Greater application force, immediate response

[VW Alignment Tech]

More braking questions for A5 exam.

When the brakes are applied, which type of brake offers immediate response? A)drum B)disc C)both A and B D)neither A nor B.

I answered A because of the metering valve (assuming the vehicle has one) as it delays pressure to the front disc brakes to allow pressure to build up in the rear drums first. (of course, I'm assuming the vehicle has disc in front and drum in rear).

Which type of brake requires greater application force and is commonly used with power-boost units? A)drum B)disc C)both A and B D)neither A nor B.

I answered D. It seems to me that both drum and disc brakes are used with power boost units but that drum brakes require greater application force (and I'm guessing at this one).

[TheSilentChamber]

Question 1) assume nothing.

Question 2) Disc brakes require more pressure then drum brakes so (although I dont know a single thing made in the last 30 years without power brakes) they would be the ones with the booster.

[UncleBob]

not a very well worded set of questions IMO, but question one would be "neither", since with properly adjusted brakes, both types would apply immediately.

The second question is kind of an odd one. There isn't hardly any modern vehicles without power assist, and all the older ones that didn't have it was drum brakes, but I don't see how that necessarily means that drums require less effort.

My 12 pistons worth of front brakes on my bike can do a stoppy with just my pinky finger, with no power assist. Totally different application here, but the point being, the design of the entire system is the major defining point here.

So to answer what I think is the "correct" answer, I'd pick "both"

[Black Lotus]

I don't see what the trouble is with question B.
Drum brakes have been designed with a "self energising" effect (self servo effect) since Christ was a corporal.
Drums will typically take less effort to apply than discs.
The self servo effect also tends to make them less predictable than discs.

[JustSayGo]

These questions mention nothing about brake systems or type of vehicle. They are related to the history of brakes and design evolution. Avoid assuming or adding information that is not provided in the question.The correct answer to question 1) is B, the disc brake will provide more immeadiate response with less hydraulic fluid movement compared to a drum brake.

Question 2) is related to question 1) correct answer is also B, the disc brake has less hydraulic leverage force than a drum brake. Disc brake designs were explored before internal expanding shoe/drum brakes. Early disc brake designs were so ineffective that the disc brake design was tossed in the "they will never work" file by many engineers until 50 years later when vacuum power boosters were added to brake systems.

[SaabJohan]

These questions are basically impossible to answer because they are more depedant on the actual brake design than the type of brake used.

Both brake discs and drums generate torque by using the same basic principle; by pressing a pad against a rotating surface. The negative torque generated by the disc or drum is independant on pad area, but it is dependant on the friction coefficient between the two friction surfaces, the force pressing the pad against the rotating surface and the effective mean radius of the rotating surface.

Both systems can be actuated by hydralic, electric, mechanical and pneumatic devices. In very "light" applications such as bikes, mechanical actuation in the form of a wire or similar may be used. In "medium" applications such as cars, motorcycles and also some bikes hydraulics is the most common method. This may be power assisted (commonly using vacuum) or not. In "heavy" applications pneumatic systems are probably most common. This is used in for example heavy trucks and trains, and unlike lighter applications the pneumatic pressure is generated by a compressor. In very heavy applications we might go back to hydraulic systems, but then the hydraulic pressure is generated by a pump.

Both drums and discs respond as soon the pad has traveled the distance to the rotating surface. Since the hydraulic liquid isn't compressible, the pressure comes instantly, it related to force applied and the area of the brake master cylinder. But for example hoses can expand and reduce the response time.

How much force you must apply to get a certain braking torque is dependant on factors such as hydraulic gearing (how large is the piston area in the calipers vs. area in the brake master cylinder), effective mean radius and friction coefficient between the pad and disc/drum. It's independant on the type of brake system.

Why modern cars use discs has nothing to do with the above, it's just that they are so much easier to cool, hence a much better performance during repeated braking. Drums on the other hand, they are cheap to produce and tolerant against dirt/water. The latter was a reason why drums were used much longer on heavy trucks, in still is in use today, even if discs have taken over the heavy truck market too.

In several racing classes, for example F1, power assisted braking isn't allowed that's why discs of large diameters are important in those classes.

[UncleBob]

very well said johan, although I would argue about the F1 cars. They use large diameter rotors more for the cooling capacity advantage.....well, we could argue which is more crucial. Both benefits are very important. More leverage and more surface to cool off the brakes.

You stated what I was attempting to state much better than I could: The design of the system will GREATLY change the results of effort vs loss of kenetic energy. There is no inherent reason why you can't use disk breaks on a manual brake system on a car.

[VW Alignment Tech]

Originally Posted by JustSayGo

.....the disc brake will provide more immeadiate response with less hydraulic fluid movement compared to a drum brake.

.....the disc brake has less hydraulic leverage force than a drum brake.

So disc brakes have a quicker response yet require a greater application force when compared to drum brakes. Call me stupid, but I have a hard time understanding why this is so.

[JustSayGo]

The real overwhelming top priority reason that disc brakes were not used on heavy trucks and trailers was again a leverage issue. Force can be adjusted and increased by the pitch of the screw used to clamp the pads against the disc, but the brakes required frequent adjustment. Now reliable automatic adjusting slack adjusters make disc brakes much more common on commercial vehicles.

The reason disc brakes require more force is because they do not self energize like shoes are designed to. That is why caliper pistons are so much larger than wheel cylinders.

There is no reason or argument that modern disc brakes will work very well on vehicles without any type of power assist. If you research the history of brakes the answer to the questions is simple.

There is no need to drag any additional (correct) info or exceptions into a simple question. You are correct that over many years disc brake obsticles have been overcome with improved design and will continue to improve. Bullet trains, aircraft, space shuttle, and any high performance application use disc brakes because of their superior cooling ability.

Disc brakes are also more economical to produce because they have fewer pieces. Without the disc brake obsticles to overcome during earlier days, there may have never been drum brakes. Power boosters made 1960's disc brake technology practacle for use on heavy Amercan autos.

Proportioning or metering valves are used to reduce hydraulic pressure to the rear brakes.

[SaabJohan]

Originally Posted by UncleBob

very well said johan, although I would argue about the F1 cars. They use large diameter rotors more for the cooling capacity advantage.....well, we could argue which is more crucial. Both benefits are very important. More leverage and more surface to cool off the brakes.

When it comes to thermal properties of the brake, there isn't any real benefit of making discs with a larger diameter. Instead there are two factors that will determine the thermal properties of the brakes.

First we have heat capacity. This is related to the material used, and the mass of the brake rotor. Increase the heat capacity of the material and/or the mass of the rotor and we can store more energy without overheating. Naturally, we also want a material that allows the use of high temperatures without a reduction in friction coefficient.

The second factor is cooling of the disc. This is generally related to airflow in the cooling passages of the disc, the airflow over the disc surface and any use of water as a coolant, either in the caliper or as sprayed on the rotor. In a racing car like a F1 car, air is fed from an intake to the center of the disc and out through internal passages. There can also be "nozzles" directing air against the surface of the disc. To provide good cooling the wheel must also be open enough for the air to flow out. Cars fitted with fairings (the wheels covered up on the outside), as some group C cars duing the eighties and early nineties, this was often a cause for brake related heat problems.

When the brakes are used, kinetic energy is converted to heat energy temporarily stored in the disc. Some of it is cooled when the discs are braking, but most of the heat is actually tranfered from the disc between their use.

Originally Posted by JustSayGo

The reason disc brakes require more force is because they do not self energize like shoes are designed to. That is why caliper pistons are so much larger than wheel cylinders.

A brake can't self energize, that is a physical impossebility. There are really only two options availible; either the brake system generate a larger force at the cost of distance (for example making pedal travel longer) or we add energy from outside the system using a vacuum brake booster, mechanical springs or whatever that can be a source of energy. The brakes will NOT boost themselves.

[KiwiBacon]

Originally Posted by SaabJohan

A brake can't self energize, that is a physical impossebility. There are really only two options availible; either the brake system generate a larger force at the cost of distance (for example making pedal travel longer) or we add energy from outside the system using a vacuum brake booster, mechanical springs or whatever that can be a source of energy. The brakes will NOT boost themselves.

Maybe the terms are causing confusion, but drum brakes do create some of their own actuation force.

If you can imagine the rotating drum helping to pull the leading shoe into itself, then you can see the "self energising" effect.

[SaabJohan]

Originally Posted by KiwiBacon

Maybe the terms are causing confusion, but drum brakes do create some of their own actuation force.

If you can imagine the rotating drum helping to pull the leading shoe into itself, then you can see the "self energising" effect.

As I understand it, drum brake shoes can generate more or less braking torque depending if they are a primary or secondary shoe, something that is depending on the rotation of the drum and the placement of the shoe.

N_primary = (K*a) / (a/2 + μ*b)
N_secondary = (K*a) / (a/2 - μ*b)

where

K = force added to the shoe
N = normal force
μ = friction coefficient
a = length of shoe, mounting point to actuating point at a vertical line (shoe placed vertical)
b = length between mounting point and to the inside of the drum at a horozontal line

So if we got a drum brake with a single actuator with a primary and secondary shoe (as pictured below), the loss from one shoe is the gain in the other. If we use a drum with two primary shoes instead, we can get a small increase in braking force using the rotation of the drum. However, that comes with disadvantages. Sure, we can only get that increase at one certain rotation depending on the construction, but this will also makes the brakes difficult to modulate (the brakes tend to get an on/off feeling). If both shoes are designed to use drum rotation to increase the braking torque in one direction we will also get an equal loss in the opposite direction.

[KiwiBacon]

Originally Posted by SaabJohan

As I understand it, drum brake shoes can generate more or less braking torque depending if they are a primary or secondary shoe, something that is depending on the rotation of the drum and the placement of the shoe.

N_primary = (K*a) / (a/2 + μ*b)
N_secondary = (K*a) / (a/2 - μ*b)

where

K = force added to the shoe
N = normal force
μ = friction coefficient
a = length of shoe, mounting point to actuating point at a vertical line (shoe placed vertical)
b = length between mounting point and to the inside of the drum at a horozontal line

So if we got a drum brake with a single actuator with a primary and secondary shoe (as pictured below), the loss from one shoe is the gain in the other. If we use a drum with two primary shoes instead, we can get a small increase in braking force using the rotation of the drum. However, that comes with disadvantages. Sure, we can only get that increase at one certain rotation depending on the construction, but this will also makes the brakes difficult to modulate (the brakes tend to get an on/off feeling). If both shoes are designed to use drum rotation to increase the braking torque in one direction we will also get an equal loss in the opposite direction.

I agree with that apart from one piece. I believe the gain in force of the primary shoe is greater than the loss of the secondary shoe.

This is because the primary shoe gains force at the edge nearest the actuator, the secondary shoe loses force in a similar manner but at the edge nearest the pivot.
Hence the torque gained on the primary shoe is less than the torque lost by the secondary shoe.

[Black Lotus]

Originally Posted by SaabJohan

A brake can't self energize, that is a physical impossebility. There are really only two options availible; either the brake system generate a larger force at the cost of distance (for example making pedal travel longer) or we add energy from outside the system using a vacuum brake booster, mechanical springs or whatever that can be a source of energy. The brakes will NOT boost themselves.

Are you talking about drum brakes? If so, that's a pretty strange statement..
Here, let me help you pull your foot out of your mouth!

[Alastor]

Originally Posted by SaabJohan

The second factor is cooling of the disc. This is generally related to airflow in the cooling passages of the disc, the airflow over the disc surface and any use of water as a coolant, either in the caliper or as sprayed on the rotor. In a racing car like a F1 car, air is fed from an intake to the center of the disc and out through internal passages. There can also be "nozzles" directing air against the surface of the disc. To provide good cooling the wheel must also be open enough for the air to flow out. Cars fitted with fairings (the wheels covered up on the outside), as some group C cars duing the eighties and early nineties, this was often a cause for brake related heat problems.

This reminds me of this StopTech white paper I read awhile ago:

http://www.stoptech.com/tech_info/wp...lections.shtml

Anyway, the piece of information that stood out was:

Originally Posted by StopTech

The amount of heat produced in context with a brake system needs to be considered with reference to time meaning rate of work done or power. Looking at only one side of a front brake assembly, the rate of work done by stopping a 3500-pound car traveling at 100 Mph in eight seconds is 30,600 calories/sec or 437,100 BTU/hr or is equivalent to 128 kW or 172 Hp. The disc dissipates approximately 80% of this energy. The ratio of heat transfer among the three mechanisms is dependent on the operating temperature of the system. The primary difference being the increasing contribution of radiation as the temperature of the disc rises. The contribution of the conductive mechanism is also dependent on the mass of the disc and the attachment designs, with disc used for racecars being typically lower in mass and fixed by mechanism that are restrictive to conduction. At 1000oF the ratios on a racing 2-piece annular disc design are 10% conductive, 45% convective, 45% radiation. Similarly on a high performance street one-piece design, the ratios are 25% conductive, 25% convective, 50% radiation.

I always assumed maximizing convection alone was critical to maintaining brake temperature. However, if this paper is to be believed then radiation is just as important. It got me wondering if race teams do anything to improve radiative heat transfer, like a high emissivity inner rim or wheel well surface.