Temperature Vs. Cooling

[Birel99]

With high ambient temperatures in Bahrain this week, the SpeedTV broadcast said that this will effect break wear significantly.

My question is, as ambient temperature goes up, does its cooling efficiency go down at a linear rate or exponential?

Say that the ambient temperature is 20F hotter today than yesterday, do the brakes notice a 20F increase in temperature?
*Assuming we are driving on the same circuit!*

If the core brake temperature is on average about 500C (?) than a 20F increase in ambient temp is only a 2% change in core temperature.

Sorry for the confusing example!
Regards,

[ISLAMATRON]

It is neither linear nor exponential... but I cant remember the exact heat transfer properties that would relate to this situation.

[ReubenG]

The ambient temperature will affect how much the core temperature drops between braking inputs - the brake cooling is essentially convection which depends on the temperature difference between the hot surface (brake disc) and the cooling medium (ambient air).

The first order solutions for heat transfer is linear with respect to temperature difference:
Rate of heat transfer = Mass flow rate x temperature difference x specific heat of fluid.
But then first order solutions are nearly always linear...

But ultimately the brake temperature is more strongly influenced by how intense and closely spaced the braking events are - Montreal is reputed to be the circuit with the highest brake wear and its ambient temperatures wouldn't be that high.

[kilcoo316]

With high ambient temperatures in Bahrain this week, the SpeedTV broadcast said that this will effect break wear significantly.

My question is, as ambient temperature goes up, does its cooling efficiency go down at a linear rate or exponential?

Say that the ambient temperature is 20F hotter today than yesterday, do the brakes notice a 20F increase in temperature?

Well.. lets put it in perspective.


The air temperature might be as high as 40 degC... but the disc temperature may climb to over 1000 degC...

So the delta is say, 950 degrees for a hot climate like Bahrain, and just 920 degrees for a cooler climate like England... that is a relative difference of 3%...



With carbon brake pads/discs doing relatively little work when they are cool, cooling the temperature down to ambient between corners is not really desirable either.


As indicated by Reuben - Montreal is the hardest on brakes... and its usually sub 30 degC... maybe even sub 20 degC.

[riff_raff]

Differences in ambient temperatures are only really a concern with optimizing performance of CRC brake systems, and not so much their life span. One of the unique properties of CRC friction materials is that their Mu value (coeff. of friction) actually increases with temperature, unlike steel brake rotors. That is why CRC makes an excellent brake rotor/pad or clutch material, in addition to being very low density.

Of course, even high tech materials like CRC have their limits. CRC materials will begin to oxidize if subjected to temperatures in excess of about 1000degF. The thermal conductivity of CRC materials also varies greatly depending upon the orientation of the fibers. The trick to getting the best performance from CRC brakes is making sure they operate at their optimum temperature, not too cool and not too hot.

Unlike conventional steel brakes, CRC brake pads and rotors both experience similar wear rates during the course of a race. A well engineered CRC brake system will finish a race with both the pads and rotors worn down to their minimum safe thicknesses.

Regards,
Terry

[xpensive]

I think kilcoo's expanation is a very good one, engine cooling is a whole different matter though.
If SpeedTV said something to that effect, I just wonder who, it's another example of what ignorant commentary us motorsport-fans has to endure while watching TV. I can still remember John Watson claiming the car went into accelleration when sliding on the grass.

I doubt if any pundit watching a European football match would accept if the commentator was babbling about how the ambient temperature would affect the wind-resistance for the players? Or would he?

[kNt]

In general for normal surface-to-air it's linear (to the delta T), heat radiation is correspondant to the power of 4 (dT^4). It has also to be taken into account that with temperature some properties of the air are changing (e.g. less density).

I agree the temperature alone shouldn't make a huge difference for brakes as the delta T is huge.

[Jersey Tom]

The air temperature might be as high as 40 degC... but the disc temperature may climb to over 1000 degC...

So the delta is say, 950 degrees for a hot climate like Bahrain, and just 920 degrees for a cooler climate like England... that is a relative difference of 3%...

With regard to deltas, let's bear in mind what generates that change in temperature.

Slowing a car from 300kph to 150kph releases a specific amount of energy. Doesn't matter how hot it is out. Inevitably the majority of that energy goes into the brakes (there's drag and engine braking as well obviously). The specific heat capacity of the brake material will define what that deltaT is. Let's say its 1000C. Assuming the heat capacity on average doesn't change appreciably from 25C to 40C, or 1025 to 1040C, then the jump in temperature should be pretty close in either case.

Additionally, cooling rate is in a large part defined by that difference in temperature from object to free-stream, which shouldn't change much.

Air density is going to be slightly less which will drop cooling rate, absolute temperature will be slightly higher which, as kNt states, will change black body radiation.

Basically in one case you start at 25C and jump to 1025, the other you start at 40C and jump to 1040 (plus or minus, maybe 1037 or 1042.. but likely within the level of variation from braking zone to braking or lap to lap).

Difference should be pretty negligible in the grand scheme of things.

Commentators don't always get it right. There have been a couple items I've disagreed with S. Matchett about recently...

Also, for what it's worth Birel, at 500C avg temperature a 20F (~11C) jump is not 2.2% Gotta use absolute scale (K).