Why do bigger rotors stop you quicker? (physics)

Anonymous

Wrong. When more heat is generated than the brake system can deal with, then the brakes fade. They will then be less and less effective as the heat builds up, and will actually be almost entirely incapable of locking.

red040281

agreed with yeehaamcgee

i come down a 45degree hill for about 500mts doing about 5mph hard on the brakes with 185mm rotors at the bottom they were fadeing and hot to touch now if i had 160mm rotors they would get ho and fade quicker surely

mossychops

yeehaamcgee wrote:

Wrong. When more heat is generated than the brake system can deal with, then the brakes fade. They will then be less and less effective as the heat builds up, and will actually be almost entirely incapable of locking.

This is a slightly different situation, this is more to do with the heat of the pads and discs and there ability to absorb the initial heat of braking which affects the surface properties of the brake pads and discs rather than the amount of kinetic energy converted to heat through friction. They are 2 seperate equations.

Believe me, it's true and is easly proved by using the same amount of braking force when braking on ice or tarmac, you will lock up on ice before you do on tarmac.

Anonymous

Now you're really not making sense. By your theory, running 205mm discs would make you less likely to lock up on ice than if you were using 160s.

cooldad

mossychops wrote:

My tuppence:

(drilling holes in the rotor also increases the surface area)

(the larger the circumference the slower in cm/s the edge of the rotor will travel for the same RPM)

Wrong on both counts. Unless the area of the edge of the cutout is greater than the area of the cutout, which is unlikely - ie the bigger the cutout the more wrong you are.

The larger the circumference the greater speed the edge will travel at a given RPM. Think about it. Really.

ddraver

Ask him what he does when he has a bolt that's a bit tight. Does he reach for a longer spanner/allan key or does he cool the spanner down to dissapate heat and therefore increase turning power....?

Alternatively ask him to open a door by the hinges and then by the handle and see which is easier. Then cool the door down and see if he can feel a difference when his hands have stopped being numb!!

Alternatively just shake your head and be sad that you are more intelligent your old man or have a rant at the state of scientific education in britain!

cooldad

This thread is really getting a bit silly. In simple mechanical terms, the larger the rotor, the more 'leverage' the brakes will exert. The pads/callipers will however have to exert more pressure due to the increased leverage.
Providing the calliper has sufficient power, the larger rotor will be able to provide more stopping power.

ddraver

^

Yeah i was being more of a dick in an attempt at humour...

geoff93

The point of rotation is centred around the hub, obviously. The further from the centre of rotation, the hub, the stopping force is applied, the greater the braking leverage. Simple laws of levers and pivots.

cooldad

Not your post, makes perfect sense, but some peoples attempts at science makes me think Brainiac was way above them.

mossychops

cooldad wrote:

mossychops wrote:

My tuppence:

(drilling holes in the rotor also increases the surface area)

(the larger the circumference the slower in cm/s the edge of the rotor will travel for the same RPM)

Wrong on both counts. Unless the area of the edge of the cutout is greater than the area of the cutout, which is unlikely - ie the bigger the cutout the more wrong you are.

The larger the circumference the greater speed the edge will travel at a given RPM. Think about it. Really.

Sorry, I did of course type that wrong. You are correct with the cutout part (though usually cross drilled are done for this purpose and drilled are simply for breaking up water) and of course the speed is higher on the outside, but as I did say stopping force required is the same so this has no effect. Lol, really half asleep today. Apologies. :oops:

And yes using 205mm discs will help you from locking up compared to 160mm discs on ice.

In short I agree with your dad, I dont think bigger rotors (or brake discs if you are English) decrease stopping times.

cooldad

Which given your previous statements I think finally confirms that his dad is an idiot.

Anonymous

cooldad, I like your style.

supersonic

n short I agree with your dad, I dont think bigger rotors (or brake discs if you are English) decrease stopping times.

It decreases the amount of force needed at the brake lever for a given decelleration though.

If you have a disc brake and you cannot lock the wheels up even when squeezing the lever as hard as reasonably possible, provided there is no contamination, a larger rotor can help decrease the force you exert.

The flip side of this is that sharp brake that locks up quickly will lock up with even less lever effort. Some find this lessens how well the brake can be controlled.

But the facts cannot be disputed - larger rotors provide more braking power for a set lever force.

02gf74

yeehaamcgee wrote:

This has all been sorted in the first four responses.

No it hasn't - the replies have been addressing :

re: I'm saying that bigger rotors stop you quicker and it's all to do with leverage.

no - it also has to do with the amount of pressure the pads exert on the braking surface, the friction coefficient between pad and disc, temperature, pad area and so on.

If we take mountain bike and compare performance between a 160 mm and 183 mm front disc, all other things being equal, as you probablyimplied, the larger disc will stop you quicker for the same amount of pressure you would exert on the brake lever, as per you statement = levers. therefore you are correct but let's see if we can get on to page 4.

mossychops

cooldad wrote:

Which given your previous statements I think finally confirms that his dad is an idiot.

If thats intended the way it comes across, it seems a bit mean and out of order. Everyone else here is just having a bit of fun. You have spent more time insulting people and criticising a thread, you continue to read, than making inteligent input (and seem to think leverage is the only factor involved an not heat, friction, coefficient, surface area or anything else).

Anonymous

mossychops wrote:

You have spent more time insulting people and criticising a thread

To be fair, it's not entirely unjust to do so, is it?

cooldad

mossychops wrote:

cooldad wrote:

Which given your previous statements I think finally confirms that his dad is an idiot.

If thats intended the way it comes across, it seems a bit mean and out of order. Everyone else here is just having a bit of fun. You have spent more time insulting people and criticising a thread, you continue to read, than making inteligent input (and seem to think leverage is the only factor involved an not heat, friction, coefficient, surface area or anything else).

Take it any way you want. The OP has stated that the original post has been answered to his satisfaction, and if you can find any factual errors in my posts, which sometimes did actually relate to the question please point them out.
In point of fact I insulted no one except the Op's dad.
If by inference you have taken it personally, MTFU.

mossychops

yeehaamcgee wrote:

mossychops wrote:

You have spent more time insulting people and criticising a thread

To be fair, it's not entirely unjust to do so, is it?

I'd say if you find a thread "silly" then just stop reading it. I think some people just need to "CTFO".

cooldad

mossychops wrote:

yeehaamcgee wrote:

mossychops wrote:

You have spent more time insulting people and criticising a thread

To be fair, it's not entirely unjust to do so, is it?

I'd say if you find a thread "silly" then just stop reading it. I think some people just need to "CTFO".

That would be your opinion, obviously you find it difficult to accept that some peoples' opinions may differ from your own.
I like reading silly threads, it's amusing when people make fools of themselves.

I would suggest that the only person here who needs to chill out is you.

Anonymous

TUnit

Using a bit of engineering knowledge:

The braking system exerts a force on the wheel. The larger the area, the lower the pressure is on the surface for the same force (important for hydraulic systems). The pressure is applied by the brake levers. The frictional force between the pads and disc is the coefficient of friction multiplied by the applied force. The coefficient of friction of ceramic brakes does not fluctuate much with temperature, until you get to about 370C and then it starts to drop quickly. So you'd need some pretty hard braking to get to that point. The higher the surface area of the discs the faster the energy can be dissipated and the cooler the pads will stay.

Additionally it isn't exactly correct that the wheels will lock when the coefficient of friction between the ground and the tyre is lower than the disc and the pads. It is also to do with the relative radii (linked to torque).

The scalar form of the torque (time derivative of the moment of momentum) is force multiplied by the radius. This is similar to deceleration. The higher this is the faster the wheel will decelerate. To increase this the radius or force can be increased.

Based on first year engineering knowledge (at nearly 2am though). Hopefully that makes sense and is correct.

The Rookie

Perhaps we can go back to O-level (GCSE) Physics, friction = mu X mass, you'll notice there is NO correction at all for sliding speed and no correction for surface area (of contact so pad area not disc area).

So a larger disc generates no extra friction by dint of the faster pad/disc surface speed at all (try it in a car, hold the car on a hill with low engine revs, keep the cluch constant and raise the engine revs, the car doesn't move as there is no more friction. So that is NOT a way larger discs generate more braking). Nor is surface area (so the effects of holes or slots) relevant (if you increase surface area you decrease the mass per unit of surface area so it is self correcting).

Larger discs do have more fade resistance if (as they usually would be) the disc is heavier, but the primary benefit is leverage, the same friction force is generated on a larger radius, hence a larger brake torque is applied, assuming the rim diameter is constant you get a larger brake torque at the wheel so slow faster!

Simon

Gizmokev

Having read through this silly thread it is obvious to me that it simply black magic that causes the wheel to stop.

In all honesty though lets think;

IMO A bigger disc will not give you more braking force..... :shock:

Assume that you are applying the same force (A) to the rotor via the pads and the coefficient of friction (B) is the same (Frictional Force = A x B or rather FF= AB). Yes you will be applying that force at a larger distance from the hub (torque= FF x distance from hub or rather T=Fd) but lets consider angular momentum (Angular Momentum = mass x velocity x distance from axis of rotation) into this...the further from the axis of rotation the faster it is moving hence you will have more momentum to overcome. This is especially true when you consider that larger discs have a larger mass as well.

What a bigger disc will do it prevent the brakes from fading during prolonged braking....due to increased heat dissipation.

supersonic

The torque and extra heat created at the rotor are both opposite sides of the same coin. You cannot have one without the other when decellerating if the wheel has not locked. For the the system to be able to create the reverse torque, it has to grab the rotor and 'convert' energy, in this case the form of heat. A larger rotor allows a greater torque and more heat. If the wheel locks, the tyre dragging on the floor dissipates the energy.

supersonic

Gizmokev wrote:

Having read through this silly thread it is obvious to me that it simply black magic that causes the wheel to stop.

In all honesty though lets think;

IMO A bigger disc will not give you more braking force..... :shock:

Assume that you are applying the same force (A) to the rotor via the pads and the coefficient of friction (B) is the same (Frictional Force = A x B or rather FF= AB). Yes you will be applying that force at a larger distance from the hub (torque= FF x distance from hub or rather T=Fd) but lets consider angular momentum (Angular Momentum = mass x velocity x distance from axis of rotation) into this...the further from the axis of rotation the faster it is moving hence you will have more momentum to overcome. This is especially true when you consider that larger discs have a larger mass as well.

What a bigger disc will do it prevent the brakes from fading during prolonged braking....due to increased heat dissipation.

But you have the mass of the bike and rider driving the wheel around. The mass of the rotor is neglible compared to this. I think people are assuming just a spinning wheel rather than the system as a whole.

The Rookie

supersonic wrote:

But you have the mass of the bike and rider driving the wheel around. The mass of the rotor is neglible compared to this. I think people are assuming just a spinning wheel rather than the system as a whole.

Indeed that is missing the Elephant in the room, an extra 60grams of disc compared to a bike weight of circa 12+Kg and a rider of circa 60+Kg kind of pales into insignificance!

Simon

cooldad

Elephants need humumgous rotors due to their weight.

Gwaredd

I think it's safe to assume that everybody who's not an idiot realises that larger discs = stronger braking.

FWIW, I run a 203 on the rear, 180 on the front as I like a snappy rear brake & use it 80% of the time over the front. It's an old motocross habit that I can't seem to break.

In summary, your Dad is an idiot If he's into cars at all, remind him how bad the original mini coopers were on 7" & 7.5" disc brakes compared to when they moved up to 8.4" (which necessitated the undesired fitment of a 12" wheel to accommodate them)

It had nothing to do with brake fade or heat build up, as you couldn't lock the wheels on a original cooper with 7" discs even from cold!

Anonymous

Gwaredd, Some would say that haivng a more powerful rear brake puts you in the same intelligence camp as the OP's dad.