RE: abs braking...

["Dave Eaton" <Dave.Eaton@clear.net.nz>]

remember that there is a difference here between the normal brake
proportioning system which sends more braking power to the front, and
removed it from the rear (thus preventing pre-mature rear lockup and
thereafter oversteer), and abs which prevents any wheel locking up, and is
able to modulate the braking power applied (through the proportioning
system) *during* the braking manoeuvre *provided* you're continuing to mash
the anchors.  this is the reason why driving courses teach you to panic
brake using abs by standing rapidly and fulsomely on the middle pedal until
all movement has died.  in practice during these sorts of driving days, it
is always instructional to see the actual difference really mashing the
pedal makes to your stopping distances.

this is further complicated by the awd dynamics.  with a locked rear
differential, you have no method of telling whether one tyre or the other is
slipping which throws your brake proportioning all to hell.  with a locked
centre on the old (generation 1) quattro setup, the abs can't figure out
ground speed reliably, so won't work.  this is the reason why abs is
disabled for the old quattro with centre-locked and also why a vc-based awd
system must have a lock-out for braking.

otoh, the old quattro system with centre locked, by functioning as a
front/read speed-difference-limiting device (but not of course a torque
limiting device, hence the increased understeer), can operate as a "poor
mans" abs system...

hth,
dave
'95 rs2
'90 ur-q
'61 mb fintail

-----Original Message-----
Date: Tue, 18 May 1999 10:08:56 -0700
From: "Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>
Subject: RE: New BMW television campaign

... you could probably come up with something that worked with a 50/50
braking distribution by putting a bunch of weight behind the rear axle too
... :-)

The thing that bothers me a bit about assuming that ABS is going to save
your bacon on the F/R distribution is what happens if for some reason the
ABS system fails?  Now you are forced to either live with having the rears
lock up or reduce the brake pedal pressure to keep the rears from locking
and suffer the reduced braking from the wheels that do most of the work.  My
preference would be for a system that was balanced properly without ABS, and
then to have ABS to handle panic stops and any situation where the friction
under one or more wheels was compromised ...