quote Originally posted by pmbrunelle: Piston balancing alone is not a perfectly acceptable method to set bias on a street car. Optimal bias depends on the traction available. Piston balancing alone sets a fixed bias determined solely by the calipers. Such a system optimized for dry weather would lock its fronts prematurely in the wet. Conversely, a system optimized wet weather would lock its rears prematurely in the dry. A proportioning valve allows a variable bias ratio, dependent on brake pressure. The higher the brake pressure, the more the bias is shifted frontwards. On low traction surfaces, brake pressure is lower, because the driver modulates brake pressure to avoid lockup, which occurs at lower brake pressure in a low traction situation. On high traction surfaces, brake pressure is high because the driver is standing on the pedal. Even with a proportioning valve though, you still need the proper caliper piston balancing to get you in the ballpark.

I did some calculating after my last post, regarding how well the brakes would work if we didn't have any sort of proportioning system.

so I considered the brake balance to be 50/50. The weight distribution 60/40 (rear/front). The weight 1230 kg, the CGH at 500mm (it's actually 495mm on a stock fiero according to what I was able to find via google).
At a deceleration of less than 0.5G the weight shift resulted in a 50/50 weight distribution. This is the point at which all four wheels would lock if there wasn't enough grip. And we're not braking very hard yet. Braking harder would result in the rears locking first, if the traction was reduced for some reason (patch of ice, water or maybe a slight bump in the road). And this is with infinitely stiff suspension, so it would be even worse in real life.

Lowering the center of gravity and biasing the brakes more towards the front greatly improved the results of the calculations.
I know my calculations lack details such as the amount of traction each wheel can produce and the effect of the suspension moving, but they clearly show why we do not really want a 50/50 brake bias (and why I don't think the GM engineers thought that would be a good thing).

on a slightly non-related side-note, how often does the combination valve cause problems? What would happen if the lines were connected to the wrong outlets or if the valve was contaminated?

I came across the same value for the height of the CG in my research.

However, for the front/rear location, someone on the forum weighed a Fiero Formula with manual transmission on 4 corner scales.

With this data, I found that the weight distribution:

43F 57R @ 0g

64F 36R @ 1g

Any other value can be calculated with linear interpolation.

The stiffness of the suspension is pretty irrelevant. The CG barely moves much.

Vehicle speed though will cause air drag on the car, changing bias needs.

Tire friction non-linearity is a big factor.

If the prop valve's seal was deteriorated then there would no longer be proportioning, if this resulted in pressurized brake fluid on both sides of the piston.

[This message has been edited by pmbrunelle (edited 10-21-2009).]

quote Originally posted by pmbrunelle: Lets try an analogy. Imagine you loosened the rear brake bleeders. Your front brakes would still grab, but the rears wouldn't. Agree the bias changed to nearly 100F, 0R? As the brake pressure increases, the springed piston "gives" more. It's not as extreme as a completely open bleeder, but it has the same effect.

yes, but a bleeder "blows off" the pressure. there is no blow off on the distrobution block. as far as I can tell, that springed piston does nothing but either absorb a slight amount of the initial pressure, or create a "after pressure" when the brake pedal is realeased.

there is no pressure blow-off, or pressure restriction whatsoever in the distrobution block. I cannot see how it can do any form of regulation of brake pressure.

quote Originally posted by pmbrunelle: I came across the same value for the height of the CG in my research. However, for the front/rear location, someone on the forum weighed a Fiero Formula with manual transmission on 4 corner scales. With this data, I found that the weight distribution: 43F 57R @ 0g 64F 36R @ 1g Any other value can be calculated with linear interpolation. The stiffness of the suspension is pretty irrelevant. The CG barely moves much. Vehicle speed though will cause air drag on the car, changing bias needs. Tire friction non-linearity is a big factor. If the prop valve's seal was deteriorated then there would no longer be proportioning, if this resulted in pressurized brake fluid on both sides of the piston.

I think the manual states a 55/45 weight ratio or something like that. I used 60/40 simply because it was easy and close enough
Anyway, I agree

I ended up putting in a proportioning valve in my 84 track car. I picked up the standard Jegs unit with the two ports, one in and one out. I then took the combination valve, cut it directly in half right at the sensor location and milled it flat. I welded up the unused ends and with that created a factory thread T fitting without having to mess with any adapters or making new lines. The same went for the rear. Rather than making new lines I just ran the existing rear half of the combination valve that was welded up. Saved a lot of time and kept the front and rear separate in case of any failures. Only problem is no brake light with doing this. If you can suffice getting rid of that, this may be a simple way to approach the problem. If you would like pics I can snap a few for you.

quote Originally posted by MoeBawlz: I ended up putting in a proportioning valve in my 84 track car. I picked up the standard Jegs unit with the two ports, one in and one out. I then took the combination valve, cut it directly in half right at the sensor location and milled it flat. I welded up the unused ends and with that created a factory thread T fitting without having to mess with any adapters or making new lines. The same went for the rear. Rather than making new lines I just ran the existing rear half of the combination valve that was welded up. Saved a lot of time and kept the front and rear separate in case of any failures. Only problem is no brake light with doing this. If you can suffice getting rid of that, this may be a simple way to approach the problem. If you would like pics I can snap a few for you.

Yes please.

This might help quite a lot. Even if I will chose the Jegs distribution block. At least I'll understand better how it works.

Ok, the car is not of any easy access at the moment, but I will snap pics when I can get to it.

Digging up an old thread...

Over the past few nights I was messing around with the combination valve from my 88 Fiero, and rebuilding a set of brakes I had on the shelf. Did a very detailed Cad drawing on the combo valve to try to help me understand how the little bugger works. It took me quite a while to finally understand what is really going on inside that block of mystery. It took an experiment to get the answer on the proportioning valve section, since the seal works in a very unusual way.

The Balance spool... or spoon as I have also seen it referred to:
In the event of a Front Right brake failure, it will cut off that brake, and keep the Front Left and Rear functioning fine and trigger the warning Light. It does NOT protect against Rear brake failure, or Front Left. The Master Cylinder isolates the fronts from the rears in the event of a failure. So if your Front left fails, you loose the right as well, but keep the rears. If the rear fails you keep the Fronts. This spool is being forced to center if the pressures front and rear are fairly close, a small aluminum collar is what aids it to the center. So, mostly, this spool only turns on your warning light if any of the 3 brake circuits fail, but offers a small measure of protection if the front right fails.

The proportioning valve was a tough bugger to understand. I know hydraulics, but the seal on the PV spool is used in a different manor then any other seal I've ever seen.

The Spring: The springs ONLY function in the Proportioning valve is to set the pressure at which the proportional valve begins to regulate down the pressure to the rear brakes. Its about a 15 LB. spring at the installed clearance. This keeps the pressures on the rear brakes EQUAL to the fronts till about 300 PSI.

The Spool: The most critical dimension of this piece was the narrow long tube section, It's 1/4" Which is the effective area of the spool. With that 15 LB spring, it would take 300 PSI to start to compress that spring, thats where I get that number. It has a vented cap on the rear to act as a reference, Otherwise there would never be a pressure imbalance to help shift this spool.

The Cavity: This also had a critical dimension, towards the bottom of the cavity, it narrows slightly to 1/2" This is the surface that the seal rides up against, which is the effective area of that seal.

The Seal: When the unit is assembled, this seal is being squeezed slightly by the spring pushing the spool into the seal against the cavity. Normally, I would have assumed that this meant that when its like this, the seal is sealing off the spool to the cavity. This was 100% NOT the case at all. When the spool bottoms out on the seal, its actually unseating it, causing it to free-flow. Otherwise the brakes couldn't ever receive any pressure. I tested the theory by assembling the proportional valve without the spring, then, I stuck a small piece of metal in the rear outlet to prevent the spool from bottoming out, I then used my air compressor to blow into the inlet. To my surprise no air was coming out of the outlet hole, but as soon as I unblocked the spool to allow it to bottom out, and it started passing air. That was proof enough.

When you apply the brakes the pressure inside the cavity and on both sides of the seal rise since its passing oil, eventually it hits the point where the pressure pushing the spool against the spring overcomes the springs pressure, allowing the spool to move towards the cap. When this happens the seal is allowed to flare out, seat itself and seal off the outgoing pressure. Which begins to limit the outgoing pressure. So heres where some math comes in.

The 1/4" diameter spool has an area of 0.0491 Square inches
The 1/2" diameter cavity has an area of 0.1963
Subtracting the two areas, since they combat each other you get 0.1472
So on one side, the area pushing the spool is .1963, and the other side the area pushing the spool is .1472
So there is a ratio of the areas, and that magic number is exactly 0.75:1

Because of the spring, the valve doesn't begin to regulate till 300 PSI, and when the incoming pressure is above that, it only adds 75% of it.

Another thing I found peculiar about the 88 brakes, is that all 4 brakes are effectively identical, save for the E-Brake. All the pistons are 1.888" in diameter, all have the same pads, and rotors. This makes calculating the braking forces very easy on this car.

In most circumstances (less then 300 PSI), the front and back brakes are working 50/50. At the most extreme pressures, (Pressure >1200) this becomes static at 55.5 Front, 44.5 Rear.

And if I plot that out it looks like this: