Say
goodbye to mechanical brake proportioning valves, and say hello to
Electronic Brake Distribution (EBD). This new technology is yet another
electronic enhancement that’s being added to many late-model vehicles.
With
conventional hydraulic brakes, a mechanical proportioning valve is used
to reduce pressure to the rear wheels when the brakes are applied.
Inside the proportioning valve is a spring-loaded piston that
determines how much pressure goes to the rear brakes.
The
reason for brake proportioning is to control brake balance. Reducing
hydraulic pressure to the rear brakes prevents them from locking up —
especially when braking hard on a wet or slick surface.
Brake Bias
What
we’re really talking about here is controlling “brake balance” or
“brake bias” between the front and rear brakes. Brake bias is the
difference in hydraulic pressure channeled to the front and rear wheels
when the brakes are applied.
The Electronic Alternative
Though
proportioning valves are relatively simple, reliable and trouble-free,
they are also obsolete. Since most vehicles now have ABS, a
proportioning valve really isn’t needed because brake bias can be
controlled by the ABS system. The ABS system can control hydraulic
pressure to any of the four brakes, so with a little reprogramming of
its control logic, it can also manage brake bias much better than a
simple mechanical proportioning valve.
The
key to all of this is monitoring wheel speed and vehicle dynamics. The
ABS system uses inputs from its wheel speed sensors (WSS) to detect
changes in wheel speed that signal a change in traction. On a
traditional ABS application, this occurs only when braking and only if
one or more wheels start to lockup and skid. On a vehicle with EBD, the
ABS system also monitors and adjusts brake bias any time the brakes are
applied.
How EBD Works
Inputs
from the wheel speed sensors keep the EBD/ABS system informed as to
what’s happening at each of the wheels. Additional sensor inputs may be
received from a steering wheel angle sensor and a yaw and/or lateral
acceleration (G-force) sensor. The latter keep the control module
informed about vehicle dynamics when the vehicle is braking in a turn
or making steering maneuvers.
When
the brakes are applied, the EBD/ABS system compares the wheel speed
sensor inputs from the front and rear, as well as side-to-side, to make
sure all of the wheels are braking evenly. The system is programmed to
automatically reduce pressure to the rear wheels like a mechanical
proportioning valve if it detects any change in rear wheel speed. If
less rear braking is needed, the EBD/ABS system cycles the inlet valves
for each rear brake circuit to modulate brake pressure.
Because
of this, it can optimize braking under all driving conditions,
including the braking of individual wheels (front or rear) to improve
braking, reduce stopping distances and maintain stability if the brakes
are being applied while turning or making other maneuvers. One of the
big advantages of EBD is that it can make the rear brakes work harder
as long as they’re maintaining good traction. On a dry road, EBD can
reduce the stopping distance significantly over ordinary brakes.
An
important point to note here is that EBD by itself is not the same as a
full-blown stability control system, though it comes close and may be
combined with stability control.
EBD,
on the other hand, isn’t as watchful or as interactive. It comes into
play only when the driver presses on the brake pedal. EBD can’t tell if
a vehicle is going into a turn too fast or the driver is making a
dangerous steering maneuver, unless he hits the brakes. What EBD can do
is optimize brake bias and overall braking performance when the brakes
are applied to compensate for weight shifts forward (when braking in a
straight line), or weight shifts to either side (when braking in a
turn). So, in that respect, it’s almost as good as stability control at
reducing the risk of understeer, oversteer or skidding — but only when
braking.