[Diy_efi] ABS proportioning valve

Jay Rabe jayrabe
Wed Nov 29 21:23:02 UTC 2006


On 11/26/06, dan at w3eax.umd.edu <dan at w3eax.umd.edu> wrote:
>
>
> Well, I looked it up on Howstuffworks, and according to them:
>
> It looks like the valve has three positions: open, closed, release
> (one-way
> allows pressure to bleed off from the brake-side).  They claim the
> controller
> is normally open but then releases if a wheel decellerates very rapidly
> until proper behavior decelleration is restored and then its re-openned.


No, there are 2 types, one chops out (holds back) a certain percentage
(usually a fixed psi drop, not variable) and there are variable
proportioning valves.  Im not sure what your MR2 has, most likely a variable
one.  Some cars dont have a proportioning valve.  If designed correctly
(piston sizes front vs rear) you dont really need one.

This leaves me with a few questions:
>
> 1) Whats the purpose of closed?  It seems like you are always alternating
> between release and open.


Closed would only make sense if the rear wheels came off the ground when you
brake, but wouldnt do you much good cause the rear wheels would be off the
ground anyways.  Closed shouldnt exist.


2) Can you partially open the valve resulting in a pressure drop across it?
> If thats the case then presumably you could just monitor the before and
> after pressure and run a simple control loop to control the current into
> the valve.


Yup, but i'd call that partially closed.  I think you are mixing up an abs
solenoid with a proportioning valve.  The abs solenoid pushes the fluid back
against the master cyl. side of the brakes to lessen pressure to one or both
rear wheels (depending on a 1 or 2 channel system) when the ABS computer
realizes one or more rear wheels are spinning slower by a certain %age vs.
the other tires.  I havent ever seen an electronic proportioning valve,
mechanical seems more practical and safe, but its possible.  I think usually
they try to keep away from electronics that mess with braking unless part
failure can guarantee a safe stop.
   What could be your "issue" is if the rear wheels spin slower than the
front from say left foot braking, etc. the abs might see this and kick in
"early"  where the newer ABS might allow a little more speed difference
before stepping in.

3) Is there a way to detect impending lockup before it locks up? From a
> technical point of view what exactly is threshold braking?


Threshold braking is the absolute most braking force that each tire can
handle.  In a straight line only stop, it would be possible to be able get
the front and rear forces to a maximum.  But, you dont want this in a panic
stop where you swerve or turn.  The rear end could swing out, most people
couldnt handle a car set up like this, its hairy to brake like that and its
dangerous.  Usually front brakes do 70-80% of the braking.   Ever try to
stop with your ebrake?  Its scary.
    You can hear your tires start to 'scrub' at threshold braking.  Basicly
your tires at their traction limit, depending on the tires can make a
scrubbing/scuffing sound.  Besides that and a possible speed difference
between wheels, not that i can think of.

   You might have bad wheel bearings, an air bubble, or spongey brake lines
in the rear. Bad wheel beearings can cause the rotor to tilt, pushing the
piston back into the caliper)  when you go to hit the brakes, force on the
master cyl pushes the brake fluid against the pistons in the front and rear
calipers.  Any play results in either extra brake pedal travel (if front and
rear play are equal) , or lower pressure for one circuit (if just the front
or rear has more play)   They are seperate circuits, so if there is an issue
with one or the other, the pedal can feel firm, but one of the circuits
could be low on pressure.

   Read this site, it most likely makes more sense than i do.
http://stoptech.com/tech_info/tech_white_papers.shtml

Jay




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