21 replies to this topic

[boost]

Anybody see the pit stop Montoya did at Imola?
The camera showed a gout of flame spewing out of the front left tyre, and a williams guy with a fire extinguisher putting it out.
I havent seen this before! Have i been missing somthing in all the pitstop i have seen? i know these brakes get hot but to "flame" like that was a bit disconcerting.

[guillbee]

Hi boost,

according to the french TV, it was because brakes were too hot because of the lack of fresh air (the car's stop was too long).
I don't know if the TV was right but it makes sense.

[imaginesix]

The only oddity is that other car's brakes don't auto-ignite when the car is stopped too long in the pits. That is a relatively frequent occurence, yet I've never seen it happen before.
It is possible, of course, that it was never shown before, but I tend to believe that Williams are in fact running a different brake material, as several other indicators point to that conclusion as well.

[carlos.maza]

I have read that the working temperature for a carbon fibre disk brake is around 600°C. Is this figure Ok?

[carlos.maza]

Should Williams be using a different material, it seems that it has bigger cooling requirements, as I haven´t seen a disk ignite before. But I didn´t notice any abnormally big air intakes...
Maybe a Ferrari-style cooling system could work fine for them

[david_martin]

I think we can safely say that the rotor did not ignite. Most common rotor materials have ignition or melting temperatures above 1500 degrees celcius, and the brake will never reach those kind of temperatures. What most likely happened is a combination of two things: detritis (brake dust, rubber) falling on the rotors during the tyre change and the long stop, allowing the rotor surfaces to heat up and preventing rotation that would naturally sweep the rotors clean. The result is a lot of smoke

[moog101]

There were actually flames though, perhaps brake fluid? Mind you, I guess they use Silicon fluid, does silicone brake fluid still burn? His brakes wouldn't be up to much if it was the fluid..

[Dents]

it was definitely something odd. Cars stop from a hot lap all the time and we never see such phenomenon. Think of all the blown motors, offs, and crashes... no flaming brakes there.

[Timm]

Could the flame be caused by the large amount of carbon dust which sits at the bottom of the wheel and around the upright igniting. As coal-miners will tell you, dust can be mighty flammable.

[bugeye]

A couple of things were weird about this incident IMHO. But one thing should be cleared up first.

1. Naturally the brakes don't get any hotter while in the pits for an extended stay and should in fact be cooler, once Montoya stopped in the pits, the KE had been converted to heat which would start to dissipate, there being no more heat being generated by stopping, so clearly the longer the stop, the cooler the disks should get.

Still though, the disks are plenty hot, certainkly hot enough to start a fire, but what burned?

I like the rubber theory, but I think it might have been something else, could the cook down of the brakes have forced more heat into areas which would normally be cooled by the airflow? Maybe the bearings or something? Could it be as simple as oil residue from an airgun? What struck me as odd though was that they seemed to have an extinguisher right there ready for it as if it had happened before, I realize they have alot of fire extinguishers present during a pit stop, but still they seemed to expect it, so maybe it has happened before. Could the ducting have caught fire? Maybe it's something as mundane as grease from the wheel nut spinning off. Who knows, but I'll definitely be looking at other pit stops to see if it happens again...

Maybe Montoya himself was so hot over being stuck in the pits he caused things to spontaneously combust !

[Amadeus]

Martin Brundle in the UK ITV feed wasn't at all surprised - said it was quite common, and not usually a major problem. From that we can guess it probably was detrius (grease, dust, rubber all inflammable) in contact with a hot element. Presumably the flames warped or burned something which led to him retiering.

Not in itself enough to justify the "wierd brake" conspiracy theory I'm afraid!

[Alien]

In the PSN latin american transmission there was this guy who works at asiatech and seems to know a lot about F1. He wasn´t surprised at all by the ocurrence, he said that the car had stopped for too long and thats why it was on fire, it probably is a normal ocurrence.

[david_martin]

Originally posted by bugeye
A couple of things were weird about this incident IMHO. But one thing should be cleared up first.

1. Naturally the brakes don't get any hotter while in the pits for an extended stay and should in fact be cooler, once Montoya stopped in the pits, the KE had been converted to heat which would start to dissipate, there being no more heat being generated by stopping, so clearly the longer the stop, the cooler the disks should get.

You will note that I said rotor surface heated up due to the long stop. It is perfectly feasible (and a known physical phenomena demonstrable using heat conduction theory) that the surface of the rotors will be at a different temperature to the bulk temperature of the rotor - hotter during braking because the the friction power being dissipated on the rotor surface where pad contact occurs, and cooler during periods without braking. This is because the surface is the only part of the rotor which can exchange heat with its surrounding in order to cool or heat it. The underlying reason for this is fairly straightforward: heat diffuses through solids at a finite rate, ie. if you cool or heat the surface of a solid body at a rate which exceeeds the rate of heat diffusion within the solid the surface of the body will be hotter or colder that the bulk average temperature.

Applying this phenomena to a brake rotor, when the car is stopped or travelling slowly and the rate of convection heat transfer between the rotor surface and the cooling air being forced over it by the cooling ducts will be significantly reduced. The brake continues to cool (that is to say that the total heat content of the rotor continues to reduce), but at a slower rate. The difference in temperature between the rotor interior and its surface is decreased due to conduction within the rotor. The surface of the rotor is observed to heat up while the interior of the rotor reduces in temperature. Once a new "steady state" temperature distribution within the rotor is observed, the rotor surface temperature will be observed to reach a maximum, and then it will being to fall. The rate and magnitude of this phenomena will be a function of the thermal diffusivity of the rotor material, the geometry of the rotor, and the prior thermal history of the rotor.

[bugeye]

Originally posted by david_martin


You will note that I said rotor surface heated up due to the long stop. It is perfectly feasible (and a known physical phenomena demonstrable using heat conduction theory) that the surface of the rotors will be at a different temperature to the bulk temperature of the rotor - hotter during braking because the the friction power being dissipated on the rotor surface where pad contact occurs, and cooler during periods without braking

.......
Blah Blah snip snip snip.......
......

but at a slower rate. The difference in temperature between the rotor interior and its surface is decreased due to conduction within the rotor. The surface of the rotor is observed to heat up while the interior of the rotor reduces in temperature. and the prior thermal history of the rotor.

Okay, Okay...you are right in theory that this could and does happen, but IS it your contention that this process takes say 20 seconds or so? I can't remember where in the pit stop this (the fire, I guess since we don't have any other indications) happened. But in practical terms approximately how long do you feel the rotor (surface) would be getting hotter? In a completely gut reaction, non-scientific, shoot from the hip analysis, I would imagine the effect you are speaking of would be over in less than two seconds. Having not had any practical experience with CF, but plenty of experience heating and cooling metal I am making some very generous assumptions as to CF heat conducting properties, but I assume it's better than iron.

Am I completely off base here?

So I guess what I am getting at here is that as I stated, a long stop would not cause brake rotor SURFACE temperatures to be higher(at the end, or in the middle for that matter)than a short stop. Thus the long stop...brakes overheat theory is bunk IHMO

[carlos.maza]

Montoya told PSN that since half race, a clutch warning light was on, so he had to brake harder, as he had less motor brake.
When he pitted, the clutch gave up and the engine stopped.
The brakes overheated because he could not put the car in neutral, he had 1st gear engaged during the pit stop, and, since the clutch was not properly working, he was accelerating and braking to keep the engine alive.
When he went out of the pits, he completed a lap in 1st gear and retired.

[IndyIan]

Bugeye,
I don't really understand david_martin's post but I think that carbon fibre would conduct heat very poorly. Judging from other carbon based substances like wood or me:)

I think the main advantage of CF brakes is that they don't conduct heat well but can still resist the high surface temperatures that low conductivity causes. If the discs were iron or aluminum the heat would transfer into the hub and cook the bearings and eventually melt the tires on the rim.(This has happened in NASCAR)

I forget the exact equation but the black body emissivity equation has: Energy lost (from the blackbody)= some constants * Temp^4
So as the temperature of the brake surface goes up the heat energy emitted increases on a steep exponential curve.

[blkirk]

I think what David is saying is:

When the driver hits the brakes, the surface temperature of the disc goes WAY up. This heat then flows into the core of the disc giving a more uniform temperature through the thickness. As the driver piddles his way down to his pit, the airflow over the disc cools the surface so that when the driver stops, the core of the disc is actually warmer than the surface. As the car sits with its wheels in the air, the heat flows from the core of the disc back to the surface again giving a uniform temperature through the thickness of the disc.

Did I get it right David?

My only observation is that the highest temperature in the disc will be at the instant the driver releases the brake pedal. This means that the disc temperature during a pit stop is lower than the temperature when the driver crosses the pit speed limit line.

[david_martin]

Originally posted by bugeye
Having not had any practical experience with CF, but plenty of experience heating and cooling metal I am making some very generous assumptions as to CF heat conducting properties, but I assume it's better than iron.

Am I completely off base here?[/B]

Some allotropes of carbon, such as Diamond and DLC, are rather good conductors of heat. Diamond is, infact, one of the best heat conductors of all crystalline solids. This is why you see diamond heat sinks on microprocessors. The kind of carbon composites that are most often used in brake rotors have rather poor thermal conductivity and a fairly low specific heat capacity, certainly lower than cast iron. I believe most rotors are produced using a layered 2D Radial reinforcement matrix, which introduces considerable internal thermal resistance in the thickness direction of the rotor, which is the predominantly direction of heat flux in the rotor during braking and cooling.

All of these properties are, of course, their great advantage - they do not conduct a lot of heat into the wheel bearings, they heat up to an optimal surface temperature rapidly, and the thermal penetration depth from a single braking event is small. This makes them relatively easy to cool. Heat will, of course, penetrate into the rotor, albiet slowly. Over the course of a series of hard laps the rotor mean temperature will rise to a rather high "steady state" value. Whilever convection remains the predominant heat transfer mechanism (which is most of the time) I would expect that when convective cooling ceases (ie the car stops or slows to a speed where forced convenction becomes negligible), the rotor surface temperature could well continue to rise for as long as several minutes after, depending on the thickness of the rotor and its prior thermal history.

[david_martin]

Originally posted by carlos.maza
Montoya told PSN that since half race, a clutch warning light was on, so he had to brake harder, as he had less motor brake.
When he pitted, the clutch gave up and the engine stopped.
The brakes overheated because he could not put the car in neutral, he had 1st gear engaged during the pit stop, and, since the clutch was not properly working, he was accelerating and braking to keep the engine alive.
When he went out of the pits, he completed a lap in 1st gear and retired.

If the wheels do not rotate, then no power is developed (and hence no heat evovled) at the contact between the pad and the rotor. Simply holding the car on the brakes won't change their temperature (especially the fronts, which was where the smoke came from). But it does explain what the source of the smoke and flames were - the organic binder in the brake pads! If the pad was in contact with the hot disc for a long period then the pads themselves will heat up to temperatures high enough to burn the organic "glue" which is used to bind the friction materials (like metallic oxides and cermets, kevlar, etc) together. The longer the contact, the more heat goes into the pad and the more likely it is that the pad will burn.

[bugeye]

Okay, I'm
wr....(um, ahhh)
wwwrrrr.....
rrr-rrr-rrr (internal struggle)
wwwwrro..
WRONG !

There I said it...Phew!

Anyway, I always thought that CF conducted heat very quicky.

Thanks

Dave

[palmas]

If anithing else besides rubber and carbon dust had burned, would they let JPM out of the boxes?

[imaginesix]

Originally posted by david_martin
But it does explain what the source of the smoke and flames were - the organic binder in the brake pads!

Well spotted, d_m. Now THAT makes sense.