Hard or soft braking

[The Real Bev]

I recently read (don't ask me where, it was a couple of months ago) that brakes last longer with harder+shorter pressure than with softer+longer pressure. Opinions?

[John_H]

The amount of energy that needs to be dissipated as heat via the brake components will be exactly the same however harder braking will result in higher temperatures as the heat is applied over a shorter period of time.

The question then becomes whether brakes wear quicker at higher or lower temperatures?

There are also other factors to consider, particularly the pad material and its suitability for the conditions that apply (consistent hard braking from high speeds requires a different material to the conditions encountered in normal city driving).

[Arlen Holder]

I agree that is a fundamental question related to the OP's concern. o Do hot brakes/rotors wear down rotors/pads faster than cooler ones?

Me? o I use Silicon Valley finesse when braking - not brute redneck force.

What I do is coast to a stop whenever possible for a variety of reasons.

1. I put the gears in neutral when I can see the red light ahead.
2. Then, I apply the brakes harder at first, so as to scrub momentum.
3. Then I coast, gently, still in neutral, to the stop light.

While this works whether or not it's a manual transmission, I do agree that some people feel shifting an automatic is (somehow?) bad for the automatic, but I've been doing this for decades, and haven't destroyed one yet.

The _advantages_ of this "coast to a stop" method are, IMHO, mainly: A. It saves on gas mileage (as 20% of the time you never need to stop) B. It saves on pad-deposition (since the rotors have a chance to cool down)

As a bit of finesse, I make sure that if, in the 80% of the time I _still_ have to stop, I simply "almost stop" a car length or two behind the stopped car in front of me, where I then expend the last bit of braking heat, and then coast to the final stop and lift the pedal off (if it's flat).

That way I don't get a pad "footprint", which, IMHO, is a major cause of brake-related vibration at highway speeds (which is a topic we've discussed in infinite detail in the past on this newsgroup so I won't belabor it).

[The Real Bev]

I would guess that heat is always the enemy. Do they get cumulatively hotter with hard sporadic braking or soft longer-lasting braking?

In traffic I just take my foot off the gas as soon as possible. Hubby says I don't do it soon enough, but I hate having someone dive in ahead of me because I left enough room for them to do so. In my defense, some assholes haven't a clue about making left turns in heavy traffic -- they stay behind the limit line until the light turns red and then dive through -- requiring everyone behind them to wait through an additional cycle. If done properly TWO cars can go through on each cycle -- maybe three if all three are quick.

Brake pads are cheaper and easier to replace than clutches/transmissions. I'll put the car in 2nd if coming down the mountains trapped behind some jerk who hasn't a clue about mountain driving -- long ago I used my brakes far too much in a similar situation and had serious fade down at the bottom. Some experiences cast long shadows.

Fine in traffic.

I'm mostly concerned with mountain and highway driving, which is what I use my car for most.

[Arlen Holder]

As someone stated, we have to expend the same amount of energy in slowing down the car no matter how we brake - almost all of which is heat energy.

Seems to me that the "temperature" is a function of how hard we brake and how fast we stop, where, IMHO, stopping slower "spreads" out that heat, such that the "temperature" has a chance to not rise as much.

Does it work? o Dunno.

I do it mostly for two reasons: a. Save fuel, and, b. Prevent pad footprint deposition.

Hehhehheh... that happens to me all the time.

I can see their point, as they likely feel I'm "slowing them down"; but they also likely don't realize that I'm coasting because I _know_ there's a red light up ahead (or stopped traffic) - where my goal isn't to stop - but to coast right through the light when it finally turns green and the traffic in front of me clears out.

hehhehheh... I often "need" to go through a yellowish-red light, where suddenly, my visor comes down to cover my face from the stoplight cameras, if any. :)

(Hint: They need to correlate the driver with the license plate, I think.)

Yes. Much. o I've heard this all the time.

But that assumes putting the transmission in neutral somehow "damages" the transmission, or, getting it back in gear while coasting damages the transmission.

The question for this ng is whether that's the case, or not. o I don't know the answer - but I've been running the test for a long time!

Are you in a manual or automatic? o I don't feel much engine braking when using an automatic downhills.

And I travel a _lot_ of downhills!

Many miles every day in fact.

Where my tires take a beating like you can't believe.

Almost always, I'm in neutral because the road is twisty curvy so you're limited to between 20 and 35 mph almost the entire way downhill (or up).

Me too, as I live in and around the mountains of Silicon Valley.

See details here: o Does the macadem road surface have a great effect on tire wear?

And here: o How would you run a lateral acceleration test in a vehicle on twisty roads at no more than 40mph?

[The Real Bev]

Our City took those out after only a few years. They didn't get a big enough cut of the resulting fines, although they claimed it was because of the increase in rear-enders.

Doing something different puts wear on the thing that's doing something different.

Auto. The Corolla engine-brakes fine, and so did the POS 88 Caddy.

The Corolla rolls really nicely. I think I could easily go over an edge if I left it in neutral.

Highway 330 and 18 to Big Bear. 40-80 depending on where you are and the direction you're going :-)

[Arlen Holder]

Maybe. Maybe not. I really do not know, and don't claim to know.

My assumption is that transmissions were meant to be shifted from D to N and back; but that they weren't designed to do that while moving.

Does doing it while moving "hurt" the transmission? o I don't know.

Maybe someone here, like Xeno, knows "how" that can hurt a 2wd front or rwd transmission?

[Xeno]

Transmissions will do that without complaint. What is at issue is whether it is a good idea to be in neutral whilst on the highway. My viewpoint is that it is not - whether the trans is a manual or an automatic. The issue here is *safety*, not trans damage.

Doing it whilst moving will not, in the short term, damage the transmission.

[Arlen Holder]

Hi Xeno, I very much appreciate your technical acumen.

I would like to ask that we ignore the "danger" factor to humans, as that isn't my technical question in the least.

You've made your point on the "danger factor" so that _others_ will be scared enough to have second thoughts - but the "danger factor" isn't even on my list of the least of my worries (I've explained why in the other post).

My key _technical_ question I would love to know the answer of is this: Q: *Does constant daily prolonged engine braking cause transmission wear*?

[Kevin Bottorff]

Arlen Holder wrote in news:r45ra7$qd7$ [e-mail]:

The amount of wear would be unnoticeabe in the life of the car. As stated being out of gear on a down hill drive would be foolish for any number of reasons. One being not using free engine braking. (on a auto, manual could be a different set of wear factors.) KB

[Arlen Holder]

Hi Kevin, I appreciate your input as I'm trying to obtain comparative facts on o Wear due to friction in the transmission, versus o Wear due to friction in the engine given there is no other way to slow down on long mountain roads.

So we only have two possible choices: a. transmission friction b. brake system friction And, I argue, the need for sudden acceleration isn't even remotely an issue on these single-lane roads that have almost zero traffic on them, and where acceleration would put you instantly at the bottom of a cliff, if not into a tree.

Hence the question is only of the relative wear tradeoff between: a. transmission friction b. brake system friction

To that end, thanks for the observation that the wear of the friction on the transmission necessary to reduce the potential energy of the car traveling on a 9% grade for miles on end would be insignificant during the life of the transmission.

Likewise, I've noted that the wear on the pads is also insignificant, given how trivially simple & inexpensive it is to replace brake pads on today's vehicles (about twenty five to thirty bucks every few years on average).

Given I can't even envision a situation where you'd need to accelerate (as, even if you did accelerate, you'd end up at the bottom of a cliff), and given the logic of the _last_ reason to "obey a law" is because "it's the law", then it seems to be a simple tossup. o Trade transmission-based engine braking for brake-system braking, versus o Trade brake-system braking for transmission-based engine braking.

It seems, to me, based on those facts, that it's a tossup between the two given in both cases, the impact of friction-related wear is negligible. a. transmission friction b. brake system friction

As always, I'm wide open to facts since I can change my mind in an instant if and when compelling facts are introduced, since my belief systems are always based on facts as much as humanly possible - and not on intuition.

[Arlen Holder]

Ooooops. That last sentence was a thinko...

The facts seem to be we only have two exactly equal choices: o dissipating potential energy via friction in the _transmission_ o dissipating potential energy via friction in the _brake system_

We have no other choices given I start in neutral so there is zero acceleration other than that due to the potential energy of mg sin theta.

Either via brakes or the transmission... "something" must dissipate the potential energy of a few thousand pounds on a 9% grade for miles on end.

In both cases, transmission or braking system, the potential energy has to be turned into friction which itself results in heat dissipation.

The question is which is damaged more by the same amount of heat dissipation, the braking system or the transmission.

If the wear is negligible in both cases, and if the safety is, as I argue, not in the least an issue (given you likely can't accelerate even if you had wanted to as you'd end up against a cliff or at the bottom of one if you did).... then...

It seems to be a tossup between the inherent wear of... a. Dissipating potential energy as heat in the braking system, versus b. Dissipating potential energy as heat in the transmission system.

I _love_ facts; so if folks have more facts, please let us all know.

[Kevin Bottorff]

Arlen Holder wrote in news:r48flc$gq1$ [e-mail]:

so you are ignoring the friction of the entire drive train not just the trany friction. also consistant drag of the drive train/transmission can make keeping control more consistant than in neutral. also miles long brake drag is NOT designed into most systems, and glazing of the pads can lower the brake efficiency which is a safety issue not just a wear factor.

again accel is not the only reason to have the vehicle always in gear.

to reiterate it is not a trany vers the brake wear the only thing at issue here. KB

[Arlen Holder]

Hmmmmm.... this question is a basic philosophical technical question, where you bring up a good point about friction in the drive train.

I readily admit I had not priori considered what you're calling the "drive train" (e.g., the differential in a RWD car, the U joints, the wheel bearings, etc.,).

However, thinking about the "drive train" for a moment, if we keep our thinking hats on, and stick to facts, my first question to you is how is the action of the drive train any _different_ with or without engine braking?

Hmmmm... take a look at just _one_ of these curves on this road:

Again, you bring up a good point, where you don't know that the speeds attainable on this road are never going to ever be greater than about 35 to

40 mph at the absolute fastest - where once you brake hard on these turns (many of which are 180 degree hairpins) - you're almost down to walking speeds, and by the time you're at the next hairpin - you're only at around 20 to 25mph (give or take).

So, while you're correct that you go from about 25 mph entering each turn down to around 5 or 10 mph at the apex of the turn, we're not talking huge changes in speed.

Although I do agree, we go from walking speed to 25mph back to walking speed back to 25mph to walking speed back to 25 mph, constantly, instead of going a steady 10 mph (which nobody does).

Thinking about it a bit (I hadn't considered the constant-speed issue prior), I don't think _anyone_ would _ever_ go at a constant speed, simply because the only constant speed possible would be either too slow or too fast.

So you're gonna be braking no matter what, with or without engine braking (which I've said prior but I hadn't considered that constant speed is likely nearly impossible until you just made me think about it).

Do you really feel constant speed is even possible at any speed other than about walking speed (which is just far too slow to be practicable).

Not gonna happen.

I flatly state that if you think there is miles long brake drag, then I didn't explain the situation properly.

Nobody is gonna be on the brakes the whole way down. Nobody. o If you were, you'd never make it to the bottom except at walking speed.

You brake when you enter the turn. o Then you coast to the next turn, gradually picking up speed.

If anyone thinks there's gonna be constant brake drag, then it's my fault for not explaining the situation properly.

It's just never going to happen, even with the most sophomoric of drivers.

But I do appreciate the questions and comments because the whole point is to figure out, intelligently, whether it's a viable tradeoff of... o dissipating potential energy via friction in engine braking, or, o dissipating potential energy via friction in the brake system.

The amount of heat dissipated will be the same in either case: o The question is which system is best designed to slow down the car.

It seems to me. logically, that it's a no brainer which system is designed to slow down the car, where, using the engine to brake the vehicle seems almost like abuse, in that the whole purpose of the braking system is to slow the vehicle down - but the purpose of the transmission is NOT that purpose.

But folks here seem to feel the transmission is designed to slow down the car, which is why I ask the question to flesh out why they think that.

[Xeno]

Anything in the drive train connected to the transmission output main shaft, and being rotated by it even in neutral, will be a friction drag at *all times*, even under cruise or acceleration.

Given the nature of your road, and the requirement to constantly change speeds, you should be using engine braking *all the time*.

So, use engine braking to maintain a relatively constant speed *between* slow points, short jab on the brakes or press on the accelerator at all other times.

There are a lot of places where I am desirous of maintaining a constant speed on a long downhill run. Engine braking makes it easier to maintain a constant speed with selection of an appropriate gear.

Indeed, there will be moments where you need to slow down more for a sharp corner. There will be straight points between those slow points where your car will naturally want to increase speed on the descent. You should be using engine braking for those straight bits thus requiring only the lightest touch of the brakes at the slow points, if at all. The brakes don't overheat and you don't experience brake fade.

Why is it, do you think, that trucks are required to halt at the top of a steep hill and engage low gear before making the descent. It is all in the interests of maintaining a *safe* speed and preserving the brakes for when they are needed. At the speeds you are travelling, your brakes likely will have insufficient time to cool between bends so you will be getting a lot of heat soak into wheel hubs, bearings, hydraulics and the like. Long term, that may well be detrimental to your brakes. On that point, you only need to experience brake fade once to really appreciate keeping the brakes cool. Brake fade happened to me just once, way back in the first couple of years of driving and in a car and at a point where I didn't think the brakes were even getting hot. I was lucky that time as there was a run-off conveniently located on the bend I was about to negotiate - or should I say *attempted* to negotiate since I had no option but to go straight at the speed I was travelling. I was relying

100% on my brakes and, at the critical moment, they were ineffective - full pedal but zero stopping power. I survived unscathed and learnt a valuable lesson that day. Been an advocate of effective engine braking ever since. Never assume your brakes will always be there.

Engine braking in top gear on my car drops off below 60kph. To maintain engine braking at a lower speed than that requires selection of a lower gear. That gear will depend on the speed required.

Doesn't need to be miles long. Repeated braking will have a similar effect.

Both have their purpose. Overuse your brakes at your peril.

I beg to differ. The golden rule of driving is to come down a hill in the same gear you go up it. In driving, the transmission is designed so you can keep the engine RPM at a level where it produces maximum torque and the transmission will multiply that torque as required. When coasting, ie. on the overrun, the trans acts as a brake. It is a

*feature* of gearing and one that proves very useful downhill as much as going uphill.

[Arlen Holder]

Hi Xeno,

Thank you very much for edifying me on the particulars, which I appreciate.

Thanks for reaffirming what I had thought on the drive train friction, although I readily admit I hadn't thought about it prior to the discussion here.

Even on the manuals, I coast in neutral 'cuz I'd rather replace brakes than the clutch any day of the week. o Woo hoo. Just got my first clutch kit - for replacing my first clutch

As you know, I do all my own work lately, where I admit the clutch has only been replaced about every 80 or 90K miles on the SUV that is a daily drive.

My thought process was that I'd rather replace brake pads than clutches is the same though on both the automatic and the manual transmissions:

Understood. Agreed. Instead of drive on the automatics, use the appropriate lock ratio for the most engine braking, and then short jabs on the brakes at the hairpins.

Much appreciated your advice on finesse and wear and tear.

I should clarify that the 9% downhill road is long, but there are so many turns that I'd estimate over a hundred of them in a few miles, so the distance between hairpins is not much more than a few hundred feet. At no point is there ever a straightaway.

Absolutely. These are 180 degree turns where you have to assume there is a truck coming up the other way which is taking up the entire width of the road. Or bicyclists. Of course, 999 times out of 1,000 there will be nothing on the other side, but you can never assume that so you have to brake at every turn no matter what method you use. There's no other way.

I don't think this happens on this road with the people in drive because they are _accelerating_ in the distances between curves.

Otherwise, they'd be going too slowly. (It's rare to have a car behind me but it happens mostly in the morning and evening at the beginning and end of a work day, where I pull over and let them pass.) Even then, if I can help it, I leave the vehicle in neutral, but it does take a few extra seconds to get the car rolling at speed again. :)

I completely understand this concept where safety is their concern given their brakes overheat (personally, I've seen a truck inside tire explode from the heat after the driver pulled over and got out of the vehicle because it exploded as I passed by it parked on the side of the road so I stopped to see if my vehicle was damanged, which I can explain if you want more information).

There is no way anyone is going to overheat their brakes on this road. I'm sure if they really tried (e.g., riding the brakes the whole way), they might, but as I said, most people are accelerating between turns so they're not riding the brakes. Neither am I. I tap them as needed.

Understood. However, we're braking at no greater than about 25mph speeds to start with, so it's not like a 60-to-0 stop at the bottom of an exit ramp, which, as I've noted, I also shift into neutral if I'm at the bottom for a while (e.g., at a light) and I make sure the pad footprint doesn't stay in one place on the rotor.

This happened to me once, long ago, in an old 1970 Dodge Dart.

The damn brakes just didn't work on a steep short hill at an intersection in the city.

I don't know why. They just had no friction. I was pressing as hard as I could, but there was just no friction. It's as if the power brakes gave up instantly, but, get this ... the engine didn't quit so afterward it was fine and it never happened again on that vehicle.

To this day I don't know what had happened, but luckily, the city traffic just honked their horns angrily as I mowed into the intersection, and mosied on down to the next one.

To this day, I don't know what happened, as the engine was running the whole time so I don't know what happened to the brakes at that moment.

This speed is about 25mph on about a 9% grade.

Understood. I think the speeds are slow enough and the braking intermittent enough that brake fade isn't going to happen under those circumstance.

That's an interesting rule I've never heard, but on the manual 2WD SUV, we generally are in first and second the whole way up. Obviously we're in first on the curves, and second for a short time in between curves.

It's impossible to be in any gear higher than 2nd on the way up.

For a manual though, things are easier, but I like that rule of matching the uphill gears to the downhill gears. I have another grandkid to teach how to drive in about six months, where I hope you don't mind that I steal your advice and use it as if it was my own! :)

("Grandpa, do you know everything" she used to ask me when she was younger, to which I said "Why yes. I do, and if keep an open mind, you will too!".

:)

In summary, I very much will keep an open mind, and try to learn from my fellow Grandpa Xeno's advice!

[Xeno]

Proves that you don't need to be doing high speeds. It's all about the heat absorption rate versus the heat dissipation rate.

If the booster had failed, for any reason, you would still have brakes but very high pedal pressures. With brake fade, you have *no brakes* regardless of the pedal pressure. That's because the pads are gassing and the gas builds up in a layer between the pads and the disc. Hence, zero friction. When the pads cool down below gassing point, the gas dissipates and brake function returns to normal. Now you know that it takes very little braking at city speeds to cause brake fade. The same applies to your longish downhill run. It simply amounts to more heat building up in a given time than can be dissipated. Whilst on that topic, the thinner the disc (through wear or machining) the less it is able to hold heat and the sooner it will reach the point of brake fade.

Intermittent braking can also overheat brakes because the intervening periods of time are to short to allow adequate cooling as are the minimal air flows available to the disc.

So, given that level of steepness, the *rule* informs the gear you should engage on the way down.

[Arlen Holder]

Hi Xeno, Much appreciated all your kind help & advice! o I'll pass it on to one of my granddaughters who will be driving soon!

On that topic of the amount of metal, I'm one of the (apparently rare) shade-tree mechanics who owns calipers and dial gauges such that I check rotor thickness, runout, and brake drum diameter. :)

I do realize a lot of people on Usenet "talk" that they check rotors and drums, but IMHO, without pictures, it didn't happen (most don't seem to even own the proper tools with which to check them, particularly those who spout that their brake rotors 'warped').

I replace a rotor or drum when it's due, but not before it's due.

As I said, I _love_ your "golden rule" of being in the same gear in both directions, which I will steal (with your permission) to use with my upcoming driving lessons for one of my grandkids.

I thank you for your advice, where today I ran a tracking program:

On the way downhill, the average speed was 21.81 mph which is a bit lower than the 25mph I would have guessed, and the maximum speed, surprisingly, was a bit higher than I would have guessed, where it was 35.48 mph.

Thanks for all your helpful advice over the years, where I'm a sponge for facts, and hence, I love when others deal with logic, sense, and facts!

[Steve W.]

I hope that isn't the caliper you use to measure drums. The jaws are not long enough to reach the friction areas inside the drum for a good measurement of wear. Same thing with a set of rotors, you need to check thickness in at least three areas on the friction surface and to do that you need longer jaws on the calipers to reach to the depth of the hat.

[Arlen Holder]

Hi Steve,

Thanks for that purposefully helpful advice, which, I hadn't thought of until you mentioned it, which I appreciate, where, yes, while I have a few calipers, that was my largest set of calipers, and, I do _agree_ (fully) with you that the jaws don't go deeply enough to check the deeper brake drum wear areas.

BTW, did anyone notice the two long metric bolts sticking out of the drum? o They are one of the most useful tools to have in your brake tool arsenal!

As for the rotors, I have all sizes of micrometers (as I'm sure most of you do too), where my normal one-inch mic usually works best but your point is well taken that most micrometers don't necessarily have a "deep" enough "C" shape to get any deeper than about an inch or two over the rotor.

As for my dial gauge, I have a block mount on a bar, a magnetic mount and the weird looking "S" shaped clamped mount, where I generally clamp that S-shaped rod to the springs and measure runout on the rotor that way.

As you noted, the tools are NOT what a professional mechanic would use, but they are, I posit, better than what I think most shade tree mechanics use, as I've heard too many times on forums the oft-asked question... o *How do I know when to replace my drums and rotors*? Which seems, in retrospect, to come from those with no mics or calipers.

Also, I've heard the similar question often of... o *Do we replace the rotors after every brake pad or every second pad*?

That question seems to be most often asked by the same people who claim that their rotors "warped" (which is almost impossible to actually happen in a passenger vehicle, even under extreme circumstances, due simply to the temperature required to melt steel being almost impossible to attain, AFAICR).

In summary, I appreciate and agree with your statement that even my calipers and mics are not the right tools that a professional will use for a typical brake job; but I maintain that these tools are more than most will use (in my experience on the automative forums).