Heavy or Light Braking

[MiketheEngineer]

Will you get more brake and tire wear if you quickly apply the brakes or "drag" them coming to a stop say from a set speed of 50 mph??

Obviously - the slower method allows aero and engine braking to help out longer but all things considered - does it make any difference? The theoretical work is the same - so would not the wear be the same??

Just a dumb old Structurual here that likes fooling around with old cars.

 

[evelrod]

As a practical application of such a "shock wave" effect...

Elsewhere in another thread, I posted that at one time I lost a LR wheel on my MG-TD and stopped to pick up all the brake shoes and parts to reassemble the LR wheel/drum. Well, there is always a bit more to the story...This instance happened just over the crest of a road bridge over a RR track...four lanes. While I was quickly putting it all back together I could hear (over the crest of the bridge) a series of screeching brakes and loud collisions. Several. I'm pretty sure they never knew what caused the stoppage.

Now, today, as a result of this and many other such cases, I always keep an eye on the mirror. It's a religion born from many years as a race car driver, in part. As a driver of the Socal freeway system, to boot.

In racing, I have a reputation of going into a corner deeper than my competition...Perhaps it's true, dunno. I do know that I am NOT a proponent of very late, heavy braking, race track or street. It creates handling problems on the track and excessive wear and tear on the street...In addition to the increased possibility of being rear ended!

Rod

 

[IRstuff]

That's precisely the point. You are not driving in an empty universe. If the idiot behind is in fact an idiot and not paying attention, then, yes, it's his fault for rearending you, but you're the one with the whiplash. And when the judge hears that you waited until the last instant to brake, you're going to get most, if not all the blame, and no monetary recompense for your own injuries.

The law, in California anyway, allows assignment to the car in front, if its deemed that the front car's actions or inactions directly caused the accident.

TTFN

FAQ731-376

 

[patprimmer]

In Aus, the law is also very clear.

You must travel at a safe distance. If you rear end someone who was not travelling backwards, you where clearly not travelling at a safe distance.

I also use the mirrors a lot. In fact I found the most enduring unsettling aspect of driving a LHD (being used to RHD) car was not seemingly instinctively knowing exactly what was going on behind me as the subconscious mirror glances found no mirrors.

My point is that driving styles that save time also often increase other operating costs and the drivers and possibly other occupants time is a significant cost.

Of course consideration of others (if they are present) should be taken into account even if it has no impact on the costs, which is what this thread is about.

Regards
Pat
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[evelrod]

If you out there by your lonesome, Pat, why bother to brake at all? I just went up to a race with a long time friend. A young man that has followed my racing career all his life. I spent the entire 420 miles on the edge of my seat. Changing lanes, following too closely and, braking at the last possible instance...LEAVING ABSOLUTELY NO ROOM FOR ERROR!!!

It's just not the most sensible method to consistently get from point A to point B without the emotional impact.

I'm not a saint. I learned the hard way and I'm just pointing out that after a 52 year racing career and somewhat longer driving career, I learned a couple things. Not trying to be preachy...lord knows I have little room for that. Just "fore warned is fore armed" kinda thing.

Rod

 

[patprimmer]

Rod

I think it's all a matter of degree and on the time and place and how much time might be saved and how much it costs.

Regards
Pat
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[evelrod]

I've been retired for over 12 years now Pat. One of the very first things I did was to put my wristwatch away. Haven't worn one since. I really must tell myself from time to time that I have no place that I just MUST be. Besides, I'm not all that great of a driver on the street. Just got back from Costco and my wife spent most of the ride telling me "stop lights, stop lights". Duh.

I love my racing. I love the competition. I love being twenty something even if it's only for a half hour. I really dislike the long tow to the races...but...I think I dislike riding while someone else is driving even more.

Oh well.

Rod

 

[IRstuff]

" matter of degree and on the time and place and how much time might be saved and how much it costs. "

Possibly, but I'd argue that most of these are invariably mitigated by planning and allocating sufficient time. I won't deny that I'm sometimes guilty of procrastination and insufficient planning, but I've decided that these risks just aren't worth it. In most cases, we're talking about a few minutes difference, and that allowing for sufficient time and contingencies makes for a more serene and less hectic trip.

It seems that too many people thrive on the adrenaline high of being late and having to rush. But cutting corners is bad in business and bad in your personal life.

TTFN

FAQ731-376

 

[patprimmer]

The OP was about costs, not social attitudes or road safety.

Time is money is part of that equation.

How much time is saved at what cost is the point. I am not saying how that nets out, in fact I think I said something like I don't know how it nets out and it will vary with circumstance, but it is an economic argument that should be considered in a purely financial analysis. What constitutes a good driver or a comfortable or safe trip is not part of the financial analysis.

Regards
Pat
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[SomptingGuy]

Pub talk. Should be moved to a suitable place. Nothing here about engineering going on.


- Steve

 

[HRHtex]

I am inclined to agree - but interesting nevertheless.
What happened to the exchange on motorcycle "stoppies" etc.?
Was that this thread? Where did the replies go? Did I hallucinate the whole thing?

 

[kpt4321]

"Traffic engineering has long recognized that vehicle flow resembles shock waves near critcal velocity; one person stabs the brakes, and each of the following vehicles has to stop progressively harder as the reaction time cushion is used up, until some poor sap, ten vehicles behind, and who was following at a reasonable distance is presented with a total speed change that is cumulatively impossible to handle."

The basis for this claim is false. There is no such thing as a person, following at a safe distance, being presented with a speed change that is "cumulatively" impossible to handle, because the speed change is not cumulative. If you are following somebody, they may need to come to a full stop at any point in time (the aforementioned kid in the street, or wild animal, or otherwise). You need to leave space for your reaction time coupled with the maximum rate at which they could decelerate. If you do this, no occurrence in front of you will cause an accident.

Furthermore, Pat (and others) was not recommending (I do not think) that people practice late braking in the form of full-on, traction limited, ABS-active braking for every light, stop sign, and curve. Rather, he was (presumably) advocating simply braking somewhat later and somewhat harder than average, still leaving a significant margin for increased braking if/when necessary. This is a far cry from your "example" of somebody pounding the brakes and causing a pileup.

There is really no reason for the high horse regarding driving style.

 

[MacGyverS2000]

kpt,

It depends on your definition of "safe driving distance". Is it defined as enough distance to come to a complete stop should the person ahead of you slam on the brakes, or is it defined as enough distance to come to a complete stop if something were to suddenly appear in front of you?

Two situations:
1) You're at distance 'X' from car ahead of you. They slam on the brakes and skid (or slow down) over some distance 'Y'. You waste a bit of reaction time before hitting your own brakes, but you come to a stop without hitting them. Distance 'X' is a safe distance. Distance 'X' had enough buffer space in it to take care of your reaction time.

2) Same distance 'X' between you and the person ahead of you. A concrete pipe rolls into the road. The car ahead hits it and basically stops dead in its tracks. You hit your brakes after the same reaction time, but this time you slam into the back of them.



In case #2, I would call that being "presented with a total speed change that is cumulatively impossible to handle."

Just a thought...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Compositepro]

I certainly don't practice this normally, but the rules of safe responsible driving are that the driver should drive at a speed and distance that he can stop in time to avoid any object or hazard in front of him. A deer jumping out from behind bushes right in front of the bumper could be considered foreseeable in some cases. If you do not follow this practice then you are taking a risk. Everyone will do their own risk/benefit analysis. Speed limits and laws on safe driving spacing are in some ways an attempt to set a commonly accepted risk/benefit ratio. The ones who have a lot of accidents are those who don't believe that they are fully responsible for their own safety. The accidents they have are always someone else's fault.

The scariest situations I've experienced involve a sudden drop in visibility on freeways due to rain burst or brush fire. You should slow down immediately, but you know that if you do, someone will rear end you.

 

[patprimmer]

I once had a complete and sudden electrical system failure while drifting through a corner on a dark moonless night on a winding unlit, poorly marked country mountain road. That was a real heart starter.

Two urgent decisions.

1) Where the *&^% is the road ahead vs where is the rock face and where is the ravine.

2) Where can I SAFELY park this thing within coasting range as all engine power went with the lights.

Turned out the battery cable had gone open circuit between the wire and the battery terminal fitting.

It all turned out alright so maybe we sometimes have more safety factor than we think. Also sometime a bit of luck helps.

Regards
Pat
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[HRHtex]

This is also known to Morris Mini drivers as the "Mr. Lucas - Prince of Darkness" effect.

 

[evelrod]

It happened to me, Pat, in 1961 on a moonless night coming down out of the mountains near Carlsbad, NM. 60 mph, 1947 Ford Coupe, curvy road...NO LIGHTS! Serious "pucker factor"...second only to the 50+mph, 1949 Mercury, downhill to the end of "freeway", light controlled intersection, TOTAL brake failure...There have been others, but none that got THAT level of my attention so quickly.

Rod

 

[Quickmode]

Id like to add a couple points to this interesting convo.. Brake component wear, or any component wear for that matter comes down to how much heat is being generated.
So, if you stab the brakes for a short enough time without generating the heat necessary to cause wear, then would that not be easier on the friction surfaces then lightly holding the pedal long enough to cause heat buildup?

Late braking is up to the driver, unless you're not being considerate. ie, coming up hard on someone at a light, or a pedestrian at a crosswalk. This can be very stressful because they're not sure if you're going to stop or not.

As far as someone behind you; its completely the responsibility of that driver to maintain distance. If they can't see what's going on and that you're intending to brake late, they're not paying attention, watching your bumper and not the road, or tailgating a higher performance vehicle beyond their vehicles capacity, as in motorcycle. Either way, the onus is on the following vehicle..
Don't tailgate!

 

[NormPeterson]

The amount of heat generated should remain relatively constant for a given speed change. The rate at which it is developed is what varies. The original "parameters" of this discussion included "50 mph", where things like "braking" due to aero drag and rotor cooling shouldn't be significant during the few seconds involved.

I doubt that bikes are inherently better-braking machines, given similarly grippy tires (and rider skill with respect to managing/avoiding stoppies). More that the sort of person who rides a bike in the first place tends to drive in a shall we say more enthusiastic manner, and that it is more difficult when following one to tell just how rapidly it is shedding speed. A bike simply doesn't get as "wide" visually as you approach it as anything else you're ever likely to be behind in traffic.


Norm

 

[BiPolarMoment]

I think Norm hit the nail on the head with the perspective issue on motorcycles. I ride and I never assume that I'm shedding speed faster than the car behind me but rather they are not attentive enough (in general) to judge the rate of my deceleration and thus they eat up their own buffer making that judgment before deciding to brake. Precisely the same reason I've run some number of lights never knowing if the person behind me is going to try to make it (I just assume "yes").

 

[potteryshard]

I would agree that the amount of speed to be shed invariantly determines the total amount of heat created. However, I would suggest, however, that the rate at which the heat is generated directly influences wear.

The kinetic energy is converted to heat at the interface between the moving and the stationary friction surfaces; that heat then soaks into the bulk of the lining and rotor materials.

Because all materials have a thermal time constant, the bulk of those materials can't instantly be at thermal equilibrium with the interface surfaces. From this, it seems that the higher the rate of heat input into the interface surfaces, the higher the temperature of the thin layer of lining or rotor material closest to the heat input, and consequently more ablation.

Yes; total heat input into the lining and rotor is based on change in kinetic energy, regardless of rate of change. Average temperature of the materials some time after braking action has ceased is probably likewise independent of rate of speed change.

The instantaneous temperature of the infitesimal layer of friction materials making up the interface, and consequently wear, is likely strongly dependent upon rate of heat input.