Silly question about "big" brakes

[irrotational]

How does having a bigger pad area or more pistons make brakes work better?

 

If standard brakes are strong enough to lock a wheel then you have used all the available grip from that wheel - having bigger brakes or whatever will not increase that amount of grip?

So how are they better?

 

I am sure they are - i am just too stupid to see why?

 

Can someone put me out of my misery?

 

Is it purely overheating/brake fade?

 

secondly - what difference does "venting" discs make?

 

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this is a local sig for local people!

[Gambo]

The big brakes will work better for longer, as they can disipate more heat, thus not over heating.

 

RED 2.0 HPC 230BHP here

[Tony C]

Agree with Gambo. Brakes work by converting Kinetic energy into heat energy - the trick then is to dissipate the heat back into the atmosphere (Global Warming). Ventilated discs help with heat dissipation. The cooler the brakes when being used the more heat they can generate. If they get too hot they can't convert as much kinetic energy (fade) and you might physically damage the discs or pads or boil the fluid.

 

BRG Brooklands SV 2.0L Ammo Duratec 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery)

[Billyboy]

As has been said its all longevity of the effectivness 😬

 

BIG BOAT BIG ANCHOR

 

DRIVE AN SV today ❗

[Sheds Moderator]

If you look at a std disc it's solid, like a dinner plate. A vented disc is like 2 plates with an air gap between (if you look round the edge you will see metal-air-metal-air so that the disc doesn't collapse when it is squeezed) and this allows cool air to flow from the middle of the disc to the outside.

[Rob Walker]

Heat dissipation is only part of the story. the big brakes have a larger piston and pad cross sectional area so if you apply the same pressure you will have a greater breaking effort.

[Tony C]

That's a potentially misleading

 

Indeed you can get the same braking with less pedal pressure with 'bigger' brakes, but no significant increasing in overall braking efficiency.

 

If you grab the disc harder for more friction you'll lock the wheel and the heat generated between tyre and road is less than within the brake. The wheel must be driven by its friction against the road to generate the heat in the brake - a function of weight.

 

Maximum straight line braking is achieved when the wheel is going round about five percent slower than it would be if the brakes were not applied - any more skidding than this will decrease braking efficiency.

 

The only advantage of 'bigger' brakes on your 7 is the improved ability to dissipate heat.

 

BRG Brooklands SV 2.0L Ammo Duratec 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery)

[Sheds Moderator]

You also need to consider relative areas of cross sectional area in master cyl and slave cyl. If your master cyl is 1cm sq and you are able to push down with 750N of force (this force is equal to that required to support a man's 75kg body on earth) (assume no mechanical advantage at the pedal, ie a 750N push from the foot = 750N on the cylinder) then you generate a pressure of 750N per cm-sq. (OK so far?).

 

If your slave cylinder (at the caliper end) has an area of 10cm sq then the force at the brakes is 10 times that at the foot end, since the pressure must be equal in a closed nydraulic circuit. This means 7500N force on the brakes. Now if the area is bigger than that, there will be more force. The downside of this is that because it's a closed system 1cm movement at the foot end equals, in our original system, 1/10cm movement at the brake end. This is known as the not getting anything for free principle. The bigger the area at the brake end the worse this gets and you can get to a situation where you have sensitive brakes at the price of having to push the pedal a long way down. Obviously there is a compromise.

[Rob Walker]

Tony C,

 

I have never heard such bolloxk. " the only advantage of the big brakes is the ability to dissipate heat " Have you tried using the uprated brakes from 140mph ?

 

 

[mav]

 

 

Chomp

 

Chomp

 

Chomp

 

😬

[Tony C]

Physics, just physics

 

The friction of the wheel against the road drives the brake disc and you apply friction between the rotor and stator of the brake. The weight of the car, and to a certain expent, the grip of the tyre on the road is the driving force. If you apply too much friction in the brake the wheel can no longer rotate the disc and the tyre skids - end of story.

 

D = V sqrd / d,

where d = F/m

 

(D = distance, V = speed, d = deceleration, F = net deceleration force and m = mass)

 

BRG Brooklands SV 2.0L Ammo Duratec 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery)

[Rob Walker]

Tony,C

 

You are missing the point. Sure at low speeds you can lock up the wheels with little peddle effort with both the standard and the uprated brakes everbody knows this you do not need to quote the theory at me. Where you get the big advantage with the uprated brakes is when you brake from high speed and really brake hard when the wheels and brake rotors are turning at their highest speed, as the speed is retarded you modulate or come off the brakes as you get down to your desired corner speed and before you lock up a wheel. The standard brakes even with exotic high friction pads and pressing the middle peddle for all you are worth cannot give you this level of retartation. I run with uprated brakes all round on my car.

 

Rob

[neil.cavanagh]

and do you need the extra heat dissipation of big brakes? Many of the graduate and academy race series run with bog standard brakes and they don't ever seem to get brake fade.... A Caterham is light, so will never have as much KE to dissipate as a big heavy car. Yes someone like Rob running slicks and with lots of BHP might start to push the limits of the standard brakes on track... but most of us won't.

 

Even in my lardy SV I never had fade or over heating of the front brakes with the standard set up. I have upgrades to Alcon calipers on the standard discs, which give a more consistent pedal feel than the standard brakes. I still have the standard discs and even in my lardy SV have never had any suggestion of brake fade (queue Rob telling me to drive faster!! 😬 😬 😬)

[Rob Walker]

Neil,

 

It seems that I am still confusing people. It is not about brake fade either, problems with brake fade can be cured by a change in friction material.

 

Its all about the sheer amount of breaking effort that you can apply, obviously you need the grip but with uprated brakes it is possible to brake confidently much later and harder before a corner from high speed.

 

Rob

 

 

[Jason Plato]

the uprated option of AP brakes front and rear provide more feel and modulation during braking in my opinion. The std rears go off very quickly.

 

Neil - if the stock brakes were ok then the r400/durateccsrs would run with them. The Grads etc are entry level and as such use std cars. They dont have slippy difs or a many of the extras available - that doesnt mean the extra kit isnt a nice to have.

 

At the end of the day you dont actually need any of the uprated gear but it will improve your enjoyment and sometimes safety.

[Tony C]

Perhaps we are approaching the problem from opposite ends here.

 

Optimum straight line braking is achieved when the wheel is retarded by 5%, so it is slightly skidding - maximum kinetic energy transfer to heat in the brakes, once this state is achieved you cannot improve the efficiency of the brakes. The heat generated in the brakes must be dissipated to the atmosphere at a rate sufficient to prevent the maximum operating temperature of the pad material from being exceeded. If the pad temperature exceeds their maximum they lose efficiency - brake fade.

 

Big brakes can dissipate more heat - QED

 

If a brake fade problem occurs with standard brakes, big brakes will reduce that problem. But if you can achieve optimum braking with standard brakes, big brakes will make no difference to the stopping distance.

 

See, we agree'd all the time

 

BRG Brooklands SV 2.0L Ammo Duratec 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery)

[Rob Walker]

TonyC,

 

Anyone can quote from a book its not big and not clever. You stick with your standard brakes and your text books if it makes you happy. Have you informed your insurance company that you have fitted a 2L Duratec and done nothing with the brakes ???

 

Rob

[Sheds Moderator]

Rob, with all due respect you need to read critically what Tony is saying. Standard brakes will stop any Caterham from 150mph regardless of the engine fitted, once. The rate of braking is the same in either case as the std brakes will lock the wheel *at any speed* and will therefore achieve this notional 5% slower than road speed figure for max retardation. You may have to shove the pedal harder (see earlier) but there you go.

 

Where the difference comes in is that big brakes will do this 10 times over and still come back for more. Std brakes by this time will be red hot and that much closer to boiling fluid, overheating pads and brake fade. This is only a problem if the brakes are to be used heavily and repeatedly - IMO if you get a 7 to brake fade on a public road then your licence/life will fade a lot faster than your brakes. On a circuit you have more chance of overheating, having said that the 140 bhp 7s use std brakes for racing and report no fade at the front.

[Rob Walker]

Boss,

 

Thankyou for the lecture, it may surprise you that I can read, but I cannot spell.

 

I would like to see you lock a set of front wheels fitted with slicks at 150 mph with standard brakes. I have to concede that if enough pressure could be applied to the brake peddle it should in theory be possible to lock the wheels but in my experience this would not be possible in practise unless your name is Arnold Swarzenagger.

 

I have deliberately refrained from getting into the technical advantages of the big brakes but another factor is discs size. The AP disc rotors are bigger in diameter than the stock disc as is the swept area this gives greater braking effort through increased leverage and friction material contact patch.

 

Rob

[Tony C]

Sorry Rob, not trying to be big or clever, it just so happens that I teach Aircraft Performance to ATPL students and braking efficiency is part of the sylabus, so I happen to be familiar with the physics involved.

 

It's a free country and you can go on believing that bigger brakes, in and of themselves, will decrease optimum stopping distance. I'm not disputing that big brakes improve brake efficiency from the point of view of brake pad fade, but if standard brakes don't lose efficiency due to overheating and you don't run out of foot force, fitting bigger brakes won't help.

 

We all "believe" some things to be true, but you can't beat cold hard facts.

 

BRG Brooklands SV 2.0L Ammo Duratec 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery)

[Billyboy]

There is only one good reason for the brig brakes is they look cool the black caterham caliper with CATERHAM in silver and the big vented disc make you feel like you are in a real fast car and you are ready to tackle any motoring eventuality with NO brake fade!! and thats a good thing as we all need less to worry about in this fast paced ruff and tumble world of LOW FLYING................ 😬

 

But it is all about the heat or lack of its desire to to hanging around your fronT brakes for longer than it needs, without all the mumbo jumbo theorising SEE

 

here

 

 

 

HEY MAV share your popcorn Want a Coke! or a hot dawg mmmmm

 

 

 

Bill

[Rob Walker]

I wish I had been given all this knowledge before spending loads ov money on brakes and exotic pads. When all I needed to do was to push the peddle harder, its so simple a concept its brilliant. 😳

 

Edited by - Rob Walker on 17 Jul 2007 12:44:08

[Colin Mill]

Can I just chuck a few figures in here for contemplation.

 

Standard front discs have a working area that has an ID of 110mm and an OD of 200mm and 10mm thick. Assuming cast iron this weighs almost exactly 2kg. So the two front disks weigh 4kg and have a thermal capacity of 2kJ per degree C.

 

The KE of a AUW 600kg car at 150mph is 1.35MJ so, if we assume that 80% of the braking effort is on the front wheels and ignoring the losses in tyres and aerodynamic drag or any cooling from the airflow the rise in temperature of the disks will be 0.8 x 1.35MJ/2kJ = 500 deg C

 

From what I can find organic pads are OK up to about 800 deg C so on the face of it the single stop should be OK. However, a slightly more difficult sum is perhaps in order to calculate the temperature of the disc surface during braking as the heat will not diffuse instantly through the full thickness of the disc so, initially the middle of the disk will be cold with the dissipated heat in the 'skin' of the disc.

 

Colin

[Sheds Moderator]

Rob, I suggested you read it *critically*. I know you can read, but on the basis of your replies it looks to me like you aren't being too critical of *exactly* what is being said. I think all of us readers would agree that that's a different skill.

 

With regard to "not stopping a 7 from 150 mph" I have stopped a Triumph Spitfire from 100mph a few times with these very brakes. This has a KE of about 1MJ, compared to our 7's 1.35MJ at 150mph. Somehow I think that the Triumph engineers built in a bit more of a safety factor than 1/3 so doing it once would be possible, bearing in mind that Tony's aircraft physics, Colin's calculations and my fag packet sums all agree. Bear in mind also that the Spit brakes were one of the few bits of that car that were better than average and stood up to racing.

 

You are absolutely right that the big brakes come into their own after a few heavy braking sessions, but this is about heat dissipation, not one-off braking force. I don't doubt also that the pedal feels nicer as you are approaching your corner at 150mph, but this is not the point.

[Jason Plato]

all I know is that when I drove Red Daves car on std brakes, I came charging into the hairpin at Pembrey doing 120mph and forgot that I was driving his car, I stood on the brakes and looked over my shoulder at the apex going past me , then on the next corner the rear brakes locked up before the fronts on trail braking into the bottom hairpin .

 

for whatever reason the big brakes slow you down far faster than the little brakes.

 

Maybe I should write to the F1 guys and suggest they use Sierra calipers instead of huge AP's and save some costs 😬

 

Edited by - Dave Jackson on 17 Jul 2007 16:41:08