Cooling your brakes

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.

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...

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.

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.

The brakes on that F150, like all other "half-ton" pickups, are not much heavier than passenger car brakes. I towed the small trailer in the sig below across the Rockies last Fall, and used the methods mentioned above to apply the brakes when needed. Used tow-haul, kept the speed to a safe and reasonable rate, downshift manually, and applied the brakes moderately for very short periods (2-3 seconds). It maintains control well.

BUT, those cheap thin rotors got too hot and warped. The front brakes pulsed noticeably afterwards, and when I got back home I had to have the rotors turned. The brake pad material was fine. Just scuffed up the pads on a flat board with sandpaper, and they were good to go.

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

45Ricochet wrote:
It's really a shame we get such non sense advice for someone really asking about a safety issue like braking. How is the OP going to weed out the BS from real world experience :H
I also drove loads up and down from LA to Seattle. That was even before front axle brakes mind you :E , and Jakes were just coming onto the market. If you ride the brakes down a pass.... that is exactly why they made the " Run away truck ramps" :E
I just don't get some posters demanding "their way" is the best because that's what I do mentally. Grow up, quit making up stories about your hollowed trucking career. Any DOT manual will spell it out, look it up boys.
As said though, the gasser will slow down when geared down much better than the diesel.
Up the hill at 45, down the hill one gear lower or 35.... stab when needed.
And I have that 1150F rotor temp link if someone needs it :B

Believe what you want.

DMV PA CDL wrote:


5.4.6 – Proper Braking Technique (SNUB Braking)
Remember. the use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine.
once the vehicle is in the proper low gear, the following is the proper braking technique:
1. Apply the brakes just hard enough to feel a definite slowdown.
2. When your speed has been reduced to approximately five mph below your "safe" speed, release the brakes.
(this brake application should last for about three seconds.)
3. When your speed has increased to your "safe" speed, repeat steps 1 and 2.
For example, if your "safe" speed is 40 mph, you would not apply the brakes until your speed reaches 40 mph. You now
apply the brakes hard enough to gradually reduce your speed to 35 mph and then release the brakes. repeat this as
often as necessary until you have reached the end of the downgrade.

Link Page 10 or so

How many states CDL links do I need to list? The old school braking method has been changed over the years when they proved it was the wrong method. Mostly due to unbalanced braking systems which most RV's are.

Thermal mass...

That the rotor assembly has enough mass to absorb the heat expected to be generated
and at the rates it is generated

This then has a low impedance path to the centrifugal vanes to reject that heat

On some rotor designs, there is a heat dam between the rotor and the path to
the hub/bearings. So that the heat will not greatly affect the bears and stay
where the cooling vanes are

Automotive brakes using a sacrificial material (friction material, both
disc and shoe) convert kinetic energy into heat.

That type of braking system has thermal dynamic attributes (laws of
physics) for each component and the sub-systems to the whole system

Thermal inertia (mass) and thermal rejection are the main elements of
managing the ultimate specification of that braking system. As are
the material specifications for each/all components (thermal conduction
rates, or impedance), efficiency of rejection, thermal performance
ranges, etc

Friction material is the main sacrificial component/material. The cast iron
is also, but to a lesser degree and dependent on the composition of
the friction material(s)

Heat generated at the point between the friction material and cast
iron will have that heat conducted to both the friction material and
cast iron. 'Some' of the heat is rejected by the outgases from,
mainly, the friction material

The out gas has a rate of conversion from solid to gas and the
rate of braking the key determination. FPM, PSI and a few
other attributes dictate that rate vs the friction material
composition/spec

That out gas rate can so high, that it will 'lift' the friction material
off of the cast iron and is the biggest portion of 'fade'. Why some
friction material will have slot(s) cut through them as escape
routs for those gases. Ditto cast iron slots and holes...out gas
escape routes



The more kinetic energy being dissipated, the more heat generated
Mainly rates of deceleration factored by stuff like speed, mass,
incline, etc. Meaning the harder you brake, the greater the amounts
of heat generated...ultimate temperature too. BTU and temp are two
different things in the same arena



Rejection (the removal or transferring of heat) has many components
and attributes to them

For our automotive brakes, ambient air is what we have as the recipient
for rejected heat. How it gets there and how much is the basics of this
topic

Very few disc brakes are solid rotored these days. Most are centrifugal
vaned rotors. RPM and tip swept dia are the main components of these fans.
The configuration of the vanes and the intake also throttle the rejection
rates

Some have more vanes, some have thicker vanes, some vanes are
straight, others curved.

Meaning that the faster the rotor spins, the more the centrifugal vanes
fling off air that has gone through the rotor innards to capture heat

Drilled rotors are mainly for track. Every day and especially when driven
HARD, has a rich history of cracking. Cast in holes do NOT have the
stress raisers that drilled holes do.

Slotted does not have as great a tendency to crack, as they do not
have the types of stress raiser that drilled holes do

Drilled holes and cast holes can have +40% reduction in material and
surface area, both detrimental to braking. Again, drilled holes are
mainly for track usage

Both holes and slots main function is to allow outgasses to NOT float
the friction material off of the mating cast iron surface

The other types of braking mentioned are good topics, but ancillary to
the OP's truck and application.




Now back to the OP's question and some of the very dangerous
advice...

First, this is a freebie forum and worth the price paid... I've taken an
extended break from this forum(s) and come back occasionally. Found
that too often was referencing Proverbs 26, verse 4...which applies to
several posts on this thread

The braking hard and coast to cool the brakes is the best, IMHO

By constantly holding or riding the brakes will NOT allow them to cool
down enough to become more efficient in rejecting the heat.

Temperature differential is the biggest component in the rates of heat
rejection. Why allowing the brakes to cool so important.



There is much, much more, but think this is the gist of it for this point
in this thread

Am also assuming within the OEMs ratings for your TV, the truck is
in good order and that you are not speeding...

coolbreeze01 wrote:

DirtyOil wrote:
I have holes cut in the floor so the wife, grand kids and I can smooth out the callouses. If that doesn't slow us down I have a anchor I toss out the window.


The advice of down shifting is good but you have to keep and an eye on the tach, high rpms can kill that gear/tranny in the auto transmission. So, if the rpms are at or near the redline you might want to up shift and apply more braking power.


my 2 cents...

Don't do this, recipe for disaster.

I agree to an extent with both DirtyOil and coolbreeze01. Apply more brakes. But don't upshift. The speed will come down easier by staying in a lower gear, and redline will quickly drop to reasonable levels for the original gear chosen.

Like I said earlier, drop to a lower speed altogther while you are braking and continue at the safer lower speed. It won't matter a lot if one uses easy steady braking or hard intermittent braking to disippate indentical kinetic energy, but the laws of physics say steady braking works better.

Wes
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