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Question Physics Question about Brakes

How is it that a larger brake caliper (e.g., Big Reds) can stop a car faster than OEM/smaller caliper? The hydraulic pressure is generated in the master cylinder. Therefore, even with a larger volume caliper piston and larger pad, the total force against the rotor should be the same, right?

I thought that I remember from high school physics class that the net friction doesn't change with the surface area. It's a function of pressure and coefficient of friction, is it not?

Stated another way, doesn't a 4 square inch brake pad produce the same amount of friction as a 6 square inch pad, as long as the pressure applied via the brake line is unchanged and the coefficient of friction of the pad material is the same, assuming same rotor diameter? (Does the big red conversion use larger diameter rotors too?)

(This one should really set the Los Alamos crowd off!)
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Old 02-03-2004, 07:00 PM
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So close....

You are on the right track...

1. Co-efficient of friction..what types of pads you use
2. Pressure created by the Master....
3. Size of rotor
4. Clamping force (can vary according to piston size, number of pistons)
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Old 02-03-2004, 07:07 PM
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I thought with a larger rotor you had a larger surface area making contact with the brake pads per revolution of the wheel.
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Old 02-03-2004, 07:19 PM
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Okie dokie, my turn to take a shot at this.
Remember form high school physics (or sometimes in chemistry), when you guys talk about hydraulics, the larger the surface area of the secondary cylinder, the more pressure it will create.
However, it will also require more travel of the smaller cylinder to create this pressure.
So, 4 piston calipers and things like that are able to apply FAR more pressure to the brake pad by having a larger surface area on the inside of the hydraulic cylinder(s).
Also, the area of the brake pad has nothing to do with the friction it creates. You are correct there. However it does have something to do with durability.
The biggest other difference is, when people upgrade their brakes, they also upgrade the rotor size. Like ae1969 said, size of rotor does make a big difference.

So, I hope I didn't confuse you or anything by trying to answer your question.
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Old 02-03-2004, 07:19 PM
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The total piston size at the calipers will increase total braking power. Swept area will NOT. Larger diameter or thicker rotors do not help stopping distances, they are only capable of handling more heat. This does not come in handy (or shouldn't anyway) for ONE stop from 60. For that the only variables are the mechanical(/hydrolical) advantage from the pedal to the pad/rotors (piston sizes basically), rotor/pad material, and tire grip as well as suspension geometry/weight shift, and brake bias. For practical purposes, beyond that nothing matters.
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Old 02-03-2004, 07:30 PM
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Oops, looks like I was mistaken there....

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Old 02-03-2004, 07:42 PM
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can't the smaller surface area still stop the car just as fast?

the reason i ask this, is because, the main idea is to push the brakes all the way to the limit before they lock up... so as long as you don't lock up, isn't the stopping time/distance the same?

wouldn't bigger brakes lock up easier, the only difference is they need less brake pedal pressure to stop for the same distance?

this is what confuses me on the subject...
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Old 02-03-2004, 09:46 PM
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You guys have it all wrong!

It stops faster because it's RED!
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Old 02-03-2004, 09:47 PM
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I believe larger rotors do make a difference. It's like using a breaker bar on your socket wrench. The farther out you are, the more torque you create with the same amout of force.

Yes, whether you lock up your tires with big brakes, or little brakes, it will still stop just as fast. That's why for every-day driving I have not upgraded my brakes. They only need to last through one emergency stop at a time, not several in a row (I hope!!).
But at the same time, it will take less pedal pressure, and sometimes stock brakes cannot lock up the wheels, like if you have racing rubber on a very heavy car. Quite often you need to upgrade the brakes if that is the case.
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Old 02-03-2004, 09:49 PM
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Quote:
Originally posted by ds475
can't the smaller surface area still stop the car just as fast?

the reason i ask this, is because, the main idea is to push the brakes all the way to the limit before they lock up... so as long as you don't lock up, isn't the stopping time/distance the same?

wouldn't bigger brakes lock up easier, the only difference is they need less brake pedal pressure to stop for the same distance?

this is what confuses me on the subject...
You are correct.

Assuming the "smaller" brake system (rotors, calipers, pads, hydraulics, etc) has the ability to lock the wheels, then the maximum braking that can be achieved is limited by grip of the tires. If you brakes can keep the tires right at the limit before lockup, then you will achieve the maximum braking performance.

Other folks have mentioned that larger calipers provide greater moments of torque and so on. Also the brake "feel" is affected by things like SS brake lines and things of that nature. Everything they say is correct, but all that does is get you to the point of locking your wheels more quickly.

Again, if your brake system can lock the wheels in a given set of circumstances, then fitting larger brakes will not decrease your braking distances in the same set circumstances.

So why fit larger brakes?

Brake systems lose the ability to lock the wheels when they get hot. Brake fluid boils, pads glaze, etc.

Where "bigger" brake systems improve braking performance is that they provide a larger capacity to absorb (and dump) heat. This allows them to maintain braking performance over a larger number of braking operations within a given period of time.

Plus they look great!

The downside to fitting larger brakes is that they increase unspring weight (unless you fit exotic PCCB brakes) and so affect handling.

Karl.
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Old 02-03-2004, 10:57 PM
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Quote:
Originally posted by Zero10
I believe larger rotors do make a difference. It's like using a breaker bar on your socket wrench. The farther out you are, the more torque you create with the same amout of force.

Yes, whether you lock up your tires with big brakes, or little brakes, it will still stop just as fast. That's why for every-day driving I have not upgraded my brakes. They only need to last through one emergency stop at a time, not several in a row (I hope!!).
But at the same time, it will take less pedal pressure, and sometimes stock brakes cannot lock up the wheels, like if you have racing rubber on a very heavy car. Quite often you need to upgrade the brakes if that is the case.
I fully agree.

Karl.
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Old 02-03-2004, 10:59 PM
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Quote:
Originally posted by Zero10
when you guys talk about hydraulics, the larger the surface area of the secondary cylinder, the more pressure it will create.
pressure is constant, force is what is increased or deacreased: Bernoulli's priciple.

i thought the benefit of larger rotors was that the brake pads travel farther. for example if you move from 10" to 11" rotors the brake pad would travel an extra 6" along the outside of the rotor per revolution. requiring fewer revolutions to slow the car.

im not sure if brake pads on a rotor is a torque
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Old 02-03-2004, 11:48 PM
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Yep, any rotational force is torque.
It's because the pressure is applied farther from the point which the rotor revolves around.

It's like using a longer socket wrench on a stuck bolt. Except the bolt is turning, and you want to stop it.

Although, I am not exactly sure about the effect of having the extra 6" along the outside of the rotor. Since that means the rotor would be spinning faster, and the coefficient of kinetic friction goes down as the velocity squared goes up. Although the increased mechanical advantage more than makes up for that.
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Old 02-04-2004, 05:08 AM
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I don't see how bigger caliper pistons will apply more force if the hydraulic pressure generated by the master cylinder is the same. I do understand how a larger rotor has more leverage to stop better.

Assuming that your OEM braking system can bring the car to a lock up as fast as an aftermarket system, single stop would not be any quicker/shorter with the aftermarket. Rather, you're limited by tire adhesion (leaving ABS performance issues out of the equation).

As we know, brakes work by converting the kinetic energy to heat (via friction). I don't want to rekindle the discussion of drilled rotors, but the heat dissipation is a big part of the key for repeated braking performance. So a rotor/pad that can dissipate the heat faster would be major way of boosting stopping power for repeated braking, i.e., race conditions.

Btw, a 11-inch rotor doesn't turn any faster or slower than a 10-inch rotor. Rotational speed is based upon the diameter of the tire. I would agree that the outer edge of the 11-inch is traveling faster, however.
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Old 02-04-2004, 08:01 AM
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Yes, my explanation was a little unclear, when I said it was moving faster, I meant the outside edge.
The reason it creates more clamping force is because the clamping force is created by pressure in the fluid.
The greater the surface area this pressure acts on, the more force it applies. However, at the same time, the more travel is required on the master cylinder to move the slave cylinder. (or rather cylinder in the brake caliper)
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Old 02-04-2004, 08:33 AM
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Hahah!
My turn. Actually I asked a lot of these questions at a tech session last weekend that had reps from Brembo (they brought some sweet GT/GTP kits). A lot of what has been said is true for brake system upgrades. What you have to answer is why you're doing the upgrade. If it's just for looks, whip out the caliper paint.

OEM brake sysytems are designed to work in all expected driving conditions and are balanced to the car's weight and performance to stop it efficienlty, but.... expected driving conditions do not include autocross, time trials, etc. Also modifications to the engine performance (hopefully an increase) require upgrades to the brake system to compensate. This is typified when the 951 was equiped with the 928 4 piston per caliper system including larger rotors that required 16" wheels to offset it's marked increase in horsepower.

If you plan on doing repeated braking in a high performance environment, then you need more than the OEM system, as it was not designed for this purpose.

1. Calipers - not necessarily larger, but individual pistons vice the single sliding frame OEM version gives more direct control and instantaneous action both in application and release. Also allows application of more and more evenly applied pressure to the rotor to reduce hot spots and warpage.

2. Rotors - 2 issues, rototional size and heat dissapation. These must be balanced with car's performance and use. Larger rotors allow more brake torque, and faster slowing, but also require more horsepower to accelerate (rotational mass). Slotted rotors are normally designed for race applications; they improve cooling ,allow gas escape under high braking, but also expend more pad material through refacing. This inhibits the affects of pad glazing, but will require new pads at each performance event (i.e. not a good system for street applications). Drilled rotors have a huge impact on cooling. This directly impacts your ability to use the brakes in a high performace environment, while preventing brake fade due to overheating, warping, and some glazing. Also it will not cut through pads like slotted rotors, so it is better suited for street cars that participate in performance events. Unfortunatley combining both slots and holes does not result in better braking, but a weeker brake system that will require pads more often and possible failure due to overly weekening the rotor's integrity. As far as cryrogenic treatment goes, it improves flat OEM rotors to performance specs near drilled rotors, but shows almost no improvent in the performance of drilled or slotted rotors.

3. Pads - Always a trade-off (i.e. you can't have it all). Reducing noise and dust requires giving up performance. Increasing performance means dust, and possible noise. Also using metallic or carbon compound pads will increase braking performance (better friction and heat dissapation specs), but will wear the rotors quicker. Brembo recommends replacing rotors every third replacement of the pads. BMW has brembo make their OEM pads and rotors matched for wear, so that rotors and pads must be replaces simultaneously.

4. Brake lines and fluids. - The biggest problems is heat. boiling the fluid due to overheating = loss of braking. Also OEM rubber lines become more flexible with heat leading to loss of braking pressure and failure in extreme situations. Performance driving require high quality brake fluid and braided stainless steel lines.

Combined together for a 944: Turbo 4 piston calipers+drilled brembo or zimmerman rotors+HP pads (PBR, Hawk, Pagid)+HP brake fluid+Braided SS brake lines = top of the line braking for AX, TT, or DE events

As always just my educated $.02
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Last edited by lousailor; 02-04-2004 at 03:04 PM..
Old 02-04-2004, 08:54 AM
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Larger or multiple pistons also do a lot better job spreading the pressure evenly across the pad causing less pressure points on the pad. I.E., a single piston in the middle will focus it's pressure in the middle of the pad, leaving the sides to flex and not apply as much.

But this still doesn't matter if you still have the same traction on road.
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Old 02-04-2004, 09:17 AM
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Silver is your car in ID? What mods have you done? Any pics? I'm Navy in warmer but not quite so sunny today SoCal
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Old 02-04-2004, 09:25 AM
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Yea, I'm only in Iceland for a few months. Mods are GURU MAP, Tial wastegate, Konis, 968 M030 rear sway, Cups w GY Eagle F1GSD3, stock PDs with Hoosiers, and a lot of other small things. Sorry, I don't have any pics here. I used to have some of me at the Bogus Basin Hillclimb but they got taken off the website.

I can't wait to get out of this country and get back to Idaho (I never thought I would say that!).

Oh, back to brakes. With cross-drilled rotors you can't absorb as much heat. You may be able to dissapate that heat a little faster but your braking will suffer under sustained rapid use. So for an autox or tight road course cryoed rotors are probably the best.

In conclusion of all these posts: If you want to stop FASTER get better tires, get a better left foot(unless you are one of those cheaters with ABS), and get your brakes biased correctly.
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Old 02-04-2004, 11:48 AM
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Hmmm,
The guy from Brembo was recommending drilled over cryo, unless racing and then he rec'd a floating slotted rotor.
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Old 02-04-2004, 12:20 PM
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