Cooling your brakes

[Padlin]

Being from the east coast I've yet to encounter my brakes heating up enough to require pulling over to let them cool down, at least not that I know of. This summer/fall we're doing our first trip out west and may well encounter such. The questions are, how do you know they are in danger of over heating and how long does one need to pull over to allow them to cool down?

Pulling a 4k lb bumper pull with an F150 6 spd.

[RAS43]

Great response Fred. That is how I drive all over but especially in our Colorado mountains, sometimes with doubles. Never had to stop to cool the brakes and my present truck doesn't have an exhaust brake. And I have not done any brake work on a tow vehicle or trailer in over 15 years.

[coolbreeze01]

mpierce wrote:
Forget about how to cool your brakes.

DO NOT GET THEM HOT IN THE FIRST PLACE!

Gear down, do not use the brakes much, if at all.

No problem to solve.

If you have to figure out a way to cool your brakes, YOU Are DOING It WRONG.

I agree. Well said.

[Golden_HVAC]

Now that you have three pages of advise, I hope that you have your answer.

Basically you really don't need the brakes very often going down a hill if the engine is downshifted to the correct gear, and you start off the downhill around 45 MPH. Of course a small highway might have 35 MPH curve signs, and those would dictate the speed limit while on such a curve. But the yellow signs that say 35 MPH does not mean the whole highway is limited to only 35 MPH, it probably will have a speed limit of 55 mph and the 35 MPH yellow sign is "Caution, curve recommended speed is 35 mph". More for a speed rating of fire trucks than anything else. Cars many times will go about 10% faster than the recommended speed. Yet if there is snow or ice, sticking to or going slower than the recommended speed would be prudent.

I prefer to apply the brakes for a short time, then let them cool while still moving. 35 MPH breeze will cool them fairly quickly. I would recommend a slightly less than firm application of the brake pedal, where "Firm" is what would be used while stopping at a stop sign.

If you can select the gear - 1 to 6, then I would first slow to about 55 at the top of a mountain while on a interstate, then shift to 5th or 4th. Then if the curves are not to wild, I might shift up to 5th and let the speed gain up to say 60 MPH. However if you are trying to control speed due to curves in the roadway, or a sign that says 7% downgrade, you would want to slow to say 45 MPH and then shift down one more gear to keep the engine above 2,500 RPM, all the way up to something like 3,800 to 4,200 RPM range. If moving downhill, and the engine speed gets up around 3,800, then apply brakes enough to slow down to about 3,400 - 3,500 RPM range, then let the speed build up again.

Fred.

[mpierce]

Forget about how to cool your brakes.

DO NOT GET THEM HOT IN THE FIRST PLACE!

Gear down, do not use the brakes much, if at all.

No problem to solve.

If you have to figure out a way to cool your brakes, YOU Are DOING It WRONG.

[45Ricochet]

smkettner wrote:


BTW Baker grade is weak. Maybe a test of the cooling system in summer but hardly the brakes.

Your right! I was talking about brake temps in hot weather. My graph from PUT's shows that too with the Davis and Eisenhower grades. It's mostly just a long pull or small % downhill but riding em down that what, 20 mile section, in 110F heat is going to warm them up a bit :W

I should have used a better grade for an example.

[BenK]

Turtle !?!?!!!!!!!

You've got a GREAT ability to envision stuff mentally....and a healthy
dose of free associative abilities (thinking very thin layer of molecules)

Yes, the 'thickness' of the pressurized air molecules would be extremely small,
even smaller than you thought

A typical molecule is measured in Angstrom's: 10 to the minus 10 m
(one ten-billionth of a metre) or 0.1 nm. CAN NOT be seen using the
visible light, as that wave length is longer than a molecule. Meaning
that if you could get a microscope that could magnify down to that
level...you'd have to use another type of wave length other than what
our eyeballs are designed for (human eye is sensitive to wavelengths
from about 4000 to 7000 Å...AKA visible light)

Even hundreds of thousands of air molecules would be less than a
thousandth of an inch

The 'shock wave' manifests itself with that compressed air molecules (preceding)
and travels faster than the speed of sound. Otherwise the air molecules would
get outa the way to not become compressed into a solid (adiabatic compression)

This kind of stuff fun for me and thank you for the opportunity to jaw a bit

So back to the OPs question and your question on hydraulic PSI

The MC to Caliper PSI is still the same, but the out gas PSI is HIGHER to bend
the caliper bridge apart a 'few' angstrom's...and/or compress some other
component, etc

That then has an 'air bearing' between the friction material and cast iron surface

Since the friction material no longer touches the cast iron, there is next to
no friction. There will be some spots/areas that do touch, but the total area
is greatly reduced. Because the friction material will NOT outgas
evenly across it's surface

Why cooling the brakes (friction material, cast iron, etc) will slow down the
out gassing or stop it

It will come back sooner, as the friction material and cast iron is
already heated up above ambient...they why of stabbing the brakes and
let off for them to cool while no braking is happening

Why continual braking will continually keep the friction materials hot enough
to out gas to float the pads off the rotor and the subsequent reduction in
braking

[Terryallan]

45Ricochet wrote:
I was trying to be civil and not quoting anyone, but since I am now being quoted Terry here is another example of the number of " taps of the brakes" as in snub braking. Notice they mention " taps" not just ride em all the way down the mountain.
Geeeze guys, how long is the Baker grade in S Cali and how hot is it there in July :H
Oh yeah not trying to beat up Ford here, just the facts of the braking tests for the exhaust braking, and number of "taps" rather than riding em. Notice the rotor temps :E the more you tapped the fat peddle.

Just giving real world situations, and not paper ones. Just saying what the best drivers do and how they taught other drivers. If and when you can say you had more than 20 years of safety awards, and then 10 years of accident free independent driving. I'll do it your way. Till then, I'll trust him.
But then as I don't trailertruck any more and my "hollowed" truck driving career is over. And you couldn't pay me to own a diesel. I don't have to worry about it, as my gas truck has all the engine braking I will ever need.

BTW. Bumble bees can't fly either. Just ask a scientist

[time2roll]

Padlin wrote:
Being from the east coast I've yet to encounter my brakes heating up enough to require pulling over to let them cool down, at least not that I know of. This summer/fall we're doing our first trip out west and may well encounter such. The questions are, how do you know they are in danger of over heating and how long does one need to pull over to allow them to cool down?

Pulling a 4k lb bumper pull with an F150 6 spd.

I have been over the Rockies, down 9 mile 10% grades, Death Valley etc. with less truck and twice the weight with no issues. Original rotors at 180,000 miles. Enjoy the trip. Just don't ride the brakes. Use the transmission and go a little slower on the big stuff.

BTW Baker grade is weak. Maybe a test of the cooling system in summer but hardly the brakes.

[45Ricochet]

I was trying to be civil and not quoting anyone, but since I am now being quoted Terry here is another example of the number of " taps of the brakes" as in snub braking. Notice they mention " taps" not just ride em all the way down the mountain.
Geeeze guys, how long is the Baker grade in S Cali and how hot is it there in July :H
Oh yeah not trying to beat up Ford here, just the facts of the braking tests for the exhaust braking, and number of "taps" rather than riding em. Notice the rotor temps :E the more you tapped the fat peddle.




On edit HERE is the link

[Turtle_n_Peeps]

Hmmmm, solid air. Wow, never did think of it that way. Great analogy, and I see what your saying!

I wonder how many thousands of an inch the gas wedge is? I bet it's super small? Like maybe a though or two?

Anyway thanks for the thought for the night. Maybe for the week? Now I have to get myself an adult beverage and think about what you typed some more. Thanks again! 🙂

[jaycocamprs]

carringb wrote:

3) The "stab" part of the "stab-and-coast" method ensures all brakes receive even pressure which distributes heat more evenly. Under light braking, brakes may not be braking evenly, which can cause individual corners to overheat sooner.

And the more brake ends you have the less chance of having just the right balance of brake adjustment. Not so much of an issue with the hydraulic brakes. Very much an issue with air or electric brakes.

[BenK]

Thanks Turtle for the kind words !

Here is something to noodle...

An explosion and that 'shock wave' doing the damage is just air, or a gas

The rapidly expanding gases from that explosion has them move so fast, that they
collide with the gas molecules in the way...and so on and so on...till they
stack up as a 'solid' wave expanding from the center of the explosion

That shock wave is just gas molecules smashed together into a solid...that then
hits whatever to then impart their stored energy.

A fire cracker, stick of dynamite, etc has gas pressure that can move and/or
damage. Ditto the ignited mixture inside our combustion chambers...it moves
the piston that is connected to an offset crank to turn the crank...

The caliper assembly will have the connective material between both
sides bend...AKA the bridge material between the piston side and anvil

On a single piston and anvil...the center will bend just like a multi piston caliper

Bend ever so much or enough to reduce the opposing hydraulic PSI on the friction material

That is what GM found in the report from Bosch (who the hired to consult on
why their GMT400 calipers were not doing well). They found that the calipers
were not stiff enough and bending apart...

[Turtle_n_Peeps]

Good post Ben, and on the money.

I always found it interesting that a gas (compressible)can force the pad away from the rotor with an (uncompressible)fluid pressing it towards the rotor. :h Doesn't seem possible that a gas can do this, but as you stated, it sure can.

[AH64ID]

OH48Lt wrote:
I was surprised at the difference between F150 brakes and the brakes on my former F250.

Like I mentioned earlier brakes are designed to stop GVWR not GCWR, and 3/4 tons often share the same brakes as their 1 ton brethren. That means the brakes are designed to stop at least 50% more weight, plus a larger safety factor/average weight based on being a 3/4 or 1 ton.

The truck brakes should also only be stopping the GVW, if they are stopping more than the GVW the trailer brakes are out of adjustment or not being controlled properly.