[Petrochemicals (OP)]

I notice that brake discs on cars get bigger the flashier/heavier/faster a car is. Why precisely is this?

[alancalverd]

Brake discs mostly cool by radiation. The bigger they are, the more heat they can dissipate, so the heavier or faster the car that they can stop without overheating and distorting.

[Petrochemicals (OP)]

Im imagining that the energies from the car motion transfer to the car brakes? Or is it a mere case of the brakes just need a bigger area to avoid slippage for the energy of motion from the car to be put into the road?

[paul cotter]

It is an energy dissipation issue. Virtually the entire kinetic energy of the vehicle has to be lost to heat in the disks. If the wheels lock up then the energy burns the tyres and tarmac. The momentum of the vehicle is what is transferred to the road, not the energy.

[Petrochemicals (OP)]

Thank you Paul. Is there a theoretical maximum for the speed of braking for the friction of the tyre? Obviously as the speed decrraces the energy the car has to loose decreaces. Would it be possible to stop the car dead?

[Halc]

I notice that brake discs on cars get bigger the flashier/heavier/faster a car is.

As others have pointed out, the larger brakes (disk, drums, and pads) are needed for greater energy radiation.  It's why trains don't have disk brakes, because there's more energy to dissipate than any disk can radiate. So it uses dynamic brakes (resistive), which radiate at the top of the locomotive instead of near the wheels.
Regenerative braking is another form of dynamic brakes, used by say electric locomotives and cars.

Virtually the entire kinetic energy of the vehicle has to be lost to heat in the disks.

Most of it, but some through the pads as well, but yes, the design of both is to put as much heat as possible into the disk, not the pads.  The disk conducts heat into other parts: wheel, axle, etc. Not all of it is radiated.  You can check your brakes on a long downhill by stopping and feeling each wheel hub.  If it's hot, wait a while before continuing.  This check is for instance mandatory halfway down Pike's peak, a road I've traveled but the once.  Our car passed with colors. Cold wheels.

Is there a theoretical maximum for the speed of braking for the friction of the tyre?

The limit is the static friction between the tyre and road.  If a force greater than that is exerted by braking, the tyre skids and the energy is dissipated to it and the road, at far greater expense of wear and symmetry, and efficiency, since skidding applies less force than does static friction, which is why cars theoretically stop faster with anti-lock brakes.  I am personally not a fan of anti-lock brakes since they would have got me into at least two accidents had I not overridden them in time.  I had the luxury of time to think both times, which is often not the case.  Boo safety features!

Would it be possible to stop the car dead?

That's what brakes do, no?  They stop the car.  Dynamic brakes (both resistive and regenerative) do not, so some kind of friction brakes is needed at the end to come to a complete stop.

If you're asking if it is possible to stop any extended thing in zero time, the answer is no, at least not with a limited number of locations where an infinite point force is applied. Not even crashing into a bridge abutment will stop a car instantly, and a good thing too.  Yay safety features!

[paul cotter]

I too do not like antilock brakes. My first car with abs was ~25 years ago and I was soon horrified to learn that if one wheel was on ice virtually all retardation was lost. I am not up to date with vehicle technology but it seems to me that hydraulic pressure should only be reduced on the wheel which locks and not on the whole system. No car I have owned has any way to defeat the abs that I am aware of. It would be a simple matter for me to fit a kill switch but this comes with it's own problems. In the event of a serious accident this switch would be seen as an unauthorised modification and could have implications for insurance cover. Quite apart from this I doubt if I would have the presence of mind to use such a switch as the window of opportunity in an accident scenario is usually very short(I have been in a number of smashes as driver and passenger).

[alancalverd]

You can check your brakes on a long downhill by stopping and feeling each wheel hub. 

This works for drum brakes, which are part of the wheel hub, but wouldn't be much use if you have inboard discs which are a long way from the hub. And of course hub caps and the like reduce the sensitivity of the test.

[Bored chemist]

The manufacturers of anti lock brakes would tell you that, if you can't stop safely, you are going to fast.
The definition of "too fast" depends on circumstances, an is likely to be much slower on ice than on clean dry tarmac.
One might imagine it would be simple to fit an ice sensor and couple it to teh breaks in such a way that it disables ABS on ice.
But, on ice, ABS works quite well.
It's when only 1 wheel is on ice that there's a problem. Individual ABS would be better, but more expensive.
It won't be long before a self driving car is cheaper than 4 individual ABS systems and that will address the "driving to fast" problem.

Would it be possible to stop the car dead?

No, but the closer you get top a "dead" stop, the closer you get to being guaranteed dead.
I'm not sure I believe that most of the heat loss from brake disks is from radiation.
I think it's advection; that's why some disks have holes punched in them (in addition to helping shed water).



If your brakes are really clever, they recharge your battery.

[alancalverd]

I've often wondered about the advective value of disc brake cooling. Many "domestic" cars seem to have two discs joined by a honeycomb of slots. I can see how air could be expelled from the slots by centrifugal action (if anyone still uses that adjective) but the  last time I removed one, at least 30 years ago, there seemed to be nowhere for fresh cold air to enter. Or is my memory faulty?

[Halc]

The manufacturers of anti lock brakes would tell you that, if you can't stop safely, you are going to fast.

Apparently <1 km/hr was going too fast on one of my cases.  I was headed for somebody's (stopped) driver door, from about 2 meters away, going waaay below a walking pace, with one wheel on glare ice. Could not stop without yanking the parking brake.  The other case was similar, a multi-car pileup on a bridge, going about 15 km/hr (on a major divided highway) with lots of room to stop, and would have gone under a semi trailer had I not shut off the ignition, thus disabling the ABS.
I guess I was going too fast in both cases and the ABS was serving only the purpose of significantly lowering the speed that was considered 'safe'.

I'm not sure I believe that most of the heat loss from brake disks is from radiation.
I think it's advection

Advection?  That would only work if the disk melted and dripped away the head.
Indeed, not radiation, (which is the primary method in space), but rather conduction of the head to the air. That's not advection since the air is not part of the car.  Anyway, larger surface area of disk equates to more area to conduct to the air, and all those holes and interior hollows seems designed to increase the airflow over the hot surfaces.

This works for drum brakes, which are part of the wheel hub, but wouldn't be much use if you have inboard discs which are a long way from the hub

I've had the hub get plenty hot with disk brakes.  I've used the test to find faulty callipers for instance. The hub gets quite hot.
If I had hubcaps (most don't these days with the popularity of alloy), the place to feel is the lug nuts.

[Bored chemist]

Advection?  That would only work if the disk melted and dripped away the head.

The disk heats the air and then throws it aside.
If it melted then that would be losing heat to the latent heat of fusion which would be interesting, but a different question.

I'm curious; assuming it's a pumped, hot- water system, how do you consider central heating to transfer heat?
Physics classes told me there are three ways heat is transferred; convection, radiation and convection

Let's assume it's a conventional with a "boiler" (which doesn't actually boil the water), a set of pipes and a pump, and then "radiators" (which mainly transfer heat by convection).
How is heat transferred from the boiler to the radiators?


Is it convection, radiation or convection?
(In my house the boiler is above the ground-floor radiators).

[paul cotter]

I would say the heat is transferred to the water in the boiler by conduction, the water is then pumped and the heat is transferred from the water to the radiators by condution , again. Not exactly what you requested but then I am quite pedantic, as you know.

[Bored chemist]

https://en.wikipedia.org/wiki/Advection#/media/File:Heat-transmittance-means2.jpg

[paul cotter]

Hi BC, advection was a term I vaguely remember from lectures ~50 years ago. Having read that wiki article I agree with you regarding the description of the overall process.

[Halc]

I'm curious; assuming it's a pumped, hot- water system, how do you consider central heating to transfer heat?

The water is part of the system.  It's movement is due to the pump, not so much the heat, so advection, not convection.  We both seem to have such a system.

Physics classes told me there are three ways heat is transferred; convection, radiation and convection

Advection (as well as convection) is a form of transport more than a transfer, the difference apparently being what moves the medium.

How is heat transferred from the boiler to the radiators?

All sorts of processes involved. Heat generated by combustion, moves hot air to the jacket (convection), transfer to jacket (conduction) through the jacket (conduction) to the water (conduction) through the circulation pipes (advection) to the radiator (conduction) to the air (more conduction) to moving the air throughout the room (convection).  There is also significant radiant heat transfer to the jacket and from the radiator directly to line-of-sight portions of the room, but that was more noticeable with my old steam radiator setup long ago.  The water is part of the closed heating system, whereas the air cooling the car brakes is not.

Does this mean that a convection oven doesn't have a fan?  I've never had one, so I don't know. I have an air fryer which does have one, and it's called a convection fan despite it technically not being convection, and I suspect the ovens also have one.

[Bored chemist]

The difference between advection and convection is the pump.
If heat can travel by convection then it can travel by advection.

Without the pump, most of my house stays cold; the heat is not transferred to it.
The pump is a vital part and the only thing it has to do is advection.

[Petrochemicals (OP)]

The limit is the static friction between the tyre and road. 

So i am thinking then if we theorise that the tyre road interface IS static, the braking is governed by the breaks ability to transport heat away providing that the static friction between the road/tyre is not overcome? If you could transport heat away fast enough the small flexure in the rubber of the tyre could provide  enough buffer to lower the energy in the car to 0.

Not even crashing into a bridge abutment will stop a car instantly, and a good thing too.  Yay safety features!

Yes everything other than the tyre at the road would continue, the counterforce to the cars motion would be extrodinary, but providing that the car didnt disintegrate it sould come to a stop.

[diverjohn]

The rotating brake disk cools by airflow entering near the axle and flowing through the vents that are cast into the iron. Sometimes holes are punched or drilled into the disk to aid cooling and to help dissipate water, but sometimes they create sounds similar to woodwind.
The heat is ideally restricted to the pad and rotor and some manufacturers use a calliper piston made of phenolic material instead of iron to protect the brake fluid from reaching its boiling point.

[evan_au]

Regenerative braking is another form of dynamic brakes, used by say electric locomotives and cars

During braking, my hybrid car puts the car's kinetic energy into the battery (with some losses).
- As Halc noted, this can't bring you to a complete stop - but it works down to quite low speeds
- When you know that you will have to stop (eg because the traffic lights are red), you can take your foot off the accelerator, and "coast" (slowed by air resistance, any uphill slope and gentle regenerative braking), until it is time to pick up the speed again.*
- Fuel efficiency is "gamified" in my car: The car has a little report card at the end of every trip - hard acceleration and hard braking yield a lower score.

This raises a question I had for Chris:
- In one podcast it was mentioned that full-electric cars would cause more wear on brake pads (and particulate pollution) because the battery made the car heavier.
- However, regenerative braking means that the mechanical brakes should be used only rarely, and this should produce less particulate pollution?
- The tyres would still produce particulate pollution for both electric and petrol cars (more for a heavier car)...

*Note that coasting in an electric vehicle is not the same as putting a petrol car into Neutral and coasting; in the electric car, the engine is still engaged with the wheels.