Technical Braking Theory/real application

OK, so if my rear brakes lock up (once I rebuild them) with the set up I have i can add an adjustable proportioning valve to the rear to reduce the braking force the keep from looping end around on a hard stop.

I don't have an issue with the front locking up under any loads so far so it looks like no metering valve is needed.

 

You got it....

 

Yep.

Although it may take a little playing to get it right, you don't want the fronts to lock up either, makes it really hard to steer.

In a perfect world all 4 wheels will lock at the same time, and you should be able to do that even if it is not optimum in most situations.

 

AFAIK, this is a proportioning valve, no trickery inside it. This is a first for me too but it looks ***y.
I never used a brake bias adjuster on a stock car, just one more item to complicate things. I guess it could come in handy to compensate for track conditions, loss of weight (fuel usage) and tire variables.

 

The thing to remember is that any stock car, like your '59 for example comes with a myriad of options but one brake setup. The brakes at least on our older cars were designed to work with any configuration the car could possibly have. Your '59 for example could have an FE, a Y block a 6, air no air, radio no radio, heater no heater, stick, automatic. Granted some of the options make minor changes to the weight distribution of the vehicle and others make major changes. Yet it comes with one basic braking package.

So lets suppose that the brake optimized around the 6 just for an extreme and you dropped in an FE. When you hit the brakes the nose is going to dive more in theory, so you would ideally want to loose a little clamping pressure in the rear. Perhaps we are choking on cream of ricel here but you catch my drift right?

Oh I have trouble with most acronyms ( truth always have had) humor a simple minded old fart here will ya? What does AFAIK mean?

 

Go and test you car on a wet road or gravel road.
If the rears lock up ,you need smaller rear cylinders to get the balance correct.[or larger front cylinders/pistons]

Then..... you add a proportioning valve to prevent rear wheel lock up in panic stops.
Get the front rear balance sorted first

If don't believe me, then go ahead and balance it with just an adjustable proportioning valve, then test it on a dry road .
Then go and test it on a wet road.[ the fronts will lock up easier ]
If you do it the other way round, you'll get rear lock up on a dry road.
In road racing, the braking is done on the “conservative side of having an accident”, every time the brakes are used it is at the absolute last moment with maximum braking
[ If you don’t brake like this, some hot shoe will be p***ing you under braking ]
So the front and rear balance is dialed in for maximum G’s [ we add more rear bias for wet/slick conditions ]
AFTER the balance is set up correctly ,an adjustable proportioning valve is used for changing conditions during the race [ fuel load, track getting stickier etc ]

 

I have never seen or heard of adjustable metering, also called hold-off. Metering simply prevents any front disc pressure until about 100 psi or less, as previously stated by fms427. These valves are normally part of a combination valve, and feature a push ****on or pin to allow byp***ing during low pressure bleeding.
Proportioning is much more involved than simply metering, as many believe. Prop valves start to limit rear pressure at their fixed, or adjusted crack or knee points, then deliver a percentage of master cylinder pressure from that point on as M/C pressure rises.

 

When I set up a system the car usually does a Brake-Dance !!

 

Yea I forgot the smiley, sorry.

 

I'll throw this out there as food for thought...

I mentioned that some of the Jag brake systems don't have any metering/proportioning valves installed from the factory; in fact, they don't even have a brake warning light that shows if you lose pressure on one side or the other as is typically found (in spite of the fact that it's supposed to be federally mandated). So what did they do?

What they did was design a master cylinder with a two-piece piston. The 'rear' piston is 'conventional', i.e. moves as soon as you press the pedal. The 'front' piston has a gap between it and the rear piston (with a spring to maintain that gap when not applied), so when the brakes are applied, there's a momentary delay before the rear piston contacts the front one and the front brakes get pressure. If using a 'split' brake system with two single-circuit masters and a balance bar, you could mimic this by installing a spring between the front master cylinder body and the balance bar, leaving a gap between the actuating rod and the piston. Of course, this explains why they don't have a pressure-differential warning light switch, as the switch could 'blink' every time you apply the brakes. Play with the spring tension and the gap to adjust 'metering'.

Oh yeah... Jag did install a 'brake warning light', but it warns of low brake fluid....

 

After thinking about this, with some creative machine work you could modify a typical dual-circuit master to mimic the Jag unit. Just don't try to run a brake warning light....

 

PORKN******: "Oh I have trouble with most acronyms...What does AFAIK mean?"
As Far As I Know. I think it's 'text-talk'.

...really good brake info here!

 

I disagree with your one brake system. They used different diameter brake drums, different width shoes, different master cylinders, different wheel cylinders and anything else they could think of. It was a running joke when ordering parts that you had to know everything about it, even the paint code. I worked at a Ford dealer and you had to have a VIN number to get any part. We used to say that Ford never made any 2 parts the same.

 

Why do people install an adjustable proportioning valve where it is not easily accessible like under the hood and you have to get out of the car and open the hood, or on the frame under the car so you have to crawl to get to it? Why not mount it so the stem comes up through the car floor with the knob just above the floor/rug/mat/what ever so you can reach it from the driver's seat? About even with front of seat works well... Hell you can put it in the dash and with a suitable knob no one would know its there...Its not against the law to put it with in reach..

 

Unless you have a race car, you'll more than likely set it once you get it sorted out and that's that. I can't think of the plumbing woes that would go with installing it in places other than close to the master as possible.

 

It is here in New Zealand

 

I'm still confused, as I have similar problems. My rear drums lock up and my front discs dont pull the car up like I expected. Some as on this thread say change rear wheel cylinders and others say install a proportioning valve. While it is currently apart I also want to put in residual valves as master cylinder is under the floor.

 

Both work, it is just two different approaches to the same problem (there are other ways as well).

 

Not even just the stem and knob up through the floor?

 

If its got a knob, somebody is gonna play with it.

 

Pull the car up - what's that mean ?

From your description it sound like the rears get way to much brake advantage before the fronts start doing their part.

If that's the case, it seems to be a gross bias difference, a parts change first would get you much closer to where the proportioning valve can fine tune it.

 

Master cylinders are designed to pressure one end before the other using springs. This is where various applications are different. On a circle track car we once used two single master cylinders mounted side by side applied by one pedal with a connection that could be moved closer to one master cylinder than the other to put more pressure on the front or rear brakes as needed.

 

"Pull the car up" means stopping it/ braking. Obviously we say things a little differently here. Thanks for your other comments, it is what someone else with experience has suggested.

 

especially the stem and knob!

 

You can get a motorsport exemption for them. [ Rally cars, and Road Race cars that are also road registered ]

 

There is 3 different ways to remedy this:

1: put an under floor pedal box in your car . That is a brake pedal with an adjustable balance bar and uses 2 master cylinders [Wilwood are cheap]
This is the best method and you end up with dual circuits , with infinitely adjustable line pressures.

2: put smaller rear slave/wheel cylinders in the rear to reduce clamping pressure [ and add a proportioning valve in the rear ]. Note that even with a tandem cylinder all line pressure is equal throughout the brake system. It is where line pressure is converted to clamping pressure that the adjustment is made [fluid mechanics]
This is the next best method

3: Add an adjustable proportioning valve in the rear to limit maximum braking only. These do not proportion more to the front, they only limit the maximum pressure in the rear.
If you were driving on gr*** ,mud or ice and gently apply the brakes you will still have too much rear brake bias .

An adjustable proportioning valve works quite well in road racing because the front braking is way higher than normal with racing tyres in dry conditions so there is a lot of weight transfer off the rear. Understand the cars are pretty well balanced prior to fitting an adjustable proportioning valve.

The adjustable proportioning valve is used to tune weight transfer , not poor design/ balance

 

If you've ever rebuilt a master cylinder, one spring is stronger than the other so one will compress before the other allowing one piston to move before the other. Once hydraulic pressure starts to build, it forces the other piston to move. Pressure differential is very low but, for instance, enough to ensure that any free play in rear drum brakes is taken up before pressure is applied to the front disk brakes.