I was thinking about improving my front brake by putting a bigger disc on but a moments thought left me puzzled. I would be using exactly the same caliper with exactly the same pads exerting exactly the same force on exactly the same area of disc. How will this improve braking ? All I can see is that the disc should stay cooler as it would have a greater braking surface area. Can any science types shed some light on this?
Bike Forum
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Why do bigger discs brake better?
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Posted 1 year ago #
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Leverage. A bigger rotor means that the caliper and braking surface are further from the centre of rotation (axle).
Posted 1 year ago # -
turning moments.
try to push a door open with one finger near the hinges, then try it again with the same force, but your finger near the handle.
Posted 1 year ago # -
Er, FOG, did you skip physics classes at school?
Posted 1 year ago # -
omin, our physics teacher was nicknamed Beaky,never understood a word he said!
Posted 1 year ago # -
my (teacher) dad's nickname at skool was also Beaky, or Captain Beaky from his more reverent pupils. He didn't teach physic though.
Back on topic, as above it's about leverage. And the other thing is the larger rotor conducts more heat away from the pads and disspiates/radiates it out to the air more efficiently, meaning it is a bit further down your enormo alpine descent before your brakes overheat and stop working.
Posted 1 year ago # -
Similar principal to Torque=Force*Radius
Posted 1 year ago # -
Don't forget you'll lose some modulation too.
Posted 1 year ago # -
Turn your bike upside down and spin the front wheel slowly.
Stop it by poking your finger through the spoke near the rim.
Spin it again, this time stop it by poking your finger through the spokes near the hub.Now do you get it ?
Posted 1 year ago # -
Don't forget you'll lose some modulation too.
Err no you won't.
If anything it'll be better as you won't have to pull as hard to get the same braking force, making it easier to control.
205mm discs front and back for me always. Miles better!
Posted 1 year ago # -
Haze - Member
It's exactly this principal!
Similar principal to Torque=Force*Radius
Posted 1 year ago # -
Thought it might be, I was relying on memory from years ago and thought it may be a little less straightforward...
Posted 1 year ago # -
Because they look moto
Posted 1 year ago # -
So what we really need then is for someone to invent a brake that works on the rim!
Posted 1 year ago # -
So what we really need then is for someone to invent a brake that works on the rim!
Buell m/c's do or did this. Good idea, but never really that good when I tried them.
Posted 1 year ago # -
superfli - Member
> So what we really need then is for someone to invent a brake that
> works on the rim!
Buell m/c's do or did this. Good idea, but never really that good when I tried them.Whoosh!
Posted 1 year ago # -
Even better would be one that works on the surface of the tyre.
Posted 1 year ago # -
avdave2 - top idea.
Posted 1 year ago # -
i am i right in thinking that with larger discs they just need less force (lever pulling) to apply the same stopping power as smaller ones?
Posted 1 year ago # -
I'm just beginning to get the whole 29er thing now...
Posted 1 year ago # -
So what we really need then is for someone to invent a brake that works on the rim!
Buell m/c's do or did this. Good idea, but never really that good when I tried them.
you mean like this
Posted 1 year ago # -
I always thought it was because a larger disc had a greater circumference so therefore increased the breaking area per wheel revoloution. Does this not play arole in the process?
Posted 1 year ago # -
isn't braking area constant ie the size of the pads? Altho I'd guess the longer circumference helps with heat dissipation
L_P 8pot and giganto rims? that just looks silly.
Posted 1 year ago # -
yes pad area [and force applied ???] is constant. The area that the pads cover, for one turn, will be greater with a bigger rotor than a smaller one due to larger circumference. Not sure whether leverage or greater surface area is the main factor tbh
Posted 1 year ago # -
leverage is the main factor. Simply put - at a greater radius, less force is required to decelerate the wheel by the same amount.
So - if it took 10 Newtons to cause a deceleration of 10 metres per second per second at a radius of 10cm, then it would only take 5 Newtons at a radius of 20cm to cause the same deceleration.
Posted 1 year ago # -
modulation does decreace with a bigger rotor,
Say you can apply a force of 1-10N wit your finger
Well with a 6" rotor you might need 10N to lock the wheel.
With a 12" rotor you only need 5N, so you've halved the 'modulation'.
Posted 1 year ago # -
I'm not disputing any of the physics, but on a bike surely isn't it modulation and heat dissipation that are the benefits of bigger rotors.
Posted 1 year ago # -
I always thought it was because a larger disc had a greater circumference so therefore increased the breaking area per wheel revoloution. Does this not play arole in the process?
Only with respect to heat dissipation, it's the moment of the force that has an effect (the distance from the axis of rotation).
isn't braking area constant ie the size of the pads?
Doesn't matter - it's a misconception that the surface area in contact has an effect on the force of friction between two solid surfaces.
Posted 1 year ago # -
cheers all
Posted 1 year ago # -
Doesn't matter - it's a misconception that the surface area in contact has an effect on the force of friction between two solid surfaces.
No physics behind it but that sounds all wrong. Bigger tyres = more grip, hold something between my hands = more grip than holding it between fingertips.Or am I missing something blindingly obvious?
Posted 1 year ago # -
"it's a misconception that the surface area in contact has an effect on the force of friction between two solid surfaces."
Is that why skinny tyres are just grippy as fat tyres?
Posted 1 year ago # -
At 20 mph, the linear velocity of the disc where it contacts the pad is greater on a 8" disc than a 6" disc. It travels 33% faster, and the circumference increases by 33%.
Ignoring leverage, I can never work out what additional effect this has. Does this increased speed mean more friction?
Perhaps it heats up quicker, but the increased surface area improves heat dissipation by a bigger factor?
Posted 1 year ago # -
now I am back to confused
Posted 1 year ago # -
Bigger tyres = more grip, hold something between my hands = more grip than holding it between fingertips.
Bigger tyres run at lower pressures give larger contact patches and more grip. Dunno re the finger one!
Posted 1 year ago # -
Ignoring leverage, I can never work out what additional effect this has. Does this increased speed mean more friction?
Does it make any difference? The pressure (=force applied/pad area) applied will be the same (so there won't be any additional friction assuming the same pad material); a bigger disc may have the happy side effect of dissipating heat faster due to having a bigger surface area though.
Leverage is what's important, I think. My mechanical physics knowledge is a bit rusty, unlike the gas laws/flow etc...
Andy
Posted 1 year ago #
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