Challenge Questions

The front brakes do most of the work stopping a vehicle because that's where the weight goes when braking, so the main difference is that they have more stopping power (larger disk/drums), but not necessarily different types of brakes.

Many vehicles have the same type of brakes on the front and rear, but the front brakes are larger. More common is disk brakes for the front and drum brakes for the rear.

Front brakes larger than rear? Not necessarily - e.g. 911s and Boxsters (15 years is probably enough for a student to count as many). You also omitted to mention that quite a few years ago it was deemed necessary for front and back systems should have independent hydraulics in case one of the systems is compromised - so maybe the son noticed a trail of brake fluid on the road behind.

Even if the systems are different, you are not "lucky that the engineers decided ..." only that the independent systems were made adequate.

But GA in general.

Sometimes the hydraulics are split up with one front and one rear opposite side so I didn't think this was the issue. Thanks for the vote.

Was this common with the older systems implied in the challenge? (My unthought assumption was that steering could be problematic without ABS)

There are specific cars that have unusual brake systems. The Porsche has a rear engine, thus much more weight on the rear and a low center of gravity and tight suspension, thus less wieght transfer. Also, when is the "type' of brake the same and when is it different? Is disc in front and drum in the rear different or is a vented disc in front and a solid disc in rear different. And what about the right front brake on a sprint car?(there isn't one) Is that different? Real race cars (Top Fuel Dragsters) go to the extreme: no front brakes, rear discs whith multiple calipers and way back, a parachute! -- JHF

I have had two older cars that developed cracks in the brake lines. In both cases, I lost all braking. Fortunately, I was able to avoid hitting anything both times.

Since we are supposed to have independent systems, can anyone tell me why BOTH ends failed when only the line to the rear brakes leaked?

Because you were driving a car with only one circuit. Or one circuit had already failed.....

What year and model of car?

First was a Ford van. I don't remember the year, but it happened in '77 and the van was old then. I traded it later and got $100 from the dealer.

Second was a '97 FOrd Taurus.

In both cases, the steel line to the rear brakes was replaced.

I am assuming your cars had dual circuits, which has been the norm since (IIRC) 1968. If these cars were older than that, then they had a single circuit, in which your only recourse was the emergency brake if there was a hydraulic leak.

The dual brake circuits are not perfect. The master cylinder can loose its seal between circuits. The warning light can burn out, so that you may have been driving with one circuit failed, and not have had an obvious problem until the second circuit failed. Wheel cylinders (and brake calipers) can seize (i.e become essentially welded together with corrosion) so that one set of brakes might not have been working at all, which was not uncovered until the more obvious failure. If one wheel cylinder has accumulated a lot of air, (needs to be bled) that problem could go undetected until the other circuit fails, and the sponginess may be enough to render one circuit very ineffective, with the pedal hitting the floor before meaningful braking occurs.

All these things can happen, from age and/or wear, at about the same time, so that, in practice, your experience is not completely unheard of. Of course, preventive maintenance would dictate that brake lines are replaced well before they start to crack.

I'm going to guess that the master cylinder seals had failed. The light did come on, and only the brake line was replaced.

Thanks for the reply.

Once the fluid leaked out the rear line, only the fronts were working. That would have given you a very low pedal, but still stoppable. When you lost all of the fluid in the rear braking section of the master cylinder, much of the fluid required for the front braking section of the master cylinder was also gone. Just as soon as you got an air bubble in that section of the master, you lost that section also.

How do you know? Such speculation is perfectly valid in the absence of a pre-existing satisfactory answer to the challenge. In this case it was clear that the posting was not necessarily intended to be directly related to the challenge, even though it was close enough that it was not marked off-topic.

More important than the brake type is the driver type driving the car - a race-driver needs harder brakes than a sunday afternoon or an allday driver!

I think you hit the nail on the head, HUX. Good answer!

I like your answer, but I feel like I'm missing something, too.

I thought the 'change' that occured, that the challenge referred to, was to use front disk + rear drum brakes vs the previous designs that used drum brakes on all wheels. Disk brakes have a big advantage over drum brakes under conditions where brake fading may occur, and it makes good engineering sense to use disk brakes on the front wheels which do most of the stopping. But a stop from 35 mph is ordinarily not likely to cause fading.

It is common, but hardly iron-clad (sintered pun), that the front brakes are disk or at least larger trailing pivot drum brakes, with higher stopping power in the forward direction, and the rear brakes are drum with shoes on a common pivot (one leading, one trailing), for cost savings and a simpler emergency/parking brake that works uphill and downhill.

If your front brakes are disks, the exposed surface can more easily get saturated with slippery substances like car wash detergent and wax, and fail to contribute as much to your stopping until they shed these lubricants.

On an car with trailing pivot drum front brakes, the rear brakes might be more effective in reverse, too.

Mechanical engineers are more often into fluid flows, so they were probably Automotive engineers, but this varies depending on the engineering school more than the title of the major. At Union College in Schenectady, it was steam in GE turbines, but as I recall, somehow we also got into the bolts of turbines.

You, being a typical EE, did not notice the pedal had gone to the floor and you used the emergency brake (Thank God you have a 1937 Plymouth coupe which uses the big handle).

The amount of braking done is different on each axel! 70%+ is done with the front axel and the rear axel does the rest.

This percentage is changed depending upon the weight distribution of the car, British minis of the 60's had over 90% of the effective braking done by the front brakes only. Sometimes the rear brakes rusted up and nobody noticed......till the next MOT of course!

50:50 would be the best arrangement if achievable - very difficult. Some Porsche's achieve 60:40 I am told......as the engine being in the rear helps a lot....probably they would have the same size brakes at the front and the rear then......but I have no exact infos on that point......

I found the original post difficult to understand quite where he was going, there are even differences between the USA and Europe for example as I believe the US market was slow to take up changes/advances in brake design in the 50, 60 and 70's......

Perhaps he would be so kind as to define his "question" a bit better for some of us not on the same wavelength - sorry!

"probably they would have the same size brakes at the front and the rear then..."

Actually, the fronts are larger than the rear (min3 are 350 mm front and 330 mm rear).

Don't confuse that proportioning valve number with actual stopping power distributed to the axles, either.

First, look at the size of the rotors front and rear and you will see not only a difference in diameter, but a difference in size of the brake calipers and in some cases the number of pistons. This isn't limited to the Porsche 911.

The front wheels see the bulk of stopping power (and wear - ask me how I know) because the weight is biased forward under braking conditions. Actually, it is contact force of the tires on the road.

As the rear contact force lightens up, less clamping is required on the rear disks to prevent premature lockup.

On my car, as in just about all cars, the physical hardware front and rear is carefully sized to provide a more equal stopping force front and rear. The proportioning valve just dials in or fine tunes the the rest.

I actually meant that if a car actually had a 50:50 brake balance, then the brakes could be the same size.......

I actually mentioned that even some well balanced cars like some of the Porsches, seldom have a better than 60:40 balance, that does not mean that the brakes can be the same as there is a significant difference of 20%.

I probably did not write what I meant accurately enough....sorry.

Surely that is a 1.5:1 ratio between the required mechanisms? I'll give you that it only represents 20% of the total.

"I actually meant that if a car actually had a 50:50 brake balance, then the brakes could be the same size......."

I can see your point, but from a reality perspective, that would never be the case, so I was approaching it from that perspective. The reason that you can not have a 50/50 split is for two primary reasons:

1. When a vehicle stops the weight distribution shifts forward. The reason it shifts forward is because the center of gravity is above the road surface and the bottom of the tires are the friction point. You always have a shift forward when you apply the brakes.

2. All vehicles have an ideal weight for their designed purpose. You could, in theory, shift the center of gravity of the vehicle to yield a 50/50 split when braking, but the magnitude of the shift in weight when braking is completely dependent on how hard you are braking. So, where do you want to set that point?

If that point is set for maximum braking effectiveness (which depends on tires, road surface, temperature, and surface conditions), then your vehicle begins to look like a dragster. Personally, that is one vehicle I would not want to parallel park.

The other way around the problem is to add mass to the rear of the vehicle. But the rub with that is that the more mass you add, the harder it is to stop. Add to that, your fuel economy goes out the window and your vehicle is subject to the dreaded Porsche lift-throttle-oversteer in a turn. I think the Corvair had that problem, too.

Now more mass does not translate to better braking just because you get more weight on the wheels. Look at 1200 lb F1 cars. They can generate nearly 4 Gs of deceleration when braking hard.

Two brake related inputs:

The rear brakes activate first then the front brakes. This is to prevent nose diving and possible skids.

The large advantage of disc over drum was not having to adjust the brakes. Improper brake adjustments often lead to pulling when the brakes were applied. Disc brakes were a great invention, but why did drums remain on the back?

I speculate that it had to do with the mechanical parking brake. Very few early designs or parking brake calipers were very effective or functional for long.

"The rear brakes activate first then the front brakes. This is to prevent nose diving and possible skids".

Well, the problem with this is if the vehicle is in almost any angle away from straight line motion, the vehicle will swap ends (spin out).

You need front activation in the vast majority of applications for safety reasons. Initial rear activation may be needed to induce oversteer for specific situations (such as on a dirt oval track, or perhaps a dirt/gravel rally course).

Yet another back-to-front statement (the third). Front-brake locking is far more likely to cause a spin. As every pedal-cyclist knows from bitter experience, rear-wheel braking (locked or otherwise) causes under-steer.

(I'm just wondering where you people get your information. but perhaps it's just as well car systems are designed to be as independent of the driver as possible)

Well, I don't where YOU get your information from, but I can provide examples of just what I'm talking about:

(1) Dirt track stock cars (at least in MY universe, maybe not yours) typically use rear brake bias (that is, rear apply before fronts) to promote a slide on many surfaces, such as a tacky clay surface. A dirt track car in this environment moves faster with a bit of angle to the direction of motion (this is analogous to a sailboat and the angle of the wind direction to travel direction).

(2) A classic maneuver called the "bootleg turn" is performed by applying the rear handbrake and simultaneously cranking in steering wheel input. When performed correctly, the rear end will swing around, allowing the driver to move in the opposite direction.

(3) I'm not a bicycle guy, but I can testify as an trained engineer and someone with vast experience in professional auto racing that front wheel brake bias (front wheel braking earlier) is used in paved track auto racing as a stabilizing device to promote understeer and driver confidence, for those drivers who don't like a lot of oversteer. Also, really skilled and top rated drivers who like oversteer often use rear brake bias to promote a bit of oversteer.

My statements are based upon experience, training, and the fact that my drivers won a large number of major auto races and championships.

I'm not saying you are entirely wrong, but just try to think outside of the box a bit instead of automatically assuming that if someone says something you may not agree with is explicitly wrong. That's not always the case.

Basically I agree with you. I've run enough dirt roads at speed to get the gist of it. If you lock up the rear, and the rear is not straight behind the front, then the guy in the rear is in the front and the guy in the front is in the rear pretty quickly. You can do the moonshiner's turn on hard surface too, but you ought to disconnect one side of the emergency brake. It's fun to try in empty parking lots, especially after a light rain.

A more subtle effect than a hand brake turn comes about from left foot braking, which is usually used on front wheel drive cars to reduce understeer or provoke oversteer at will. At about the point when the front would drift unacceptable wide, you apply both brake and throttle. The throttle cancels out the braking effect on the front wheels. The rear wheels, however, become over-loaded with the combination of braking and cornering, and start to drift wide (oversteering). By modulating the amount of brake and throttle, you can go around in an understeering, oversteering, or neutral attitude. Goof up on the modulating, and you can also depart the road front first, rear first, or even merry-go-round style... or for greatest drama, on the car's roof.

I never tried that! New York cabbies do that, but not for drifting.

I will add a comment about left foot braking; it is primarily used in racing to allow the driver to play the brakes against the throttle. By using careful modulation of the brakes vs. the gas pedal, a finer degree of control can result. Also, we reduce the time lag by having to switch one foot from the accelerator to the brake pedal.

One more thing; in the mid 1990's, there was an instance of a Indy car which had a unique engine package. The team discovered that by using a very gentle "brush" of brake application in the turns, lap times were reduced. The other cars with different engine packages were basically "flat" (on the throttle all the time with no braking in the turns) at all points on the track.

The gentle application settled the car in the turn and promoted greater speed through the turns. This package set a course record during this time.

The unique characteristics of the engine, with regard to c of g, as well as weight distribution and transfer led to the discovery of this driving technique and its successful application in this unique circumstance.

It can work (or not) both ways.

On a standard road car, moderate braking of front wheels will promote over-steer, and of rear wheels will promote under-steer. If you brake hard enough that the braked wheels lose grip you may expect the effects to reverse.

There are a number of causes for the effects at moderate braking; I think the least obscure is via the castor of the front (steering) wheels. It could be that the different optimisation of track cars results in a different response at moderate braking.

The effect when the rear wheels lose grip is more obvious:
. when you lock the back, the front wheels continue to move along their initial track, and the lack of grip at the back means that momentum dominates and the back can go into the lead - and probably further round.
I'm not certain whether locking the front wheels has the same effect under all circumstances - I think at modest turn rates it will cause under-steer, but I wouldn't be surprised if there was a threshold beyond which the car is unstable.

The point you are making about using the rear brakes to promote a slide is valid but it is to initiate a turm ie you are using the brakes to aid turning not using them to stop. If you want to stop the purpose is to be able to apply as much braking force as possible before a skid starts after which braking doesnt do much anyway. As a vehicle is slowed it transfers force to the front. Watch motorcycles to really see the effect. as weight moves to the front it comes off the rear. As we all know friction is related to the weight exerted on the contact. the heavier something is the more friction is genrerated trying to pull it along. the higher the friction then the more difficult it is to initiate a skid. if you want to brake brake with the front, if you want to slide a turn brake at the back, but only until the turn starts. you dont brake hard while turning unless you really can drive. ABS is fitted to make cars brake while turning or on slippery surfaces because most people aren't good enough do be able to do this on their own

I am a pedal cyclist. I know full well that rear-wheel braking does NOT cause understeer, nor could it. Understeer is a condition wherein the vehicle in question attempts to remain pointed in the same direction in spite of steering; oversteer is a condition in which the vehicle rotates at a rate faster than the change in the direction of travel. Put simply, an understeering race car will hit the outside wall nose first, and an oversteering race car will hit the wall back-end first.

Understeering can only happen when the front (steering) wheels have less traction (as in a skid) than the rear wheels. Oversteering happens when the rear wheels have less traction than the front. So rear wheel braking on a bicycle can cause oversteering if the wheel locks, but not understeering. If the front wheel locks, with most bicycles you are far more likely to go over the handlebars than get a front-wheel skid.

I can relate to your point of view accurately....

You're correct.

When I was a kid, "coaster brakes" were common on kids bicycles. You'd pedal backwards slightly to apply the brake, which was only on the rear wheel. Every kid I knew would have fun applying the rear brake hard to induce oversteer, with the rear wheel coming around to pass the front wheel. (Same effect as in a car "hand brake turn.) Ordinarily, you'd stick out one foot to catch the bike before it fell over, but if you were skilled you could reverse direction without touching your foot to the ground, and smoothly pedal off in the new direction.

This maneuver was done intentionally, of course, and the rotation (in yaw) was helped by flicking the handlebars one way or the other. If you did not do this flick, and if you were reasonably awake, you could make very straight stops with the rear wheel fully locked and skidding.

"Yet another back-to-front statement (the third). Front-brake locking is far more likely to cause a spin. As every pedal-cyclist knows from bitter experience, rear-wheel braking (locked or otherwise) causes under-steer."

This statement couldn't be more wrong. Front brake locking will cause a slide. Rear brake locking will cause a spin. This is factual. You don't have to be an engineer to know this, every true car person does. Try pulling your emergency brake handle and see what happens. It works best in an empty parking lot covered in ice or snow. But if you really want to challenge this concept do it on the highway at about 60 mph. We can read about it in the police blog/obituaries.

BTW, this applies to your bike as well. Your confusing a slide with a spin, likely caused by an issue with your weight vs. the bike.

(I'm wondering where you get your information. I knew of this before I even had a drivers license more than 30 years ago. It's just as well that you don't design car systems.)

"The rear brakes activate first then the front brakes. This is to prevent nose diving and possible skids".

It may be a problem in your eyes, but it is the way US cars are designed. The rear brakes are applied first as braking is applied. This gives good feel to the brake pedal, and brings the vehicle speed down in a slow, gradual manor. Just what one would want as one was cresting a hill with limited sight, and desiring to be prepared in the event that something over the hill required a change of speed.

If there was an obstruction that needed to be avoided, the driver would push the brake pedal harder, raising the pressure in the master cylinder. This would now overcome the pressure limiting valve allowing the front disc brakes to activate, slowing the vehicle at a much more aggressive rate.

If the brakes were working well, and the weight of the vehicle was near the design specifications, the rear wheels would just lightly lock up before the fronts. At that point the anti-lock system would monitor the wheel lockup to prevent skidding.

Please note that the comments in italics are from comment #13. These are not my words.

I would like at this time to provide quotes from Paul Van Valkenburgh. Mr. Van Valkenburgh is regarded within the automotive industry, both with passenger cars and high performance/racing vehicles as one of the true authorities with regard to automotive engineering. He is a much published author, with a number of books and many professional papers for engineering societies. He has worked for General Motors, several major (and successful) racing organizations, and has developed many of the ideas accepted by knowledgeable authorities in automotive engineering.

These quotes are taken from his work, "Race Car Engineering and Mechanics":

"When reliability and stopping power are adequate, balance or proportioning must be optimized. Distribution of braking forces to each wheel is fixed. But the load on each wheel is not, so there comes a time when one wheel is more lightly loaded and will lock up more easily. The most common example is the inside front tire when braking into a turn. When the load transfers away from it, the limit on braking will be that tire, unless it is to be skidded and flat-spotted. This is less of a problem at the rear, where a drive axle - and usually a locking differential - connect the inside and outside wheels".

"The effect of longitudinal load transfer is even greater since maximum braking is done in a straight line. It is well known - often from personal experience - that if the rear wheels lock up first in braking, the car will be highly unstable. So it's important that under all circumstances the front wheels should lock first - and yet not so much sooner that the rear brakes don't do their share of stopping also".

"Proper longitudinal braking distribution is an important design consideration and is a function of center of gravity height, the wheelbase, the total weight, the front and rear weights, and the coefficient of friction."

Note that these comments refer to rear wheel drive vehicles. A comment about FWD:

"FWD has created some unique problems to solve in front/rear brake balance. Most obvious is the extremely light rear weight in braking, which may approach 15 - 20 percent at the limit. Not only is this a stability problem in straight line braking, it almost guarantees inside rear wheel lift and lockup when braking into a turn. (However, it may be that such lockup is not an important concern, because the light or zero load creates minimal flatspots)."

Please note that the italics and bold print are my edits.

Again, Mr. Valkenburgh is a recoginized authority in automotive engineering, with much success in professional motorsports attributed to his knowledge and concepts. I present this information as being proved and documented reliable.

Last paragraph; "recognized authority"...my mistake in spelling. Thank you.

Mid seventies training classes with GM led me to believe that rear before front was the desired goal. I have no desire to try to match my training with Mr. Van Valkenburg, or any one else. I was just trying to add to the discussion.

Van Valkenburgh seems to have a generally good grasp of the situation (at least as it was some years ago), but the language used is a bit sloppy, it seems.

His statement: Distribution of braking forces to each wheel is fixed. But the load on each wheel is not, is completely wrong.

Braking force is that force that retards the car, measured in units of force (such as pounds or newtons) and acts at the tire contact patches. (The fact that braking force and the car's CG are not at the same height is what causes the car to pitch forward on braking.) Ordinarily, one would not say force "to" each wheel, but instead "at" each wheel. One might say hydraulic pressure "to" each wheel, and one might say that the distribution of hydraulic pressure is, in a race car, changed by the balance bar. But in ordinary driving and racing, braking force is not equally distributed from wheel to wheel, or axle to axle. In a straight line stop, the braking force at each front wheel is often fairly close, and significantly greater than braking force at each rear wheel (for reasons that seem to be clear to him in other things he has written). In anything other than straight line stopping, each wheel contributes different amounts of braking force, and the distribution of those forces is anything but fixed. (The is one obvious reason for having four channel ABS.)

Perhaps he meant that braking pressure distribution is fixed -- but of course it is not. On ordinary cars, the front/rear pressure distribution changes according the the action of the proportioning valve, and in race cars the pressure distribution is changed as necessary, prior to each race (and during races, when the car is equipped with driver balance control).

Re one wheel locking in a turn, he seems to be saying that pressure is fixed from side to side at one axle. This is true, in simple brake systems.

Every reasonably sophisticated race car is set up so that relative front/rear brake hydraulic pressures can be easily changed to alter the f/r balance of braking force (which depends upon hydraulic pressure, total piston area per wheel, disc/pad friction coefficient, the effective disc radius at the center of the pad area, the effective tire contact point radius, the friction characteristics between tire and road, and the tire loading.)

Any race car can be set up (with an ordinary balance bar connecting the two master cylinders) to cause the fronts to lock first or the rears to lock first, and different courses and different drivers require different settings -- this, obviously, being the reason that f/r bias is always adjustable on race cars (unless class rules prohibit such in the name of keeping a car closer to a production car.)

The usual case in race cars that must turn both right and left is that, for a given pedal force, the hydraulic pressure is the same from side to side. The hydraulic pressure is not the same in the front circuit and rear circuit, however, unless coincidentally. In ordinary engineering of race cars, the brake components are sized (in respects such as piston size, effective disc radius, etc) so that with equal hydraulic pressures f/r, the front brakes will lock at a slightly lower pedal effort than required to lock the rears. Then, the balance bar is set to the preference of the driver to fine tune the balance.

The situation is not a lot different in production vehicles, because a standard dual master cylinder has (typically) equal sized pistons, and that would produce equal front and rear pressures for a given pedal force (if there were nothing added to the circuit to change that). So the components are first sized with that in mind, and then a proportioning valve is used to 1. fine tune the f/r pressure balance to fine tune the braking force distribution, and 2. to dynamically change the proportioning to suit the load on the rear tires. In practice, this means that front wheel drive cars have small rear brakes and big front brakes, and typically have ventilated rotors on the front but not on the rear, (or use drums on the rear, etc.)

Much of the other stuff Van Valkenburgh writes is essentially true. However the bolded statement (that if the rear wheels lock up first in braking, the car will be highly unstable) is exaggerated. In a panic stop of an old tech car without ABS, EBD, proportioning valve etc, it is not uncommon that the rears lock up a couple milliseconds before the fronts. The car is not highly unstable, and continues, typically, in a fairly straight line and without a lot of yaw, with all four wheels skidding. For yaw to occur, there must be an unbalanced force to cause that yaw. (Even on a motorcycle [which relies on the gyroscopic effect of spinning wheels for stability] the rear wheel can be locked and a straight stop can be made. With all wheels locked while the car is in a corner, the situation is different, and rear lockup prior to front lockup will catch most drivers off guard, with the result that the car yaws dramatically, and can go off the road backwards if the driver fails to react correctly. Rear lockup on a turning motorcycle is even harder to deal with quickly enough.

in discussing the challenge question, a phrase like "locks up first" is perhaps misleading, because it suggests that there is some time delay element in the brake system. If we are to think of the stop as an ordinary fast stop with a competent driver, then a 'fifties era car would do just fine, with a single circuit master cylinder, no brake proportioning valve, and drums on all four wheels. A moderately skilled driver would modulate the brakes to avoid locking up any wheels. If, on the other hand, we are to think of the stop as a true "slam on the brakes panic stop," then all four wheels would lock at effectively the same instant in time, and the vehicle will travel in a generally straight line. If the rears lock 5 milliseconds earlier than the fronts, it doesn't matter, because that time is insufficient for any meaningful yaw to develop. Anyone who was around in the days of primitive brakes can attest to the fact that cars went straight in panic stops, even if you were trying to will them to turn. (This tendency is what brought ABS into wide usage -- imagine... turning and stopping at the same time!)

Given that the son is in the back "sit" one could assume the question designer was trying to suggest something about weight distribution, and that the proportioning valve would have helped the car stop in a straight line. (But does this valve fit with "different types of brake systems, one for the front wheels and one for the rear wheels"? I don't think so. Any proportioning valve must fit between the front and rear circuits to do its job of "proportioning".)

I would like to make some points about this situation:

In reference to:

"His statement: Distribution of braking forces to each wheel is fixed. But the load on each wheel is not, is completely wrong".

Not necessarily. The reference to "load" refers to static and dynamic weight distribution. Any variance from a perfectly distributed load of 25% per wheel implies an asymmetrical weight distribution. Obviously, almost any four wheeled vehicle will have asymmetrical weight distribution, and dynamic weight distribution will obviously change as operating conditions change. Two obvious examples are some oval track sprint cars which lift the left front wheel off of the surface, as well as some vintage road racing cars whose roll center is positioned in such a way that the inside front wheel lifts during cornering. Obviously, this is a extreme example of weight distribution changing as conditions change. It is my understanding that this is what Van Valkenburgh refers to.

Next:

"Perhaps he meant that braking pressure distribution is fixed -- but of course it is not".

Actually, for an instantaneous moment in time, the pressure distribution is fixed. It certainly can be changed manually, or can change if some variable in the system changes that can affect performance. However, for all intents and purposes, it can be assumed that if a moment in time is isolated and analyzed, the pressure distribution is indeed a fixed value. The distribution can of course vary front to rear, and some special instances side to side (re, late 1990's F1 McLarens with twin brake pedals allowing the driver to apply more pressure to one side of the braking circuit to help set the car into a corner), but again at a instantaneous moment in time, the pressure distribution is fixed.

Finally:

"However the bolded statement (that if the rear wheels lock up first in braking, the car will be highly unstable) is exaggerated."

I do not necessarily agree with your response. I will admit that there are circumstances which might not be "highly unstable", but I will argue that dynamic response of a vehicle under severe braking, with rear wheel lockup will tend towards instability. Why? Because in most circumstances variables will most likely come into play which will tend to promote instability; weight distribution, driver input to the steering wheel (direction of front wheel travel different from rear), braking forces and pressure distributions, and friction coefficient at the tire/surface interface to name just a few.

Thank you for your input!

I agree with a great deal of what you have written.

Re: "His statement: Distribution of braking forces to each wheel is fixed. But the load on each wheel is not, is completely wrong".

My argument is not with the second part (But the load...), which is is completely true. (In fact, I am committing the error I am accusing him of... namely, over- generalizing.) It is the first part that is untrue, partly because the second part is true. Braking force and the distribution of braking force is not fixed, because braking force is measured at the contact patch, where many things are continually changing. Perhaps he meant brake pressure distribution is fixed, but it is not fixed in the ordinary use of the word.

To say that pressure distribution is fixed at an instant in time robs the word fixed of useful meaning, I think. Any changing system of any type can be thought to be fixed at some instant (and it is such thinking that allows many engineering calculations to be done). You can say that a car on a drag strip, at some instant, passes through 60 mph, and that, at that instant, its speed is fixed at 60 mph, as you can show by a stroboscopic photo of its speedometer. But you cannot say that the speed of a car accelerating down a drag strip is "fixed" in any ordinary use of the word.

Certainly in the general sense, VV is correct. But he presents particular cases as if they are universal. One can accurately say "If the rear wheels lock up first in braking, the vehicle is likely to become unstable." But if you say "if the rear wheels lock up first in braking the car will be highly unstable" (as VV does) then one need only show one case in which the car is not highly unstable to show that the statement is not always true. (Such statements are what my old English teacher would call glittering generalities.) A car stopped only by yanking on the emergency brake is quite directionally stable, because the retarding force is behind the CG.

But in any case, I am only verbal sparring for the practice. Your general points, and VV's are correct -- it is always better, in the interests of stability to design things so that the fronts lock up first, as pedal pressure increases.

Thanks, too, for your input.

This has been a fascinating discussion, hasn't it?

I admit that I have oversimplified the discussion concerning braking forces, pressure distribution, and so forth, especially regarding using an instaneous moment in time; a senior citizen moment, I think!

I'll try to clarify what I was saying; I think we can all agree that the forces at the contact patch will change as dynamic inputs/outputs occur. Certainly, with weight transfer as we accelerate/decelerate and transfer weight left to right and back, forces will vary significently. I'm just stating that at a particular point in time, pressure distributions are, using the term "fairly constant"; and, if I expand this period of time linearly, for example cornering in a constant radius curve and my driver maintains a "fairly constant" load on the brake pedal, my forces at the contact patch at each wheel will vary and change with time, but my pressure distribution will remain "fairly constant".

I participated years ago in a series of preliminary tests with a instrumented race car and a championship winning driver; these confirmed much of what I present in the preceeding paragraph. It would have been interesting to expand this research, but it seems the organization was more interesting in winning the race that following weekend - go figure!

Thanks again for the discussion! Have a great day, and the same to all of our fellow CR'4ers!

All the brake limiters that I have seen and worked on, on normal everyday cars, reduced the pressure to the rear brakes to make sure that the rear brakes do not lock up under light loads......not a timing function.....

I have never seen a system that did not apply all brakes simultaneously either......if what you said was true and you braked into a corner with the rear brakes only, you would be off the road immediuately......

Brake balance front to back is what percentage of the braking effort goes where, not "when".....

When they say "first" they probably mean depending on braking strength (not timing). Rear brakes only would be applied at braking rates that are so low that the brakes cannot lock. That would have a couple of advantages: it tends to lengthen service intervals by evening out tire and pad wear between front and back, and on most production road cars it induces slight understeer (which is better for unskilled drivers than oversteer). Once the braking pressure exceeds some critical value at which there is any possibility of the rear tires losing grip the proportion applied to the front needs to be increased.

Personally, I think it's a completely dumb idea - unless there's a machanism to switch it off when it's cold (as ABS etc. cut in too late to do much if it's icy).

Please try to picture a small cylinder in the brake line going to the front wheels. Inside the cylinder is a spring loaded piston trying to keep brake fluid from leaving the master cylinder and going to the front wheels. When sufficient pressure is forced against the spring, the piston will move to the far end of the cylinder, allowing brake fluid to go to the front brakes. Until that pressure is reached, fluid is only going to the rear wheels.

If the spring requires 1,000psi to compress, the fronts would almost never do any work. But if the spring only required 50 psi to compress, the effect would be very subtle. That was the way it was explained to me.

Have you ever experienced a brake fluid loss of only one end of the car? When the front fluid is gone, the pedal stays high, and firm, but stopping sucks. Usually heavy pedal is required, along with rear wheel lockup. When the rear fluid is lost, the pedal suddenly goes lower, but stops reasonably well. The reason for the low pedal is the spring loaded piston needing to move before the breaking begins.

Again this was from a GM training class in the early seventies when discs were new to most of us.

US market I can believe (I'll believe anything there!), but I have never seen that on either a European or Japanese car.....so at least there I beg to differ.....

The systems I have seen in my life (Mitsubishi, Ford, Mercedes, BMW, Nissan, Renalt, Mini, Sunbeam and a few others) either had a dual master cylinder or a single, mostly depending upon the age of the vehicle in question.....(my experience is over the last 50 years, even before dual brakes were the norm).

What I have seen is that mounted usually in the rear of the car is a mechanism that adjusts the amount of hydralic fluid/pressure that is allowed to get to the rear brakes....this is often mounted so that as a car is loaded, the amount of fluid/pressure is allowed to increase, proportional to the load....

So rather than preventing fluid getting to the front brakes (a dangerous idea as the front brakes perform the largest part of the braking on well over 90% of modern cars.(I personally have not even seen a car that achieves anywhere near to 50:50 front back balance, but there could be one like a F1 or Indy car.... In fact it could even be called suicidal to reduce pressure to the front brakes.....) the pressure is controlled/reduced proportionally to the rear brakes once they have ALL been applied.

All four brakes need to be in use at the same moment in time, the front brakes being the most important of all.....otherwise the car will be unbalanced and either show excessive under or oversteer.

The only vehicles that I can recall seeing the load sensing braking valve on was a Ford Taurus station wagon, and some trucks.

The pressure that is restricted to the front brakes must be very little, because everything you say true, and the breaking would be poor. I hope that there is someone that is more current on the subject that is reading this. I for one am going to have to make it my business to put one of my clunkers on stands and try this.

And PLEASE, no begging.

Pretty please?

If this is truely how US cars are designed then that could be the reason that they only like going in straight lines and baulk at any hint of a corner.

The electrical engineer was talking about how stupid the mechanical engineers are for dragging a car to stop by using friction to dump useful energy.
Electric breaking could generate electric energy into a super capacitor for rapid exhilaration. Batteries and solar electricy not included.

Two good reasons off the top of my head...

1. To allocate greater braking effort where the weight transfer accumulates, i.e. into the front wheels so that the back brakes don't lock up when they start to lighten up;

2. So that a single point failure in the brake system doesn't wipe the both the front and back brakes at once, at least ensuring that you still have some brakes, and which should still feel balanced through the steering wheel.

I always figured the front brakes were larger than the rear brakes so that if you brake hard enough to break traction, the front wheels will lock up before the rear wheels do. If the rear wheels lock up and the front wheels maintain traction, the back end of the car will whip around and almost guarantee a loss of control. If the front wheels skid while the rear wheels maintain traction, the car will stay pointed in the direction of motion, thus ensuring that when the front wheels grab again the driver will have control immediately.

Of course, this is only effective in that narrow margin where one pair of wheels locks up and the other doesn't. I don't know how often this happens.

But if the front wheels lock up, you can't steer, also resulting in loss of control.

See msl's correct reply #28 (to an identical comment made in post #27)

I gave you a GA because I think that is exactly what happened. Dad applied the brakes too hard and the front brakes locked with the front wheels skidded and the rear wheels continued to rotate controlling the direction of travel until the car stopped.

But the kid was wrong if he thought that it was because of a different type of braking system that they did not also skid in the rear. Without anti-lock brakes, it is possible to make all four wheels skid if they are all disk, all shoe, or disks front and shoes rear even at 35 mph.

In the early 1970's I worked as an auto mechanic at an imported car dealership and we abused tested the different makes of cars that we found on the used car lot to their limits. Rapid acceleration (burn rubber), panic stops (full skid), four wheel drifts, etc. were part of our test drive. With some of the imports (Jaguar, Ferrari, Aston Martin, etc.) it was difficult to make them skid before they stopped on a dry road. But if the road was wet, you'd feel as if you were accelerating at the transition from braking to skidding. In those days I did not know that the sensation was caused by changing from a high rate of deceleration to a lower rate of deceleration.

this is absolutely Business, long ago, we do product regardless of safety now all PRODUCTS even to the smallest ones adhere's to strict safety standars, excpet for those product coming from country with low quality standards, of course they have cheaper products, but in the long run quality itself dictates durability, & users conformity rules, well that's really businees.

With airbags who needs brakes?

But motor bikes have disk brakes front and back, even do most of the weight during braking is at the front. i think the biggest reason why most cars had only disc brakes in the front is due to cost. Cheaper manufacturing means more profit!

RTFQ: different types. That is not at all the same as the different functional requirements you are writing about.

Maybe the son's answer was: "You electricals couldn't have afforded a car with discs all around"

Actually, I did read the FQ. It specifically asks about "types", front vs rear. The reason for different types (i.e. disc vs drum) should and does, if the system is engineered, have to do with differences in function (and related cost benefit, etc) -- the different types are not there merely for styling. Disc brakes function differently than drum brakes, but not in any way that would make someone "lucky" to be able to stop quickly from 35 mph.

In stopping a car from 35 mph, the type (or even subtype -- such as how many shoes lead or how many pistons are used, or ventilated vs non-ventilated) makes no difference. Either type can provide enough braking force to overcome the traction of the tires. ABS, pressure proportioning, number of circuits, etc., all have nothing to do with differences in front/rear brake types.

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.

Nature or nurture? (silly question perhaps - when did you last meet a dog that could mew convincingly).

I agree. A single stop from 35 MPH is no big deal. I think we are missing something.

All we know is that car as a different type of front and rear brakes. Unless there is something exotic or esoteric here that means discs and drums. And the car has a front and a rear sit (sic).

Any ideas what I'm missing?

Drums expand with heat and breaking stopping power fades. Disc brakes do not fade until the rotors and breaks heat up and glaze. Rotors are generally lighter than drums. Air brakes on trucks generate great breaking using large drums + shoes, and break arm has long travel to compensate for drum expansion. I think disc brakes might have less mechanical time lag and are more responsive. All these brakes are a wasteful drag.Again the stupid thing is not capturing the energy. It does not matter what kind of brakes you have if you can stop the car safe. A parachute for a dragster is ok with me.

Safety is the reason you have two braking systems with three activation systems.

Two Hydraulics systems were engineered, one each, for the front and real brakes. This allows more breaking power in the rear breaks verses the front breaks. This also reduces a hydraulic failure from failing both the front and rear at the same time, it can still happen, it just reduces the chance.

The third and final system is the emergency breaking system which uses a cable to activate the rear breaks in case of a total hydraulic failure.

Entirely production tooling and cost driven...... or at least it should be.

It may be conservative prejudice and stupidity driven as much engineering was at one time...

The main reason is simple. When cars were first made we did not have all of the fancy eletronic or hydraulic things we have now. At one time brakes could and did lock up. If you only had rear brakes and they locked up, you would SPIN AROUND 180 degrees! Front brakes do not do that, but if you were going backwards then the same thing would happen.

Simply try using you parking brake on an icy parking lot, with noone around, while going forward. You will spin out of control.

The weight shift is a secondary thing. That's why the poportioning valve is generally adjusted to about 1/3 to the rear and 2/3 to the front.

I don't think so. With rear brakes only the car would remain stable. With front brakes only the car *might* become unstable (i.e. spin) whether or not they locked.

Think of the car as if it is suspended from the rear wheels (rear brakes only). Since the CofG is below the rear wheels (in almost all cases) the car would remain stable.

If the car were suspended from the front wheels (front brakes only) the CofG would be above the front wheels (again in almost all cases). Then the car would be stable only as long as the CofG was along the center line of the support.

If brakes (back) lock up, then usually the car will spin, unless you are a particularly good driver......

No, as in #27 quoted "If you only had rear brakes and they locked up, you would SPIN AROUND 180 degrees"

This is incorrect, if you had "only" rear brakes, the fronts will "free-wheel" thus there would be nothing trying to slow the front of the car, but locking up the rears, would still cause drag between the tyre and the ground.

So, therefore, you would have the fronts rolling and the rears dragging, even if the back end were to change direction or the steering was to move to another direction, the back end would re align and still trail the front wheels, if on ice and you got it to spin 180, then it won't stay there, it would keep going around until the back end is trailing the front, and everything would stabilize.

Very simple and safe way to try this.

Get a stick, walk along and drag it on the ground, your legs are the front free-wheeling wheels and the stick on the ground are the locked brakes, no matter what direction you turn, the trailing stick would attempt to follow you.

now if you change ends, and the fronts lock up and the rear goes free, the car will attempt to change direction so as the dragging wheels are behind.

Skid steering, or "cutting Brakes" when you activate that brake, you'll never get that wheel trying to over take you and go in front, it will always attempt to drag back and be at the rear most, take a look at how "skid-steer" dozers work, the skidding side never attempts to overtake.

The only way for the car to "spin" is to get into a turn and hold the turn while the back brakes are locked, the friction on the rear wheels will break loose, drag will minimise (as if your on ice) and the back will flip around, *BUT* steering back into the direction that the car is travelling (or wants to travel) the car will once again realign back to have the back trailing the front

Braking systems are designed to try to balance maintaining directional control with maximizing braking capability over a wide range of operating conditions at an acceptable cost. Different brake types, sizes and configurations, proportional valves and active brake controls all improve the performance envelope and predictability of braking while maintaining stability and control by trying to match braking forces to the available traction under varying conditions of weight distribution, road surface conditions and rate of application. In addition, it is desirable to match brake heat absorption and rejection capacity with the braking force distribution to prevent one set of brakes or the other from overheating prematurely under hard or repeated use. Continuing improvement in tire technology has increased available friction (and weight transfer) leading to increased rates of heat generation during hard stops. This was one of the main reasons for differing brakes front and rear, and the adoption of disk brakes with their larger areas exposed to cooling air. It is also why brakes suitable for daily driving are generally inadequate for racing or high duty use such as heavy towing.

The two sides of the discussion argued in posts above on braking vs. stability are both correct - depending on the amount of braking being applied. The stability of the vehicle changes as the braking force is increased to near the limits of adhesion of the tires. As long as the rear tires do not skid the car will remain stable - stability being defined as a tendency for the vehicle to dampen motion caused by transient disturbances without active input. When the rear tires skid, lateral control of the vehicle without active input is lost, and a lateral disturbance (or momentary reduction in traction during cornering) can result in a slide or spin. This is because the rear tires can provide no additional lateral forces behind the center of gravity of the vehicle to dampen lateral sliding or rotation about the vehicle center of gravity, and the driver must steer the front wheels to regain rear tire traction. Failure to act correctly and quickly will result in loss of directional control and/or control of position on the road.

At low braking forces, with moderate side loads on the tires, rear braking is stabilizing. At high braking forces (or cornering forces), stability can only be maintained if the rear tires operate at a lower slip angle than the front tires, making them capable of providing the self-straightening lateral inputs that maintain stability, and reduce slip angles when disturbances occur. As tire slip angles increase, the excess friction capacity reduces and the margin of stability diminishes until stability is lost, and the driver must provide the inputs to limit slip angles. All the current mechanical and electronic controls strive to increase the stability and increase controllability near the braking limit, making control easier under a wider range of loading, road surface, braking and cornering conditions. The anti-lock systems are active controls, and are designed to restore lateral friction capacity by ensuring the individual tires do not remain locked, and therefore permit lateral control at the braking limit. They are not capable of ensuring control under all cornering and braking conditions however.

Regarding the comments on braking forces versus loading, braking forces are limited by mechanical input and the mechanical brake components, and force distribution is fixed under any steady state loading condition unless active systems redistribute the hydraulic pressures to each brake. Tire loading and road surface conditions vary continuously and change the friction available and the braking force that may be applied, but do not change the braking force. The friction force available at each tire is basically controlled by the weight on the contact patch and coefficient of friction (tire construction, tread design and suspension geometry also contribute). This must be split between braking forces and lateral forces. As cornering forces increase, braking must decrease, and as braking increases, cornering forces must be reduced or loss of lateral control will result. Ignoring active control systems, the braking forces are controlled by brake system input, and the tire loads (and available friction forces) are a function of the vehicle weight distribution, suspension geometry and vehicle dynamics. Skidding occurs when the combination of lateral and braking forces applied exceed the available friction at any tire contact patch. This is why high performance driving requires constant modulation of the brakes to maintain directional control while transitioning from straight line braking to cornering, changing lateral acceleration and with varying road surface conditions.

GA

With high performance driving, you want to brake before the corner and accel thru and out of it to reduce wasted time & increase exit speed

The proportioning valves I have worked on all took into account the load on the rear axle, this means that the heavier the load, the higher the braking effort that went to the rear wheels.

I personally have never seen one that was totally "fixed" in its percentage, but I do expect that they exist, but that is not a good method at all.....

The reason a car spins around when the rear brakes lock up is because the front end is slowing and the back end is not. All four wheels have brakes, so while the back wheels are sliding the front wheels are slowing. The only way to show that a car would or would not spin without front brakes would be to remove the front brakes, something only the brave and stupid would do intentionally.

Unfortunately the parking brake trick (colloquially, a "brodie") does not prove one way or the other, because often the parking brake is only applied to one wheel. Instead of a difference in deceleration between front and rear, you have a difference between left and right, with the same result: the car spins and you're prepared for a career in running moonshine.

I agree that the weight shift is secondary; it is a function of the car's total deceleration, not of which set of brakes were applied first. The proportioning valve is to compensate for the weight shift, not to mitigate it.

Save all that trouble of removing the front brakes. Locked up rear wheels will generally try to pass you whether the front brakes are on or not, primarily because they are no longer tracking. Even rear wheels that are not locked up will try to pass the front if they get a chance. Rear wheels spinning will try to pass the front. Rear wheels just rolling along at 60 mph in 5 inches of snow will sneak to one side and then the other trying to pass the front end of the car. Fortunately steering was invented to keep the rear tires in the back.

Where do you get this from ?

At no time will the rear wheels try to pass the front if they are trying to move slower than the front, if your hard on the Accel, the rears will try to push past the front (Fish-tails)

Locking the rear wheels, as long as they are still dragging will always pull behind.

*IF* the front wheels try to go slower than the rears that have locked, then you will get the car to spin into the opposite direction.

Driving in Snow, the fronts are trying to "push" through the snow, if front wheel drive, the fronts will try to climb on the snow, the rears will just be dragged along, if Rear wheel drive, the backs will be pushing, and the fronts will be pushing the snow (making the front try to move slower than the rear) and you can get the rear to try to overtake the front.

How many trailers have you towed that have either blown a tyre or lost the wheel completely? the trailer will always stay behind, and the lost wheel will try to be at the furthest position behind you, pulling your car to 1 side, but NEVER to try and over take

Lots of experience! For some personal hands on experience you could try this.

Get a front wheel drive car (someone else's of course) up to about 40 mph on a straight road covered with a sheet of ice. Apply the emergency brake until the rear wheels lock and continue straight down the road. See how far you can make it before the rear bumper passes you like you are sitting still. If you continue to accelerate slightly you can make it a little further.

In theory the dragging stick scenario is true. That's theory. In practice there are many other variables at play.

The amount of friction on the rear tires of a front wheel drive car or a truck is negligible, at best. Lock them up so they no longer track and you will have to work (counter steer) to keep the rear end behind you.

I don't proclaim to know all of the physics involved in why you won't make it very far down that straight icy road with your rear wheels locked, I just know the rear end won't drag along like some obedient stick on a leash.

I hate to say this (but I am not alone either!) but WHERE did you get these ideas from?

They make nonsense at all....

Have a great day anyway.....

In Europe, at least since the late 50's (and some older cars that I have worked on also) I have never seen a car that the handbrake that only worked on one wheel.....at least not when in a good condition!!

It could be possible in the USA of course, but it sounds very unplausible.......please correct me if I am wrong (at least on the US cars).....

You may very well be right. I have driven several cars wherein pulling on the handbrake would make the car pull to the left or right, and I always figured it was one of those with the parking brake on one wheel, but it could be simply inequal pressure, maybe worn pads from somebody driving around with the brake on.

I stand corrected, thanks.