Safe temp. range for BFG-R1 tire?

attila · 10-19-2009, 08:18 AM

Lucid,

Dress warmly! Around 40F outside temp. is still possible to overheat our brakes. I am sure you are not planning to use the stock pads like me, so you will be fine. On dry surface the BFG-R1 was not bad, but we had some wet tarmac, even standing water.

lucid · 10-19-2009, 09:33 AM

Yes, front rotors run north of 500C if you start pushing the car. A 20C-30C delta in air temperature is irrelevant to rotor/pad temperatures for the most part.

Dascamel · 10-19-2009, 11:15 AM

If we had better ducting on the brakes cooler weather would help. Also, running nice sticky tires with the stock brake setup isn't exactly ideal.

lucid · 10-19-2009, 11:34 AM

Convective heat transfer is directly proportional to (t_rotor_surface minus t_ambient). At 30C, that delta is 470C. At 0C, the delta is 500C. Not a major difference. Ducting does not change that. It increases the heat transfer coefficient significantly, which is the other variable in the equation.

Dascamel · 10-19-2009, 11:39 AM

So an outside drop in temp from 25-30 deg C, to 5 deg C won't help cooling by even 15-20%?

lucid · 10-19-2009, 11:51 AM

Quote:

Originally Posted by Dascamel

So an outside drop in temp from 25-30 deg C, to 5 deg C won't help cooling by even 15-20%?

Nope. If you use the general convective heat transfer equation, a drop in ambient temp from 25C to 5C for an average rotor temp of 500C should result in roughly 4% increase in heat transfer into the environment. That assumes everything else is a constant in the comparison, meaning the heat transfer coefficient is the same; forced cooling of the rotors due to wheel rotation, etc. The largest potential for gains is to increase the heat transfer coefficient by using more forced air flow (by using ducting, etc). If it rains, that's different though as you would introduce conductive and evaporative cooling and so on.

attila · 10-19-2009, 01:08 PM

Quote:

Originally Posted by lucid

Nope. If you use the general convective heat transfer equation, a drop in ambient temp from 25C to 5C for an average rotor temp of 500C should result in roughly 4% increase in heat transfer into the environment. That assumes everything else is a constant in the comparison, meaning the heat transfer coefficient is the same; forced cooling of the rotors due to wheel rotation, etc. The largest potential for gains is to increase the heat transfer coefficient by using more forced air flow (by using ducting, etc). If it rains, that's different though as you would introduce conductive and evaporative cooling and so on.

I really like Lucid's explanations. Anyway, "my experiment" showed two things.
1. The outside temp. has little effect on brake cooling, but
2. Has signifficant effect on cooling the driver with open windows. I got very high heat transfer coefficient through the window right onto my neck!

attila · 10-19-2009, 01:11 PM

I almost forgot: no PS fluid issues around 40F!! Eventhough the car was not running at much cooler temp.

10-19-2009, 08:18 AM	#23
attila Captain 83 Rep 628 Posts Drives: 2015 Mclaren 650s, 2015 GTS4, Join Date: Dec 2008 Location: Omaha, NE iTrader: (0)	Lucid, Dress warmly! Around 40F outside temp. is still possible to overheat our brakes. I am sure you are not planning to use the stock pads like me, so you will be fine. On dry surface the BFG-R1 was not bad, but we had some wet tarmac, even standing water.
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10-19-2009, 09:33 AM	#24
lucid Major General 374 Rep 8,033 Posts Drives: E30 M3; Expedition Join Date: Apr 2007 Location: USA iTrader: (0)	Yes, front rotors run north of 500C if you start pushing the car. A 20C-30C delta in air temperature is irrelevant to rotor/pad temperatures for the most part. __________________
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10-19-2009, 11:15 AM	#25
Dascamel Lieutenant Colonel 48 Rep 1,664 Posts Drives: 2008 e92 M3, 2010 e91 328i Join Date: Jul 2007 Location: Bakersfield, CA iTrader: (0)	If we had better ducting on the brakes cooler weather would help. Also, running nice sticky tires with the stock brake setup isn't exactly ideal. __________________ 2008 E92 M3 Jerez Black,DCT,Fox Red ext,Prem,Tech,19", ipod/usb, CF roof and trim 2010 E91 328i Space Gray,Black int, M sport, most options 2007 Montego Blue 335i (retired)
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10-19-2009, 11:34 AM	#26
lucid Major General 374 Rep 8,033 Posts Drives: E30 M3; Expedition Join Date: Apr 2007 Location: USA iTrader: (0)	Convective heat transfer is directly proportional to (t_rotor_surface minus t_ambient). At 30C, that delta is 470C. At 0C, the delta is 500C. Not a major difference. Ducting does not change that. It increases the heat transfer coefficient significantly, which is the other variable in the equation. __________________
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10-19-2009, 11:39 AM	#27
Dascamel Lieutenant Colonel 48 Rep 1,664 Posts Drives: 2008 e92 M3, 2010 e91 328i Join Date: Jul 2007 Location: Bakersfield, CA iTrader: (0)	So an outside drop in temp from 25-30 deg C, to 5 deg C won't help cooling by even 15-20%? __________________ 2008 E92 M3 Jerez Black,DCT,Fox Red ext,Prem,Tech,19", ipod/usb, CF roof and trim 2010 E91 328i Space Gray,Black int, M sport, most options 2007 Montego Blue 335i (retired)
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10-19-2009, 01:11 PM	#30
attila Captain 83 Rep 628 Posts Drives: 2015 Mclaren 650s, 2015 GTS4, Join Date: Dec 2008 Location: Omaha, NE iTrader: (0)	I almost forgot: no PS fluid issues around 40F!! Eventhough the car was not running at much cooler temp.
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