Downhill braking

[lostagain]

http://www.crashforensics.com/papers.cfm?PaperID=36

Interesting paper on the reasons why stab braking is the preferred method over slow and steady braking while going down hill.

Even though both methods create the same amount of heat, the fact that brakes are rarely balanced from one axle to the other, or even from one side to the other on the same axle, hard braking helps spread the load evenly to all brakes. Stab breaking is the standard now.

JC

[gus]

This argument goes on and on and on and..............

[HB of CJ]

Yep.  Forty five years ago we were taught to apply steady treadle pressure to reduce speed about 10 mph, then get off the pedal.  Also to pretend there was an egg between our boots and the brake.

[digesterman]

You can always smell the brakes on the semi's going down a long  grade with their brakes applied "slow and steady"

If you can smell hot burning brakes they aren't going to brake nearly as well

[Hobie]

This article is for trucks with trailers.  Yes, there are different crack pressures for tractor vs. trailer valves. Stabbing will bring more pressure into the system and use all wheels for braking better. 

However, no so for busses.  Without opening the whole can of worms,  I like to keep light treadle pressure to keep the brake shoes in contact with drum to keep shoes cool.  Not constant pressure but hard pressure to slow then just enough to keep shoes cool.  If you smell the brakes, you need shift down another gear, too much treadle pressure, or mis-adjusted brake.

[Tony LEE]

My sources refer to it as Snub braking. Crash Forensics does too.

Basic technique applies to all vehicles from bicycles to road trains regardless of type of brakes. It is a speed management technique that relies on proper selection and operation of transmission, brakes and auxiliary brakes all in a coordinated fashion.

[kyle4501]

I wish they had done a better job of explaining how excess speed builds heat exponentially.

Ever heard the expression E=MC2

Energy = mass * speed (squared)

Since the energy goes up much faster than the speed, as little as 10 mph can double the energy.

For example, letting the speed build to 30mph & then stabbing the brakes to slow to 10 mph requires the brakes to dissipate more heat than maintaining 25mph.

If your RV brakes don't evenly share the load, maybe it's time to fix them. 

Every situation is different, but if you smell your brakes, you are going too fast.

[luvrbus]

All the casino bus drivers here are taught to stab the brakes even with Jakes and retarders on the bus so I don't know why it is a no/no in a bus.

I know they are running a 50 year old bus they are all fairly new except 1 that has a 2000 ? model Dina with ABS disk brakes even it has the stabbing process in the operators manual.

We have a 11 mile 7% grade coming down from the Black Mountains into Bullhead and Laughlin they need all the help you can get   

[Tony LEE]

I wish they had done a better job of explaining how excess speed builds heat exponentially.

Because it is not that simple a calculation.


As for having all 6 brakes sharing the task equally during constant light brake application, or even both sides of an axle sharing equally, that is never going to happen. Never!

[Jon]

Rather than opinions which may be correct or incorrect I think it would be great if someone that has internal TPS with temperature readouts to take the time to do a couple of long downhill runs using steady light constant pressure or the stab method. That way we would have data to support which method is best and which drives up brake temps.

I do know with a retarder or jake, proper gear selection, and speed it is possible to descend 7 and 8% grades without touching the brakes.

Far too often we all pass on information without knowing if it is correct or if it is old wive's tales.

[HB of CJ]

I will do it.  Stabbing brakes and ice conditions do not mix?

[luvrbus]

Jon,coming off that pass in 118+ degree weather it will fry a B500 with a retarder and they stab the brakes they always have. I come off the pass with my Jakes at 45 mph the posted speed limit in 4th and stab my brakes 5 or 6 times coming down with a 60 series and B500 but I don't use the retarder on mine just the Jakes  

Over the years I would say I have seen at least 10 bus brake fires on that grade no telling how many truck brake fires
Fwiw you can take the Toolbox software for the Wabco ECU and pull that information on brake temperatures it will show the temperature range of applied brakes at any time the outfit JC drives for does it all the time so do most accident inspectors and lawyers      

[Jeremy]

Ever heard the expression E=MC2

Energy = mass * speed (squared)

Actually C doesn't stand for 'speed' it stands for 'the speed of light'. Since the speed of light is a constant the purpose of the equation is to demonstrate the Special Theory of Relativity which states that mass and energy are different manifestations of the same thing, or, looking at it another way, that mass contains energy. And because the speed of light squared is a very, very, very big number, you can get a lot of energy from a very small amount of mass. Which is very useful if you run a nuclear power station, but not so much if you're trying to slow down a bus.

Jeremy

[luvrbus]

We get the physics involved on this we are all doomed  

[kyle4501]

It seems some have missed the point I was trying to get across, so I will try to add some clarity. 
How is this -
Kinetic energy = half the mass times the velocity squared . . . . . (assuming vibrational and rotational effects are negligible)


It does not matter how many brakes are or are not working on your vehicle, it is still a fact that kinetic energy increases by the square of the speed.

The point of my first post is that a "simple calculation" demonstrates how a 10mph increase in speed can double the heat load.
For example, an increase from 20 to 30 mph more than doubles the kinetic energy (ie: required brake capacity).

So, the stab braking method allows better uniform loading of the brakes & reduces the chance of unpleasant surprises. However, doesn't let you go faster.


No matter how you use your brakes going down hill, if you have allowed your speed to get to the point that the kinetic energy exceeds the braking capacity, you are in deep do do . . . .