Braking Methods..
07-28-2007 | 04:20 AM
#16
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Joined: Apr 2003
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From: Cummins Technical Center, IN
NO NO NO!!!! Ahh theoritical Physics - why God invented engineers
First, your statement of constant brake pressure is only true if the downhill grade remains constant - which it doesn't. Downhills are not uniform in grade, nor are they straight - both an impact to braking depending on load and balance. Constant brake pressure may keep you at 25mph for part of the time but it will surely bring you to a stop OR speed you up as changes in grade occur. Grade is an average.
Second, you assume that the braking ability on all wheels is the same. In reality this is not true. Every rotor and pad or drum and lining are unique with different braking properties after usage for awhile. For example, a rotor or drum with a hardspot will generate far higher temperatures and be much more susceptible to fade (as temperatures increase over optimal - typically higher than 450F) and reduced braking efficiency. You want all of the brakes working some of the time, not some of the brakes working all of the time.
Third, the engine is a brake that dissipates temperature build up much more efficiently than the rotors and pads on your wheels. There is a reason downhills have signs all over the place "Trucks use lower gear". Watch the pros. The trucks are in lower gears and the brake lights go on and off. They'll typically drop around 5 mph and repeat the cycle. This is called snub braking and it is now the current teaching in the CDL manual, having replaced steady application of brakes. This is specifically true with pneumatic brakes and large trucks and buses. The PA DOT also recommends use of snub braking by schoolbuses in their training manual.
Fourth, higher speed provides more cooling airflow on the wheel brakes dissipating the temperature buildup faster than at a slower speed, particularly disc brakes. Snub braking became the recommended method of downhill braking after tests by the University of Michigan Transportation Research Institute showed that controlled or snub braking resulted in the same average brake temperatures for well balanced brakes and snub braking in poorly balanced brakes resulted in more uniform brake temperatures.
Fifth, putting a trailer on the back of your pickup changes the entire equation. Trailer brakes are not the same design as the Dodge brakes nor are they sensitive to pressure changes on the brake pedal (at least in what I tow). My trailer brakes are electric and either on or off unless I change the Tekonsha settings. They are also drum brakes which dissipate heat more slowly than the brakes on my truck. If they fail, I will very likely exceed the braking performance designed into the truck in a long steep downhill (which is why they make run-offs).
First, your statement of constant brake pressure is only true if the downhill grade remains constant - which it doesn't. Downhills are not uniform in grade, nor are they straight - both an impact to braking depending on load and balance. Constant brake pressure may keep you at 25mph for part of the time but it will surely bring you to a stop OR speed you up as changes in grade occur. Grade is an average.
Second, you assume that the braking ability on all wheels is the same. In reality this is not true. Every rotor and pad or drum and lining are unique with different braking properties after usage for awhile. For example, a rotor or drum with a hardspot will generate far higher temperatures and be much more susceptible to fade (as temperatures increase over optimal - typically higher than 450F) and reduced braking efficiency. You want all of the brakes working some of the time, not some of the brakes working all of the time.
Third, the engine is a brake that dissipates temperature build up much more efficiently than the rotors and pads on your wheels. There is a reason downhills have signs all over the place "Trucks use lower gear". Watch the pros. The trucks are in lower gears and the brake lights go on and off. They'll typically drop around 5 mph and repeat the cycle. This is called snub braking and it is now the current teaching in the CDL manual, having replaced steady application of brakes. This is specifically true with pneumatic brakes and large trucks and buses. The PA DOT also recommends use of snub braking by schoolbuses in their training manual.
Fourth, higher speed provides more cooling airflow on the wheel brakes dissipating the temperature buildup faster than at a slower speed, particularly disc brakes. Snub braking became the recommended method of downhill braking after tests by the University of Michigan Transportation Research Institute showed that controlled or snub braking resulted in the same average brake temperatures for well balanced brakes and snub braking in poorly balanced brakes resulted in more uniform brake temperatures.
Fifth, putting a trailer on the back of your pickup changes the entire equation. Trailer brakes are not the same design as the Dodge brakes nor are they sensitive to pressure changes on the brake pedal (at least in what I tow). My trailer brakes are electric and either on or off unless I change the Tekonsha settings. They are also drum brakes which dissipate heat more slowly than the brakes on my truck. If they fail, I will very likely exceed the braking performance designed into the truck in a long steep downhill (which is why they make run-offs).
To your first point, well it's hard to disagree with the observation that hills aren't ramps and actually vary in inclination.
To the second, yes, brake proportioning is something that you're stuck with. Until manufacturers start offering smart brake proportioning that adjusts braking power to available traction. I didn't assume that it's the same at each wheel-- rather that it's CONSTANT. Big difference. Braking power obviously varies with the load on the tires (weight), hence pin weight and such affect available braking power because braking power is related to traction and load. And you want ALL the brakes working ALL the time, because when the workload is evenly distributed and drawn out over time, the heat loading of the wheel brakes will never exceed design spec-- hence, we validate the assumption.
ON the third point, obviously diesels have very little engine braking because of the lack of a throttle plate. You assume that engine braking is a considerable factor, when if it really was all that effective, we wouldn't be having this discussion because the wheel brakes aren't your primary method of losing speed. I mean, would you apply the brakes without lifting off the loud pedal? I assumed that the typical driver is smart enough to maximize engine brake FIRST, though a diesel without an exhaust brake has less retarding power available.
On the fourth point, yes faster speeds cool the brakes more, but the increase in energy the the brakes have to dissipate because of the higher speed MORE than offsets the additional cooling available. On NASCAR cars and other vehicles with purpose-built brake cooling ducting, the airflow at speed adds considerable cooling to the brakes, and there is VERY high speeds involved. The additional cooling to the brakes at higher speed is just not a considerable factor compared to the heat load of maintaining 20mph down a 10% grade compared to maintaining 50mph down that same grade.
On the fifth point-- get a Brakesmart so you can have smarter trailer brakes.
While I respect engineers, keeep in mind that professional expert engineers built the Titanic, filled the Hindenburg with hydrogen, and designed the o-rings in the boosters on the Space Shuttle Challenger. At one point nuclear power was just the theory of a mad German scientist, and all the "experts" said it couldn't be done.
JMO, take or leave as you see fit.
jh
07-28-2007 | 07:36 AM
#17
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From: New Hampshire
Just kidding, my formal background is theoritical but my experience since is practical. Yup engineers can mess up royally and will continue to do so forever - comes with pushing theoretical limits - sometimes with truly unfortunate consequences. But I'm always up for an engaging discussion.
Two follow-up points. First, our trucks don't have dynamic brake proportioning. You push the pedal and move an incompressible fluid to each wheel. If you have a hard spot on one of your front rotors, that rotor can heat up to 1000F rather than hover at the 450F level on a long downhill. Under constant pressure, that forces the other rotor to assume more of the braking responsibility, thus raising its temperature higher than it should be also. Overall braking effiency is reduced and you now have to push harder on the brake pedal to maintain status quo. Empirical evidence shows that snubbing actually better equalizes the brake temperatures on vehicles that do not have equal braking on each wheel - which covers about 99.99% of us consumers. Don't forget that engineers have designed consumer brakes for longevity and low cost, not necessarily braking efficiency on long downhills and so the tradeoff is fade at higher temperatures.
Second, comparing our brakes to racing brakes isn't a good comparison. NASCAR brakes are made with different materials that have different coefficients of friction. NASCAR brakes have to be heated up to reach maximum braking efficiency - they are designed and built to operate at much higher temperatures - they do not bite until they reach operating temperature. Consumer brakes are designed to bite cold and operate at much lower temperatures.
regards,
ajg
Two follow-up points. First, our trucks don't have dynamic brake proportioning. You push the pedal and move an incompressible fluid to each wheel. If you have a hard spot on one of your front rotors, that rotor can heat up to 1000F rather than hover at the 450F level on a long downhill. Under constant pressure, that forces the other rotor to assume more of the braking responsibility, thus raising its temperature higher than it should be also. Overall braking effiency is reduced and you now have to push harder on the brake pedal to maintain status quo. Empirical evidence shows that snubbing actually better equalizes the brake temperatures on vehicles that do not have equal braking on each wheel - which covers about 99.99% of us consumers. Don't forget that engineers have designed consumer brakes for longevity and low cost, not necessarily braking efficiency on long downhills and so the tradeoff is fade at higher temperatures.
Second, comparing our brakes to racing brakes isn't a good comparison. NASCAR brakes are made with different materials that have different coefficients of friction. NASCAR brakes have to be heated up to reach maximum braking efficiency - they are designed and built to operate at much higher temperatures - they do not bite until they reach operating temperature. Consumer brakes are designed to bite cold and operate at much lower temperatures.
regards,
ajg
07-28-2007 | 09:57 AM
#18
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From: Phoenix
I was the first person to use the brakes in NASCAR as an example for regular stopping. When Awol posed the question of if it was better to apply a constant pressure to the brakes or more of a pumping action, I simply used the NASCAR example to clearly, and accurately show that when applying breaks at a constant pressure, the rotors on the 24 would glow orange. However, when the 48 applied the breaks in a more aggressive, pump of the breaks, the rotor did not experience as much heat build up, I said heat, not temperature.
Regardless of what type of components were in installed, aside from the fact they are a purpose built system, the systems on the two cars could, for all practical purposes, could be considered identical, pads, calipers, rotors, and cooling ducts. The stopping power, component type, and longevity of the racecar system was never compared to the systems on our trucks, however, a correlation between stopping style and heat build up was made. I stand by my observations because the vehicles were both doing the same amount of stopping work, but it was done in different methods. One method of doing the work produced more heat, while the other probably produced higher temperatures.
Regardless of what type of components were in installed, aside from the fact they are a purpose built system, the systems on the two cars could, for all practical purposes, could be considered identical, pads, calipers, rotors, and cooling ducts. The stopping power, component type, and longevity of the racecar system was never compared to the systems on our trucks, however, a correlation between stopping style and heat build up was made. I stand by my observations because the vehicles were both doing the same amount of stopping work, but it was done in different methods. One method of doing the work produced more heat, while the other probably produced higher temperatures.
07-28-2007 | 01:14 PM
#19
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From: New Hampshire
I was the first person to use the brakes in NASCAR as an example for regular stopping. When Awol posed the question of if it was better to apply a constant pressure to the brakes or more of a pumping action, I simply used the NASCAR example to clearly, and accurately show that when applying breaks at a constant pressure, the rotors on the 24 would glow orange.
07-28-2007 | 01:28 PM
#20
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From: Phoenix
AJG, I know where you were going with the comment and I took no absolutely no offense. I just wanted to clarify, like you did, that the mention of the NASCAR braking systems seemed to take a different turn and started comparing components instead of comparing a scenario.
07-28-2007 | 01:45 PM
#21
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From: New Hampshire
Thanks Cheese. Your observation is pertinent. Any braking system has a temperature point after which performance degrades or things actually begin to come apart violently (most often rotors) and as UMich has determined, different operator methods of applying the brakes can mitigate temperature increase under hard braking conditions.
07-29-2007 | 04:24 PM
#22
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From: Cummins Technical Center, IN
Originally Posted by hohn
To the second, yes, brake proportioning is something that you're stuck with. Until manufacturers start offering smart brake proportioning that adjusts braking power to available traction.
As for snub braking, it's very effective and there's a reason it's taught. The problem is that most people go too long between "snubs". A professional trucker won't let his truck get up to 45-50mph, rather he will snub at 25-30mph (usually less) and drop 10mph or so and repeat.
By contrast, the amateur driver descending a hill will coast up to 65-70mph or so, then brake down to 45-50 or so. Now as most people know, 10mph is NOT 10mph when it comes to braking. Kinetic energy is 1/2 MV^2-- the speed is squared. That means you put a LOT more heat into the brakes going from 70 to 60 than you do going from 40 to 30, even though both are just reductions of 10mph.
I'm well familiar with the braking performance of racing brakes. The Porterfields I had on my Nissan would actually brake harder as they got hotter. It was kind of nice, because it cold weather (icy) I couldn't skid the car very easily at all. Yet in hot GA weather, I could flat spot the tires at will once they were up to temp-- those brakes would yank your fillings from your teeth! But you are correct in that those brakes are designed to very different criteria than our OEM truck brakes.
As posted, I wasn't the one who first mentioned NASCAR brakes. Rather, I referred to the NASCAR braking example to make a point, then related those conclusions to a towing scenario.
Justin
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