proportioning valve effectiveness - was monkeylads

[Livolsi, Stephane]

That's right there is only one channel going to the rears!  I am
enlightened!

Now taking this one step further (if you don't mind) ...

Just how effective (accurate?)can the proportioning valve be?
My first experience with a proportioning valve was on my 82 Toyota 4X4 where
I had to make some modifications because of the lift kit I installed (2"
lift)  I learned a bit about the system then and I recall thinking this is
pretty goofy.  Under braking, the relationship between the body and the axle
changes due to the nose of the vehicle dipping and I guess the rear lifting,
which activates the lever and reduces the brake fluid flow to the rears so
they don't lock up.  Seems pretty imprecise.  Looking at the valve on my
5ktq, it isn't a whole lot different in design than the toyota.



Stephane

> ----------
> From: 	Bernard Littau[SMTP:bernardl at acumenassociates.com]
> Sent: 	August 19, 2003 4:16 PM
> To: 	Livolsi, Stephane; 'Audi Quattro List'
> Subject: 	RE: Monkeylad Madness! - thanks
>
> >
> > Bernard, your comments got me to thinking that maybe the valve is
> > redundant
> > anyway, because wouldn't the abs system take care of any wheel lock up
> > under
> > the circumstances you describe?
>
> Yes and no.
>
> In the extreme, yes, the ABS system will detect the lock up and modulate
> the
> rear wheels.
>
> The rear brakes are both controlled by one ABS channel (on the 5ktq), so
> if
> either wheel locks, both are modulated.  In straight line braking, you
> might
> be ok trading the proportioning valve for ABS as far as function goes, but
> triggering the ABS is decidedly more ugly than proportioning the rears.
>
> In a corner, where the inside rear wheel is un-weighted and much more
> prone
> to lock up, the single channel ABS would reduce the brake effectiveness of
> the outside wheel unnecessarily.  So no, the ABS would not have equivalent
> function to the proportioning valves.  The two proportioning valves reduce
> the brake pressure to the inside wheel with much less affect on the other
> wheel than ABS.
>
> Best,
>
> Bernard
>
>