that (and the rear) for Shimano Freeza rotors - these have aluminium sandwiched between the steel of the braking surface, which acts as a heat sink. Never experienced overheating with them as they operate at a lower temperature under the same braking forces due to dissipating the heat better. Still 140mm front and rear on two different bikes.

    3. Yes, there is a weight penalty for larger discs, but really that is irrelevant - the brakes are there to stop you, if a rotor overheats then it will fail to do so. So surely reliable stopping should be the number one concern? The weight difference is minimal. And as for aero losses - again negligible and irrelevant between 140 and 160mm discs, even less so if you are not racing against the clock.

    4. Holes in the disc do not compromise integrity. If you designed and sold a disc in that way you would end up liable for the inevitable accidents! The holes are there to help dissipate heat and water from the braking surface. They also help dissipate any small debris trapped between rotor and pad. Obviously on a bicycle they also help to keep the weight a bit lower too.

    5. Answered already - yes they can overheat. It depends on a number of factors such as rider and bike weight, speed at application and how hard you apply the brakes, cooling time between applications etc etc - so long, steep descents, with lots of hairpin turns and straights in between will mean repeated heavy braking from high speeds which will put more and more heat into a disc unless it can dissipate it at a sufficient rate.

    6. Pad material can affect heat transfer into a disc rotor.

    7. It’s not just rotors overheating though - you can get the caliper very hot too. Any small bubbles of air in the system can then expand due to the heat and the brake lever gets spongy, possibly even travelling to the handlebar without maximum braking force!

    8. The fluid doesn’t help keep the disc cool, that’s not it’s job. It is however designed with a boiling point that will have been tested to try to ensure it won’t boil until a temperature above what should be achieved under normal conditions. A higher boiling point fluid is just that - it’s not to cool a rotor, it’s to prevent it boiling until that higher temperature. The caliper and therefore fluid within getting hot is simply a byproduct of heat dissipation through the caliper, which is transferred from the pads. You will notice some pads have finned backing plates - again to improve airflow and increase surface area to assist cooling.

    The worst combination is a heavy rider, descending said steep hill with lots of heavy braking from high speed into repeated hairpin bends. A lighter rider, (assuming both use correct technique) may well be able to use smaller rotors and get adequate braking, when a heavier rider may need a larger disc to achieve the same, all other things being equal.

    Dragging the brakes downhill is the worst technique as heat will continually build...Good technique is to let the bike run, then brake hard to scrub the speed off before a bend, then let the brakes off and allow the bike to build speed again before the next application.

    PP

    Thanks, mostly what I thought. For the first time time I'm on hydraulic discs and my wife and I have a cable disc brake tandem, for my purposes, I've never experienced over heating brakes and that can include feathering brakes, to stop picking up speed in the first instance.
    My main point summerised was/is if such an issue and for the little differences, I would've thought they'd just be supplied with the biggest rota's.
    Ah re different BP of rluids, I couldn't see how this would help, as again never had levers/hoods heating up due to heat transfer from the disc, that in itself must be some serious heat.

    Naturally descending down big slopes at high speed, would create air flow, I guess I've never ridden abroad l, at high speeds down long slopes 🤣
  • pilot_pete
    pilot_pete Posts: 2,120
    andyh01 said:



    Ah re different BP of rluids, I couldn't see how this would help, as again never had levers/hoods heating up due to heat transfer from the disc, that in itself must be some serious heat.

    Err, who said anything about levers/ hoods heating up? That’s never going to happen.

    The caliper can and does, hence the fins on the back of some pads to increase surface area and aid cooling.

    If the caliper heats up so will the fluid in the caliper and the attached brake line. If there are any tiny air bubbles in the fluid trapped in the caliper they can expand and make your brake feel go spongy. As the caliper/ fluid cools it goes back to normal feel. That is an indication that your brake needs a more thorough bleed.

    PP

  • lakesluddite
    lakesluddite Posts: 1,337
    ...and don't forget folks - if you are intending to fit larger rotors, you'll need a 'spacer' inserted under the existing calipers (sorry if this is teaching granny to suck eggs).
  • pilot_pete
    pilot_pete Posts: 2,120

    ...and don't forget folks - if you are intending to fit larger rotors, you'll need a 'spacer' inserted under the existing calipers (sorry if this is teaching granny to suck eggs).

    Most that I have seen (flat mount) come with a mount bracket. You simply turn it one way for 140mm rotors and put it the other way around for 160mm.

    PP