Technical Braking Theory/real application

in a traditionally styled car...with a disc drum set up.

What would determine is a front metering block was needed?
What would determine if a proportioning valve is needed?

I have a 38 front axle on my 36 with a speedway disc brake kit, a 57 Chevy drum rear.
I am going to rebuild the whole rear brake set up, new drums, shoes, springs wheel cylinders
Disc set up is a few years old

I have a ch***is engineering pedal to mount a mustang style master cylinder to the x member like the stock one was. a Ford disc drum master, 2lb res valve for the front, and a 10lb for the rear as the master is below the floor, this below the calipers/wheel cylinders.

I do not have a metering or proportioning valve on at all, what is the need, or what would i see as a problem to determine if I need them?

If this is too new or OT sorry please delete

 

I can't say for everyone else, but for me a proportioning valve is for fine tuning the brakes.

If your car sets high when you hit the brakes it is going to unload the rear wheels and you need a brake bias that is less to the rear then say one that sits stock. if you have a car that sits low and level then you can apply more brake to the rear thus increasing your stopping power that is what the proportioning vale is for, at least that is what I use one for.

Anytime you go with a disc drum setup the easiest setup is with metering block, it gets dicey when you are mixing and matching unless you use an adjustable metering block. I like a metering block when I can use one, otherwise I just depend on my check valves and drive it to see if it needs fine tuning.

Does that make any sense?

 

If the rear wheels lock up under hard braking, you probably need a proportioning valve.

 

Yep, that is a clue for damn sure.

I usually install one in my car if I am building one that needs to stop better then a stock vehicle. I have discovered that if you can add to the rears without them locking up the car will stop better.

 

Agree with ******-I always install an adjustable one and then go try it and adjust till it feels/stops right.

 

I've been reading on a couple of T bucket sites that they often use an adjustable valve on the FRONT line to limit brake forces to the skinny front tires.
Always thought that would be a no-no, but apparently it is commonly done by a couple of bucket manufacturers!
Any thoughts on that aspect of this thread!?!?

 

Before you hit the order ****on, let's tune your brakes the old-fashioned way, to get the setup as close to ideal before add-ons.
First, of course, it's gotta be a running, driving bolide.
Panic stop the ******. Which end locks up first??
Front?? Decrease the rear cylinder size, you need more power there. If it's the rear first, increase cylinder size. I stick with rear, as, frequently, the rear has more options for tuning. GM cylinders in particular come in sizes from 3/4" to 1", possibly larger.
Leverage is the principle here, larger cylinders decrease pressure, smaller, increases.
The first application of a proportioning valve that I am aware, was the 1965 T'bird, and the reason was never more apt: a simple way to tune an already near perfect setup. Many have been disappointed when adapting these valves (from the T'bird) to find that they do not have a large range of adjustment. Well, they never were supposed to have a large range, they were to tune a system that was already set near correct.
And you, too, will be happier if you start with a near-perfect system, then add a proportioning valve (if even necessary).

Cosmo

 

Yup, just because someone DOES it, does NOT make it right. It just makes it done.

Cosmo

P.S. It's kinda like going hang-gliding with an Erector set, three rubber bands and a bedsheet. You can DO it, but that doesn't mean you aren't gonna die.

 

You'll be fine if your picnic table isn't too tall. I probably wouldn't go off the roof though.

The theory sounds good, try and put most of your breaking on the widest contact patch. It will only work in the real world if your front doesn't dive and unload the rears. One thing to remember is that some of those companies used to build fad Ts without front brakes.

In my mind a T with scooter tires on the front should only be run for 1320 ft at a time and have a chute pack and a roll cage. But what do I know I have been informed that if I say something I am pushing my lifestyle off on someone else.

 

"They say that braking-up is, hard to do..."

 

But if they say brake it down you should jump up and bust a move.

 

I'll let you study up on why you need a prop valve. Drums are "self energizing" by design and therefore require less pressure to actuate. The pinching action of discs require higher pressure by proportion.

 

This post confuses what is true for a master cylinder, but just the opposite for a wheel cylinder. A smaller M/cyl piston increases brake line pressure for a given amount of pedal pressure. A wheel cylinder is weaker with a smaller piston diameter. Think about it...say we have a 1 sq in wheel cylinder area. Whatever ever the line pressure (say 900 psi) is what force we have at the wheel cylinder. Now, increase that wheel cylinder piston area to 1 1/2 sq in and you will have a force multiplier and will have 1350 lbs of force from 900 psi.

You were on the right track though. Front lock up first requires either less brake at the front or more brake at the rear. But, except for T-buckets with bicycle tires on the front, front wheels locking up first is considered desirable in maintaining directional stability. It is rear lock up that puts you sideways into another lane or off the road or ???. Of course the best scenario is no lock up, rather keeping the tires just at the edge of their traction limits. Hence, antilock braking on modern cars.

Weight distribution, brake component sizing, tire size and rubber compounds and more, all factor into it and obviously will differ from hot rod to hot rod. In my opinion, and experience, with some careful planning and good (lucky ?) guesstimating, a proportioning valve often isn't necessary. As Cosmo suggested, trading out wheel cylinder size can help fine tune brake balance.

Ray

 

Two words - bore size.

They went over bore size ratios in 8th grade.

What I find hard is finding the bore sizes published.

Now, if you can get close because of part combinations a proportioning valve can fine tune and fill the gap.

Center slide metering block is a great thing. Since it "meters" flow, if you loose one system the valve slides and closes off the open side due to pressure differential. A real life saver in certain citations and a mystifying demon possessing brake fluid if you are unaware of its function and it sticks.

 

A proportioning valve is simply to change the brake bias from one brake to another or in the case of most cars from one end to another.

Look at it this way a general rule of thumb is that your front brakes do 60% of the braking (just humor me OK). That is considering an older Detroit tank not a new car with all the bells and whistles. If you lift the car when you launch it transfers more weight to the rear tires, but when you break it works in the opposite direction and transfers most of the weight to the front tires. If you consider a 60/40 bias on this lifted car then your rears are going to lock up and your fronts are going to be overridden by the weight of the car. the cure? you change the brake bias and you do this with a proportioning valve. Now perhaps instead of having 40% of your braking pressure going to the rear you maybe only want 30% so with the twist of a knob you have changed your brake bias from 60/40 to 70/30.

Basically it changes the amount of fluid that is going in one direction and pushes in in the other.

 

There are different ways to change brake bias. One way is by limiting pressure (by changing master cylinder bore size, pedal ratio, proportioning valve adjustment, for example), another is by changing how much "work" that pressure can do (by changing wheel cylinder size, or brake rotor or drum size, for example), another is by changing the weight balance of the car (moving the CG forward or backward, or lightening one end, for example)

 

Very interesting. I have this in line to my rear brakes. Do this do that?

 

nope. The one he's talking about has more than two plumbing connections.

 

No...that is a proportioning valve. Spring inside to regulate at what pressure it allows p***age of brake fluid, adjusted by the screw.

The valve 31 Vicky refers to is a 'shuttle valve', which allows p***age of fluid to separate lines for front and rear brakes as long as fluid pressure exists on both sides of the 'shuttle'. If one circuit has no pressure/resistance due to a line or other failure on that system, the shuttle valve is then moved to block that circuit, preserving pressure and fluid for the remaining system to stop the vehicle. Often there is a semi-proportioning valve incorporated with the shuttle valve body, resulting in what the manufacturers often refer to as a 'combination valve'.

Ray

 

Most detriot land sharks as long as they stay somewhere close to stock really don't need much in the way of tuning. well maybe if you are building a canyon carver or an auto crosser.

What kills us in general is that we choose to build something out of the ordinary, some of us lower 'em other lift 'em , we rake 'em change the brakes around and mix and match. None of us can normally look at a car on this site and say this is what you need to do because there are seldom any two that are the same.

Let me give you an example: I have a brake system that I have been building on for a couple of years. It consists of 12" buick drums for the rear and twin piston bendix calipers with 11" rotors for the front.


I have worked out my master cylinder or pretty close to working it out, I have a metering block that I am sure will work with what I got going on. The system is not yet on a vehicle and I cannot collect much more for it other then a proportioning valve, which I may not need. My final setup won't happen until it lands on a car because its final resting place has changed a couple of times since its inception. if it goes on the current vehicle it will be on a vehicle that is pretty light and sits up fairly well, but before it gets used it could go on a very light car that sits pretty low. For me a proportioning valve will probably be the easiest solution but if I posted it on here I would have a dozen answers and none of them would be the perfect solution.

its a can of worms and almost everyone who builds a mix matched system goes though the same problems just different cans.

 

Blow by, I don't think so

Bendix calls them a Combination valve.

 

That's a true statement, but a proportioning valve doesn't address that. They don't add braking pressure, they can only take away. If the rear wheels lock before the front, then a proportioning valve is called for. If not, then one isn't needed unless the fronts lock too quickly, then you need to limit brake pressure to the front brakes. A properly-biased brake system won't need a valve; as an example, none of the series 1-3 XJ/XJS Jaguar brake systems used one. Those 'combination' valves used by Detroit were primarily there to allow separation of the front/rear brakes in the event of a failure, with 'proportioning' being a minor secondary reason.

I've installed proportioning valves on two different cars (due to 'common knowledge' that you 'need' them), and both times ended up getting the best braking with them adjusted wide-open.

 

Absolute dangerous “Cowshed Engineering” done by patch mechanics that cannot do Brake Torque calculations.
[Or are too stupid / pig-headed to actually ask for the correct knowledge ]



You need to understand how a proportioning valve works! It is a pressure limiting valve that won’t allow more than the [pre]set pressure to p*** through.
Below that pressure and the proportioning is a factor of fluid mechanics ,and not actually doing any proportioning.
The only time a vehicle needs the same brake torque on all 4 wheels is when it is parked. When the rate of deceleration increases [G’s] the more front bias is needed.
eg 1G of braking will get more weight transfer to the front than 0.5G
So a slower braking vehicle will have more rear brake bias than a faster braking vehicle.
Now you need to start from the point of Traction and work backwards.
In wet slippery conditions a car isn’t capable of braking the same as in maximum dry conditions so a car with too much front brake bias will lock up the fronts well before the limit [ on road race cars we dial in more rear brake bias in wet slippery conditions ]
So in wet conditions the speeds are a lot slower and the braking G’s are a lot less ,hence the more rear brake bias.
On a road car the braking speeds [and conditions] are a lot less than a race car, so the manufacturers proportion a lot more rear brake bias into a vehicle [trying to achieve all 4 wheels braking correctly in less than ideal conditions or slower speeds.]

So in these situations the proportioning valve is not actually doing anything, it is the calculated bias of the front and rear [ fluid mechanics ], But when a panic stop is required the proportioning valve will limit the amount of pressure to the rear brakes [ any excess pressure from the pedal goes to the front wheels ,while the rears stay the same ]
On a T bucket a pedal box with a balance bar and 2 master cylinders would be better [if somebody doesn’t know how to mix and match pistons/slaves]

 

Dooley- let me try to answer your specific questions very simply-I designed brake systems for GM cars for many years ( many years ago ...[emoji3] ) so simple is somewhat hard sometimes....

Metering valve essentially holds off pressure to the front brakes at low line pressures (60-90 psi ). Since disc brake pads ride right on the rotors, they start braking right away. Drum brakes have to overcome the spring pressure holding the shoes off of the drums, so they do not start braking right away. Metering is used for brake wear balancing ( in really light stops -like city traffic, front brakes do all the work )- metering moves wear from front to rear brakes. Also, metering prevents front wheel lockup on very slippery surfaces under light braking. Both are basically fine tuning issues, and unless you see issues in the above areas, metering valves are not usually used in our modified cars.

Proportioning valves are a different issue. They are needed to remove rear braking force as braking effort increases to compensate for two things: 1) weight balance shifts from rear to front due to CG shift under heavy braking, and 2) modern drum brakes are "self energized" - one shoe essentially applies the other-making output increase at an increasing proportion at higher pressures. Simplistically if your brake has one wheel cylinder it is self energizing (57 Chev) , if two wheel cylinders it is not ( old Ford ) . Bottom line is you should use a prop valve - adjustable is good to let you tune your brake balance to your car, since it is affected by many things- weight distribution, tire sizes, brake sizes, etc. In the industry we have a very big computer simulation and a lot of testing to determine these things .... Give me a PM if something is not clear of I can answer any other questions.

 

I admit a reversal of fortune, thank you all for not raking me over the coals, OLD HAMB style.
Yet, I am glad the discussion turned to doing brakes right, instead of band-aiding them. It costs more to do the brakes right, but it pays off in the long run.

Cosmo

 

Good info

 

This is what I'm using on my '59 Ford

 

is the above a proportioning valve?

 

That is what CPP and Speedway call it. 2 lines from the master go into the top, 2 ports out to the fronts and 1 out to the rear. I did away with the factory br*** block. Excuse the ****py attached picture.

 

The question becomes is it an adjustable metering valve or a proportioning valve? Both?

I have seen those and from what I understand they do work well. I have not used one yet.

I used to knew a guy with a roundy round car that had proportioning valves in the cabin ( ****pit?) with him and he would adjust his bias from one side to the other depending on track conditions. He only used his brakes on a long track so most of the time he never touched them at all. he said that you really couldn't always get things the way that they needed to be on hot laps.