[Closed] Why do bigger discs brake better?

Leverage. A bigger rotor means that the caliper and braking surface are further from the centre of rotation (axle).

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.

Er, FOG, did you skip physics classes at school?

omin, our physics teacher was nicknamed Beaky,never understood a word he said!

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.

Similar principal to Torque=Force*Radius

Don't forget you'll lose some modulation too.

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 ?

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!

Haze - Member
Similar principal to Torque=Force*Radius

Thought it might be, I was relying on memory from years ago and thought it may be a little less straightforward...

Because they look moto

So what we really need then is for someone to invent a brake that works on the rim!

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.

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!

Even better would be one that works on the surface of the tyre.

[img] [/img]

avdave2 - top idea.

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?

I'm just beginning to get the whole 29er thing now...

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

[img] [/img]

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?

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.

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

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.

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'.

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.

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.

cheers all

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.

Or am I missing something blindingly obvious?

"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?

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?

now I am back to confused 😕

Bigger tyres = more grip, hold something between my hands = more grip than holding it between fingertips.

Bigger tyres [u]run at lower pressures[/u] give larger contact patches and more grip. Dunno re the finger one!

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

To add to the confusion......if you are being pedantic then it doesn't matter the area of the pad when thinking about contact area. A large pad with a given force will not have any more area physically in contact with the disc compared to a small pad with the same force applied. The smaller pad will have more intimate contact therefore meaning the surface area is identical.

A large pad with a given force will not have any more area physically in contact with the disc compared to a small pad with the same force applied.

Counterintuitively, this is correct (Amonton's 2nd Law) - in which case, why bother with 4 or 6-pot brakes? Why not use ever-bigger rotors with normal 2-pot calipers?

Andy

friction has nothing to do with surface area, only coefficient of friction and normal force. However as you know heat build up/contaminants do affect the friction coefficient, so a large area pad increases the likelihood of contact and is more efficient as dissipating heat.

*shakes head and walks away*

Oh Amonton's 2nd Law, why didn't you say so in the first place of course 🙄

Bigger tyres run at lower pressures give larger contact patches and more grip

Amonton's 2nd Law

God Bless Wikipedia 😛

Andy

You gotta love it. It also gives us this

Contrary to earlier explanations, kinetic friction is now understood not to be caused by surface roughness but by chemical bonding between the surfaces.

L_P 8pot and giganto rims? that just looks silly.

😯 Wash your mouth out young man (woman?).

Actually very clever mounting the disc on the rim, moves all the braking forces off the hub and onto the rim. Means the spokes can be thinner and lighter as they have much lower forces applied to them. Lighter wheels is good 😀 .

good idea, but it's on a buell, ugh!

tyre pressure/size wise - for a given pressure the contact area will be the same.

a 3" tyre at 50 psi, under 100 pounds weight will have a 2 inch square footprint.

a 1" tyre, at 50 psi, under 100 pounds weight will have a 2 inch square footprint.

the footprints will just have a different shape (same for thin/wide car tyres). the shape of a footprint [i]may[/i] affect the grip, depends on the tyre.

friction has nothing to do with surface area, only coefficient of friction and normal force.

Only for perfectly flat surfaces.

Are you saying a 3foot/metre/ mile wide tyre at 100 psi would also have the same footprint with a 100 lbs force?
i need this explaining - not saying it is wrong but i need a reason as it seems , prima facie, to be wrong.
Amotons law is about the force required to overcome inertia for two , equal weight /material, objects with different footprints- same force required to move each object.
Does this really apply to braking
Anyone?

In my uneducated opinion is it not the brake manufacturers finding a happy medium between braking performance and unsprung rotational forces? The bigger the disc the harder it is to turn in? Being into the motorised version of two wheeled fun as well as unmotorised and haven riden a Buell or two in my time with the daft rim mounted disc I have to say they handled teribbly compared to a similar streetfighter style road bike like say a triumph speed triple, or it could have been that the Buell was a shite handeling bike in the first place and nothing to do with the rim mounted disc?

haven riden a Buell or two in my time with the daft rim mounted disc I have to say they handled teribbly compared to a similar streetfighter

Amotons law is about the force required to overcome inertia

It isn't. Newton's laws of motion describe inertia (these are relevant to braking in that applying the brakes generates a negative force along the axis of travel, causing negative acceleration). Amonton's laws relate to friction.

Are you saying a 3foot/metre/ mile wide tyre at 100 psi would also have the same footprint with a 100 lbs force?

[url= http://en.wikipedia.org/wiki/Contact_patch ]The all knowing oracle[/url] suggests that the two most important factors in determining contact patch are indeed tyre pressure & load, and that this is limited by tyre geometry.

Andy

Contact patches on tyres and brake pads are one of those areas where the simplistic view is too simple to be useful - like using Newtonian mechanics to deal with Quantum physics. It's not that the simplistic model is close enough to be be fine for rough calculations, it's way out.

I've just spent a good 15 mins trying to dig out an image of a trials bike which put the rotor (aluminium I think) on the rim. It was a custom one-off. My search lead me to thinking it was the Ruthless Switchblade, but the site seems dead and I can't find any images 🙁

Anyway - I think things like this should be explored. It's fun.
This m/c seems to use very small calipers - maybe even bicycle ones.

[img] [/img]

The modulation thing seems in debate. Most people report better modulation with larger rotors, I'm wondering if this is because we find it easier to adjust force at the lever when we're dealing with small forces, rather than big ones.

Im thinking: gears in reverse, larger rotor = less lever force required but giving a lower more gradual braking force.
Smaller rotor = pull hard on lever to get harder, more instant stopping.
Bigger rotor will give more feel/control. Small rotor will tend to lock up.

are motorbike rim-mounted-rotor brakes only a step away from a bike using Magura rim brakes?

well that wikki link surely applies to the same tyre with the two most important factors being

* The larger the load on the tire, the larger the contact patch.
* The larger the inflation pressure, the smaller the contact patch.

Well for the same tyre I can see why this is clearly true - more weight = more contact reduce pressur e= more contact and vice versa It cannot be the same bettween 2 diifferent tryes thoogh. If we apply the 100 n force to a 100 psi tyre to a 3 inch tyre and 3 metre tyre. It is obvious which must have the greatest footprint - the bigger one.

surely the reason people don't use massive disc rotors near the inside of the rim is because they'd be really heavy and have bad effects on acceleration and handling?

It is obvious which must have the greatest footprint - the bigger one.

Ok, lets say you had a slick 26"x2.1" tyre,inflated to 60 PSI, and a slick 29"x2.1" tyre, also inflated to 60 PSI. If you loaded them up with the same weight, they would have the same contact patch area. It would differ in shape however, the 29" tyre would have a longer, narrower contact patch whilst the 26" tyre would have a shorter, wider contact patch.

a 160mm disc has a radius of 80mm.

a 200mm disc has a radius 100

that's an increase of 25%.

bigger discs mean more braking oomph, or the same oomph for less squeezing.

(oh, and while i'm on, 4 pot brakes are completely pointless for mountain bikes - they make sense on cars, where there is limited room between the hub and the rim, so 4 or 6 pots can be used to the same effect as 2 big ones - without taking up the same radial space - they also mean a thinner (radially) disc can be used, which weighs less - this is why 4/6 pots are used on motorbikes)

poppa yes I agree in that example but that says nothing about my stupid three metre wide tyre v 3 inch wide one.

Fog and everybody else, if what Fog said...... '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'...... then he's doing bugger all but changing the disc and nothing else. He'll actually make both acceleration and braking worse ie: more mass to both start and stop rotating. Is this what you mean Fog, just changing the disc? if so save your money (but send me half as a consultancy fee)

poppa yes I agree in that example but that says nothing about my stupid three metre wide tyre v 3 inch wide one.

Same thing will happen, except the big tyre will probably weigh a lot more which will contribute to the loading.

Only for perfectly flat surfaces.

Sorry, wrong!

The modulation question is one that interests me. Modulation, the ability to vary the force nicely (linearly?) from 0 to 100%, does not change with leverage ratio changes such as disc size. The only reason modulation seems to change is because you don't have control over your fingers as well as you think you do.

When you're used to grabbing a handful, then you switch to a more sensitive system, grabbing a handful will bin you. Simple. Like left foot braking in a car - the brake pedal doesn't change it's braking ability, but your left foot is used to pressing a very firm, coarse control pedal. So when you use it to press the brake pedal with careful control you end up eating windscreen.

There is no inherent reason why you can't get exactly the same modulation with a large disc as you can with a small disc, you can still vary brake pressure from 0-100%. Humans are not digitised creatures, their output force is continuous, not discrete.

Humans are not digitised creatures, their output force is continuous, not discrete.

Two scenarios.

a) Someone asks you to exert 1Nm on a lever, then 2Nm.
b) Someone asks you to exert 0.0001Nm on a lever, then 0.0002Nm

Assume you have a digital readout or somesuch letting you know the force you uare exterting. Which one do you think you could do more accurately?

The difference is you're not talking orders of magnitude, you're talking 25% extra radius/torque at the most. So the difference is more like 1N then 2N, or 1.25N or 2.5N. You get bigger variations in modulation from one end of the brake lever to the other, or by braking with one finger or two.

Make sure your frame / forks are rated for the larger disc as it can be very expensive and potentially dangerous if they crack due to the larger braking force. The extra braking acts on your frame as well as the wheels and at an angle the frame was not designed for.

Same thing will happen, except the big tyre will probably weigh a lot more which will contribute to the loading.

It is seems to me to be clearly not true

What is your reasoning/proof behind this thought? As far as I can see it is true! Is there a physical explanation why not? Quite often reality flies in the face of 'common sense'!

why dont we all just ride skinny road wheels if we get the same grip footprint whatever size tyre we use?

This bit doesn't make sense to me!

On road we want minimal rolling resistance, therefore require high pressure. Narrow tyres can more readily/safely be inflated to higher pressures for various reasons, and so are better suited to road riding. You get a small contact patch due to the high pressure. On a smooth surface grip is not strongly related to the contact patch size.

Off road, because the ground is bumpy, a lower pressure, wider tyre rolls better. A lower pressure also gives a larger contact patch, and hence more grip (because both the tyre and ground are not smooth).

So, in summary, you get nominally the same contact patch for any two tyre diameters/widths [i]at the same pressure[/i]. The thing is, nobody runs large tyres at high pressures because it defeats the point of having large tyres. Nobody runs narrow tyres at low pressures because it defeats the point of having narrow tyres.

@coffeeking

Makes sense!