http://www.crashforensics.com/papers.cfm?PaperID=36 (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
This argument goes on and on and on and..............
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.
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
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.
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.
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. :o
Every situation is different, but if you smell your brakes, you are going too fast.
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
QuoteI 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!
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.
I will do it. Stabbing brakes and ice conditions do not mix?
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
Quote from: kyle4501 on September 15, 2015, 07:45:26 PM
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
We get the physics involved on this we are all doomed ::)
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 . . . .
And this is why I'm getting my Jakes to work right - they've never worked properly since I got the bus. I want to keep my brakes as cool as possible at all times. Heck, I would also mount a Telma if I could! Mind you, even with Jakes I don't descend quicker than I would if I didn't have them - I don't ever want to get into a position that they're the only thing that saves me. They're for slowing, not stopping, and definitely not an excuse to drive downhill like an idiot.
John
There is a real good reason Jake Brakes are built into engines now-because they work. Many engines like the DD13, 15, 16 and the Cummins ISX15 have dedicated cam lobes just to operate the Jakes-that work very well and very quietly. The trick going down any hill is to be geared down to a speed that the Jake brake can just about hold the speed with you tickling the brakes (light application for about 10 second [I don't believe in stabbing]) to bring the speed back down.
My bus weighs 31,000lbs and with my M/B in tow, weigh in at 34,750lbs. I can go down the Grapevine northbound at 35mph with just my Jake brake (8V-71) having to switch from two heads to one head to keep speed up-never touching the brakes. When I went exploring in the Sierra Nevada Mountains, I was down in 1st gear-which will lockup the torque converter when pulled down to 1st range. The Jake brake was very effective at those speeds. Good Luck, TomC
Clifford,
Here is some context for my use of the retarder and gearing.......
I know heat is the enemy and that the retarder has the ability to generate a lot. So my management of speed begins before the top of the hill. I am slowed down and geared down and in my 2 coaches I use 55 in 4th as my top speed. I crest the hill in 4th at 45 MPH or less. My retarders have the stalk on the steering column and from experience if I am on a 7% grade I can maintain my target speed or less using the 2nd retarder position and control my retarder output fluid temps to 230 degrees or less. If the speeds creep up I can go to the third position, but in addition to speed I am also self imposing a 230 degree temp limit. If I have to because the grade steepens and there is no flat spot in sight I do stab my brakes and take it back to 40 to 45 MPH, but that is when the grade is not constant but has steep sections and flatter sections.
I have been to Las Vegas twice using the route discussed with a 97 coach at 46,000 pounds and my 06 at 53,000 pounds. The 06 measures temps at a different location and has a more effective transmission cooler but the methods I used stayed the same with the same results.
STAB braking is an entirely different concept and is used in emergency situations.
QuoteStab braking: Use only on vehicles without anti-lock systems. Apply the brake all the way. Release the brakes when the wheels lock up. As soon as the wheels start rolling, put on the brakes fully again.
Using Air Brakes And Stopping Distance | High Road Online ...
www.truckingtruth.com/cdl-training-program/page46 (http://www.truckingtruth.com/cdl-training-program/page46)
SNUB braking is the widely-recognised technique for descending steep grades under control
QuoteSnub braking is now the recommended method of downhill braking. This method works by: first, choosing the correct gear for the hill; second, allowing the truck to speed up to the maximum safe speed as it descends the hill; third, applying the brakes hard to slow the truck down 5 mph; and then repeating this process to the bottom of the hill.
Who woudda thunk there was such a big difference between STAB & SNUB.
For the benefit of those who aren't in the business, a clear & concise definition usually helps others understand.
I donno about " applying the brakes hard " . . . . A hard brake application for me is just shy of leaving skid marks ;)
A firm application is quite noticeable, but passengers aren't thrown against the seat belts. :)
Does SNUB braking really want that version of hard braking?
Really doesn't make that difference with the newer stuff the ABS controls your braking anyways
Quote from: luvrbus on September 18, 2015, 08:20:49 AM
Really doesn't make that difference with the newer stuff the ABS controls your braking anyways
How does ABS know if you are loaded, overloaded or empty ;)
i never was a fan of ABS.
The only thing ABS does is sense a wheel going slower than the others which it interprets as impending lock up, and the modulator valve closes the air path to the brake chamber, and exhausts the air in the chamber. As soon as the wheel speed matches the others the modulator valve closes the exhaust path and maintains open flow again to the brake chamber.
LOL you been to Bendix school also I just read the exact quote from my Bendix book
Quote from: Jon on September 18, 2015, 09:35:56 AM
The only thing ABS does is sense a wheel going slower than the others which it interprets as impending lock up, and the modulator valve closes the air path to the brake chamber, and exhausts the air in the chamber. As soon as the wheel speed matches the others the modulator valve closes the exhaust path and maintains open flow again to the brake chamber.
still don't like them ;)
I was reading where Honeywell and Wabco has made a lot of technology advances on the ABS systems in the last 2 years but I am not a huge fan of taking that much control away from a driver
The Wabco system when the ECU detects a lockup then it will activate not before according to the Wabco manual section 1 page 3 I questioned my Bendix instructor on that one
When I read the title "Downhill braking", I thought DUH? :o Of course it is downhill ---Who would brake on an uphill?! ;D
I know ---my sense of humor is strange ;D
Steve Toomey
PAbusnut
Clifford, speaking of less driver control,the Freighliner Cascadia Evolution I was driving (I quit a couple of days ago, but that is another story not for this forum), has Adaptive Cruise Control. It senses a vehicle or an object moving slower than you in front and reduces your speed. If you get close enough, it applies the brakes. I didn't test it, but I assume that the ABS would take care of a lock up if necessary. Now with Lane Departure Warning, Automated Manual Transmission, GPS and other electronic doo dads, it is not such a big leap to driver-less vehicles...
JC
wheel holding nanny state, led by the insurance industry, litigation happy lawyers and geeky programmers... i am so happy for me, having lived in the past and getting sorrier for my grand kids...
mini rant over, smh.
With no job now JC maybe see you in Jan @ Quartzsite ?
Yep. We are trying to get away early November and travel down South for 2 or 3 month.
JC
Good, bring me a gal of that good Canadian honey and I'll pay you that was good stuff thanks a bunch
Why is everybody getting all caught up regarding features such as adaptive cruise control, ABS, automatics, etc. and the trend toward driverless vehicles. We have had them for yours, mostly used by drunks, texters, internet surfers, etc.
I guess the vehicles really weren't driverless, its is just that the drivers weren't driving.