| Author | Message | ||
| Tim Brandt (Timb)
Registered Member Username: Timb Post Number: 88 Registered: 10-2003 Posted From: 66.165.176.60 Rating: N/A |
First my thanks to RJ and Buswarrior for the article they linked to on the MAK board regarding the application of the emergency brake. As a bus newby this has prompted a couple of questions. 1. ACC valve. Since the Flxliner that I just purchased has the Johnson bar to actuate a drum in the driveline for parking I am assuming the valve located on the side bulhead just applies whatever air is left in the system to the rears in the event of catastophic failure correct? (yes I do also plan on retrofitting spring brakes but would like to understand the function of what is there) 2. Buswarrior wrote that using the wrong braking method coming down a long grade can cause issues due to overheating and expansion of the drums. What is the proper way to brake coming down a grade to prevent this? Although my coach has jakes I would like to understand the right weay to brake assuming the jake didn't exist. Thanks again RJ and Buswarrior for your efforts to educate those of us that are new to these machines Be gentle  Tim | ||
| Tony LEE (T_lee)
Registered Member Username: T_lee Post Number: 1 Registered: 11-2007 Posted From: 58.165.54.86 Rating: N/A |
"What is the proper way to brake coming down a grade to prevent this?" Ideally, you should be in the correct gear so the engine does most of the work and the brakes do the minimum. There was a thread elsewhere regarding technique - and while there are some who advocate riding the brakes to maintain a set speed, it is hard to find any official sanction for the method. Snub braking is most widely accepted as the correct method. This means getting into the correct gear at the top and then, if that gear doesn not quite hold the speed, allowing the speed to build up to say 105% of the target speed and then applying reasonable braking to bring it back down to say 95% and then releasing the brake and repeat the cycle. Other sources say to cycle the speed by about 5mph. If the vehicle is in the correct gear, this cycle will only need to be repeated at say 15 second intervals and the brakes are either fully released, or on. Any sign of getting away from you and you should stop and engage a lower gear - or at least slow down enough that if you miss the downshift, you can still stop. Pumping the brakes rapidly is poor technique and runs the danger of running out of air. (Message edited by t_lee on November 15, 2007) | ||
| Tony LEE (T_lee)
Registered Member Username: T_lee Post Number: 2 Registered: 11-2007 Posted From: 58.165.54.86 Rating: N/A |
http://www.e-z.net/%7Ets/ts/downhill.htm is one take on the subject http://www.johncglennon.com/papers.cfm?PaperID=36 is another | ||
| Jerry Liebler (Jerry_liebler)
Registered Member Username: Jerry_liebler Post Number: 298 Registered: 3-2005 Posted From: 67.140.168.194 Rating: N/A |
Tony Several months ago there was a long thread on this topic over on the 'MAK' board. The reason 'snub braking' is advocated is because more pressure is used and that assures that all brakes share at least some of the heat. This is an issue with tractor trailers particularly because they use multiple 'relay' valves which may have different minimum pressure. The 'ride the brakes' to maintain constant speed results in less total heat but because of the various valves it may be concentrated on one axle resulting in hotter brakes on that axle. The best situation is to go slow enough and in a low enough gear that the engine does all the braking on long downgrades. I personally use the ride the brakes in my bus but I'm sure all wheels are sharing the heat because I've checked all the drum's temperature after a long downgrade. Regards Jerry 4107 1120 | ||
| Gus Causbie (Gusc)
Registered Member Username: Gusc Post Number: 538 Registered: 11-2005 Posted From: 63.97.117.21 Rating: N/A |
I use the same system as Jerry on my 4104 with 4-sp and no Jake. Did the same when I drove an 18-wheeler. | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 295 Registered: 8-2006 Posted From: 76.168.69.233 Rating: N/A |
Tim, Further on braking techniques, and construction. Yes, the ICC valve just sends full reservoir pressure to the rear brake chambers. And the brakes WILL release when the air fully leaks off, so it can't be used for parking. My father favored snub, or cycle braking, as a result of driving many OLD mechanical braked cars and trucks, so I guess I do too. But, much more importantly, is the requirement to descend the grade in the same gear you ascended it, or would have ascended it if you had come that way? You should also determine the maximum speed your bus will go in all of the lower gears, and memorize it. Do this by starting out in 1st and flooring it, and remembering the maximum speed the bus reaches, as the governor limits the engine speed. Do the same in 2nd, and 3rd. If the bus will go 17mph in 1st, you will not be able to downshift a non-synchronized trans into 1st at over 17, and you will over-rev the engine if you let the downhill speed in the lower gears exceed the engine governed speed. Does this make sense? All of the above posts are correct IMNSHO. HTH, George | ||
| Tony LEE (T_lee)
Registered Member Username: T_lee Post Number: 3 Registered: 11-2007 Posted From: 138.217.147.45 Rating: N/A |
And regarding Jake and similar engine brakes. Since they shouldn't be used in icy conditions or even in heavy rain, and since they do fail, it is better to learn not to rely on them too heavily and always keep a good safety margin in reserve. http://www.newbiedriver.com/ is another site with practical tips and links to other resources. Reading through this and other similar forums shows pretty plainly that while particular individuals may be confident that their brakes are all operating uniformly, many are obviously not even close to ideal, so advice to newcomers should be pitched on the conservative side. | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 296 Registered: 8-2006 Posted From: 76.168.69.233 Rating: N/A |
Tony, I appreciate your advice on conservatism, a year ago I was told, "Quit scaring the newbies" when I said to descend a grade in the same gear you ascended it in, even though that is in the state truck drivers manual, etc. Oh well.... I would like to pose a question about Jake brake use though. When we stop and think about the amount of rertarding force developed by Jake brake, it is MUCH less than the power developed by the engine, and MUCH MORE less (sorry) than the effort developed by the brakes. A bus engine can't spin the dual rear wheels going uphill at full throttle in the lower gears in the rain, which is a WHOLE LOT more torque being applied than Jake being on in higher gears going downhill. Considerably more braking effort is applied to the front tires while braking, (with less weight on them) than is developed by Jakes on the dual rears, with the weight of the engine, trans, and differential on them. I know you are going to want to post back and say "Its going to [want to skid more] with the COMBINED braking effort of Jakes and air," which is correct, but how much more? In MY opinion, very little. What happens if you are descending in the proper gear with Jake on and it fails? All your drums are cold, and you can certainly maintain control while you either slow down some more and descend in a lower gear, or slow down and pull off to see what broke? This is just food for thought, go ahead and jump on me! George | ||
| Mark R. Obtinario (Cowlitzcoach)
Registered Member Username: Cowlitzcoach Post Number: 181 Registered: 4-2001 Posted From: 204.245.250.171 Rating: |
I hate to disagree with Jerry and Gus but continually riding the brakes should not be done with today's brake lining material. In the old days when brake lining material was manufactured with asbestos in the mix, the brake lining materials could take a whole lot more heat and dissipate the greater amounts of heat quicker. Without the asbestos in the brake lining material the new brake lining material can actually catch on fire if it is subjected to the same amount of heat. You must give the linings an opportunity to cool off and get rid of excess heat. The only way that is possible is if you let up on the service brakes and give them a chance to cool off. As long as the brake lining is in contact with the drum you are creating heat. If the lining is in contact with the drum the only way in which the heat can dissipate is by heating the drums up. When the drums heat up the diameter of the drum will grow. As the diameter of the drum grows the brakes have to travel further to maintain the same contact. The further the brakes have to travel the less force can be applied. Heat fade then becomes a serious problem. Stab or snub braking is what is recommended by brake lining manufacturers, by state department of licensing CDL driver's licensing manuals, and by all commercial driver training schools. The recommendation has more to do with heat than with equal braking pressure between different wheels and axles. All things being equal, when the service brakes are applied, the same amount of air pressure is applied to every brake can every time. The proportioning valves will take care of balancing the pressure between axles. If you are not getting the proper application pressure then you need to have the brakes inspected and repaired. Relay valves are used to put air pressure to the rear axles first and the front axle last. The amount of delay built into the relay valves is not very much but it is enough so that the front axle won't lock up first. And every air brake equipped vehicle has relay valves. If Jerry's bus has brakes that are equally hot between the front of the bus and the back of the bus then he has to know his relay valves are working correctly. The key to safe braking on downgrades in large vehicles is to go slow at the top so that you can stay going slow all the way to the bottom. Halfway down the hill is not the time to decide you started out a little too fast. For the newbies who don't know their vehicle, going too slow at the start is not a bad thing. It is of particular importance when a newbie in an unfamiliar vehicle is presented with a downgrade of unknown length or grade. Ideally, when going down a downgrade, the service brake should not be doing very much braking at all. The lion's share of the braking should be done by the engine supplemented by the occasional stab/snub to get the speed back down to the target speed. Auxillary braking systems are great because the use of them precludes the necessity of using the service brakes. But by definition, auxillary braking systems are auxilliary and should not be depended upon to stop the vehicle. They should also not be a license to travel faster downhill just because you don't have to worry about getting your service brakes smoking hot. In the old days drivers were taught to go downhill in the same gear you went up the hill. With the new power packages available the new wisdom is to go downhill in at least one gear lower. Since most of us don't have 500+ HP under the hood, going downhill in the same gear you went up the hill is still a good practice. I know I may have tread on some toes with what I have just written but what was a good practice in the past is not necessarily good practice today. If all things were equal, I would agree with the practice of long steady pressure for braking on downgrades. That was how I was trained to drive 35 years ago. But long steady pressure will create more heat than the new brake lining material can handle. You must give them an opportunity to "breathe". And that is why as a commercial driver trainer I now train newbies the stab/snub braking practice. Just my two cents worth on a Friday afternoon. Mark O. Castle Rock, WA | ||
| Tony LEE (T_lee)
Registered Member Username: T_lee Post Number: 4 Registered: 11-2007 Posted From: 58.170.198.24 Rating: N/A |
George, not having Jakes, and living in a country where you have to drive a long way out of your way to find ice on the road, I would be the last one to give practical advice on Jakes. Having occasionally driven vehicles with Jake and other types of engine braking, I was always a bit disappointed with their performance and regarded some of the claims I've seen on boards with a degree of scepticism - as comes out in your take on the subject. I was just emphasising the OP's comment "Although my coach has jakes I would like to understand the right weay to brake assuming the jake didn't exist." because regardless of what people actually do - either deliberately or in blissful ignorance -- conservative advice seems to be not to use engine brakes or cruise control when the roads are slippery. Then it becomes even more important to know how to drive and brake conservatively. Even here we do get very heavy rain and I recall a bus driver/mechanic relating the story of a coach pulling in complaining that the engine was stalling in heavy rain coming down the hills outside of town. As unlikely as it sounds, they put it down to using the Jake and having the wheels lock up in sections where there was a LOT of water running over a slippery surface.Once the wheels locked, the Jakes were enough to keep them locked even after the brakes were released. Tony | ||
| Gus Causbie (Gusc)
Registered Member Username: Gusc Post Number: 540 Registered: 11-2005 Posted From: 63.97.117.12 Rating: N/A |
Mark, Your points are well taken but my theory is that if you maintain the same average speed that the exact same amount of heat will be developed simply because the forces are the same. I like constant braking because that theory is that the brake pads will have a constant path of heat transfer to the drums and axles which have much greater mass and can cool much better than the enclosed pads. Since this should be a light pressure the heat generated is not that great. I've never had overheated brakes on my ancient Int 9670 with 80,000 lb gross or with my 4104 and we've crossed the Rockies three times in the past two years. Jake on the 9670 but not on the 4104. Every time you let the speed increase above the average it generates more heat to slow it back down again, simple physics. When I got my CDL in the '90s steady braking was recommended. Have brake linings changed since then? Do you have any written references on less heat tolerance by the new brake linings, I've never read this anywhere so I may be behind the times? We both agree about topping hills at low speed, that is the key no matter what braking method you use. Don't worry about stepping on toes, friendly disagreement and discussion is what this board is about. | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 297 Registered: 8-2006 Posted From: 76.168.69.233 Rating: N/A |
Mark, that was more than two cents worth, several bucks at least, AND the cost to value far exceeds the actual amount! Tony, what I threw out was food for thought, and I appreciate your comments. Can we think about your last paragraph? I know you were hearing the story second or third hand, and it would had to have come from someone not up on Jake brake operation. Please continue to humor me for a minute, and I will try to explain as well as Mark did above. As you know, Jake brakes operate by opening the exhaust valve at the top of the compression stroke, and releasing the compression pressure so it doesn't push the piston back down. Thereby energy is consumed in compressing the air in the cylinder on the compression stroke, and is not re-invested in the engine by pushing the piston down on the power stroke. Anyhow, in a mechanical engine, Jake operation is controlled by the dash switch, and then the buffer switch which actually feels the fuel rack. The buffer switch will only allow Jake operation when the rack is in the no fuel position, which only occurs when the driveline is turning the engine above the throttle setting. If a rear wheel slid, the engine would slow down, and the governor would move the rack to keep the engine at idle speed, which would turn off the Jake before the engine died. Now that I have taken it away, let me give it back. IF the buffer switch is not properly adjusted, the engine will die at idle, or it will put out black smoke during operation, because it is on while the governor is trying to have fuel injected. George | ||
| Don Evans (Doninwa)
Registered Member Username: Doninwa Post Number: 90 Registered: 1-2007 Posted From: 66.45.165.58 Rating: N/A |
Why I use stab braking. My problems with using constant pressure braking are: 1. How do you tell when you are using to much? Don't want to get to the point that brakes are fading to find out. 2. How do you keep from increasing pressure unconsciously? My bus does not have an application gauge yet, only a pucker factor gauge. When the hill gets unexpectedly steeper, the traffic gets heavier or the conditions of the road change the factor can increase quickly. 3. How many of us and newbies alike drive the bus enough to be really familiar with brake feel after driving thousands of miles and/or months in a car and then jumping in the bus? (I know, I know, I need to spend more time in the bus!) 4. Just can't see myself riding the brakes for mile after mile. Why I stab: 1. If I stab brake it would be more obvious to me when an application is not as effective as the first or most recent and easier to recognize that I need to do something to correct it. Not that I can do a lot about it with all of 4 forward gears. Gotta love those Spicers! Gotta get those Jakes too!!! 2. The length of time between stabs gives me a very good indication if I am in the right gear before brakes ever get warmed up. Works for what ever vehicle I find myself in, familiar or not. What ever grade, familiar or not. 4K roller skate or 80K semi. Nothing works better than starting downhill in a conservative mode. Works for me. YMMV Don 4107 | ||
| larry currier (Larryc)
Registered Member Username: Larryc Post Number: 148 Registered: 2-2007 Posted From: 207.200.112.41 Rating: N/A |
When you stab brake, oxygen gets between the shoes and the drums. It does not matter how old or new the shoes are. If you let oxygen get to the shoes they will smoke and fade. If you are lightly loaded or mt, stab braking is perfect. If you are loaded heavy and you try to stab brake you will gain speed everytime you let air to the shoes and have a runaway situation. Stab braking works for "bed bug" haulers running light. Constant pressure braking is the only thing that will work if you haul real freight. Most bus situations are closer to the bed bug hauler senario. I use constant pressure with my Eagle. To do this I have to carry alot of speed off the mountains or I will slow to the point that stab braking becomes manditory to keep from stopping. Each pass takes a different technique. Willamett Pass is flat half way down, then steep again. With a heavy truck, (no bedbugs), you need slow at top, fast speed before the flat spot, with constant pressure correct, you will be slow again just before the last half of the grade, ready to put some more heat to the drums without having let air get to the shoe surface so they don't start smoking or burning. If you can stab brake, you are not heavy. | ||
| Jerry Liebler (Jerry_liebler)
Registered Member Username: Jerry_liebler Post Number: 301 Registered: 3-2005 Posted From: 67.140.168.194 Rating: N/A |
Mark, You are simply WRONG. Lining temperatures will ALWAYS be HIGHER with snub braking. It has to do with physical laws of motion nothing else. The reason for snub braking is exactly as I said earlier. Brake fires happen from only one cause GOING TOO DAM FAST!!!!! regards Jerry 4107 1120 | ||
| Jerry Liebler (Jerry_liebler)
Registered Member Username: Jerry_liebler Post Number: 302 Registered: 3-2005 Posted From: 67.140.168.194 Rating: N/A |
George Todd, Check your facts an 8V71's Jakes can absorb over 500 hp that's more than even uncle sam gets out of their hot rod dual turbo versions. Regards Jerry 4107 1120 | ||
| Tony LEE (T_lee)
Registered Member Username: T_lee Post Number: 5 Registered: 11-2007 Posted From: 58.167.43.14 Rating: |
Perhaps those who are advocating constant pressure braking or talking about oxygen getting between the lining and the drum could do some research and give us credible sources that can justify recommending the practice over SNUB braking. I can't find any credible sources, yet there seem to be plenty advocating snub braking. I'm also not sure that STAB braking is the same as what is referred to by SNUB braking. To me, stab braking is very rapid on off application of the brake that is advocated under certain conditions and is similar to what occurs automatically and more controllably with ABS brakes. I wouldn't think this mode would be ideal for controlling a heavy rig down a long incline. I can't see that the size or weight of the rig makes any difference to what sort of braking method should be used. The key to responsible driving is to be in the gear that allows the engine to do most of the work so the brakes are there for "emergencies". For any weight, there will be a hill steep enough that a certain gear will need to be selected to control the speed using the engine and there will be a hill long enough and steep enough to cause runaway in too high a gear no matter whether you use snub, stab or constant braking As for Jake brakes absorbing 500 HP. The Jake system itself is not absorbing any power, it is the engine itself, so the theoretical limit is dictated purely by the engine configuration acting as a pure compressor. The power put out by an engine - say 300hp - is due to the burning of fuel which releases a lot of energy in a small space and creates a pressure to keep sustained pressure on the piston as it is forced down. The same size engine acting as a pure compressor is only absorbing whatever power is needed to compress pure air - surely less. As an example of the difference, years ago when VW Beetles were on their last legs, it was common to convert them into an air compressor by running two cylinders as an engine and converting the other two to act as a compressor. Ran pretty rough but worked well. Unless the two cylinders using petrol generated more power than the other two took to compress the air, the system wouldn't work. Still, Jakes do help a lot and I was quite disappointed to find that the switch marked Jacobs Brake on my panel wasn't actually connected to anything. Yes, the story of the coach stalling the engine was perhaps a load of rubbish, although once a wheel is skidding under heavy application of brakes resulting in aquaplaning (and this hill was literally a multi-killer until it was bypassed), the long time lag between letting off on the pedal and the physical releasing of the brakes, could (perhaps) cause the engine to stall. (Message edited by t_lee on November 18, 2007) | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 298 Registered: 8-2006 Posted From: 76.168.69.233 Rating: N/A |
Jerry, I just got my ribs taped from the cracks incurred reading the post above yours, the bruise on my behind from the fall will stop hurting in a while, and I'll be ok. (ROTFLMAO) I agree with you at least 99% of the time, and respect your knowledgeable opinions, but I would like to respectfully disagree on this one, for the following reasons. Under full fuel conditions, cylinder firing pressure is several times the compression pressure, right? Consequently, to my way of thinking, more torque is developed by the power stroke than is absorbed by the compression stroke, and its accompanying release of pressure at the top? Now, we have to think about what makes up horsepower, (the amount of work performed in a certain time.) I think we both agree that the same amount of work performed in a lesser amount of time would require more horsepower. I don't know if anyone has ever run a bus uphill at full throttle, in a gear that is not against the governor, and recorded the speed and elapsed time for several miles, then turned around and headed back down, to see if the Jake will hold the bus to the same or higher speed, in the same gear, which would indicate to me greater horsepower absorption than produced under power. I just can't cope with the scenario from down under, and I know that the poster wasn't the driver or the mechanic, and I'm NOT referring to him. To my way of thinking, a sliding pair of duals, while going downhill with Jake on, resulting in a dead engine, would bring on a no charge light, a low oil light, loss of power steering (if equipped) AND the Jake would drop out even if the buffer switch wasn't adjusted properly, due to low oil pressure, before the engine stopped entirely. So the bus in question went thru some water on the road and just slid along with a dead engine and both pairs of drive wheels stopped until the bus stopped? I think that mechanic believes that engines run hotter with no thermostats also! And here I always thought brake shoes smoked because they got hot from rubbing on drums! Regards, George MC6 7337 (I started on this before Tony's last post, so please forgive the duplication, he said it better than I did.) (Message edited by George Mc6 on November 18, 2007) (Message edited by George Mc6 on November 18, 2007) | ||
| Tony LEE (T_lee)
Registered Member Username: T_lee Post Number: 6 Registered: 11-2007 Posted From: 58.167.43.14 Rating: N/A |
"So the bus in question went thru some water on the road and just slid along with a dead engine and both pairs of drive wheels stopped until the bus stopped?" Yes, I agree that this would be unlikely but what if the driver had Jakes on and was heavily on the brakes when he hit deep water over many metres of road. Anyway, it is all a bit academic, especially for my situation, but it still remains that using Jakes in slippery conditions is said to be not a good idea, so obviously there must be some adverse consequences to not following the advice | ||
| Tony LEE (T_lee)
Registered Member Username: T_lee Post Number: 7 Registered: 11-2007 Posted From: 58.167.43.14 Rating: N/A |
In my continuing education, I found an interesting reason to use snub braking instead of continuous braking. From http://www.ntc.gov.au/filemedia/Reports/EngineBrakeSafeHartJan2006.pdf "There is apparently some basis to the claim that heavy vehicle drivers are sometimes booked for using the service brakes continuously whilst descending a steep hill. Service brake use is apparently taken as a sign that the driver did not select the appropriate gear at the top of the hill. (The brake lights coming on provide a convenient indication of service brake use)" That same site confirms that properly fitted Jake brakes can provide 100% braking power so I've learned something today. I suspect it only applies to turboed engines, but even so, they talk about holding speed on a 3.4% grade with a butterfly-valve type exhaust brake and a 2% grade on engine alone, so the improvement is significant. With a Jake engine brake the improvement is even more marked. (Message edited by t_lee on November 18, 2007) | ||
| Don & Lynn Tetreault (Lyndon)
Registered Member Username: Lyndon Post Number: 2 Registered: 10-2007 Posted From: 68.147.142.222 Rating: N/A |
With all due respect to previous posters, the idea that brake drums will get hotter under steady downhill braking pressure is utter nonsense. I saw the word "physics" used in this thread. Let's consider the physics. When climbing, extra power is required -- energy is put into the system -- to overcome the vertical component of the motion (namely, gravity). Since the engine has a limited amount of power, less energy is available for the horizontal component of the motion and the vehicle slows. The energy used for the vertical component is not lost, but rather stored in the form of the potential energy of gravity. The potential energy stored when climbing is released as the vehicle descends the other side of the hill. That energy must either be used up doing work -- going faster - or dissipated in the form of heat. Just like the engine has a limit in adding energy to the system, the brakes have a limit in removing energy from the system. Brakes drums are designed to dissipate heat over time, and the only factor determining whether (properly serviced) brakes are up to the task is speed. The amount of potential energy to be dissipated in the form of heat cannot be changed, only the rate of dissipation. The drums will be unable to dissipate heat quickly enough at too great a speed and WILL overheat, causing fade, failure and maybe fire. It will not matter what braking technique is used. With regard to "snubbing" as described previously, the brakes are forced to reduce vehicle speed with each application to remove additional energy gained (additional speed) from the system, dissipating greater heat in surges, than a steady application at a steady speed. At the bottom, both methods have dissipated the same amount of energy -- that's just physics -- but if I'm paying the maintenance bill, I'll take the steady application anytime. Ideally, of course, you're in the right gear for the Jake to dissipate all of the heat (through the engine's cooling system) and the brakes don't get used at all. No Jake? No problem with a steady application, as long as you start down in the correct gear. After 15 years of heavy loads, hills and mountains, let me assure you that I smoked the brakes just once (the result of mechanical failure affecting two of six axles) and it wasn't something you want to try twice. I never have (and never would) "snub" brakes on a serious grade. Of course, the typical bus weighs much less and leaves a more room for error, which probably explains the diversity of opinions here. Please be safe! Don | ||
| Don & Lynn Tetreault (Lyndon)
Registered Member Username: Lyndon Post Number: 3 Registered: 10-2007 Posted From: 68.147.142.222 Rating: N/A |
To put the excerpt from the article mentioned above into full context, it goes on to say: "...(The brake lights coming on provide a convenient indication of service brake use). Any regulation that bans service brake use on steep hills is inherently misguided because the natural retardation of the vehicle together with the engine compression brake retardation may be insufficient to control the speed downhill." http://www.ntc.gov.au/filemedia/Reports/EngineBrakeSafeHartJan2006.pdf (My italics). The statement sounds anecdotal at best, fails to mention jurisdiction (Australia, perhaps), and is clearly not anywhere near the Rocky Mountains. Nevertheless, the author is correct about any such regulation being misguided, but says nothing to support "snub braking." The other articles refenenced discuss snubbing as a compensation for various brake "imbalances" and problems. Running the mountains with brake "imbalances" and problems is dangerous and illegal in every jurisdiction. All references concede that the same amount of energy must be dissipated, either way. Do you want to risk heat surge damage? Overworking the compressor? Gaining more speed than you can control? Not me. Don | ||
| RJ Long (Rjlong)
Registered Member Username: Rjlong Post Number: 1328 Registered: 12-2000 Posted From: 67.181.166.160 Rating: N/A |
Tony - Since you're in Australia, you're probably not familiar with this road, but WB I-80 between Reno, NV and Sacramento, CA has nearly 70 miles of 4, 5 & 6% grades once you crest Donner Summit and start down. (You should be able to see this route using Google Maps or Google Earth.) This is a mountain freeway whose curves are designed for 70 mph traffic, except in a few spots, which are clearly marked. Anyway, the point I want to make here is that back in my charter days, I used to drive GMC 4106s, 4905s, MC-8s, MC-9s and MCI 102A3s over this route weekly, often loaded with 35 - 47 passengers (depending on the bus) and full baggage bins. All of the company's coaches were equipped with Jake brakes, both the stick-shift GMCs and the automatic-equipped MCIs. With the Jake brake on, every one of those coaches, fully loaded, would hold 60 mph in high gear down the steep grades without ever touching the service brakes. Which, of course, leaves a HUGE safety factor in an emergency, because your service brakes are cool. On the shallower grades (4%), I'd actually have to feather the throttle slightly to maintain speed, as the Jakes would continue to slow the coach. The 6V92-equipped buses would sometimes require a little light snub braking on the 6% grades, but the 8V71s never did. In snow/slush conditions, I'd keep the Jake off and use 2nd or 3rd gear to come down the hill - the same two I used to go up - to keep everyone on board safe. So Jakes work on a bus, and they work well. If everything's in good condition, they rarely fail. And if they do fail, you revert to Plan B - bring the speed way down and grab a lower gear. (I must add that in my 10+ years of driving charters, out of a 25+ - year career in the bus industry, I NEVER had a Jake brake fail on me - company had an excellent maintenance dept.) Highly recommended, IMHO. As for the snub vs continuous braking method, I know that here locally our CHP Motor Carrier folk (the guys who pay attention to the commercial fleets on our highways) about 15 years ago did a comparison study on what we call "the four lane", which is a stretch of CA Hiway 168 NE of Fresno. They took a truck loaded to 80K lbs and descended the grade using both braking methods, with the trucks all instrumented to see "what's what". At the end of the day, at the bottom of the grade, the temperatures of the brake drums were the same, regardless of the method used. Now, there are some caveats here. First, the truck was equipped with a Jake brake, so that's a factor. Second, both braking runs were done in a gear one higher than used to climb the hill (a 6% grade, btw). Third, they used a 5 mph "window" for the snub braking - i.e. they only allowed the truck to gain 5 mph before bringing it back down to the target speed. Obviously, the larger speed "window" used, the greater effort must be utilized to snub brake back down, and the greater the heat generated. As for me, I prefer using the Jakes with light, gentle snub braking within a 5 mph window on dry pavement, if necessary. In the slushy stuff, it's Jakes off/correct gear/light gentle snub braking. I say gentle snub braking because I'm used to hauling passengers, not freight, and I don't want to send somebody sprawling. Everyone has their own opinion, which leads to spirited discussions like this one, which is good, because folk learn from it. If you want to read a very spirited discussion about brakes, here's a good one - be sure to follow the extra links in the thread as it progresses: http://www.busconversions.com/bbs/index.php?topic=1489.msg12432#msg12432 FWIW & HTH. . . | ||
| Richard Bowyer (Drivingmisslazy)
Registered Member Username: Drivingmisslazy Post Number: 2013 Registered: 1-2001 Posted From: 75.108.83.178 Rating: N/A |
quote "Jake to dissipate all of the heat (through the engine's cooling system)" Don, I have always heard that the heat is not dissapated thru the cooling system. In actual experience my DD always ran much cooler on the downhill side while using the Jakes. Am I in error? Richard | ||
| joe padberg (Joemc7ab)
Registered Member Username: Joemc7ab Post Number: 167 Registered: 6-2004 Posted From: 66.38.159.33 Rating: N/A |
Richard It runs cooler because you are adding very little fuel to the mix. Joe. | ||
| Don & Lynn Tetreault (Lyndon)
Registered Member Username: Lyndon Post Number: 4 Registered: 10-2007 Posted From: 68.147.142.222 Rating: N/A |
This is a great discussion, because many subtle, but important, points have been addressed by people with many years of experience. My interest in this important topic has led me to wonder where this idea of snub braking has become so prevalent when the billions of miles of combined mountain experience of so many of us "old timer" truckers says otherwise. As near as I could find, the source is a Technical Report contracted by the US Federal Highway Administration in 1992. So I downloaded and read the original report -- try http://deepblue.lib.umich.edu/handle/2027.42/960?mode=full -- and came to the conclusion that the FHA should recommission some extensive new studies. This report has several problems, contradictions, obsolete methodology and subjective conclusions. Let me list a few: 1. Two trucks, one stretch of mountainous highway; not exactly a large sample space. 2. After being careful to inspect and test all braking systems, the brakes on one truck were "smoked" when trying to establish baseline speeds. Any subsequent measurements using those drums/pads is highly suspect and could have affected the results. 3. Two thermocouples gave different readings on the same drum! Assumptions are made as to the reasons, which could have affected the results. 4. The use of automatic slack adjusters could have affected the results. (This is considered and discounted as a factor). 5. While application air pressure is considered, supply pressure is not. This could become a critical issue if snubbing is excessive. 6. Compressor temperature is not mentioned and possibility of compressor heating/overheating is not considered. Again, a critical issue is ignored. 7. No measurements were taken of stopping distances at the peak speed when snubbing vs. the lower speed when dragging (this one should be obvious, of course, but would seem to be an important consideration). Stopping distances were measured after 3/4 descent, however, and one important conclusion seems to be buried at the bottom as insignificant: "In some circumstances, with combined misadjustment and imbalance, the snubbing strategy was found to lead to hotter brakes for the misadjusted brakes, which in turn leads to an additional increment in stopping distance after a mountain descent." This little fact does not seem important enough to mention in the recommendation to update the wording of the CDL text. At the end of the day -- or bottom of the hill, in this case -- wouldn't the ability to stop in the shortest possible distance be a primary goal? 8. The authors state: "The average temperature per 100lb of brake drum is practically equivalent whether the light dragging or snubbing strategy is used for controlling the speed of heavy trucks on long steep downgrades." That is true, on average, but may not be true at some critical point when the driver needs to stop, especially if that point is near the peak speed when snubbing. Looks to me like some cubicle dwellers might have been a bit quick to accept some highly debatable conclusions. I would love to see an extensive new study, using modern instrumentation such as infrared temperature measuring, GPS, computer controlled braking (remove the human factor), etc. Don PS, It's important to remember that the "dragging" technique is relevant only in the absence of a Jake, or when the Jake cannot hold the vehicle back. If the vehicle speed is stable or decreasing, a driver (obviously) would release the brakes, as with the stabbing technique; I think this is what rjlong is correctly describing above. | ||
| Don & Lynn Tetreault (Lyndon)
Registered Member Username: Lyndon Post Number: 5 Registered: 10-2007 Posted From: 68.147.142.222 Rating: N/A |
"Don, I have always heard that the heat is not dissapated thru the cooling system. In actual experience my DD always ran much cooler on the downhill side while using the Jakes. Am I in error? Richard" Good point, Richard, and you are correct. I didn't mention the exhaust temperature, which would be a big factor using a Jake, energy converted to motion, and the possibility that your speed may be a bit higher on the downhill side, providing additional air cooling and drag. In any event, all of the energy has to be accounted for: 1. Engine brake dissipates heat; 2. Service brakes dissipate heat; 3. Drag and rolling resistance (again, energy lost in the form of heat, but you probably aren't going to need a shuttle heat shield at highway speeds); 4. Potential energy is converted to kinetic energy (speed is maintained or increases without adding fuel energy). Some combination of these 4 must add up to the total energy lost in the descent. Don | ||
| Jerry Liebler (Jerry_liebler)
Registered Member Username: Jerry_liebler Post Number: 304 Registered: 3-2005 Posted From: 67.141.39.247 Rating: N/A |
All, Jake brakes dissipate heat out the EXAUST as very hot air. Remember that air was heated by compression and was hot enough to rapidly burn Diesel fuel. The reason a jake often can produce more braking horsepower is the energy released by the jake is normaly only stored and returned, much like a compressed spring stores energy. This normal energy flow in and out of the compressed air in a cylinder is quite independent of the energy added by fuel's burning. The fuel burn adds energy that must be taken out as useable power and waste heat. I once saw actual performance data which showed 500 hp braking for an 8v71 at 2100 RPM, unfortuntely the site I bookmarked no longer exists. Regards Jerry 4107 1120 | ||
| Gus Causbie (Gusc)
Registered Member Username: Gusc Post Number: 542 Registered: 11-2005 Posted From: 63.97.117.12 Rating: N/A |
Nothing here has convinced me that constant braking is not the best method and Don's posts make me even more positive. He did the best research. Why the Jake brake discussions? The original question clearly excluded Jakes?? This just confuses the whole issue and doesn't help Tim at all. | ||
| Don Evans (Doninwa)
Registered Member Username: Doninwa Post Number: 92 Registered: 1-2007 Posted From: 66.45.165.58 Rating: N/A |
Don T "That is true, on average, but may not be true at some critical point when the driver needs to stop, especially if that point is near the peak speed when snubbing." If I snub brake and use 5mph above and below the speed that I would use with constant pressure for the same average speed down the hill and approximately the same total heat generated, I have a 50% chance of being at the same or lower speed should a sudden stop be needed. If at 5mph below the average speed, I might be able to stop quick enough to avoid a problem when someone at the average speed may not. You pays your money, you takes your chances. To ALL. I plan on traveling far and wide with the new bus. There is no way for me to know the profile of every grade to know how much pressure I would need to use constant braking. The time between snubs tells me all I need to know about how steep the hill really is and how my rig is handling it. In reality with my bus (4107) and jakes I should not need much brake. On the trip home (empty shell) sans jakes I went down most of the hills North of Grants Pass on I5 in 4th with little need for brakes. When you are pushing a brick through the air drag increases with the square of the speed. Once the bus is finished and loaded up, and jakes installed I will laugh at 6% grades. Bottom line, if you are anywhere close to overheating you brakes anytime, you are pushing to hard. Don 4107 PS: Snowing here! Better go turn those jakes off!!! | ||
| Laryn Christley (Barn_owl)
Registered Member Username: Barn_owl Post Number: 92 Registered: 10-2006 Posted From: 72.66.180.128 Rating: N/A |
"AND the Jake would drop out even if the buffer switch wasn't adjusted properly, due to low oil pressure," I have read over several months now that Jake brakes can cut out due to low oil pressure. I am not a Jake expert, but while I was fixing mine, I would activate them with a 12v jumper while at idle and they would work. My bus seems to have low oil pressure compared to some other 8v71s I read about, so I have concluded from my experience, that the buffer switch seems to be the sole controller of the jakes. Am I wrong? Where did the low oil, Jake cut out thing start anyways? I haven’t read anything about it in the Jake publications that I have seen. Could someone post a link? Thanks, Laryn (Message edited by barn_owl on November 18, 2007) | ||
| Don & Lynn Tetreault (Lyndon)
Registered Member Username: Lyndon Post Number: 6 Registered: 10-2007 Posted From: 68.147.142.222 Rating: N/A |
Don E. I always seem to agree with your "bottom lines". (Just not what comes before!) But it's worthwhile discussion and I hope we all learn something. Don T. | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 301 Registered: 8-2006 Posted From: 76.168.69.233 Rating: N/A |
Laryn, Jake brakes work by using engine lube oil pressure to push the exhaust valve open against its spring. There is no oil pressure cutout switch, or an exact set pressure, when the oil pressure drops to the point where it can't overcome the valve spring tension, the Jake will just stop operating. Your test probably killed your idling engine before the pressure dropped enough for it to become ineffective? George | ||
| Richard Bowyer (Drivingmisslazy)
Registered Member Username: Drivingmisslazy Post Number: 2014 Registered: 1-2001 Posted From: 75.108.83.178 Rating: N/A |
With the engine hot, my oil pressure at idle was about 5 pounds. Not enough to operate the oil pressure actuated solenoids that open the exhaust valves to provide Jake operation. I had to adjust the idle up to about 800-1000 rpm to get enough oil pressure to actuate them and test the Jake operation. I suspect that many of the older DD's are in similar worn condition and the idle oil pressure is not enough to actuate the Jakes. Richard | ||
| Buswarrior (Buswarrior)
Registered Member Username: Buswarrior Post Number: 1112 Registered: 12-2000 Posted From: 76.68.122.192 Rating: |
Oh Boy! Please remember folks, in trying to help others, What are the consequences of an inexperienced driver getting it wrong? Those of us with many miles under us, know what we mean, those with none, our words might as well be a foreign language. "Use the same gear going down as you used going up" does not help a new person in any way, and you are only repeating the same ineffective instructions of the person who tried to train you. It doesn't help someone who hasn't climbed the hill yet!!! None of us had a clue what was meant by this the first time we heard it, and neither does a new driver today. It means something after some experience, if you live to reach an understanding... More recent recommendations for training techniques are trying to put the novice in a safer position to discover the realities. There is more than one safe way to get down a grade, and there is more than one way to help a new person do it without damaging their brakes and safely. Who taught us to drive? Who are we, the warriors of the highways? It is still left to less than properly trained trainers to try and pass on the laws of physics, when none of us were any good at that science! More useful contributions might answer these questions: "How do I know I've picked the right gear?" "How do I know I've picked the right speed?" "What do I do if I've picked the wrong gear/speed?" "Where might I find the evolving research as time goes by?" There are no laws/regulations on the continent that I know of that specifically regulate that mechanics, on performing a regular inspection, must check cracking pressures of relay valves or check brake application timing and pressures, which will dramtically affect a steady light application pressure on a long downhill grade. In other words, there are no laws or regular maintenance investigations or procedures to combat the imbalanced conditions that are commonly found on North American heavy vehicles, that advocates of snub braking are trying to defend against. Many mechanics won't even know what you mean if you ask them at what pressure a relay valve will activate. And if they do, they won't have the tools to measure it. Those brave souls who are doing European style brake system maintenance research are scaring themselves silly with the variations found in North Amercian accepted maintenance practices, in brake replacement techniques, and in the performance of new from the box brake valves. Reality is, until proven otherwise, do not expect your bus conversion, or tractor trailer, to apply the intended pressures under a light brake application between all the axles. The foot valve, the relay valves, the mismatched piping, fittings, hoses, brake chambers, brake linings, drum surfaces, all have variables that common North American maintenance practices ignore. Never mind those previous owners, or ourselves, who do unorthodox things in component replacement or maintenance! Snub braking has become the preferred recommendation to combat the reality of the less than perfect performance of the braking systems and their maintenance as we find them today out on the road. NOT in the labratory or high school science class. And it has a built in evaluation for a new driver to check on their decision making: off the brakes for many times longer than you were on the brakes to slow it down, with your firm snub. Otherwise, slow down some more. Choose too high a speed using steady pressure braking and how will you know? And then what? If we are able to confirm that the air is actually distributed as intended to all wheels, and that the friction that is intended is really generated at all wheels, the brake linings having not been destroyed by some previous shenanigans or under use storage.... And we figure out a way to explain to a new driver how to figure out the safe speed to descend a grade, the first time, and every time afterward...and it works.... I am sure that steady pressure braking on downhill grades will be the method employed as Jesus is driven down the mountain in the Second Coming, no doubt, aboard a classic bus conversion! However, until that day, I will, for the sake of the safety of new folks, advocate snub braking. happy coaching! buswarrior | ||
| Tim Brandt (Timb)
Registered Member Username: Timb Post Number: 90 Registered: 10-2003 Posted From: 74.244.14.221 Rating: N/A |
Thanks again to all who have responded...I must admit I was waiting to see what RJ and Buswarrior would weigh in with. Rest assured being overly cautious is my goal until I'm comfortable with the coach | ||
| John MC9 (John_mc9)
Registered Member Username: John_mc9 Post Number: 548 Registered: 7-2006 Posted From: 66.217.101.130 Rating: N/A |
"[If we] ....figure out a way to explain to a new driver how to figure out the safe speed to descend a grade, the first time, and every time afterward..." From experience? This may sound boring, but..... The posted speed limits can be used to determine the safest speed to navigate the roadway during: -poor weather conditions -through turns -on steep downhill slopes The posted limits will provide the absolute safest -maximum speed- you can travel under those conditions. I never used "snubbed braking", nor ever instructed any new driver to use that method. Why? Because an inexperienced driver may "snub" too often, losing too much air pressure. One former school bus driver, on a charter using a fishbowl, lost his air coming down Hunter Mountain (upstate NY), and ran into the rear of the Eagle he was following (no severe damage; the Eagle held him back fine) (Intercounty Motor Coach; 1960s) The driver claimed he was trying to slow the bus on the slippery surface, by stabbing at the brakes, rather than use constant pressure. (The bus was an automatic) When he lost his pressure, he said he hit the "emergency stop" button, assuming that would stop the bus! Holding the bus to the posted speed limit by using a low gear and some constant brake pressure, can make for a safe descent. If you're going to speed, all bets are off. | ||
| niles steckbauer (Niles500)
Registered Member Username: Niles500 Post Number: 804 Registered: 11-2004 Posted From: 72.91.168.92 Rating: N/A |
FWIW - if you have to "stab" or "snub" the brakes more than once on any downhill run "you are traveling TOO fast" - this is true no matter if you have jakes or not - this is a "rule" not a matter of "experience" or "opinion" - quit giving YOUR "experience" or "opinion" - the "rule" works for every situation - if this does not make sense to you, it does not apply - BUT DO NOT LET THE NEWBIES BE MISINFORMED AND THEREFORE AT RISK BASED ON YOUR PAST AND QUALIFIED EXPERIENCE - HTH | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 302 Registered: 8-2006 Posted From: 76.168.69.233 Rating: N/A |
BW, Descend a hill in the same gear you ascended it in, or would have is pretty straightforward! "It doesn't help anyone who hasn't climbed the hill yet." Well how the heck does a newbie get to the top of the hill in a bus he is going to drive down, if he doesn't go up it? I think you have been smelling too much brake shoe smoke, sorry. | ||
| Don & Lynn Tetreault (Lyndon)
Registered Member Username: Lyndon Post Number: 7 Registered: 10-2007 Posted From: 68.147.142.222 Rating: N/A |
The good news is the one thing everyone seems to agree on, which is starting your descent slowly, conservatively if you like, especially when you are unfamiliar with your vehicle's response or the nature of the grade. That's good, because it's the most important thing. How slowly? If we are told that the long-standing rules-of-thumb, those being "same gear" -- or with current big powered engines, "one gear lower" than the gear required to climb it -- is not such a good rule for newbies, what then? Real slow? Slower than real slow? Let me stick my neck out and provide a number or two. I cannot think of a long 8% grade where 20 m.p.h. would get anyone in trouble, assuming ideal weather, no overload, and all brakes adjusted and working properly. So there's a starting point I wouldn't recommend starting down any 6%, longer than a mile or so, at greater than 30 m.p.h. I think these are conservative numbers. Someone may think they are not conservative enough. Others will say they are "silly-low". So I'm sure I'll start a big debate (which is why I think I'm sticking my neck out and probably why we have rules-of-thumb). This thread started with the assumption that "the Jake didn't exist". Tim makes a wise assumption; all it takes is a broken wire or bad switch. So conservative from the top is best, even with a Jake. What gear? From your starting speed you will want to choose the gear which allows your RPM to be within the normal operating range of the engine, preferably toward the high end (typically 1800 or so, which still gives a bit of head room). You will get maximum braking effect from the engine at higher RPMs, even without a Jake. Remember that its possible, but extremely difficult, to downshift once you are rolling down a steep grade. That's why you want to slow down and select a lower gear before starting the descent. Upshifting is easier, but still dangerous. If you miss a gear, your are coasting! Save it for near the bottom. (Imagine trying to brake, clutch and adjust your revs all at once, halfway down a long grade). What if you started out in the wrong gear? Too high a gear? Try to stop on the shoulder before your brakes get too hot and fade. Wait until they are cool enough to touch and start down in the right gear. Some manuals recommend trying to brake down 10 or 15 mph and drop a gear, but I would recommend that only in desperation because it's pretty hard to catch that gear! Better to stop if you can. Too low a gear? Stay as much to the right as you can, turn on the 4-ways and enjoy the scenery until you can see the bottom, unless you are very comfortable with your shifting abilities. So that leaves us with braking technique. Here's the other good news. As outlined previously, either method of braking will generate nearly the same amount of heat, on average, so all things are nearly equal! Well, not quite, but I think I've already made my case for the the nice, light, steady application (another long-standing rule-of-thumb we are asked to ignore). Just a little aside here: It been suggested that snubbing will somehow help you decide if you have chosen the correct speed (yikes!). I just don't get it. If you are descending much too slowly, I don't think technique matters -- it would be logical to ease up or come off the brake. If you are descending much too fast, the first or second 3-second "snub" application will generate heat so fast, you will see smoke in your mirrors (I've seen it more than once, but only once in my mirrors). Worse, after recommended 6-second release you will be going EVEN FASTER! On the other hand, too fast with the drag technique, you might see smoke before reaching the bottom, but I wouldn't wait that long to try to get stopped and cool things down. Either way. So take a good look at your options. Consider the wisdom of some 80 years of trucking experience, passed down from one old-timer to another. (I learned in the '70s from some real high-milers before running well over a million of my own). Consider that braking systems are highly- engineered and well-balanced, when serviced properly. (And I can guarantee you there are laws against running down the road without properly serviced brakes! For example, fines for the very common slack adjusters out-of-adjustment are handed out like candy in mountainous regions like mine.) Consider that snub braking has not become "the preferred recommendation" everywhere. (I can't find it mentioned anywhere here in the Alberta or in the B.C. professional driver manuals. These provinces straddle the Canadian Rockies, where safety standards are very high!). Finally, even if you don't like science, you can't ignore physics when it comes to mountain descents. Don | ||
| JC Alacoque (Jc_alacoque)
Registered Member Username: Jc_alacoque Post Number: 51 Registered: 7-2006 Posted From: 74.127.205.107 Rating: N/A |
This thread is way too academic. Make sure the vehicule you are driving is in good mechanical repair (it is your legal duty). Whether it be a commercial bus or your private conversion. If you're not comfortable driving it, go out and practice, driving conservatively. Practice shifting it untill it is second nature. Uphill and downhill. Get a good feel for the brakes. If you can't get there, bus driving is not for you. Once you are one with your bus, through the seat of your pants, you will know how to descend a grade safely. Only the idiots that drive too fast get into trouble (drive off the road). And they are in the great minority. | ||
| RJ Long (Rjlong)
Registered Member Username: Rjlong Post Number: 1329 Registered: 12-2000 Posted From: 67.181.166.160 Rating: N/A |
Think we also need to briefly discuss the difference between "stab" and "snub" braking. Personally, I think of "stab" braking in the same vein as "pumping the brakes" - rapidly on/off. Excellent way to exhaust the air supply and leave you in "Oh S**T!" mode. "Snub", to me, is a gradual application sufficient enough to keep the vehicle within the 5 mph speed window I've established, then release, allowing the speed to creep back up. If you're in the correct gear, this doesn't happen very often. Some vehicles, especially 18-wheelers, have application pressure gauges on the dash, so you know how much pressure you're applying. Most buses don't, unless installed by the owner. JC makes a very good point: "If you're not comfortable driving it, go out and practice, driving conservatively." Amen, brother! This has been a great thread! And thanks, Buswarrior, for some "reality check"! FWIW & HTH. . . | ||
| Tom Caffrey (Pvcces)
Registered Member Username: Pvcces Post Number: 1148 Registered: 5-2001 Posted From: 65.74.67.83 Rating: N/A |
I'm with the snub braking crowd. It's in the CDL manual in Alaska, but it wasn't until the steady method was published and a great big furor was created over it. One other way to look at snub braking is that it causes the iron skin on the inside of the drum to get to a higher temperature while snubbing. Then, during the time the brakes are released, there is some air circulation between the shoes and the drums. This surface heat then penetrates the rest of the drum. The amount of iron and the amount of braking are what control the drum temperature, which in turn, controls when the brakes will fade. I think that this combination will get rid of a little more heat than a steady application would. However, I think the big benefit to snub braking is that you can measure, through the seat of your pants, how well the brakes are slowing your rig. With a steady application, you don't have as good of a feel for how the brakes are doing. I think that you could get the brakes dangerously hot without realizing it with a steady application. Stopping under those conditions might prove nearly impossible, so I'm sticking with the snub braking method. For what it's worth. Tom Caffrey PD4106-2576 Suncatcher Ketchikan, Alaska | ||
| John MC9 (John_mc9)
Registered Member Username: John_mc9 Post Number: 549 Registered: 7-2006 Posted From: 66.217.103.49 Rating: N/A |
Hmmmm..... Re: ""Snub", to me, is a gradual application sufficient enough to keep the vehicle within the 5 mph speed window I've established, then release, allowing the speed to creep back up. If you're in the correct gear, this doesn't happen very often. " That's essentially what's done, with the "constant pressure" method! The brake is applied while in the correct gear for the downgrade, and the brake pressure is regulated for the amount of braking needed to keep the bus within the speed tolerance (window) desired. Only a slight "hiss" of air release is heard. The amount of pedal pressure changes, according to how much braking is needed.. Fully releasing the brakes, then re-applying them, would be the same as "stabbing" the brakes, regardless of how soft the application is. It kinda' sounds like this debate is an "Apple & Oranges" thing. The main point to any "newbie", is to observe the speed, and never outdrive your stopping distance. It may sound corny to say that the posted limit should be observed, however.... Even if you desire to ignore the posted limit during "good times", when the weather's bad, or you're driving on an unfamiliar road, if you stay to the posted limits on the turns and downgrades, you will -never- have a problem. How you keep to the limit, is your choice. Spend some time driving your bus; driving that 40', 11 ton vehicle should be second nature to you, prior to a major trip. Cheers! | ||
| Gus Causbie (Gusc)
Registered Member Username: Gusc Post Number: 544 Registered: 11-2005 Posted From: 63.97.117.11 Rating: N/A |
niles, Experience and opinion are hardly the same thing. I go with experience every time and opinion hardly ever. There is far too much opinion posted on these boards and not nearly enough experience. Who made these rules?? I wasn't even aware of any rules concerning braking? After reading these great posts it appears to me that both systems work very well. Your whole post appears to me to be your personal opinion. I fail see how you have any authority to tell anyone here what they can or cannot post. There is some good stuff here and anyone is free to read take or leave any of the advice. Just because a person is a bus newbie doesn't mean he is a complete dummy and can't do his own thinking! |
