When coming to a stop, is it better on your brakes to apply constant pressure until stop? or to kind of pulse them with greater pressure??

 

Very interesting question. I have always applied steady pressure until I stop but a couple weeks ago I was watching NASCAR, as I always do, at New Hampshire I think, and they were commenting on how Jeff Gordon was getting his break rotors glowing red from heat but his teammate Jimmy Johnson wasn’t having break issues. The race announcers then commented about how Gordon would apply less pressure over a longer period of time but Johnson would apply more pressure for a shorter period of time. So basically Gordon was applying steady, light to medium pressure for a long time and generating more heat than Johnson that was applying very heavy pressure for a short period of time and generated less heat.

For those of you that don’t know, these were two cars that were built in the same shop and most likely had the exact same parts on them. The braking systems on these two cars were probably an exact clone of each other. Also, the weight of the two cars were probably within 50 pounds of each other so although there are thousands of variables in cars, this is probably a very, very good comparison.

Your question is what is better, after analyzing the information collected from the race, I would have to say that it is better to break harder for a shorter period of time because this will most likely generate less heat in the rotor. Less heat in the rotor equates to less chance for the rotors to warp and the pads to glaze over.

 

Thanks for the reply. Yea thats what i'd been doing but i wanted a 2nd opinion

 

My wife applies "very hard" braking forces about 5 ft from a stop sign. She'll almost throw you though the windshield...I guess I owe her an apology now.

 

Keep in mind NASCAR uses VERY aggressive pads and the lightest possible rotors to save unsprung weight. They also change the pads & rotors after every race, not to mention rebuild the entire car.
Your argument does not necessarily hold true for production / stock brakes. We have a LOT of mass and ventilation on the rotors. I have ALWAYS anticipated stops and braked slow and easy and have NEVER had short life out of my brakes no matter what vehicle I drove.

Some good info: http://www.centricparts.com/techsupport.htm

I'm also looking for another good article...but can't find the link right now. If I find the link, I'll post it.

HTH

Tony

 

With ABS you can stomp as you like, the computer handles pulsing the brakes so the wheels stay unlocked.
With normal brakes you have to find the edge of where the tire breaks loose and skids and back off to the point where you get your maximum braking force. Its not easy and I need a lot of practice.
recent trucks have RWAL brakes to keep the rear end from skidding and swinging around. Doesnt help much for stopping.
I think the best thing to do if youre having brakes that lock up easy is to get bigger wheels. More of a contact patch on the road means more friction to stop.
If you look at the rotors theyre solid and really thick to prevent warping under heat. You can go lighter on rotors but you need good cooling like drilled or slotted rotors give. You can run aggressive pads but youll eat the rotors a bit more. Considering how cheap both are, Id say find the setup that gives you the best braking with the loads you normally carry.

 

Found it!

Good info: http://www.stoptech.com/tech_info/wp...rakedisk.shtml

HTH

Tony

 

Regardless of what pads, rotors, calipers, and fluid is used in NASCAR the conclusion can still be drawn that by applying the break for a longer period of time produces more heat. As far as ventilation on the Cup cars go, they have air ducts and blowers that aid in cooling the brakes so they receive MORE cooling than our trucks do. I read the website you posted and unless someone has high performance pads installed, heat build up in the pad, fluid, and rotor is an issue that should be considered.

I have not found any data that compares braking styles of easy vs hard and heat build up but if someone can find it, I would be very interested in reading it. For what it’s worth, they say hard breaking uses more fuel but I think that statement should be qualified a bit. I think when the news reports that aggressive breaking uses more fuel they are not factoring in two vehicles coming to a complete stop from the exact same speed. I think the variable they fail to include is that the majority of people that break hard or aggressively, would be traveling at a higher rate of speed as they approach a stop sign, say 40mph instead of 30mph. It’s obvious that a vehicle will use more fuel going from 0 to 40 to 0 than a vehicle that goes from 0 to 30

 

It makes NO difference whether you brake hard or soft-- you will put the same amount of heat into the brakes.

The reason is that you are doing the same amount of work. If you are stopping a 7000# CTD truck from 55, you are doing the SAME amount of work, whether you take 30 seconds to do it or 5 minutes to do it.

The difference in time is not a difference in work, but a difference in POWER. Power is how fast you can do work. You can do the work of accelerating your truck with 100hp just as you can do it with 500hp, but the higher POWER requires less time!

Now, taking this back to brakes-- "power" in the braking sense is HOW FAST the brakes can absorb heat energy. This is a function of how fast the brakes can dump heat as well as how much heat they can store to begin with (usually, this is a function of how much MASS).

Think about it, braking is simply converting kinetic energy to thermal energy using friction. The total amount of energy involved depends on how much you started with. A car has a lot more kinetic energy at 80mph than at 45mph. Hence, it will produce a LOT more heat to stop the faster car. I can stop the 45mph car and make the brakes get "hotter" (higher temp) than the 80mph car, even though there's less energy being dissipated. I'd just have to take more time to do it. This is where the POWER shows up, as it represents the time required to do the work.

NEITHER has much to do with braking temps. Temperature and heat are two difference things, because heat is thermal energy (the "amount" of energy). Just as I can have 300 kilovolts and less than a single watt of energy, I can have something that's a thousand degrees in temp that contains very little heat, while a bathtub of water that's only 110 degrees might have a LOT of heat.

That's today's free physics/thermo lesson

 

HOHN, well said! I was about to head down the physics / thermo path too!

 

As a corollary to my post above--

This is also why when you are going downhill with a heavy load, you should apply constant brake pressure and NEVER pump them.

The trick is you have to brake within the "power" that your brakes have-- which means you have go very slowly down the hill to draw the time out so your brakes can dissipate the heat energy that builds up.

If you pump the brakes, you actually make things worse because the truck accelerates when you let off the brakes, and it INCREASES the energy of the truck, which means your brakes now have to do that much more work trying to dissipate that energy.

The trick when braking downhill with a heavy load is to never let the energy build up to begin with. That means start out slow and stay that way. It might be a good idea to stop periodically and let the brakes cool a bit.

The people who have brake failures are those ignorant souls that let the truck get up to 55 or 60mph downhill, slow down to 35 or so, then let gravity take them back up to 55 or 60 and repeat this all over again.

Instead, if you just "ride" the brakes all the way down keeping a steady speed of 10-15mph or so, you'll have no brake worries at all.

REMEMBER: ENERGY GOES UP WITH THE SQUARE OF SPEED.

That means that if you let your speed double, you've just made FOUR TIMES more work for your brakes. If you let the speed triple (say, from 20mph to 60mph), then your brakes now have to do NINE TIMES more work.

There's a reason you see the experienced professional truckers going down the long grades so slowly: they know what they are doing!!

 

Another big reason for applying steady pressure, is that excessive heat, with the addition of more oxygen, equals that thing called fire.

A steady application of braking while going downhill, while heating the brakes, also keeps the friction material in contact with the drum/rotor. In doing that, it actually keeps the heat transfer to a maximum. When pumping the brakes, the friction material moves away from the drum/rotor, and helps introduce oxygen to the face of the friction. If the brakes are borderline to catching fire from heat, this added oxygen will put them over that edge. Suddenly, rather than brakes, you have BBQ bricks, that are doing nothing for your current situation.

I have seen this very situation, more than once, and it isn't pretty at all. A set of brand new brake shoes, that were a rich black colour, turned literally white. They also crumbled, and resembled light coloured BBQ bricks, or at least the chunks of lining that were left in the wheel end assemblies did...

Just a little more food for thought

 

Heh, Whenever I go up to Mt washington, I usually smell one or two cars that are roasting brakes. It reeks really bad.
Some guy in a minivan last year had smoke pouring off his brakes and didnt even notice. Nevermind all the signs on the way down the hill about flaming brakes

 

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).

 

Here is an excerpt from the Colorado Department of Education website about mountain driving. The sub-topic heading is in bold with a small paragraph below. The full article can be found in the link below.

Maintaining Vehicle Control With the Brakes

For long downhill grades, maintain a safe speed with the transmission and
retarder (if equipped) so that minimal use of the brakes is necessary. Use
the brakes intermittently as needed, with time enough between
applications for the linings to cool.

http://www.cde.state.co.us/cdenutrit...%20Driving.pdf