Why do bigger discs brake better?

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  • Why do bigger discs brake better?
  • Premier Icon ratherbeintobago
    Subscriber

    Amonton’s 2nd Law

    God Bless Wikipedia 😛

    Andy

    ooOOoo
    Member

    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.

    flicker
    Member

    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 😀 .

    benjag
    Member

    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 may affect the grip, depends on the tyre.

    Premier Icon jim
    Subscriber

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

    Only for perfectly flat surfaces.

    Junkyard
    Member

    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?

    scotsman
    Member

    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?

    5thElefant
    Member

    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

    I think you might be in a minority. Bike’s Best Handling Bikes ever put Buell’s #1 and #8

    Premier Icon ratherbeintobago
    Subscriber

    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?

    The all knowing oracle 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.

    Premier Icon AlexSimon
    Subscriber

    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.

    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.

    Premier Icon cheese@4p
    Subscriber

    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.

    james-o
    Member

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

    Junkyard
    Member

    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?

    poppa
    Member

    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.

    Premier Icon ahwiles
    Subscriber

    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)

    Junkyard
    Member

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

    lazlo53
    Member

    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
    Member

    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.

    ac505
    Member

    Only for perfectly flat surfaces.

    Sorry, wrong!

    coffeeking
    Member

    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.

    poppa
    Member

    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?

    coffeeking
    Member

    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.

    teef
    Member

    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.

    Junkyard
    Member

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

    You are arguing that[ lets get more extreme here] a 3 mm tyre has the same footprint as a 3 mile wide tyre with the same load/psi applied etc. It is seems to me to be clearly not true – why dont we all just ride skinny road wheels if we get the same grip footprint whatever size tyre we use?
    As is said it is true for the same wheel but not between /across wheels of different sizes

    poppa
    Member

    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 at the same pressure. 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.

Viewing 29 posts - 41 through 69 (of 69 total)

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