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Hot Breaks

03/19/2017 10:48 PM

There is a commercial where they tell the race driver that his brakes are getting to hot and not hit them so hard.

Now am I wrong, But I am always remembering that the energy of a moving vehicle is converted into heat energy by the brakes.

So, going from 200 Mph to 100Mph is gonna generate "X" amount of heat.

Now shouldn't it not matter if the heat generated was made in 1 second vs 2 seconds is it not gonna be the same heat generated? So why would the brakes run cooler?

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#1

Re: Hot Brakes

03/19/2017 11:38 PM

F1 drivers have biasing control from the front to the rear brakes, typically they run 60% bias on the front brakes, but can adjust for conditions....also the brakes are cooled by air ducts, so slower braking allows more time for the brakes to cool....

https://www.formula1.com/en/championship/inside-f1/understanding-f1-racing/Brakes.html

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#6
In reply to #1

Re: Hot Brakes

03/20/2017 6:23 PM

In the current cars, the driver can also choose to augment the mechanical braking by harvesting more of the kinetic energy using the hybrid battery system. It's a complex juggling act with the driver making dozens of adjustments in each lap of the race.

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#2

Re: Hot Breaks

03/20/2017 1:44 AM

I'd say hitting them hard and quick heats the brake system less than easy and long.

Here's why. When you hit them hard and quick (short duration), the surface of the rotors (and pads) get very hot (think high delta T with the air) but the short duration gives less time for the heat to soak into the rotors, pads, and calipers.

Hit them easy but for longer durations, and the pads remain in contact with the hot rotors for a longer period of time allowing more time for heat to soak into the pads and the calipers.

Yes, the overall energy dissipated is the same regardless, but the difference is where that heat goes. Short duration, high delta-T allows for longer times for the components to cool between applications of brakes.

I have pretty good reason to believe this.

I had a scary experience towing an 8,000 lb trailer down into Yosemite Valley from the East Entrance because the person in a sedan in front of me was going way too slow (like 10-15 mph), and I had to be on my brakes way too much. At the first available pull-off, I was barely able to stop the truck-trailer combination due to brake fade and almost coasted right through the whole thing, and back onto the road while standing on the brake pedal and the trailer brake controller max'ed out. The first and only time I could boil water on the aluminum rims, at the tire bead!!

I'd rather pick up speed with brakes off between turns, then quickly and hard brake at the entrance to each corner. The biggest problem with my 'scare' was that we were going so slowly, there was little airflow to the brakes. That particular road wasn't particularly scenic and I could have easily kept the speed up at about 35 mph even with the trailer, but for some reason this person was really scared on this road.

I had descended grades much worse than that road in Yosemite, but with higher speeds and quicker/harder braking, and I had no indication of brake fade.

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#13
In reply to #2

Re: Hot Breaks

03/21/2017 6:25 AM

You should have stayed in a lower gear. In the UK you will sometimes see the sign at the top of the hill, possibly with a reminder along the way.

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#18
In reply to #13

Re: Hot Breaks

03/21/2017 1:37 PM

I was in the lowest range possible, but the truck has an automatic transmission and you don't get much engine braking at those slow speeds. If it were a diesel truck I would have jake brake, but no such luck.

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#14
In reply to #2

Re: Hot Breaks

03/21/2017 8:08 AM

you are comparing apples to oranges. the brakes that you have on your vehicle are made of cast iron and organic material (possibly mixed with ceramic or metal particles). those used on race cars are made of carbon/carbon material (as do many airplanes). the carbon/carbon has a high thermal conductivity AND actually have a better coefficient of friction when hot (to a limit), and the friction is created by the oxidation of the material. And as far as you losing your brakes down a hill with a load, you should have used your engine to control your speed (jake brake).

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#19
In reply to #14

Re: Hot Breaks

03/21/2017 1:49 PM

Pardon me? Comparing what? I have no idea of how carbon/carbon brakes have anything to do with my post.

The point I was making is that the longer time duration the pads are in contact with the rotor, the more heat from the rotor will be transferred to the pads and ultimately the caliper. Harder braking but of shorter duration is less likely to result in brake fade versus lighter but continuous braking.

No jake brake on a petrol fueled Suburban with an automatic transmission. Here, you try this. Get your truck heavily loaded with a trailer on a steep grade, put it in low and try to keep your speed at 10-15 mph. Please tell me how that works out for you.

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#20
In reply to #14

Re: Hot Breaks

03/23/2017 6:11 AM

"... the friction is created by the oxidation of the material. .."

.

Hmmm, oxidation of the pads is what causes friction, you say?

So if oxygen in contact with the brake were replaced with N2 or Ar, the brakes wouldn't provide the friction needed to function as brakes?

You note the formula one brakes are carbon/carbon, does your 'friction is caused by oxidation' model lead to no brake dust since it is oxidized to CO2?

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#21
In reply to #20

Re: Hot Breaks

03/23/2017 6:26 AM

"does your 'friction is caused by oxidation' model lead to no brake dust since it is oxidized to CO2?"

Definitely not, if you look at video of tyre changes at the pit stops, you will see clouds of dust.

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#22
In reply to #21

Re: Hot Breaks

03/23/2017 12:44 PM

Yeah, I asked to highlight problems with the claim that the fiction comes from oxidation.

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#23
In reply to #22

Re: Hot Breaks

03/23/2017 1:21 PM

well, the fiction has to be coming from somewhere, but I don't believe oxidation (unless we are talking catalyzed air oxidation of glucose in blood).

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#3

Re: Hot Breaks

03/20/2017 7:58 AM

The brakes are cooled by the air all the time. Braking adds heat. If you add heat (energy) faster than it can dissipate, the brakes get hotter.

The same can happen in your automobile. In driving down a mountain grade, you should shift to a lower gear and turn on the air conditioner and use the brakes as little as necessary.

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#4

Re: Hot Breaks

03/20/2017 2:04 PM

By the way it is not spelled "breaks", brake shops all over the world can attest (except the ones in Rio Linda, CA. and the one in Hot Springs, Arkansas.

It is all in the air flow rate around the brakes, and that varies with speed somewhat, but basically the brakes can apparently "make" heat from speed faster than the brakes can exchange heat with surrounding flowing air.

Maybe there is another engineering (or enginerring) advance to be made here:

Apply LN2 when the brakes are applies, but this will add up to extra weight. If you used electrodynamic braking, supposedly, the energy from winding the car down hard could be used for acceleration (or for recharging the LN2 reservoir???).

Someone smarter than me on heat flow will have to put this all in a model and douse either the brakes, or my idea. I want my cut when you make the first sale to NASCAR.

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#9
In reply to #4

Re: Hot Breaks

03/21/2017 1:39 AM

Well I think regenerative braking can be developed much further....most of the braking can be done with motor/generators that run off batteries charged with the brakes, and add to vector capable acceleration when demand for power is high...running the parasitic loads off these batteries when the vehicle is parked will keep the starting battery fully charged...

http://knowautobible.blogspot.com/2011/02/regenerative-braking-system.html

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#5

Re: Hot Breaks

03/20/2017 4:11 PM

Most of the trucks in our place runs on water brakes, but still you can smell the brake lining when your car follows it on a downhill.

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#15
In reply to #5

Re: Hot Breaks

03/21/2017 8:34 AM

I googled "water brakes" and found zip, except for when an expectant mother's water breaks. What the heck are you actually talking about? Never heard of them.

Did you mean "air brakes"?

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#16
In reply to #15

Re: Hot Breaks

03/21/2017 9:14 AM

I think he means water cooled brakes, you see the mist coming off of these on racing trucks.

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#7

Re: Hot Breaks

03/20/2017 11:34 PM

Dear Sir,

your reasoning is flawed by the assumption that the heat to be dissipated by the brakes is constant for the same change in speed - even if you did not use the brakes at all, the vehicle still slows down. The proportion of aerodynamic and engine braking (and others I'll leave out for simplicity) increases if you use the brakes less "hard", meaning that the amount of heat generated in the service brakes is *not* the same. In the case of the race car driver, this would mean lowering his average speed, as he would need to get off the accelerator earlier or travel at a lower speed in order to use the brakes less aggressively.

regards,

Paul

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#8

Re: Hot Breaks

03/20/2017 11:37 PM

Not hitting them so hard can translate to not going into the corners so hot , or if you prefer letting off the throttle earlier using the cars air drag and engine braking.

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#10
In reply to #8

Re: Hot Breaks

03/21/2017 3:18 AM

I had once a bad experience with engine braking, it ended up my clutch lining was worn out and its more expensive to replace than the brake lining.

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#12
In reply to #10

Re: Hot Breaks

03/21/2017 6:07 AM

Engine braking is undertaken when the clutch is fully engaged. That means selecting a low gear and then leaving the clutch pedal alone. If you have been slipping the clutch instead of the brakes I'm not surprised you wore out the clutch lining.

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#11

Re: Hot Breaks

03/21/2017 4:40 AM

The requirements for racing differ from those needed for economical public road driving.

In a race, fractions of a second are important. So a driver will accelerate hard as early as possible and brake hard and as late as possible. All components are theoretically replaceable so long as the car finishes the race in one piece. So a race vehicle braking system has to be designed to get rid of heat as quickly as possible, and to hell with the long-term impact on the components so long as they survive to the end of the race. A vehicle used economically on public roads does not have to endure these possibilities.

If stands to reason if the same amount of heat has to be dissipated in a shorter period of time then the equipment is going to run hotter during that event than it would if the heat is to be dissipated over a longer time.

Integrating some form of system to warn the driver that the braking is too fierce to be sustainable invites, among other possibilities:

  • the vehicle being unable to stop or slow down in time to avoid some obstruction
  • the vehicle running too fast into corners, and coming off the circuit
  • the vehicle slowing too early, allowing other vehicles not so fitted to overtake

all of which are recipes for not coming first in the race. So much for the <...commercial...>.

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#17

Re: Hot Breaks

03/21/2017 10:44 AM

The issue is one of time. The brakes radiate and convect heat away to the surrounding environment. The quantity of heat flowing out of the brakes depends on the environment and time.

Heat added to the brakes depends also on time and the force applied. To slow the car from 200 to 100, you generate the same quantity of heat, but if you use lower force, you use a longer time.

So, if you use lower force, you add heat more slowly and allow more time for the heat to radiate out, resulting in a lower peak temperature.

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