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 | | Θέμα: Brake Proportioning Valves Δευ 11 Νοε 2013 - 15:32 | |
| Brake Proportioning Valves
The True Story of a Misunderstood, Misused and Misnamed Brake System Componentby James Walker, Jr. of scR motorsports Elsewhere on this website
we have discussed (in painful detail some would add) the importance of
front-to-rear bias, or brake balance, and how its optimization can lead to
better braking performance. However, one critical factor in establishing bias –
the mechanical brake pressure proportioning valve – was left out of the
discussion. After all, one can only take so much of this brake bias talk in one
sitting. That said, we are back to
share the intricacies of just how brake pressure is distributed to the front
and rear of the vehicle. More importantly, we hope that you take away the
understanding that replacing, modifying, or just plain fiddling with your
proportioning valve can do more harm than good. While it is not quite black
magic, there are plenty of opportunities to throw the system into disarray
without even knowing it. Consequently, we offer this
quick look into these devices and their mechanical siblings. A small amount of
knowledge can yield surprising benefits, not the least of which is avoiding
nasty surprises. Rear Brake Pressure
ControlIn general, there are three
ways to deal with rear brake pressure: leave it alone, make it proportional to
the front brake pressure, or control it in a way that combines these two
strategies.
Strategy 1: Leave it AloneIf no device were used to
modify the rear brake pressure, then as shown in Figure A, the front brake
pressure and rear brake pressure would always be equal. Naturally, this is the
easiest way to deal with the issue, but in order to prevent rear bias under all
conditions, the rear brake itself would need to be absolutely tiny.  As you can imagine, this is
not a realistic solution and is not found in the real world. Strategy 2: True
ProportioningTrue proportioning, as
shown in Figure B, would result in rear brake pressures being linearly
proportional to front brake pressures under all conditions. Ironic as it may
seem, “proportioning valves” do not provide this kind of control, as they are
not the purely proportional devices their name would imply.  This type of pressure
regulation is certainly possible to achieve, but it typically requires tandem
master cylinders and an adjustable reaction linkage, the same setup found on
nearly every purpose-built racing car today.
 On vehicles so equipped,
the proportioning ratio is achieved through a combination of master cylinder
piston diameter selection and the adjustment of a mechanical reaction linkage
that connects the two master cylinders. Also known as a bias bar, the linkage
geometry determines the front-to-rear force distribution coming from the brake
pedal assembly. The significant benefit to this set-up is that a bias bar can
be designed in such a way that the driver can adjust the front-to-rear
proportioning ratio while at speed. In the example shown below,
an adjustment of the bias bar of 0.010 inches results in an increase in front
master input cylinder force from 125lb to 133lb. At the same time, the rear
master cylinder input force drops from 125lb to 117lb.
Real-time driver control
over bias makes adjustments for changing track conditions or fuel load
practically effortless. The complexity of this design makes it highly
impractical for street use, but on track it simply can not be beat for ease of
adjustment. Combining Strategies – The
Misnamed Proportioning ValveConventional proportioning
valves should really be referred to as “braking force regulators” or “brake
pressure regulating valves.” While their name might imply true proportional
control, in reality they provide a combination of the control found in Figures
A and B. Up to certain pressures,
these valves allow equal pressure to both the front and rear brakes (à la Figure A). However, once
a preset pressure point is reached (600 psi in the example), the rear brake
pressure continues to build, but at a slower rate (or slope) than the front
brake pressure. Figure C displays this for us quite clearly.  Looking at the diagrams,
one can see that it is possible to design both a Type B system and a Type C
system that ultimately give the same brake balance at the point of maximum
deceleration. (Note that Figures B and C both generate 950 psi of rear
brake-line pressure when the front brake-line pressure is at 2000 psi.) However, one can also see
that Type C systems—those that use proportioning valves—can bring us closer to
optimum balance at lower deceleration levels. This benefit is relatively
meaningless in a racing application, as the vehicle is always operating at
maximum decelerations, but it is of great advantage on the street. In so many words, the
proportioning valve allows us to drive around town under optimized
brake-balance conditions (good for front brake-pad life) but also keeps
everything in check when we need maximum braking (good for stability). Due to their compact size
and relatively low cost, these devices can be found on nearly every vehicle
which requires rear brake pressure reduction to achieve optimum brake bias.
Typical passenger cars and production-based race cars fall neatly into this
category. Height-Sensing
Proportioning ValvesSome vehicle’s
proportioning valves go one step farther, as the kneepoint on the graph can
vary with the amount of weight on the rear axle. Effectively, as the rear axle
weight increases, a linkage between the axle and the body is compressed. This
linkage acts on a cam inside the proportioning valve which increases the
preload on the proportioning valve spring. The end result is that more rear
braking (bias) is allowed as weight is added to the rear axle, helping to take
advantage of the increased traction now available at the rear tires. Figure
D illustrates this relationship quite clearly.  So, Can One Adjust The
Proportioning Valve? Believe it or not, in
nearly all cases the OEM valves are well matched to the original brake system
and should not be tampered with, as there are no parts inside that are able to
be modified by ambitious owners. Unfortunately, some are externally adjustable,
so the temptation to tinker is right there in front of us! One point to ponder is that
because they are a mechanical device, proportioning valves must be designed as
a best compromise for use under all conditions. High speed, low speed, fully
loaded, and empty vehicle scenarios must all be evaluated and figured into the
proportioning valve design. Of course if you have
modified your vehicle in a way that impacts front-to-rear bias you might be
standing out in left field! As a refresher from our bias article, we will bring
forward again the lists of modifications which can influence front-to-rear
bias. Factors that will increase
front bias - Increased front rotor
diameter
- Increased front brake pad coefficient of
friction
- Increased front caliper piston diameter(s)
- Decreased rear rotor
diameter
- Decreased rear brake pad coefficient of
friction
- Decreased rear caliper piston diameter(s)
- Lower center of gravity (i.e. lowered
vehicle)
- More weight on rear axle (i.e. loaded)
- Less weight on front axle
- Less sticky tires (lower deceleration
limit)
Factors
that will increase rear bias - Increased rear rotor
diameter
- Increased rear brake pad coefficient of
friction
- Increased rear caliper piston diameter(s)
- Decreased front rotor
diameter
- Decreased front brake pad coefficient of
friction
- Decreased front caliper piston diameter(s)
- Higher center of gravity (i.e. raised
vehicle)
- Less weight on rear axle (i.e. unloaded)
- More weight on front axle
- More sticky tires (higher deceleration
limit)
Proportioning
ModificationsWe could start this section
by clearly stating that you should not modify your proportioning valve. But,
what fun would that be? In all seriousness, making changes to the proportioning
valve to effect brake bias should be left to those with the proper tools and
measurement devices, but if you have tweaked your vehicle beyond recognition,
this may be your only solution to restore a sense of proper bias to your
braking system. We’ll start here with three
of the most basic rules regarding proportioning valve installation and
selection. 1. If you have the
deeply-rooted need to install your own adjustable proportioning valve, be
advised that they should NEVER be installed if the factory unit is still in
place. Proportioning valves in series with one another can do nasty,
unpredictable things! 2. If you have the
deeply-rooted need to install your own adjustable proportioning valve, be
advised that they should NEVER be installed in-line to the front brakes. The
effect would be to make your vehicle rear-biased before you could say “terminal
oversteer.” Front brake line pressure should always be left alone – only the
rear pressures should be considered for proportioning. 3. In all cases, the basic brake system
balance needs to be close to optimized to start with. This is the only way that
a proportioning valve can be effectively utilized. You should never assume that
simply adding a proportioning valve will address all rear-bias conditions, as
even the best proportioning valves must be well-matched to the target vehicle. Proportioning Valve
SelectionSelecting the correct adjustable proportioning valve for your vehicle
entails not only selecting the proper point at which slope limiting begins (the
kneepoint), but also selecting the proper rate at which rear brake line
pressure builds after that point (the slope). Nearly every adjustable
proportioning valve on the market today has an adjustable knee point (the point
at which the rear brake line pressure begins to be controlled), but a fixed
slope (the rate at which it builds beyond the knee point). While one parameter
is adjustable, both are critical to system performance. Note that in Figure E
the two curves have the same knee point, but the slopes vary greatly.  So how does one select the
right kneepoint and slope? Without the test and measurement resources of a
major automotive manufacturer, it’s next to impossible to say. Of course, you
could trial-and-error your way into a solution that you believe to be
appropriate, but without testing under all conditions of loading, speed, and
road conditions there might be one operating condition just waiting to bite
you. In short, if you find
yourself thinking, “I wonder how I would pick the right proportioning valve for
my car?” you probably shouldn’t be changing it yourself. Electronic Proportioning:
No Tampering AllowedAs a small sidebar to the
mechanical proportioning valve discussed here, there is a movement afoot to
replace the proportioning valve function with the hardware performing the ABS
function. While this is not yet the norm, most high-end performance cars
already have this feature as standard equipment and one can predict with
reasonable certainty that the trend will continue. Based on information
gathered from the four ABS wheel speed sensors, the Dynamic Rear Proportioning
(DRP) or Electronic Brake force Distribution (EBD) algorithms calculate the
front-to-rear slip ratio of the four tires. Then, given preset thresholds and
parameters, the ABS hardware can intervene and modify the brake pressure going
to the rear wheels automatically. Because DRP and EBD are
based on actual wheel slip and not on brake line pressure, this type of rear
proportioning is more flexible and adaptable to modifications one might make to
their vehicle. It is also less expensive, as the OEM can now remove the
mechanical proportioning valve from the vehicle and replace its function with
other hardware already on board. Naturally, the OEM does not
want owners fiddling with their front-to-rear proportioning, and as a result
there is no way for the enthusiast to reprogram DRP or EBD to suit their desires.
Of course, if the vehicle’s original front-to-rear bias is in tact in the first
place, there is no need to reprogram anyway. Plan WiselyIn summary, there is more
to the proportioning valve than meets the eye. You should make every attempt to
carefully plan and select your brake modifications so that you are able to
retain and reap the benefits of the stock proportioning valve. In other words,
pay attention to (and don’t stray too far from) the factory bias in the first
place and you will be ahead of the game. If for other reasons you
are forced to scrap the stock unit and replace it with an aftermarket unit, be
advised that selection and adjustment are not for the uninitiated. While there
is more than one way to achieve optimum balance at the point of maximum
deceleration, without the right amount of know-how you might be making
compromises under partial braking conditions that were not present with the
factory hardware. Again, remember that simply
adding or replacing a proportioning valve may not be enough to remedy
basket-case bias scenarios. Proper planning and up-front design should ensure
that the base vehicle bias is appropriate from the start, enabling the
proportioning valve to be used properly as a fine-tuning device.
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