Challenge Questions

Slightly off the original topic but...

I recently replaced the rear rotors on a '97 Subaru Legacy 2.5GT. They have a small drum cast into the disc for the parking brake. The lining on the shoes was very thin. They are designed to be used as a parking brake (i.e. no movement between the shoe and the drum and hence no wear) not as an emergency brake. Don't depend on the parking brake to stop you more than a few times. Besides the leverage is low.

Unequal pressure of a hand brake (Off Road call them cutting brakes) on to the rear wheel will make 1 wheel lock or slow more than the other, if the right hand wheel is locked, or slowed, the car will pull to the right, and vise versa if the other wheel is locked/slowed.

We use cutting brakes in offroad to turn into corners quicker and shaper than plain steering, also allows you to keep the foot down to induce slide out of the corner.

I feel like the wording of the question indicates something deeper

Ohms parallel restsistance rule ???

I don't know much about car brakes, but I suspect that the answer to the question is related to security. If the brake system installed in the front is different from the one installed in the back, the probability of a simultaneous breakdown of the two brake systems is almost null.

The systems that has been most used in Europe is either diagonal brakes on the same circuit (FR & RL + FL & RR) or Two slave cylinders on each front brake, so you get pressure on each circuit to both front brakes and one single brake.....

I personally have never seen a car where each circuit fed either the front or the back....it probably exists somewhere though!!

I GOT IT--

In the challenge, the 'You' referred-to is a FEMALE electronics engineer. And being a FEMALE driver, you have a tendency to 'ride the brake'. Your son, aware that your driving habits would cause drum brakes to overheat and fade, is grateful that car has front disk brakes and thus is able to stop from 35 mph. (Probably to avoid hitting a squirrel.)

The (young man) in the back seat could have just as easily been refering to the fact that we no longer use a rod or exterior shoe rubbing on the tire, or a stick rubbing the ground as a brake..... This would include the addition of a braking system to the front wheels.

Best answers are:

Blink, post 21, "So... There is nothing about having different "types" of brakes f/r that enables a quick stop from 35. Thus, my conclusion that both driver and son are daft -- the real question is about genetics." and

Guest, post 24, which referred to cost as the difference between the shoe and drum.

Without knowing more about the vehicle it is impossible to speculate as to the installed breaking system. And there is no telling what sort of nonsense one of kid's professors might have shared in the class room. I can think of no advantage of shoe brakes over disc brakes other than lower manufacturing cost.

Most modern brake systems have brakes at all four wheels. some early systems put a brake on the drive shaft. This worked until the drive shaft or an axle broke. Mechanical linkages from the foot pedal and hand lever to shoes to all four wheels also worked. The legendary racer Barney Oldfield would not drive a car with hydraulic brakes as his job was to make a car go as fast as he could, not stop. Some vehicles had two sets of shoes in each rear drum, one operated by the foot pedal and one by the hand lever. The E-Jaguar had IRS (independent rear suspension) with the rear disk brakes attached near the differential with drive shafts and U-joints connecting to the wheels. Some vehicles with disks all the way activate the rear brake pads using a cable from the had brake. The variations are practically endless.

So why is the student sitting in the back seat? And why he he be amazed that his dad can stop the car when it is traveling at 35 mph?

The front disc brakes do most of the stopping due to weight transfer when you apply the front brakes - the rear brakes can't take advantage of this when decellerating from froward motion. Front discs have a lot of surface area, but have no force mutiplication in their mechanism, so they need an engine vacuum booster.

The rear drum brakes are self-energising, the more you apply them, the more they apply themselves, they need no booster.

The main reason they are separate is that hopefully they won't both fail at the same time. Also, you don't want to have a vehicle where the back brakes lock while the front brakes are digging in the front - it will swap ends.

You state that "... disk brakes need an engine vacuum booster." I disagree as many cars made in the USA had vacuum boosters and shoe brakes while at the same time British cars had front disk brakes without a vacuum booster.

There is a great mechanical advantage provided by the brake pedal lever arm and there is an additional mechanical advantage provided by the differential diamer of the sinlgle <1" master cylinder piston diameter driving multiple brake pad pistons all with diameter >2".

Question, is the car in question a Front-wheel drive or Rear? as braking forces will vary significantly depending on which wheels are the "driven wheels"

Vacuum assistance is only that, its the same as Power steering, its only to assist the driver to turn or apply the brakes, some of the older model cars that didn't have power assistance used a larger brake pedal to allow you to place both feet on it at the same time in emergency situations.

Twin circuit braking is for saftey, wether its a single master with twin circuit porting, or twin masters with a bias-bar connection on the brake pedal so you can, with a turn of a knob on the dash, alter the bias from front/rear by modifying where the pivot point is in relation to the 2 master cylinders.

Twin circuit can be either FF/RR or diagonal (FL-RR/FR-RL) its a redunancy system to allow 1 circuit to fail, and the other to still help you to stop, the "Emergency brake" is a totally seperate activation independant of the main braking system (I mean it doesn't cut into the main system to activate the brakes)

The only reason to use different types of brakes front and rear is production cost (discs cost more) and weight (discs weigh more). Um the only 2 reasons to use different types of brakes...

Arg. I'm starting to sound like the Spanish Inquisition.

I would have guessed that the discs would have been lighter. Does anyone want to state some weight examples. Are they made heavier to absorb heat and stay true or something? I have seen allot of weight reduction holes and slots in them and that would definitely driving up the cost. I liked your answer and would like to learn more.

Response to "The only reason to use different types of brakes front and rear is production cost (discs cost more) and weight (discs weigh more). Um the only 2 reasons to use different types of brakes..."

This is only a personal take on the subject, open to correction, but its what I have understood over many years:-

1) Prodction costs are less for disk brakes as far as I am aware, that is one of the reasons they caught on so well as the manufacturers liked them too...

2) The lightening holes serve several purposes, when raining they help the brake pads in clearing water off the disk quickly, also they help in cooling, dynamic balancing can be done at the same time and obviously they reduce weight as well...

3) They actually do brake better under most conditions than a similar sized drum brake.

4) Its far easier to see when the pads need changing than with drum brakes, so they are safer in that regard too.....

5) Due to being mechanically simpler than drums, there is far less mechanical stuff to go wrong/rust up.

6) Even without holes, they cool better than drums, especially the "double wall" disks most often used at the front of many car types....

The weight comes from the caliper, which is a highly stressed open U. I suppose if you could support the caliper from the axle as well as round the rim of the disk, the shape might be more efficient. As i recall, a comparable disk and a drum are not much different in weight, and the pads and the shoes are not much different in weight, either. But, the drum backing plate, shoe pivot(s), springs and cylinder are generally lighter than the caliper, even if it is the one-piston low cost variant.

I'm reading an underlinig riff between a Mechanical engineer and an Electrical engineer. The electrical engineer wants his expertise to have an influence to the solution. Electronic anti lock brakes. Never mind that friction brakes can't do regenerative braking. The electrical man wants to get amped up.

ABS sorts such things out very well and with ESP on top, you have a stable braking system......

The way ABS functions (and ESP), it is immaterial what type of brakes you have, you need a sensor on each wheel to allow a small computer to see which wheels are turning and by how much. If any wheel locks up, the brake is released for a split second on that wheel.....

Almost the same for EPS, when the car is unstable as when braking and cornering at speed/same time, specific wheel brakes will be individually turned on and off to hopefully stabilize the car......its a good system, but if the driver really "over did it" then probably the EPS will not alone be enough......physics of this type always has its practical limits.....

Its fun to drive a car with EPS too fast into a corner and feel as it scrubs the speed off.....dramatic!

The front brakes do approximately 70% of the braking so they need to be larger or more powerful brakes.

...only on some makes of cars.

Its actually a product of the designed weight distribution, its not a fixed ratio as you suggest.....but it might be a reasonable average of all cars.......

In fact it varies between around 60% for some rear engined sports cars up to over 90% for the old Mini. Also the loading of the car, number of passengers, luggage in the front or back etc. all change this figure.......and I bet these are not the full extremes either.....

The reason why isn't so much with the car as it is with users. GM didn't adopt make disc brakes to the front wheels until 1969 and that was found mostly on the "A" body cars, like the Grand Prix. The fear was more due to warranty repairs because disk brakes are much less forgiving to be "ridden on" than drum brakes, i.e they can take more abuse and resist going out of "square" like rotors (warping) tend to do. Disc brakes are quick to react since they sit right on the rotor whereas brake shoes are retracted a little by return springs. Disc brakes are cheap to make compared to all the parts and assembly that go into drum brakes. Remember, most big trucks you see on the road use the drum brake set-up, not the disc brake set-up.

Brake types aside; the weight transfer during deceleration dictates that the front wheels will be doing the greater portion of the braking. An old mechanics rule of thumb is 60% front, 40% rear.

Therefore, the front wheels require larger brakes than the rear. You could always put the heftier brakes on all four wheels, but at the expense of adding more weight, that increases momentum, that could drive you to need even bigger brakes.

But the question was in fact rhetorical. The EE (therefore superior) father just wanted a coy way to insult his son's poor life choice!

It depends. If the engine is aft as in Tata Nano and you hit a strong front brake, then you are sure for somersaulting. If the engine is forward, you are sure that the engine is not lost while driving but not sure whether your luggage is lost in transit.

Then do not buy a Porsche......most of them have been rear engined......

Also most racing cars are rear engined, because that is the best for accelleration and deceleration as well as fast cornering!!!

What a surprise for you!!! It turns out to be the ultimate system for a car........rear engined!

This was an older car with rear drums and front disc brakes. The father had hit a section of road covered in water. As drums fail almost immediately when immersed in water, the front discs stopped the car.

the rear brakes are designed to have less power, to avoid rear wheels lock upon emergency stop. If that happens the car slides sidways out of control. If the front wheels are locked , the car maintains its forward direction. This brake setting is important to minimize car accidents.

One presumes the question refers to the fairly standard automotive arrangement of front disc type brakes and rear drum type. The answer to the "Why two systems?" would be economics.

Disc brakes have superior heat loss without "fade", thereby increasing stopping (tranferring vehicle momentum to heat)ability. It's desireable to have the more powerful brakes at the front of the vehicle due to weight transfer during hard stopping. The weight comes off the rear wheels/axles, giving them a tendency to lock, producing a skid. The standard ratio of front/rear braking is approximately 60/40%, making less efficient-but cheaper to produce- drum brakes perfectly acceptable for their"supporting" role in the brake system.

However, there were not always disc systems to install. The early hydraulic brake systems were all drums, and manufacturers used a combination of master cylinder valving and larger drums on front to achieve the desired effect.

Also, many cars are now available with 4 wheel disc brakes- and again, with valving,ABS wheel speed sensing technology,and differing brake rotor sizes, safe non-skidding,controlled braking is achieved.

The front to back balance of braking power is not a fixed value on any (relatively) modern car as you appear to imply, simply due to the loading or not of luggage/passengers and even how hard you needed to brake changes the actual value at the time.....

Even 60:40 (a really good value by the way, 50:50 would be the "unachievable" best!) will usually only be achieved by cars having the engine BEHIND the passengers....few front engined car can ever achieve this under normal braking conditions. As a static weight distribution yes, some front engined cars can achieve this......

Most front engined cars under braking are at least (at best!) 80:20 some are 90:10 or even worse under normal braking conditions. Emergency stops will of course be far worse......

More to the point, having to rely on basically only the 2 front tires to perform often 90% of your braking is a very sad occurence....and extends braking distances dramatically......

There is quite a good article on this subject in simple language at:-

brake bias and performance

There is another short discourse here that gives a good simple explanation of ABS:-

Brakes

What I have personally seen on every car that I ever worked on, (before ABS was generally fitted), was a brake balance sensor unit, that took into account the loading of the rear of the car and applied more of the hydraulic pressure to the rear brakes when the car was more heavily loaded (generally the rear end).

One system I noticed, actually measured the changing angle (Fords) that a heavy load in the rear made to the relationship in height of the rear to the front of the car. The "lower" the rear end, the more pressure was allowed to go to the rear brakes.

Other makes had similar systems......more rear load, more pressure to the rear barkes.....most very simple and very effective.

This "Dynamic" means that the relationship of the front to rear braking balance is changed almost from second to second as conditions are changing, as they do in reality when driving....

Remember, little or nothing in this area (on a well designed car) is "fixed" in value, a simple mistake that many people make....

I am also sure that there are plently of older cars that were never fitted with such good methods of changing the braking dynamics for the situation in hand, before ABS came along for almost all cars.....but such primitive cars/systems are outside of my personal area of knowledge.

just as in drag racing, you want as much traction as possible. How we do this is, first and foremost, a set of tires with a softer compound and side walls that wrinkle under hard acceleration, the second part of the equation is weight transfer. We put racing shocks and springs in the front, and in some cases in the back, to let the front rise as you launch the car. This puts a lot of weight on the back wheels causing the car to "hook" if you will. The braking system works in the same way.

The front and rear brakes are separated by a proportioning valve that may be adjustable or just set in the factory, giving each brake circuit the right amount of pressure causing the transfer of weight to the front of the vehicle. If there wasn't a proportioning valve, the back brakes would lock before the fronts causing all kinds of problems for the driver. If it were brake drums on the back the valve isn't needed, although most vehicles come with valves now regardless of the components used, because you can manually adjust the shoes to come on after the fronts. This can only take us so far however. The vehicle can only transfer so much weight before the brakes either lock up or brake fade begins. There have been many upgrades and, after-market components designed to compensate for these conditions, such as more pistons in the brake caliper, slots in the discs to dissipate heat, and some brake shoes have wide gaps in them to remove excess dust and heat. The final part involved in having a better braking system is the use of better tires and most importantly, the invention of ABS(anti lock brakes). ABS has done more for the brake systems on most vehicles than any other one thing. The tires can do their jobs because the ABS system keeps the wheels from locking up. Skidding you tires will lengthen the stopping distance considerably, just as in drag racing, spinning your rear tires causes slower and inconsistent elapse times.

Common sense would tell us that a dual circuit braking system would only be split front to rear due most of the braking is done by the front wheels (assuming the car is travelling forward). To split braking side to side is lethal for obvious reasons. Experimentation would dictate the hydraulic power proportioned to the front would be more than that that went to the rear IF the braking systems were the same front and rear. That is, all drums (same size), or all discs (same size rotors and pads)

Differential braking between front and rear can be obtained by:-

1) Different "types"

2) Different size

3) Pressure restrictors to the rear.

Today with ABS, things are different. This allows, thru' technology, to have full braking power to all wheels and monitor each wheel for skid or a slower rotation relative to the opposite wheel "on the same axle". Allowing the wheels to keep turning gives the driver turn control which is the reason for so many cars being steered off the road during hard braking with ABS. A car with ABS should be steered sufficient only for avoidence purposes, not "hard over" steering!

The old method (before ABS) was to have dual cylinders on each front brake, one cylinder on each front wheel and one rear brake was fed by one circuit, the rest by the other circuit. End result was that both front brakes came on at reduced rate and one rear brake....it was enough to get the car stopped.

Another method was a front brake with one rear brake, diagonally......of course!! Not perfectly balanced but better than nothing!!!!

Rolls Royce did this with disc brakes for some years by using leading and trailing calipers in the front, combined with single calipers in the rear.

They were not alone, many manufacturers did something similar.....

the front brakes must overcome the forward momentum of the car along with the weight factor. Not sure what the equation would be, but you can look it up if you are that curious.

Anyway as the car decelerates it will nose dive due to the friction of the road/brake/gravity combination working in contrast with the forward momentum. The back brakes will actually lose weight advantage as the center of gravity of the car is shifted forward. So the Front brakes must due more work than the rear brakes, this is accomplished in a lot of different ways on various types of cars. Some cars will have the disk/drum combo, others will have a size differential between front and back, still others will use a boosting system to increase the front brake pressure. Some cars even have multiple calipers on the front and singles on the back. It depends on the car. The reason being is that the front brakes have to overcome the cars momentum.

It really comes down to the application and purpose of the intended braking system.

On my old Off Road Buggy, I had a pair of CB650 motorbike front wheel calipers on the front wheels of the Buggy, and a pair of Girlock (Holden Camira) front calipers on the back wheels.

I needed most of the braking force on the back wheels as compared to the front, there was very little weight on the front wheels, and it wouldn't take much to lock them up, but as the back wheels were also driven by the gearbox/engine, they needed so much more braking force.

The front tyres were standard VW fronts, re-grooved and the backs were 32" BFG's, the new machine I'm building, I'm running Girlocks all round and 31/32" BFG's on each corner.

So, the balancing and "stopping" power needed per wheel and front/back all depends on the application, having more braking force on the front wheels than the back may not be suitable for the vehicle your driving.

This is most obvious when riding a bike, as one's weight is higher in relation to the wheel base.

When braking, most of the weight shifts to the front wheels due to the forward rotational moment during braking, so they do most of the work. Conversly, this is why we prefer DRIVING the rear wheels for forward acceleration, not the front.

HMm, they must have been in a Citroen DS car. It had full power disc brakes in front and drum brakes in rear, plus a weight sensing mechanism, so that when the back was empty, most of the braking effort went to the front brakes, but when it sensed weight in t he rear, it redistributed braking effort 55 front/45 rear. And when I say full Power , I mean just that, Power for the brakes did not come from engine vacuum but from a high pressure pump, the brake 'pedal' was a 'mushroom ' on the floor, and was basically a vavle to allow high pressure to go to the brakes. A special device in the valve made sure the pressure did not go all at once so you did not go thru the window . The French used it as a mars rover as seen here ( this is an actual picture of a 1969 citroen DS 21. H.L. Goldman livagain1@yahoo.com.

That was the same system RR used on their cars of the 1980 era. Pressurised brake fluid was allowed to travel to the wheels only when the brake pedal was depressed, but proportionally according to the pressure on the brake pedal.

Do you have the after picture of the Citroen?

Well first off if I'm not mistaken the brakes themselves are not different, but how they are used is. Most vehicles now have disc brakes on the front and rear of the vehicle. Some use a disc/drum setup for the emergency brake, and others use an integrated ratcheting caliper for the emergency brake. Front brakes on a vehicle do most of the stopping for all vehicles. First off, as they learned in the 60's, the rear wheels have a tendency to lock up and send the car into a skid if applied with the same force as the front. All of the weight of the vehicle is in the front. When you hit the brakes all of the forward momentum is applied directly to the front suspension and wheels, therfore the demand for braking in the front is more neccessary than the rear. The opposite happens when traveling in reverse, all of the momentum is applied to the rear and the brakes should compensate accordingly through the use of mechanical proportioning valves or in newer vehicles computer controlled braking, or "smart braking". If you buy a car new and drive it for a few years you will proabably notice that you have to change the front brake pads sooner and more often than in the rear.

Have any of the people replying bothered to do a very simple exercise? Take a model car - chock the front wheels using a match stick or similar and let it slide down a ramp. What happens? Chock the rear wheels and watch what happens. Simple isn't it? Lose grip at the rear & the car spins. Lose grip at the front & the car continues in its former direction regardless of steering. This is why the tyre fitters used to have a poster advising the deepest tread tyres to go on the rear to prevent the rear aquaplaning in wet conditions.

Rick

I believe the Electrical engineer needed to stop the car fast and experienced the following: He pushed on the brakes and a red "electric" light came on to designate master cylinder failure as his pedal went to the floor. His car stopped anyway because the "Electronic" Brake controller applied his "electric" brakes on the tandem axles of his toy hauling trailer. We know he was hauling a trailer with his toys (four wheelers, snowmobiles, etc) because electrical engineers make enough money to afford them, unlike mechanical engineers (like the son who just wants to ride them).

The trailer hauled the car to a stop nicely. The son, being properly belted in the back seat never saw the mechanical failure, and with confidence of youth (and mechanical engineers) just assumed only one mechanical failure occurred and the other system stopped them (because all he could see is the brake light on the dash)....

The father, being wiser (but we already said he was an electrical engineer) wondered out loud why having a single point of failure in a master cylinder could ever be considered intelligent. And he wanted to the son to explain why two mechanical systems was such a good idea when you have four wheels, and why anyone ever wanted electronic anti-lock brakes, and why insurance companies mandated them? Because if one mechanical system had failed like the son thought, the "electronic" anti lock brakes still would have given them the most control...even if all the brakes on every wheel of the car were identical.

Brakes on the Front wheel are probably Disc Brakes. Rear Brakes probably are Drum Brakes. Drum Brakes are suseptible to being made Wet by Standing Water on Streets. Disc Brakes are Not as Prone to being Wet from Standing Water on Streets. Reason: Brake Pads on Disc Brakes sit higher on Chassis then Drum Brakes. Discs Throw Water off, and Water doesn't remain on Pads long enough to Cause Pads to Swell and Loose Friction. Another Advantage of Disc Brakes is Closer Tolerances and drag between Pads and discs cause pads to remain dry and Ready to apply Friction to Braking and give a resulting Short Distance Characteristic. Though Drum Brakes have Larger Surface Area, and do not Generate as much Heat as Disc Pads, they Wear Longer, due to Larger Wear Surface while Giving the Same Power to Stop with Less Pressure applied. If Rear Brakes are Wet, the Car Will Still Stop more Surely due to Disc Brakes on the Front, when You apply Power to Your Brakes. Front Disc Brakes give the additional Safety Feature of Stopping More Surely, Summary: Front Disc Brakes Advantages: Quicker Stopping, Safer Stopping and Surer Stopping, Disadvantages of Disc Brakes: Shorter Wear Characteristics of Disc Pads due to Smaller Surface and Higher Pressure Needed to Stop The Front Brakes. The Rear Brakes have Larger Wear Surface, With Less Power Applied, they operate Cooler and Last Longer, and have a Tendency to Swell to a larger size when wet and Slip not Stopping when Operated on Streets with Standing Water. Using Disc Brakes gives you an Extra Safety Factor if You are Fortunate enough to Have them on Your Car, as Not all Cars have them, or at least the last Time I bought one they were not always Standard, in I believe 2007.

Sorry to say this but your post is a mixture of r*****h and truth.......(the missing letters are:- ubbis......) Please send me a case of the beer you were drinking while posting, it sounds like good strong stuff!!!! I will pay of course!!!

For example:-

Reason: Brake Pads on Disc Brakes sit higher on Chassis then Drum Brakes.

Only if you have different sized wheels on the front and back (radius/diameter). All brakes sit close, almost inside the wheel hubs on some models....the do not really "sit on the chassis!" They are part of the unsprung weight on 95% of cars.....

Otherwise they are the same height above ground......

There are several other "black" areas in your post, only if you wish will I point them out as most of the people here on CR4 will already know what is true and what is not......

Most modern cars in Europe nowadays have discs on both front and back.....I actually don't know of a car still being sold with drums, but I am sure there are still some. Probably some of those tiny "women's cars" may still have drums.......

I bought a chrysler in 2007 and had to search for a car with Disc Brakes on Front and Rear and is the Reason I bought the Chrysler, at the Time I bought the Chrysler, the GM X car, I believe it is Called that is So Very Modular had Drum Brakes, or I would have considered one, As I liked a lot of Features on it. I also say, Maybe the Disc Pads are Not as High as I thought, the Drum BRakes did as Long as I owned them have problems with Standing Water, the Brakes would swell up and Would be Slipping if Brakes applied after or While Driving in Deep Water, while Disc Brakes Never Did, i thought I did Describe the Reason Well, however I may Not have Been Accurate or Correct and I always look to Learn from Those who know More about Brakes or any field, as I am Interested in Many Fields, I do Imbibe or Drink but Mostly Port and I really don't Drink anything when I am Doing something which Requires Clear THougths, I noticed someone said something about Beer, Port Wine when I do drink It, I only Have one glass as it is 20 Percent Alcohol, so I could not drink more then One glass and I am Ready to Have a Rest, I love it because it like Alcohol used in Gasoline helps water go through Filters which helps Older Carburetors to Work consistently and it helps My Kidneys Filter Better the Trash we eat that is not Good For us., Thanks for Request for Drink Recipe, its just about anything with 20 % or more Alcohol but not enough to Give me difficulty Thinking. I want a Clear Mind, heh heh.

Back in the day, when all we had was drums, one technique for driving through standing water was a light application of brakes to seat the brake shoes against the drums, accompanied with an application of throttle to provide forward motion. This procedure would almost always get one through a deep water situation.

Used after the water, it also allowed one to dry the drums off by warming them up.......

Yes Indeed, Instant center of gravity is what it's cracked up to be. Applying brakes is one way to shift the mass of the car forward and to the front axle, thereby making the greater braking required on the front axle where the weight (and traction) will be proportionally higher then shift back as the car's kinetic energy dissipates.