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Post by silverdragon on Nov 21, 2012 8:42:39 GMT
Brake Fade.
I have seen this discussion on many boards, as to what CAUSES brake fade....
Take hills out of the equation, we all know Trucks have Exhaust or Jake brake because of over-heating brakes on long down-hill sections, in fact, lets take trucks out of the subject anyway, as us guys that drive them KNOW the problems already...
So sticking with Rollerskates, (cars and light vans) what causes brake fade?...
For me, Automatics are the worst, as I regularly can tell what is an automatic long before I read the alphabet Spaghetti on the boot (XLI TDI EFI etc et all) as there is that warm glow from the brake lights that tells you the driver is riding the brake pedal.....
Does that create THAT much friction that will create brake fade when you need it most?...
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Post by trakmec on Nov 21, 2012 15:05:44 GMT
Silverdragon; Not 100% on this but in those cases I believe the fading is due to a build up of brake pad dust between the pad and rotor, which allows the dust to act as a sort of dry lubricant. It could also be due to poor maintenance, leaking master cylinder= low brake fluid= softer or no brakes when needed
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Post by c64 on Nov 22, 2012 0:02:23 GMT
There are 3 effects:
1: When the brake fluid turns too hot, it boils and the vapours are compressible. This lowers the pressure applied to the brake pads. I there is water in the brake fluid, it starts to partially boil at 100°C which is a lot sooner! That's why you need to change the brake fluid regularly.
2: The hotter the surfaces, the lower the friction coefficient so you need more brake pressure to accomplish the same.
3: If the material turns way too hot, it melts lubricating the brake. That's why race cars use ceramic brakes, they can simply take a lot more temperature before melting.
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Post by c64 on Nov 22, 2012 0:10:32 GMT
I forgot:
4: Iron rotors deform and warp at around 500°C. The brake pads then start to bounce at high speeds so they don't touch the rotor in between. This can be felt in the brake pedal.
I had that only once in my race car, the effect was intense, brake performance above 100 mph was pretty poor but on a brake tester, everything felt fine and the results were excellent.
That's when I switched to partial ceramic since full ceramic is't allowed in public traffic since you need to either hit the pedal real hard or need to warm up the brakes in order to make them work. Partial ceramic can still fail but is unlikely to fail catastrophically and it still works cold.
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Post by srracing on Nov 22, 2012 0:12:15 GMT
Brake "Fade" is not the same as boiling brake fluid.
With fade, the pedal is firm, but the car won't brake. Thiis is due to gasses being released from the pad and effectively forming a fluid between the rotor and pad. Fix is, cooling the brakes, slotted/drilled rotors, higher temp brake pads.
With boiling brake fluids, the pedal goes to the floor or gets very soft. No brakes. Fix is higher temp brake fluid, again cooling, better rotors, or pads that don't transfer the heat as fast to the caliper.
On a street car/truck either is unlilkely except with missuse. (excessive downhill loads, dragging the pedal, etc. or terribly polluted brake fluids. )
Both the above are common to race car drivers and have been experienced at least once.
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Post by c64 on Nov 22, 2012 0:17:34 GMT
Brake "Fade" is not the same as boiling brake fluid. I made that "failure in general". There is another reason. Overage or burned out pads turn hard and smooth, the surface turns glass like and has a lot less friction. This is a serious problem since they can't use asbestos pads any more. A friend owns a classic car, it is 46 years old and drove 120,000 kilometers on its original brake pads which are still good! |
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Post by Cybermortis on Nov 22, 2012 1:15:45 GMT
I recall a driver on (I think) the Paris-Dakar rally back in the mid 1970's who wrote about this. He and his navigator were doing a stage on a mountain road, and the constant braking resulted in the car loosing its brakes entirely about halfway up. This cleared itself up when he simply stopped using the brakes for a while (he used the gears instead).
The driver apparently decided not to worry his navigator by telling him the brakes were gone at the time, but neglected to mention what the navigator had to say when he found out at the end of the stage.
I wish I could remember the book this story came from....
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Post by the light works on Nov 22, 2012 6:43:29 GMT
I had a company truck which left me with a spongy pedal coming into a town. on inspection, we found that the rear brake drum on the passenger side was glowing cherry red. on towing it to the shop, it was discovered the wheel bearing had failed.
I've got over 100,000 miles SINCE I BOUGHT the chassis my service truck is built on. about 70,000 of those since I put the service body on it. the tire centers still think my brakes look fine.
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Post by silverdragon on Nov 22, 2012 6:56:36 GMT
Since I wrote yesterday, I had three occasions to test my ABS in the car, I can report that its working very well.
One was someone who decided to change lanes whilst alongside me.
One was someone who decided to emerge from a side street right in front of me, and the third was a pedestrian with a death wish.....
I then got to work, did a four hour stint in the truck, and only lost the brakes twice... both were on steep hills with lunatic drivers trying to do incredibly stupid things in the wet....
Brake Fade is not the same as brake fail.
Brake fade is when you dont get full braking power for a short period of time because of overheat in the brakes.
We are kind of lucky in the UK that we dont get much of a dust build up in there, we invented wet weather here, we own the rights to Rain....
Yesterday in the wet was akin to having the underside of the vehicle pressure washed, as it was standing water in many places.
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Post by srmarti on Nov 22, 2012 16:13:21 GMT
Brake "Fade" is not the same as boiling brake fluid. With fade, the pedal is firm, but the car won't brake. Thiis is due to gasses being released from the pad and effectively forming a fluid between the rotor and pad. Fix is, cooling the brakes, slotted/drilled rotors, higher temp brake pads. With boiling brake fluids, the pedal goes to the floor or gets very soft. No brakes. Fix is higher temp brake fluid, again cooling, better rotors, or pads that don't transfer the heat as fast to the caliper. On a street car/truck either is unlilkely except with missuse. (excessive downhill loads, dragging the pedal, etc. or terribly polluted brake fluids. ) Both the above are common to race car drivers and have been experienced at least once. That's how I always understood the phenomena. |
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