Doug made a point worth repeating, there is no testimony lending credence to the assertion that removal of the proportioning valve is harmful in any way. I certainly would not advocate its removal if there were. Neither do I believe the vendors such as Steve would offer kits for the proportioning valve’s removal if it might create a liability issue for them.
Mike’s story regarding how the proportioning valve came to be is very enlightening, at least for me. It appears the brake system was designed without the proportioning valve, in other words Dallara (or someone else within the DeTomaso organization) designed the brake system with the extremely forward biased brakes intentionally! This also means removal of the proportioning valve is equivalent to removing an alteration orchestrated by Ford engineers and restoring the brake system to the engineered balance Dallara had intended. Now that doesn’t sound so bad does it?
This leads me to a discussion of front to rear brake bias. So settle in guys, pour yourselves a drink, this is going to be one of my long winded posts.
Mike described the front to rear bias of an ideal brake system as being so closely balanced that the front brakes lock-up first, and the rear bakes lock-up shortly thereafter. That is the text book ideal brake bias described by some race car engineering books, but in the real world that is seldom how brake bias is set-up in an amateur race car; and it is universal in the world of street car design that rear brake lock-up is NEVER a good thing; it is something to be avoided. It is desirable for the rear brakes to do as much work as possible, but never so much as to risk the condition of rear brake lock-up. Street cars are by nature heavily front brake biased.
And remember, the Pantera is a GT car, a high performance street car.
When braking there are 3 possible scenarios:
(1) 4 wheel lock-up - It’s fairly easy to understand why 4 wheel lock-up is not a good thing, at that point all 4 tires have lost adhesion to the road and the driver has no control of the vehicle’s speed or its direction of travel. This condition is to be avoided with no exceptions.
(2) Rear wheel lock-up - When the rear wheels lock-up the rear end of a vehicle has a tendency to swing around on the outside, unless the vehicle is traveling in a perfectly straight line. Even when traveling in a straight line the rear end will have a strong tendency to swing in one direction or the other, depending upon centrifugal force, the road surface, tire condition, etc. It can be difficult to avoid rear wheel lock-up under all conditions because of the wide range of variables affecting weight transfer, centrifugal force and the chassis’ ability to cope with them. The variables include the driver’s driving style, road conditions, varying corner speeds, shock absorber wear, tire wear, tire pressure, changes in tire make and model, varying outdoor temperatures, and varying vehicle load (passengers, fuel level, luggage, etc.). Once the rear end starts swinging it is nearly impossible to stop; this tendency is even stronger with a mid-engine vehicle … which no doubt contributed to Dallara’s decision to set up the brake bias the way he did.
(3) Front wheel lock-up – If a vehicle is traveling in a straight line when the front wheels lock-up, the vehicle tends to continue to travel in a straight line. If the vehicle is cornering when the front wheels lock-up the vehicle will begin to travel forward in a straight path towards the outside of the corner. Unlike rear wheel lock-up, front wheel lock-up is fairly easy to recover from, all the driver needs to do is lighten the force on the brake pedal, allowing the front tires to regain adhesion, and the vehicle will resume cornering. A driver becomes aware of the loss of front tire adhesion quickly because they feel the loss of directional control via the steering wheel in less than a second’s time. If they are driving an open wheel race car they can see the tire smoke from the skidding tires almost as quickly as they feel the loss of directional control. Front wheel lock-up is considered by far the best of the 3 scenarios.
Both passenger cars and race cars are set up with a bias towards the front brakes, not just because most of a vehicle’s weight transfers to the front wheels under braking, but also because it is preferable for the front tires to lose adhesion first. And although it is important for the rear brakes to shoulder as much of the braking chore as possible, vehicle dynamics make it equally important to have a margin of error built into the brake bias to avoid rear wheel lock-up under dynamic or varying conditions. In the top levels of racing the extra front bias for that margin of error is often very small, the brakes are set up as Mike described, the front brakes will lock up just before the rears under heavy straight line braking. In amateur racing the norm is more forward bias; and street cars are another thing altogether because the range of variables encountered by a street car are tenfold greater than the variables encountered by a racing car.
Fixed – linear – mechanical brake bias is “built-in” to vehicle brake systems in many ways. The size of brake disks can be varied front to back, the size of brake pads can be varied front to back, the size or number of caliper pistons can be varied front to back, and dual master cylinders can be employed having different sized pistons for front and rear circuits.
Besides these fixed means for providing front to rear brake bias, race cars usually have a means for varying the brake bias as well; this usually entails a bias bar between the dual master cylinders. This allows racers to compensate for temperatures, tracks, or tires. On top of that, some types of racing make it necessary for the driver to be capable of adjusting brake bias during the race. This allows the driver to compensate for changing weather, tire wear, changing track conditions and even varying fuel levels. In that application a cockpit mounted knob or lever attached to a cable assembly that manipulates the bias bar is often employed. I’d like to point out however, while the professional drivers use those cockpit mounted brake bias adjusting gadgets, the amateurs usually leave them alone.
With all these mechanical means to provide fixed – linear brake bias, you may be wondering why a hydraulic “proportioning valve” is needed at all. At least that’s what I’m hoping you’re wondering because that’s what I want to describe next.
A proportioning valve is not a device for setting linear brake bias; it does not proportion brake force front to rear in a linear sense, the proportioning valve we are most familiar with is a hydraulic pressure limiter. Observe the graph below.
The hydraulic pressure input equals the hydraulic pressure output until the “knee point” setting is reached, from that point onward the output changes very little. In the average car with a proportioning valve attached to the rear brake circuit this means under light braking, when there is no chance of the rear wheels locking-up, the rear brakes are allowed to receive full brake pressure and contribute significantly to slowing or stopping the vehicle. But under heavy braking the pressure to the rear wheels is severely limited and the braking bias shifts drastically to the front tires. This should indicate just how badly the major auto designers wish to avoid rear wheel lock-up, and is probably the other consideration explaining why Dallara designed the brake system with the extreme front mechanical bias it has.
I also hope this explains why anyone considering installation of an aftermarket “adjustable” proportioning valve in the Pantera’s front brake circuit shouldn’t do so. You do not want a device with a “knee point” limiting FRONT brake pressure in this way. Proportioning valves are designed strictly for rear brake circuits.
DeTomaso describes the valve I call the “proportioning valve” as a “front brake pressure control valve”. Mike described it thusly:
quote:Originally posted by Mike Drew:
The stock proportioning valve is actually a pressure-reduction valve. By having different-sized pistons inside, it takes X psi input and delivers Y psi (lower) output.
In other words, it reduces hydraulic pressure in a linear fashion; it does not function like a classic proportioning valve. I have heard this description before, I have never verified if this is accurate, but I am willing to accept it as truth for arguments sake.
Am I getting too techy for anyone, or are you guys still hanging with me? Have another drink. I’ve had several myself.
A question for Mike, has anybody truly verified the Pantera’s proportioning valve truly does not have a knee point built into it, that it is strictly a linear bias device, or is this just an assumption?
I do not agree with painting a description of the Pantera brake system with the “proportioning valve” installed as finely balanced, and removing the valve as throwing the brakes way off balance. As explained above, even with the valve in place the brakes must still have a significant margin for error built into the front to rear brake bias, to compensate for all the possible variables encountered by a street car. If the oem brake system were truly "finely balanced" then changing tire sizes or even using tires with different rubber compounds would be enough to throw-off the front to rear bias.
If the front disks had been ventilated I doubt if the proportioning valve would have ever been installed. I’m supposing that brake fade under repeated hard braking was the concern at Ford. For that reason I am willing to concede that the addition of ventilated front disks should accompany the recommendation to remove the “proportioning valve”. I’m guilty of not emphasizing that enough.
I’ve also been meaning to mention Chris’ rear brake upgrade kit, so now is as good a time as ever, I think Chris’ new kit should be added to the list of essential Pantera upgrades. I’m not aware of any other brake upgrade that involves just the rear brakes upgraded to complement the braking capabilities of the oem front brakes (sans the damn proportioning valve). Good stuff Chris.
So Adams lets amend my “essentials” list for the brakes to include Chris’ rear brake kit. I wasn’t aware of its existence until he mentioned it earlier in this thread. I know Chris and can vouch for him, he’s good people … even though he installed a Chevy motor in his white Pantera. We all make mistakes.
Ideally the Pantera’s front disks should have been ventilated. The Pantera was the most expensive car sitting on the Lincoln dealer’s lot, and it didn’t even have ventilated disks, a feature even the lowly Mustang had. If you want to know which company was responsible for the solid front disks, I'd like to point out to you the European GTS had ventillated front disks. The solid disks were a Ford spec.
If more rear brake bias than that provided by Dallara was desired Ford should have accomplished this mechanically with stronger rear brakes, not hydraulically by lowering the pressure to the front brakes. That’s just flippin’ crazy.
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The 1981 Popular Mechanics exotic car comparison test (March 1981) didn’t paint the Pantera in as good a light as it could have. Phil Hill wrote some complimentary things about the Pantera, but he also commented even though the Pantera had the most points, he didn’t feel safe driving it. You know the old saying, one Oh S#!t cancels a hundred atta boys. And the editors of Popular Mechanics stated even though the Pantera won the test, the Porsche 928 was the car they would most like to own. Good grief! In my opinion, Pantera bashing was as strong as ever in that article. Can you think of any other auto comparison test where the winning vehicle was so under-played? If the guys importing the Pantera in 1981 (Panteramerica) had purchased some advertising the negative comments would have probably been omitted. As it stands the magazine article is a mixed bag of good and bad.
Popular Mechanics March 1981, page 112
-G