Originally posted by Cowboy from Hell:
Corey’s suggestion to try the brakes both ways and make your own choice is sound advice.
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.
The vendors don't seem to be overly concerned about liability. All of them are selling brake systems that are clearly labeled by the manufacturer as "for racing use only"; at least one of them removes all those warning inserts from the packaging before sending it on to the customers. Although as a grown-up I don't feel there is any great inherent danger in using 'racing' brakes on the street, a sharp attorney looking for some deep-pockets cash after an accident might not feel the same. The vendors manage to sleep at night (presumably) so they must not think the liability risk is great enough to warrant not offering these systems to the public for street use.
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?
Sorry George, that particular argument doesn't hold any water, particularly when viewed in the context of the Mangusta, whose brake bias was a complete disaster. Those things had so much front brake bias that the rears were nothing more than ballast. Some of the cars had a proportioning valve from the factory, while others didn't. That valve was located at the bottom of the firewall on the engine-bay side; I confess I don't know if reduced pressure to the front or the rear. But clearly their engineering department left a lot to be desired in that regard.
That's why Ford had to re-engineer the Pantera system slightly, when the prototype cars failed to meet DOT standards for braking performance. The fix was a very expedient one, when a proper fix would have been a complete re-engineering of the system with bigger/better rear calipers.
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.
If I said 'shortly thereafter' I misspoke. They should lock up some time later, but not immediately afterwards. But they should lock up eventually.
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.
Agreed, with a strong caveat. Four-wheel lockup would only result from driver error (braking too hard). A proper braking system should have the capability of achieving it, and be engineered so that the driver can control his braking to approach it, but not go over the edge. Prior to the advent of ABS, virtually every car sold in semi-modern times could easily do so--including the Pantera.
I don't know of any non-ABS car that was ever designed to intentionally prevent the brakes from fully working in that regard?
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.
Very much agreed!
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.
That last part is a pretty long reach, but you raise an EXCELLENT point about the characteristics of adjustable prop valves, one that I only recently learned myself. I presumed (and their literature often seems to indicate) that they reduce pressure in a linear fashion, but the non-linear reduction you describe above makes a lot of sense when they are employed in the rear circuit. It also makes them sub-optimal when employed in the front circuit. Because the harder you brake (with one of those valves in the front circuit), the more the bias shifts towards the rear, exactly what you don't want. So if you have one of these, you have to have it adjusted rather conservatively. As you said above, you never, EVER want the rear wheels to lock first.
With an adjustable valve set up to ABSOLUTELY ensure that the rear brakes don't lock up first, it will deliver less-than-optimal pressure to the rear brakes, most of the time. However, it will still deliver substantially MORE pressure to the rear brakes, than not running a valve at all. So while it's sub-optimal, it's still demonstrably better.
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:
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.
It's true, and has been demonstrated as such by Bill Taylor (see below)
Am I getting too techy for anyone, or are you guys still hanging with me? Have another drink. I’ve had several myself.
HAHAHA!!! This is a great discussion!
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?
Good question! It is a linear pressure reduction device. Bill Taylor built a test rig and measured input/output of a stock valve; here is his graph:
While the output line is not perfectly aligned with the input line, it is close enough to linear to be considered as such. It's a pure pressure reduction valve. A full accounting of his test protocol, and photos of a disassembled valve, appear on Mike Dailey’s Panteraplace website, here:Pantera Place--Brake proportioning valve
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.
Which it does! Most people have upgraded their Panteras to use wider tires, and the rear tires are normally relatively much wider than the fronts (that is, the increase in rear size is much greater than the increase in front size). This means that the rear could potentially use even more brake bias than before, which would mean (if the stock calipers were maintained) that the front brake pressure would need to be reduced even further than the stock scheme, in order to maintain appropriate front/rear bias and allow the rear brakes to work properly.
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.
Those are unrelated arguments. A vented disc will stop just as well an an unvented disc, when at normal operating temperature. A vented disc will allow temperatures to be maintained in the useable range, and allow the brakes to continue working when they have been used aggressively. Under normal driving conditions, brakes never get hot enough to suffer heat-induced brake fade, so vented rotors would have no effect on braking bias. If the brakes are biased properly and are used aggressively, all four will heat up, so all four would benefit from venting. Putting vented rotors in the front and solid rotors in the back would probably result in the brake bias slowly shifting forward as the rear brakes overheated and the front ones didn't, under aggressive driving conditions. That's not really optimal either.
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.
It sounds like Chris has found and filled a niche that has been overlooked for far too long. However, I can't find anything specific about it anywhere. Chris, would you mind (and George, would you please allow) generating a bit of an advertisement for them here, and let everyone know exactly what George is referring to? I think for somebody trying to improve their braking on a budget, an improved rear caliper setup that would deliver proper braking without pressure reduction anywhere in the system would be absolutely the best thing ever!
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.
No--ideally all FOUR of the Pantera's discs 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 ventilated front disks. The solid disks were a Ford spec.
Vented discs were one of many OPTIONS on the Euro GTS, but did not come standard, nor did the 10-inch wheels, etc. I've worked on several different Euro GTS Panteras, both here and in Europe, and the vast majority came with solid rotors from the factory.
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.
Although we're not supposed to insult people here, I am going to go out on a limb and say that it wasn't just crazy, it was just plain STUPID. Ford and De Tomaso were under both time and cost constraints to bring the car to market, and the band-aid fix took minutes to implement while sourcing all new rear brakes and redesigning the system would have taken weeks or months. The cars have suffered ever since due to this shortsightedness (as well as similar shortcuts on numerous other systems, i.e. air conditioning).
In answer to other’s questions, yes, the standard Panteras through the end of production all came with the 1971-spec brake system. Better brakes were an option on the narrow-body cars, but seemingly not many received them. The GT5 and GT5-S came standard with vastly upgraded brakes. (And yet, curiously, I believe that they still retained the brake bias valve??? Can somebody who owns one of those factory widebody cars confirm that?)
And Doug, as to your statement that nobody has ever demonstrated that Panteras have been wrecked as a result of degrading the rear brake effectiveness, that's a rather specious argument. We all know that Panteras have been crashed over the years; some of them had received this modification. It would be next to impossible to 'prove' that an accident occurred because of this modification, and would have been avoided without it.
In discussing the argument in favor of degrading the rear brakes, I'm reminded that there are people who still, today, stridently believe that you're safer NOT wearing seat belts. That was a long-held belief for years when seat belts first came out, and there was plenty of anecdotal evidence available to support such contentions (heck, even I can provide one; many years ago a guy in Nor-Cal drove his Pantera straight into a telephone-pole-sized tree, hitting so hard that the trunk ended up touching the wiper grilles! The front was bent into a V and the headlights were only a couple of feet apart. He wasn't wearing his seat belt and was unhurt--except for injuries to his arm afterwards, when he deliberately broke the window to get out of the car).
While that mindset was prevalent back in the day, with the passage of time, more and more people have come to realize that seat belts are a good thing. Similarly, while many years ago people (including some Pantera vendors) advocated increasing front caliper performance at the expense of total vehicle balance, that idea is no longer as widely held as it once was.
As to the braking issue, while it's impossible to demonstrate that an accident could be caused by partially disabling the rear brakes, intuitively one can see that it could be true. What COULD be demonstrated empirically is that stopping distances under max-effort braking will increase if the valve is removed, or conversely, will be reduced if it is installed. Ford proved that conclusively back in 1971.
But, as many people have mentioned, times have changed, we're all running different tires, etc. etc. so the specifics of those tests are no longer perfectly valid for any of us. What is needed is a new round of objective, empirical testing. And I've been keen to do just that.
My fiancee's Pantera #1765 had stock brakes, with a blown-out master cylinder, and a badly leaking stock pressure reduction valve. The folks in PCNC got together and installed a new Wilwood master from Dennis Quella, and an adjustable (knee-style) Wilwood proportioning valve in the front system. (She has Pantera East 16/17 inch Campagnolo clones on her car with 245/45-16 and 335/35-17 Dunlop tires). The valve has been left in the full-open position (no pressure reduction) so that it effectively mimics having no valve at all.
It does not stop well.
She drove it at a few track days, and totally destroyed her front brake pads due to the excessive front bias. New Porterfield pads have just been installed in front, and new rear Porterfields will be going in shortly, along with braided stainless steel flex hoses.
Once that is complete, her car will represent a sizeable percentage of the demographic here—stock calipers, upgraded pads, braided stainless hoses, aftermarket master cylinder, and oversize wheels/tires. We will then undertake a scientific test regimen to establish the optimal positioning for the valve. There is an industrial park near my house that is largely uncompleted, offering a straight, wide road about a half-mile long with zero traffic. We will use both distance- and temperature-measuring equipment, to try to get some empirical data to support or disprove the contentions that one can only otherwise intuit.
We’ll get the brakes heated up by driving around and performing a few hard stops. Then we’ll do a max-effort stop from 50 mph and measure the stopping distance, and front and rear rotor temps. Then we’ll start dialing in some rearward bias by reducing the pressure to the front system, and repeat. By measuring front rotor temp, we’ll ensure that our data isn’t being skewed by thermal runaway (the brakes getting hotter and hotter on each stop). We’ll figure out how much cooling is necessary between tests so that front rotor temps remain fairly constant, and rear temps increase as braking increases. We’ll continue numerous times until we have gone too far (if in fact it’s even possible to do so) and the rear end locks up. Then we will back well away from that.
I am expecting that some brake restriction to the front system will result in shorter stopping distances than no restriction. It remains to be seen what will happen first after that—rear brake lockup, or front brake ineffectiveness due to excessive restriction. If stopping distances initially decrease, and then start increasing, with front rotor temps dropping and pedal effort increasing, then that will probably demonstrate the point where the adjustable valve hurts more than it helps.
Eventually we’ll hopefully arrive at an optimal, safe setting that affords the best possible braking with the constraints that the stock braking hardware produces.
And then, she’ll want to buy a complete aftermarket braking system!