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Author Topic: proper way to decend grade  (Read 23594 times)
Tony LEE
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« Reply #15 on: August 26, 2007, 05:35:10 PM »

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

Downhill Braking

John C. Glennon, Jr., BSAT

[ Reprinted from the Trucker's World Magazine, Volume 5, Issue 6, June 2001]


When the Commercial Drivers License (CDL) manual was first published, it recommended that a driver use a light and steady application of the brakes when descending steep grades. This recommendation was based on an old theory that heavy brake applications would generate more heat than light applications. This method (controlled braking) was commonly taught to drivers and, even after changes were made to the CDL manual because this theory was proven wrong, the method is still taught and practiced today.

Snub 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. To understand why this method is recommended takes some understanding of the basics about how brakes work. Slowing a truck with it.s brakes, creates friction between the brake shoes and brake drum to convert the kinetic (forward movement) energy of the truck into heat energy dissipated by the brakes. The amount of heat energy produced is dependent upon the weight of the truck and the amount of slowing desired. Assuming these two factors remain constant, the manner in which the brakes are applied, hard for a short time or lightly for a long time, will not change the amount of heat energy and heat produced by the brakes. This heat energy will be distributed among all the brakes that are working. Again, assuming all other factors constant, the more brakes the system has working the cooler each brake will be.

This explains why the old theory of light and steady braking is incorrect. However, to understand why snub braking is the recommended practice, you must also understand the basics of pneumatic balance. Trucks have relay valves to control the application and release of the air brakes. A standard truck-trailer usually has one relay valve for the tractor drive axles and one for the trailer axles. The relay valves are controlled by air pressure from the foot valve (brake pedal). This control pressure opens the relay valve allowing the desired amount of air pressure from the air tanks to pass through the valve and supply pressure to the brakes. Pneumatic balance is created by having equal air pressure at all wheel ends. Pneumatic imbalance is a result of these valves that open at different pressures. For example, a tractor may be setup with a relay valve that opens at 15psi (15psi crack pressure relay valve) and the trailer being towed may have a relay valve with a 3psi crack pressure. A vehicle setup this way would only apply the trailer brakes during controlled brake application, which typically has an application pressure of less than 10psi. However, a .snub. brake application of 20 to 30psi will open all valves and apply all brakes. This type of imbalance can also be a result of contaminants and alcohol in the air system that can cause these valves to hang-up and have higher than normal crack pressures.

Snub braking became the recommended method of downhill braking as a result of testing done by University of Michigan Transportation Research Institute. This research found that trucks with properly balanced brake systems had basically the same average brake temperature when using either the controlled or snub braking method. However, trucks with poor brake balance were found to have more uniform brake temperatures when the snub method was used. Unless pneumatic testing is performed on a truck to ensure that proper brake balance is maintained, there is no way to know if a truck has good brake balance. This type of testing is difficult to perform in most trucking operations since a tractor is usually hooked to several trailers over relatively short time periods. Therefore, for the purposes of deciding which braking method to use, it would be virtually impossible to determine if a truck has good brake balance. This is why snub braking is the recommended method.

Although snub braking does compensate for imbalances in the pneumatic system of the brakes, there is a misconception that snub braking also compensates for brakes that are not evenly adjusted. Brakes that are not evenly adjusted have a torque imbalance. Torque balance is created by having matched mechanical components that are working properly and adjusted correctly. Snub braking has limited ability to compensate for torque imbalance. A good example of this would be a truck with a six-inch slack adjuster on one side of the axle and a five-inch slack adjuster on the other side. This truck will always have an imbalance at any pressure because the brake with the six- inch slack adjuster has more leverage. The same imbalance can happen with uneven brake adjustment because the force output of a brake chamber is directly related to the brake adjustment (push rod stroke).

Since the snub braking method cannot compensate for torque imbalance, trucks should always be inspected and repaired with the following in mind. A truck.s brake system should have matched mechanical components such as the same size brake chambers and same length slack adjusters on both sides of an axle and, most of the time, on all brakes in a group of axles (i.e. tractor drive axles). When inspecting the condition of the brakes, any isolated premature wear found is an indication of a balance problem. If one brake wears faster than the rest, there is a torque balance problem and that brake is doing more work than the rest. If one brake wears much slower than the rest then that brake is not working as hard as the rest. When brakes are repaired, it is important that the cause of an identified torque imbalance be found before repairs are made. Repairs made without correcting the torque imbalance could amplify the problem causing the overworked brake to work even harder. It is equally important to ensure that the same repairs are done on both sides of an axle. If the brake hardware is replaced on the right side of an axle it should also be replaced on the left side. If the s-cam bushings are replaced on the right side they should be replaced on the left.

Snub braking is the method that every truck driver should be using. Although snubbing is a very good precautionary measure, it is still no substitute for a properly balanced brake system. Brake imbalances not only cause brakes to overheat when driving in the mountains, but also can cause instability both on slick driving surfaces and during hard brake applications. These stability problems (to be discussed in a future article) are the primary cause of jackknifes and trailer swingouts. Therefore, I recommend not only that trucks be tested, repaired, and maintained to ensure that they have good brake balance, but also that the snub braking method be used to compensate for any variances that result from interchanging tractors and trailers.
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Jerry Liebler
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« Reply #16 on: August 26, 2007, 05:36:54 PM »

Tonylee,
     Constantly applied braking also loses FAR less air because the only time air is expelled from air brakes is when they are released.  The lining glazing argument is similarly bogus.  Glazing of linings is a function of temperature and the peak temperature is lower with the constant application.  This can be proven mathematically and has been verified in many carefully set up experiments.
Regards
Jerry 4107 1120 
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Songman
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« Reply #17 on: August 26, 2007, 05:41:03 PM »

Another...

Quote
Proper Braking Technique

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 a proper braking technique.

1. Apply the brakes just hard enough to feel a definite slowdown.

2. When your speed has been reduced to approximately 5 m.p.h. below your “safe” speed, release the brakes. (This brake application should last for about three (3) seconds.)

3. When your speed has increased to your “safe” speed, repeat steps 1 and 2. For example, if your “safe” speed is 40 m.p.h., you would not apply the brakes until your speed reaches 40 m.p.h. You now apply the brakes hard enough to gradually reduce your speed to 35 m.p.h. and then release the brakes. Repeat this as often as necessary until you have reached the end of the downgrade.


From drivingrules.net CDL Study Guide - http://www.drivingrules.net/cdl/cdlsecb/b11mountian.htm

I think everyone should go ahead and use whatever method they feel comfortable with. As for me, as a new guy, I am going to trust the  trained expert who studies truck crashes for a living. There is a reason the CDL manuals have all been changed and that is good enough for me. The Bachelor of Science in Automotive Technology degree he has tells me that he knows more about heating brakes than I will ever know.
« Last Edit: August 26, 2007, 05:45:30 PM by Songman » Logged
tekebird
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« Reply #18 on: August 26, 2007, 05:55:37 PM »

CA has snub braking in thier cdl

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Jerry Liebler
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« Reply #19 on: August 26, 2007, 06:02:02 PM »

The reason 'snub' braking is being advocated is light pressure often concentrates all the heat in one brake while heavier braking forces all the brakes to share the heating.  So the argument is with poorly adjusted brakes over heating of one wheel's brake may occur so it's better to spread a little more heat among 4 or more brakes than concentrate all the heat in one brake. Take your choice.
Regards
Jerry 4107 1120
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skihor
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« Reply #20 on: August 26, 2007, 07:18:55 PM »

Jerry L.
I see lots of stuff supporting snub braking but nothing supporting your claims. Do you really expect me to believe that virtually state in the nation, that supports snub braking as the safest method, is wrong? I'll stick with the experts on this one.

Don & Sheila
« Last Edit: August 26, 2007, 07:31:51 PM by skihor » Logged
Jerry Liebler
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« Reply #21 on: August 26, 2007, 07:44:24 PM »

Skihor,
    Tonylee's post above quotes a Truckers World article that acknowledges that with a properly balanced braking system the temperature rise is 'virtually the same'  The problem of poorly balanced brakes forcing all the heat into a few of the brakes  is the reason snub braking is now advocated as it results in more even heating of the brakes even though more total heat is generated.  Hopefully a bus has a well balanced braking system unlike a tractor trailer big rig.
Regards
Jerry 4107 1120   
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skihor
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« Reply #22 on: August 26, 2007, 07:50:05 PM »

My last post was not meant to be mean. Quite the opposite. The issue here is how to decend a grade safely. I'm willing to bet you could test every one of our busses to see if ALL of the brakes function equally and not one would pass. Thus, if for no other reason, snub braking is the safest way. Not many of us, myself included, have big rig driving experience. I feel this is a very important issue in the operation of these 30,000+LB monsters.

Don & Shela
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Songman
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« Reply #23 on: August 26, 2007, 07:51:15 PM »

Hopefully a bus has a well balanced braking system unlike a tractor trailer big rig. 

That is an awfully big word to post your life on. All things being equal, I would prefer the method that makes it best if something is wrong. You never know when something could go wrong in a braking system that you don't know about.

And that is the point... In a brake system where everything is fine, both systems would equally well. So no big deal. But if everything is not fine, which is probably the case in a lot of scenarios, snub braking gives you a better chance of keeping your brakes cool and functional.
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Jerry Liebler
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« Reply #24 on: August 26, 2007, 08:36:51 PM »

The no longer recommended technique of rideing the brakes is still safer if the braking system is properly balanced which is the case if it is on a single vehicle that has been properly maintained.  But if it is on a composite vehicle, like a tractor trailer, or has maintenance issues then the now recommended technique of 'snub' braking  is safer.  So next time you come to a long downgrade try rideing the brakes down it, then stop at the bottom and run around the bus with your infrared thermometer and read the temperature of all the hubs. if they are'nt within say 10 degrees you should be using the snub technique.
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Jerry 4107 1120
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TomC
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« Reply #25 on: August 26, 2007, 09:13:27 PM »

In order to do the constant brake pressure technique, you should really have a brake air pressure gauge that reads the pressure you're putting into the system when you press on the brake.  From many years of experience, no more than 10psi of pressure should be pressed, unless you want to smoke the brakes.  Course the normal way a good truck driver comes down a grade is to come down at the speed that will allow only light usage of only the trailer brakes keeping the tractor brakes cool for emergency stops.  You can use the off-on-off method, but have to learn a very light foot to keep the brakes from overheating.  Stabbing the brakes relatively hard is a sure way to over heat them.  The best way to tell you came down the hill right is that you did NOT smoke your brakes.  Good Luck, TomC
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« Reply #26 on: August 26, 2007, 09:13:49 PM »

Going fast then going slow by using the brakes just makes them hotter. The drums get hotter simply because the vehicle is going faster. True, it takes the same amount of energy no matter which method is used but the rate of heat buildup is faster and takes longer to dissipate.

Constant braking allows the heat to constantly flow from the brake shoes. When the brakes aren't applied the the shoes don't have nearly as good a heat path as they do through the drums when applied.

Which is better, hot and not so hot back and forth or constant medium hot?

Lining glazing is only a problem when first breaking in new linings. Some types of new linings are purposely gotten hot so the resins flow and level the lining face.

Brakes are used gently all the time in normal driving so that obviously is not a problem.
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donnreeves
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« Reply #27 on: August 27, 2007, 04:44:52 AM »

I live in a mountainous region of North Jersey. While our mountains are not as big as those out west, the hills are very steep. I have been driving dump trucks and equipment haulers up and down these hills for thirty years, and I genrally use the higher gear constant pressure method. I find it gives me much more brake left at the bottom of the hill. That said, I am familiar with all the grades I go down and know what to expect and just how to approch them. For the less experianced, I think I would recomend a lower gear and the on-off -on method.Keep in mind that both methods will get you in trouble if used too hard.If you are unfamiliar with the terrain, it is best to take it slow and get it stopped at the first sign of overheating of the brakes while you still can.I can attest to the fact that it is no fun at all screeming down a hill with the brakes on fire praying that no one pulls out in front of you and the light at the bottom doesn't turn red.  Donn
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Nick Badame Refrig/ACC
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« Reply #28 on: August 27, 2007, 06:01:02 AM »

I can attest to the fact that it is no fun at all screeming down a hill with the brakes on fire praying that no one pulls out in front of you and the light at the bottom doesn't turn red

Hi Don,

This sounded like me back in April... Grin  There is always a traffic light at the bottom of a steep mountain...

Tis why I installed Jakes..

Nick-
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« Reply #29 on: August 27, 2007, 06:26:41 AM »

Just a thought here:
 I have little experience with the pros and cons of this subject, but would additional cooling help the brakes??
  I was just thinking back to my other thread about putting an air dam on the front of the bus and running air ducts back to the brakes. I will do this when I try my air dam experiment  Smiley but could some sort of air deflectors or diverters help at all? It seems that you have quite a bit of air flowing by, why not try to direct it towards the brakes?

  I duuno, just thinkin,
        Chaz
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