DavidRB

Braking response is improved because the bigger brakes are a bigger lever, so a larger caliper/brake unit can reach a given braking force with less brake pedal push than a smaller one. This is the feeling of "improved bite", the brakes seem to start braking more quickly. Better materials also build up the friction more quickly, again reducing the time that the brakes require to begin braking.

The improved sensitivity then comes from not having to push the pedal quite as hard as before (assuming the servo isn't adjusted or changed).

Fade is reduced because of the improved heat dissipation of a larger lump of metal and by using materials better able to cope with the higher temperatures.

Apart from the change in unsprung weight, altering the brakes has no effect whatsoever on the road-tyre interface. The laws of physics and the material coefficients involved don't change.

Mr Leigh

Why would AP 4 pot brakes stop my car any better than my OE 4 pot brakes, especially with uprated pads?

Trout

iTrader: (1)

This may be the wrong place to post this - BUT I am not asking which brakes to fit, but a more general question about road driving.

If I have my standard brakes (4 pot, brake bracket, braided hoses, Pagid Blue FR), on the road I can easily reach the 'limiting static friction' of the tyres - i.e. I can trigger the ABS on a dry road if I press hard enough.

I have recently had the geometry set to significantly reduce negative camber and so I am now running 0.5deg, and so my 'contact' patch is near optimum.

So....if I am reaching the maximum grip level of the TYRE on the road, with reasonably standard discs and pads - how can fitting Brembo/AP/Alcon improve my deceleration - or IS IT that fitting these will merely change the brake feel and reduce propensity to fade on the track?

Answers on a post card please - and yes I have seen Stef brake incredibly late at Donington, and it is scary!

Cheers,

R

Rich_R

R,

I'm also interested to hear the 'techie' explanation for this.

On road/track my AP's definitely stop me much much faster than my MY00 OEM brakes (I also have non standard geometry settings, bump steer removed etc.)

In the dry is it very hard to trigger the ABS with the AP's whereas with the OEM brakes it was quite easy when stamping hard on the pedal.

Any physics experts about?

Rich.

Trout

iTrader: (1)

Rich,

well that answers one part of my question - but I don't understand that standard brakes can cause the tyre/road interface to have a different coefficient of friction!

R

SDB

I have to say that this subject confuses the hell out of me...

Benefits I know to be had (from the driver's perspective) with big brakes..

more progressive *control* over the brakes
less fade
more consistency through the braking zone

I would be really interested to hear an explanation as to why you can stop quicker as I agree with rannoch that the tyre is the thing that's doing the stopping.

Maybe it is the consistency which makes the big difference.

Why on earth should smaller brakes be able to lock the wheels easier?

There must be some kind of clever physics thing going on.

I look forward to reading this later.

Cheers

Simon

EvilBevel

Rannoch, I don't think they can. Total stopping power must be defined by the available grip.

My guess would be however that these bigger brakes can a) come to that ideal compromise between max brake & locking up sooner than the standard brakes (this is important as you do most travel distance as you start to brake), and that the better kits give you more feel on where to find that ideal compromise (different everytime you brake depending or tyre temp, surface, etc...). Then of course there is the fading part.

I personally have the Prodrive 330 mm job, and if they improved anything, it is ... confidence. I can if I'm silly do 100 mph and dip the brakes in such a way that they lockup almost instantly (I realize this is silly, but it might give you an idea of how they perform), I could not do that with the standard brakes.

With the standard brakes I kept trying to find the perfect balance between serious braking and locking (I don't have ABS), with the "bigger brakes" I find it much easier to be spot on every time (and I am in no way a talented driver)


But there are so many variables (better braking is more loading of the front, so less stopping power at the back etc ...) that it is really hard to find a golden rule here. The brakes perform totally different with DS2000 versus DS3000 (I had both)

Anyways, above may be a load of bx, but hope it somewhat helps your thinking.

Mike Rainbird

Hello Peeps,
The only tests I have at hand carried out by AP, are on an Audi A6 Saloon owned by Datron.
They did 12 stops from 80mph with the standard brakes and then fitted AP's four pot CP5200 caliper and 330mm disc and repeated the test. With the standard brakes the car stopped in just over 210ft on the first run 215ft on the second 225ft on the third gradually gettin worse until the last run where it was around 265ft!

With the APs fitted, all twelve stops were done under well under 210ft with fluctuations of only 2-3ft.....

So in answer to your question, yes it does decrease the braking distance (and the pedal pressure required), but more importantly it offers greater consistancy.
Best regards
Mike R

Triggaaar

In the time I've taken to write a reply, it's all been said I think. Oh well, have it all again anyway.

If you can lock your wheels up with std brakes, then they are 'strong' enough to stop you as quick as any other brakes - however, locking the wheels is not the objective (I know you know that, I'm just going through it all) - the objective is to get the wheels as close as possible to locking, for the whole duration of the braking manover, without actually locking the wheels.

So in answer to the question:
how can fitting Brembo/AP/Alcon improve my deceleration
Better quality brakes make it easier for a driver to control thier level of braking, enabling them to keep the car closer to the limit of braking without exceeding it - as I think you've all said basically.

Rich,
You should be able to get ABS to kick in with you APs if that was your aim - if not I'd be surprised - maybe I'll have to test it tonight.

dumpsonscoobs

Is it not to do with bigger brakes means a bigger lever force to the same amount of work by your foot. This means that the bigger brakes take less effort to get the same result. The reason your stopping faster is because you are using less force with your foot which means it is more sensitive and you are able to find the transition between braking and locking more earier than when you just put the boot right in with standard brakes.
It all changes when you uprate the callipers so that they have a larger surface area of moving pot making it harder to brake again but if you compensate for this with a better lever at the pedal then you end up with amazingly sensitive brakes and find it even easier to find the transitional point.
All guess work
Martin

EvilBevel

Triggaaar, know what you mean

Everytime I post on this BBS, and go back to see how eloquently I have put my incredibly knowleadgeble <sp> response, 3 other people have put more info in less words

But to get back to the topic: if you throw in, say, road legal slicks with (for the sake of the argument) better friction; how would that interact with the standard versus "bigger" brakes ?

JayDee

I think its to do with heat dissipation

at relatively low speeds probaly not much difference, but at high speeds the smaller, ungrooved, undrilled discs would not be able to dissipate (get rid of) heat quickly enough and so a degree of fade would occur VERY FAST (microseconds).

Don't forget that all the kinetic energy of the car (proportional to the square of the speed) is pretty well transformed into heat in the brakes (aerodynamic effects excepted)

JD

cwal1

There is another factor in the argument against OE brakes, caliper spread under heavy braking. Poor quality ie OE calipers, actually spread when heavy braking pressure is applied, that is to say that the 2 halves of the caliper are forced apart by pistons during the braking action thereby reducing the anount of pressure fed to the discs and also reducing the feel and consistency of the brakes. Good quality brakes from manufacturers such as AP will have most of this flex designed out.

CW.

Geezer

I'm sure Mr Lines of Prodrive fame can get a decent explanation from one of their engineers. After all, Prodrive do have a little bit of experience in setting up cars!

Geezer

robski

Nice question R, Ive always wondered the same myself!

My guess would be that the main improvement would be at higher speed, where the aftermarket brakes work more efficiently.
Obviously the OEM brakes are not designed for "serious use", so they will suffer from fade.

This raises a question in my mind tho, do tyres generate more grip at higher speed?
If not you should be able to invoke the ABS at any speed, and Im sure I havent at high speed, but I have at lower speed?

Maybe the bigger brakes are more consistent in the force that they apply, so you can be right on the limit, but rarely over it, where as the ABS is working on the OEM brakes, which means you are loosing braking whilst the ABS is allowing the brakes to unlock the wheel?

I also wonder why a locked wheel is so variable in its braking effect. Ive seen a few tests where a locked wheel stopped the car much quicker than the ABS could (ABS disabled). It also took much longer than the ABS. Over a number of runs, at the same speed, on the same surface, on the same day etc etc, it seemed completely variable (and they did cool the tyres and brakes after each run).

WHY!?

robski

Pete Croney

Mike Rainbird has hit the nail on the head, its all about what happens after the first big stop.

From 70, all Impreza brakes will do a good stop. Try it 3 times, or try it from 120 and its a different matter. Try it on a track day or a 30 minute country lane blast and the difference is chalk and cheese.

Tyres are a big factor and its one of the reasons why slick shod cars on a track day are so much quicker. Of course they help the cornering, but give you the ability to load the brakes by a factor that could not be dreamed of on road tyres.

Beware cold slicks, as any one following me into the hairpin at Trax, will have witnessed

EvilBevel

Pete, Mike R talked about "well under". I read that as, "a bit shorter distance, but the "every" time bit is a lot more important.

This link

Trout

iTrader: (1)

So we could be bottoming it out to heat dissapation in a single enthusiastic stop - and the build up of heat from multiple stops.

I am less convinced about the leverage issue - I totally buy that it could be more consistent with more feel, but a locked wheel (exceeding the limiting static friction of the tyre/road interface) is a locked wheel. Anyway, anyone who has driven a Volvo or a Merc will experience enormous braking power from barely a shove on the pedal.

Robski,

a locked wheel is inconsistent in performance due to the relationship between the static friction and surface effect. Simply gravel has no grip and so the ABS will provide little or no braking power. However a locked wheel in gravel effectively starts ploughing a furrow which has a significant retardation effect. The same is true for snow and other similar loose surfaces.

Thanks for all your replies guys, very interesting!

Cheers,

R

Mike Rainbird

Evil,
Sorry, that's my crap self editing. I meant to put under 210ft, then noticed that most were around the 205ft mark (hence why two unders ) so put well under instead.
Mike R



[This message has been edited by Mike Rainbird (edited 19 September 2000).]

MorayMackenzie

Rannoch,

The "enormous" braking effect exhibited by some OE brake systems (Volvo, Audi A4 etc...) is often because the brakes are "over-servoed", i.e. the brake's are power assisted to a greater extent than most... so your little pedal input is aplified greatly before acting on the brakes. I found it suprising how sharp the brakes on the Audi A4 felt, but I believe they could be quite dangerous too, especially on a performance car driven on, or near, the limit.

Moray

Trout

iTrader: (1)

Moray,

my comparison with overservoed cars was rather tongue in cheek - I merely wanted to underline that greater leverage wouldn't stop you any faster but take you to the limit of grip faster.

I do accept that better leverage, with feel is a GOOD THING tm, but isn't the whole answer as to why bigger brakes are better.

R

GranTurismo

I think its because.......

Energy can not be created or destroyed, it can only change type. To remove the momentum of your car the brakes turn it into heat energy but he action of friction on the disc/pad.

Tires are in friction with the road surface as well in order to provide grip. Your tires can maintain a certain limit of friction before they skid. If the friction between the pad and disk exceeds what the tires/road can contain then you "lock up".

Bigger breaks work better because they can convert momentum to heat faster, they are still likley to lock up BUT the key thing is that due to having a larger surface area they are able to create and disipate more heat BEFORE they reach the point of lock up. Hence they are able to stop you faster.

The more heat you can generate before the friction on the pad/disc exceeds tyre/road the better.

I am fairly sure thats correct.

Tyson

A major factor is probably that most of the braking is performed by the front wheels. I.e. triggering the ABS with standard brakes in a straight line is likely to be affecting the back wheels only (which stops them from skidding to avoid reduced friction with the road). This will be evident if you lock the wheels on a car without ABS, since the back end breaks away). With a brake upgrade, you will significantly enhance the braking ability on the front wheels (increase the bias to the front wheels further) which will improve the car de-acceleration. Plus other reasons of brake pedal feel etc.

NO expert, just common sense???

GranTurismo

The back end of a car can skids away under braking becuase deceleration moved the center of gravity of the car forward. Becasuse the friction of the back tires on the road is a result of the mass acting down uppon them then the friction (grip) is reduced and you slide.
This is unrelated to waht is happening at the front (abs or not)

A tyre provides "Grip" in two main directions forward/Backward for accelleration/braking and left/right conering. When the mass of a car moves forwards there is less friction at the reat tires to support left/right forces so its slide time!

mphnw11

Off thread I know - but here goes

>A thermodynamics professor had written a take home exam for his
>graduate students. It had one question:
>
>"Is Hell exothermic (gives off heat) or endothermic (absorbs heat)?
>Support your answer with a proof."
>
>Most of the students wrote proofs of their beliefs using Boyle's Law
>(gas cools off when it expands and heats up when it is
>compressed) or some variant. One student, however, wrote the
following:
>
>First, we need to know how the mass of Hell is changing in time. >
>So, we need to know the rate that souls are moving into Hell and the
>rate they are leaving. I think that we can safely assume that once
>a soul gets to Hell, it will not leave. Therefore, no souls are
>leaving. As for how many souls are entering Hell, let's look at the
>different religions that exist in the world today. Some of these
>religions state that if you are not a member of their religion, you
>will go to Hell. Since there are more than one of these religions
>and since people do not belong to more than one religion, we can
>project that all people and all souls go to Hell. With birth and
>death rates as they are, we can expect the number of souls in
>Hell to increase exponentially.
>
>Now, we look at the rate of change of the volume in Hell because
>Boyle's Law states that in order for the temperature and pressure in
>Hell to stay the same, the volume of Hell has to expand as souls are
added.
>
>This gives two possibilities.
>
>#1 If Hell is expanding at a slower rate than the rate at which
>souls enter Hell, then the temperature and pressure in Hell will
>increase until all Hell breaks loose.
>
>#2 Of course, if Hell is expanding at a rate faster than the
>increase of souls in Hell, then the temperature and pressure will
>drop until Hell freezes over.
>
>So which is it? If we accept the postulate given to me by
>Ms. Therese Banyan during my Freshman year, "That it will be a cold
>night in Hell before I sleep with you," and take into account the
fact
>that I still have not succeeded in having sexual relations with her,
>then #2 cannot be true, and so Hell is exothermic.
>
>The student got the only A.

crusher

This has been very interesting for me to read as I did part of my degree (a few years ago!) in Physics and have also experienced the brake upgrade phenomenons first hand. It has been explained by many contributors in slightly different ways,the sum total of which I think would say it all.
The main point to remember is that friction between tyres/road and disc/pad combinations is always directly proportional to the load.
ie.IF you've got more load in the car 2 things happens.
1 the load per mm tyre/road goes up,
2 the pads can exert more force on the disc(presumming that the calipers don't spread)
before the wheels lock(more torque turning wheel)
BUT....
YOU take longer to stop because the amount of kinetic(moving) energy that needs to be converted into heat(what's actually happening at the brakes!)has gone up considerably.....
Now if you replace the weight analogy with more or less stopping area provided by bigger brakes,and then add a bigger coefficient of friction ,provided by improved braking materials,the bigger brake ,improved material question becomes clear....bigger brakes equates to better stopping distances as the frictional area is increased allowing for more force to be exerted before lockup occurs....overheating/fade is overted as the bigger brakes are also bigger heat sinks,although venting etc is cruicial as the brakes need to disipate this heat quite quickly.
The other effects such as improved feel and more progression are a direct result of more braking power .....imagine trying to stop with smaller/non vented discs!!!!!!! after 1 or two hard stops the brakes would be useless as their ability to convert kinetic to heat energy is too small,the result is overheated pad material,overheated discs/calipers and no brakes!!!

[This message has been edited by crusher (edited 20 September 2000).]

Don Palmer

Its all about heat capacity. Quite right whoever said it's all about changing energy from momentum into heat. Big brakes do it better.
Size matters.
More ability to dissipate heat is what it's all about. Carbon brakes do it at very high temperatures and very high wear rates - the disc and pad only last 200 miles!
The fact that you can do a stop once is irrelevant if you want to do a number of stops and the heat generated by one stop is not dissipated by the time you do the second stop. It follows that the brakes overheat very quickly indeed and this causes distortion of discs possibly even cracking in extreme cases.
Big ones handle more heat and get rid of it better - venting and drilling do this even better.
Don Palmer - driving development

Stef

Thanks Mr Osmond.

Stef.