i can definitely feel my car decelerate much smoother... but then with my ps2's its always been able to stop pretty well so i guess... lol

 

Yup...just like the TSIB brake pads make the brakes -feel- much more smooth and linear in stopping, but the car will still stop in _exactly_ the same distance as it did before.

 

I always find these discussions about braking systems very interesting... I learn a lot!

Kurtz

I know little about the engineering design aspect of braking systems, so I have a lot of questions for my own academic benefit... Sorry When you say, "brakes don't stop the car, tires do." What do you mean by that? Correct me if I am wrong, but without the force of the brake system the car would not stop. As for brake systems impacting brake performance; Is it really true that the braking system has nothing to do with improving stopping distance? Why then do manufacturers install larger brake systems/rotors on performance cars such as the ISF, C63, ZR1 or on heavy trucks?

I understand basic physics and the rationale behind why tire quality has an impact on stopping distances, but are there not other contributing factors that cause a vehicle to slow to a stop or how quickly a tire reaches its' break-loose point which would otherwise engage the ABS? Does improved brake torque or fade resistance not improve braking performance and thus stopping distances? Furthermore, would a larger brake system and rotor increasing surface contact area not improve performance under any circumstance? Was the only reason the recently tested IS250 F Sport had improved braking distance over stock, the tires and not the F Sport brake system?

What about the weight of a vehicle, does that not have an impact on stopping distances too? The lighter the vehicle the less inertia and thus less force required to stop the vehicle, right? Conversely, the heavier the vehicle the more force required to bring it to a stop? I don't believe a smaller braking system would stop a heavy vehicle in the same distance. What about the impact of brake materials on braking performance? Do ceramic composite brake systems (CCB's) used on F1 cars not perform any better than say a stock IS 250 brake system at stopping an F1 car or vice versa? In this instance clearly one reason performance would improve would be a reduction in unsprung weight, but I would think a higher resistance to brake fade as a result of less heat build up would be another reason.

Are brakes not designed and fitted to vehicles based on vehicle parameters/use or more simply put optimum braking is achieved when brake torque matches dynamic weight distribution? Would that not explain why slapping a larger diameter rotor on a vehicle does not necessarily transcend to better stopping distances? But isn't there more to it than that? More specifically, I would think just increasing the size of the rotor using the same stock caliper and pad set up would not increase the contact area so in essence it will not perform any better then stock. To the contrary, the added weight and thus the increased inertia may even result in longer stopping distances. However, would there be no improvement in stopping distance by increasing the contact surface area of a brake system by installing larger calipers and pads to accompany a larger diameter rotor?

Just thinking out loud here; If all brake systems are created equal, why don't all car manufacturers install 9" cast iron rotors and single piston caliper brake systems on all vehicles? Sure would save the engineers the pain and suffering not to mention the $ expended on R&D.

 

there always is benifit visting the site.. and reading long post..

 

Well, if there was -no- brake system it'd certainly impact stopping the car.

But as long as it has one that can engage ABS, which virtually every modern car built does, additional changes won't stop the car any shorter a distance.

It can do lots of OTHER things, but reducing stopping distance is not among them.

(let's be clear here- Stopping distance as it's normally defined is the distance it takes the car to perform a single stop from a normal speed (say 60 or 70) when applying maximum brake force (ABS engaged or wheels locked without ABS). That is the distance car magazines generally report, and it's the distance that generally matters to a normal street driven car, since it's a typical "panic" stop. We're not discussing stopping the car from 130 mph for the 9th time in the last 2 minutes since street cars aren't generally driven that way... but even if we were, a larger braking system would never give you a stopping distance _less_ than your best result from the stock system... it would just keep your result closer to that number longer as you made repeated high speed stops in short order).


As to why big brakes show up on performance cars there's a number of reasons, including people expect it. Just like drilled rotors show up on them despite being entirely inferior from a performance perspective. There's other reasons like durability, system feel, etc...I'll address that a bit more later though.


Once you reach ABS engagement (or wheel lock on a car without ABS), which again any reasonable braking system can do from the factory, additional brake force does -nothing whatsoever- to stop the car any sooner.

Fade resistance is about how long it takes before the stopping distance gets -longer-.... it doesn't ever make it shorter than when you started though.

And assuming we're discussing normal street car use fade resistance won't ever matter unless you're driving down the side of a mountain (where, indeed, increased fade resistance will mean the car maintains its braking distance longer before it begins to increase from fade.

Are you talking about the Sewell tests? Those were horrible tests. They didn't control for anything, they changed a bunch of things at once, and didn't test under controlled conditions. Totally useless results to be honest.

You need to be careful how you define "performance" though.... larger rotors can improve "performance" if your brakes are experiencing problems related to heat. But that doesn't generally ever happen on a street car. It'll happen to a car being used at the track though. Even then, it won't stop the car in any shorter distance, it'll just be a longer amount of time before performance starts to get worse than you started with.

Wrong.

Weight has no effect at all.

The formula for stopping distance is this:

d = -Vo2/(2a)

d is distance to reach 0 speed
Vo is the initial velocity
a is the acceleration rate (a negative value when braking)

That's it. Basic physics.

To figure out what a is we only need a couple of things, weight isn't one of them.

We need the grade of the roadway (slope basically), and we need the coefficient of friction between the tire and the road. Because the tires stop the car, not the brakes.

The final formula looks like this:
d = V2/(2g(f + G))

Where
d = Braking Distance (ft)
g = Acceleration due to gravity (32.2 ft/sec2)
G = Roadway grade as a percentage; for 2% use 0.02
V = Initial vehicle speed (ft/sec)
f = Coefficient of friction between the tires and the roadway



So long as the braking system is sufficient to engage ABS (or lock the wheels without ABS) you would be wrong. Again, see the math above. Weight of the vehicle doesn't matter.


Again, resistance to fade does not -shorten- braking distance, ever.

It simply gives you more stops, at higher speed, before braking distance begins to _increase_

Important at the track.... pretty useless info for normal street driving.

F1 cars use high-end components in the brakes not just for less weight, but because they will braking from much higher speeds than your IS generally will, and FAR more often in a FAR shorter period of time.



Nope. There would be 0 change in stopping distance with larger rotors, calipers, and pads. Because the brakes don't stop the car. The tires do.

If the stock brakes can already output enough force to engage ABS (which is the maximum force the tire/road interface can handle) how could -more- force possibly do ANYTHING?

It can't.



As I said, there's a LOT of things changing brake system parts can alter.

Feel. Fade resistance (for race/track cars, or if you live on a mountainside), how long the parts last on a given vehicle, and other items.

Stopping distance, however, is not among them.


I would strongly encourage you to read this great article by a very well known brake engineer where he goes through the major brake system parts, explains what each does, and explains what you can, and can not, change by swapping it for a different part. (Hint: the thing you can't change is stopping distance)

http://www.scirocco.org/faq/brakes/p...n/pfpage1.html

 

Agreed

Isn't the engagement of ABS essentially the same as locking up the wheels in the sense that the ABS is only engaged when the system detects that the tires are breaking free? And isn't it the kinetic engery created by the brake system ultimately what creates the heat that dictates how quickly the system can bring rotational inertia of the wheel to a stop as defined by Kinetic energy = heat F = ma? As an example a vehicle weighing 639 lbs. At 55.9 mph has kinetic energy of 90,770 N-M Therefore, stopping the vehicle at .9G takes 2.9 seconds which is equal to 42HP.

For discussion sake, okay.

So it is your contention that car manufacturers install bigger brake systems simpley because they look better or customers expect it? You're losing me here... Although I will agree that system feel certainly can be impacted by a larger brake system.

I certainly agree that any factory brake system can achieve lock up within a reasonable distance. I don't believe I ever used "force" in any of my questions, but isn't force a piece of the Laws of Motion? That said, I am sure we can agree that the output of any brake system is directly related to the coefficient of friction (µ) between the lining and the disc. Thefore, the instantaneous value of µ during any given stop is what ultimately impacts the friction coefficient. A lower friction coefficient will ultimately result in the brake system requiring a longer period to stop the rotational inertia which would translate into prolonged stopping distances.


Regardless of weight, which I will get into a minute, would you agree that heat dissipation in this case would improve fade resistance and thus improve stopping distance? Brake fade is the loss of performance resulting from the lining friction decreasing as the lining and rotor rises in temperature. If this vehicle expriences a high level of brake fade the brakes would become ineffective and fail thus it will not stop regardless of type of tires it has. Brake fade which ultimately leads to failure of the effectiveness of the brake system is a problem faced by trucks driving in the mountains.

Why would I need to be careful how a define brake performance? How do you define brake performance? Ultimately are we not talking about improved performance which ultimately can impact how quickly a vehicle is brought to a stop by a brake system?



Well, I don't think that's it... First there are the fundementals and supporting calculation for Dynamic Weight Transfer. Optimum braking is achieved when brake torque distribution matches dynamic weight distribution.


Well here again, I think there is more to it than that as described in the formula below.

Ms = The torques acting on the wheel calculated from the center of the wheel.
Ft = The force acting on the wheel (actually the tire) from the ground as it slides.
rw = The radius of the wheel inclusive tire.
Fbr = The force acting on the brakedisc from the brakepad in the caliper.
rbr = The radius of the brakedisc.




See explanations above regarding the impact of weight on braking performance.


It's well known that 90% of the braking energy goes into the rotor. If the rotor is too light it gets very hot and if the temperatures get too high it can be disasterous. There are documented cases where racing teams used to small of a rotor for the weight of the vehicle in order to save a few pounds which ultimately resulted in brake failure. This gets back to my previous comment about the impact weight has on interia. In addition this would also contradict your statement that brake rotor size does not have an impact on stopping distances.



I am very familiar with this article and its' author which has been regurgitated on this forum on a number of different occasions. I am also sure you can appreciate there are different schools of thought behind the science of braking system performance.

 

I guess the thread will move to "Suspension and Brakes" section... don't know if it is the OP's intention.

 

car looks great!!!

 

holyshyza this became a freakin mathclass lol

 

holy molly..what a discussion....math + engineering + physics = I'm lost..

 

How quickly the car stops depends on the friction between the tire and road, because that's your limiting factor. Once ABS is engaged, which we seem to agree even the factory system can do, -more- force does nothing to stop the car any shorter.

How -can- it do so, since it's 100% wasted force?


Nope... as I say later in the post there's more than just looks involved... (feel, system durability, looks too, etc)... but braking distance isn't one of em so long as it's large enough to engage ABS and handle the heat of normal street use (which generally any decent factory brake system is). Now, if it's NOT large enough to engage ABS in the first place there's a problem from the factory- but that might be the answer you were looking for about why they don't put tiny rotors on huge heavy trucks, because it wouldn't be able to engage ABS in the first place... but certainly that's not why they put big rotors on a corvette being it weighs relatively little.

Nope. The coefficient of friction is high enough to engage ABS virtually instantly on any decent factory system (we're talking miliseconds here)- hence a higher CoE won't stop the car any faster.... because, again, -added- force beyond the force that locks the wheel/engages ABS is useless force.

This is why changing to higher friction brake pads (or lower friction ones), as long as both can engage ABS, will not change braking distance whatsoever.

I can quote you a TON of sources that agree with this, including Brembo, Stoptech, and the folks who test police cars for a living... (who don't even bother testing panic stops when comparing brake pads, because they acknowledge different pads are incapable of changing braking distance as long as they can lock the wheels or engage ABS)

Again you seem to confuse "the system begins to fail, and braking distances then begin to increase" with "the system somehow brakes in a shorter distance than it could to start"

Greater fade resistance does NOT improve stopping distance.

Improve means get better.

You never stop in a shorter distance.

You simply manage to stop in the SAME distance for a greater number of stops in a short period of time if the system has greater heat capacity.

ANY decent factory system will show 0 fade for a single panic stop, which we both agreed at the beginning was our definition of stopping distance.

A system with higher fade resistance will maintain that stopping distance for more stops before that distance gets longer- but the higher heat capacity won't ever make that distance _shorter_ than what you started with.

Because you seem to keep confusing "taking longer before braking performance gets worse" with "stopping in a shorter distance than you started with"

Normal street cars won't run into brake fade in normal driving... so a larger braking system will do absolutely zero for them, ever.

Now, if they're coming down off the side of a mountain then a larger system will maintain -the same- performance for a longer period of time/number of stops... but it still won't _improve_ the optimal stopping distance that the factory system could manage the first time.



But again, as long as the car is engaging ABS virtually instantly, which the factory system does already, then larger amounts of brake torque do NOTHING to improve braking.

Once you have more force than the tire/road interface can use then added force doesn't help you.

Certainly weight figures into engineers designing the brake proportioning system, and it's why front brakes do most of the work of the system... but weight has no direct impact on stopping distances of cars that can lock the wheels/engage ABS, because by definition the braking system is already applying maximum possible stopping force, and weight doesn't matter. The only place weight comes into things is the fact mass not being 0 means that gravity is working on the car.

Galileo showed us gravity accelerates all things, regardless of mass, at the same speed. (32 feet per second per second)

Yup... and they get into all that math and more later in the pulp friction article.

He then explains to you why that math stops being relevant once you lock the brakes-

The factory system can already apply enough torque, per static and dynamic vehicle weight, to surpass the limits of the tire/road interface. So more force doesn't make a difference... and going -larger- on the brakes won't stop the car any quicker.

It really wouldn't, unless you're talking about one of two situations:

1) You put on a much -smaller- brake system than factory, so much smaller it can't lock the wheels/engage ABS. Nobody else is talking about such a poor idea though, so I can't image you are either. Certainly the factory system is plenty large enough to engage ABS, and going larger does nothing to stop the car any shorter.

2) You are racing the car and planning to stop repeatedly in a short period of time from very high speeds. Which we agreed at the beginning wasn't how we defined braking distance. Even if it was, your first stop would be the same distance with the factory system as with an over sized one.




Really?

Can you please show me some sources (from someone other than a guy selling large brake kits) that disagrees with any of the science in that article?

Brembo, Stoptech, and many others all seem to agree with him... I'd love to see who agrees with you

Again, his conclusion:


How can greater force, beyond that which locks the wheels, stop the car any shorter?

 

Look, we can agree to disagree here. The article summarizes the generalities of the physics behind braking, but it is diluted. Maybe we are talking about two different things... That said at the end of the day there is a lot more that goes into the engineering of braking systems and brake performance than tires or as described in a 4 page article for the layman. One last point and I will leave well enough alone... There is a lot of research and testing that goes into brake systems in order to comply with FMVSS 105 and 135 guidelines. The research and design incorporates the mechanical components and ingredients that go into brake pads that work in concert as a brake system. These systems are designed for a specific vehicle taking factors such as vehicle perimeters and use into consideration. As an example, if certain perimeters are changed such as replacing an OEM wheel and tire assembly with one that is significantly larger in diameter than the original factory set, it can reduce the stopping power of the brake, as will reducing the contact surface area on a rotor. Increasing the size of the wheel assembly increases the distance from the ground to the center of the wheel which conversely increases the leverage on the brake. As a result it creates an increase in the stopping distances not to mention possible thermal damage to the pads and rotors - and this holds true for modern braking systems. This is why vehicles with much larger wheels have larger diameter brake rotors or drums.

This was an enlightening discussion, you presented many valid points. At this stage I have nothing further to add and I apologize to the OP for derailing, but it's pertinent in many respects to what the OP was trying to accomplish.

 

woo feels like i'm in college physics class again. LOL

Interesting discussion tho bout how brakes work.