Brake Failure on Mt. St. Helens


#1

In the 5th segment of the most recent program available on line, there was a discussion about using the brakes when descending Mt. St. Helen’s in a rented Dodge Challenger. Surely this car has, at least, disk brakes on the front wheels, so the hotter they get, the better they work. I know that older style drum brakes were subject to fade when over used (the drums got hot and expanded), but the whole point of disc brakes is that they don’t fade. Or have I missed the point somewhere?


#2

Disc brakes perform better than drum, but will still fade if over-heated. There are other pad compounds that work better as they get hotter, but these brakes tend to be lousy when cold. Makes them dangerous for street cars.


#3

The most important thing to do when decending a mountain is to use the engine to brake the car by using the lowest gear possible.

Discs are not immune to fade. They have two heat advantages over drums, one being that expansion of the disc due to heat doesn’t fade braking ability like with drums, and two is that discs are MUCH better at shedding the heat than drums. But discs can and will fade if hot enough.

It’s not fade that you’re truly worried about though, it’s vapor lock.

-Matt


#4

LOL, surely they have some mountains in Ottawa! “The hotter they get, the better they work” only holds up to a point. As was mentioned, you can get a ceramic compound that performs very well when heated to extremes (those fantastic glowing rotors you see during night races), but they perform poorly when cool and grandma will be rear-ending everyone with the Grand Caravan if you install a set for her.

Disc brakes are just as hairy-scary when they overheat as are drums.


#5

If your brakes (either disc or drum) become very hot, the brake fluid can literally boil. At that point, the overheated brake fluid will begin to take on a gaseous/vapor form and the presence of some of that non-liquid brake fluid will lead to severe brake fade–i.e.–no brakes when you need them the most.

So, despite your belief that “the hotter they get, the better they work”, the exact opposite is true for the type of disc brakes that are installed in regular passenger vehicles.


#6

if you’re having a problem with brake fade, a good one to try is Porterfield R4S compound pads. They’re grippier than stock and don’t fade nearly as easy.

If you’re still having a problem then 1) learn to drive in the mountains and 2) get slotted rotors. NOT crossdrilled. Slotted. It’ll help vent the outgassing.


#7

True story: I was on a road trip through the interior of British Columbia in my 02 Hyundai Accent. I was driving along some mountain roads, ones with no shoulder, and a 1000ft cliff on one side, and a 1000ft rock wall on the other. There were also a number of 2km long 15% downgrades, at the bottom of which were 180 degree hairpin turns. Anyway, since my car has limited engine braking ability, and a tendency to understeer at speed, I discovered that really hot brakes on Accents fade like an SOB. I wish stock brakes got better the hotter they got… too bad reality doesn’t agree. Taking a hairpin turn at 60km/h, without the benefit of decent braking ability (engine or otherwise) is quite scary, especially when you’re no more than 5 or 10 feet away from certain death.


#8

By the way, for anyone who cares, it was highway 99 from Kamloops to Vancouver. It’s very scenic, but so is highway 1, and number 1’s a lot easier of a drive.


#9

Or is it highway 3? Anyway, the TransCanada Highway… take it instead of 99.


#10

One of the reasons for flushing brake fluid is that as it gets older it picks up moisture which lowers its boiling point . We don’t USUALLY use the brakes hard enough to notice this but when we do need the brakes and we lose them we sure do notice it !


#11

Hopefully by now we have given the OP enough reason to question his theory about disc brakes working better when they are hot. Incidentally, the vehicle in question was actually a Dodge Magnum, not a Dodge Challenger.