[Diy_efi] OT: ABS retrofit into older car...

[Bevan Weiss]

> > > I would've thought the front brakes are more prone
> > > to locking up first
> > > upon a heavy stomp, as they have more braking power
> > > to start with, and the
> > > front wheels are unloaded relative to when the car
> > > is actually slowing down rapidly.
> >
> > Actually, that is exactly why the rears are more prone
> > to locking than the fronts.  With less loading, they
> > have a much lower friction coefficient with the
> > roadway.  That coefficient is not linear in proportion
> > to load; it is exponential.
>
> Yeah, what I was *trying* to say :) was that when you're happily puddling
> along, in a well balanced car, the load on the front and rear is somewhat
> equal, but the brakes are more powerful on the front - so a violent pedal
> stomp in this situation results in more braking force being applied at the
> front, which in turn can lock the front wheels up as the braking force can
> exceed the maximal traction force. Once the weight transfers, obviously
> there's more tractive force available.
>
> So the co-efficient changes with load?
> Obviously the tractive force varies with load (and coefficient) but I
> didn't realise the co-efficient itself changes.
> In the high school mechanics classes there was never any mention of the
> co-efficient of friction changing with load - what materials does this
> happen with?

You are quite right, the co-efficient of friction doesn't change with load
(only static/dynamic ie non-slidding/slidding) however what does change is
the weight distribution of the car, and hence the normal (in engineering
terms) force acting downwards through the tire.  When this force decreases
the 'grip' of the tire will decrease, both because of the decrease in normal
force, and also because of the loss of surface area in contact with the road
that results from the slight decompression of the tire (again caused by
reduced force down through the tire).

The front tires have the reverse of this, they tend to be pressed down more
into the ground (hence increasing the normal force) and with this increase
in normal force the tire also gets slightly compressed, increasing the
surface area of the tire in contact with the road.

The sizing and power of brakes is of little effect, It's pretty easy to get
a tire to lock up, it only really depends on the characteristices of the
tire (ie pressure, profile, size, surface area, compound etc).  It's trying
to keep the tire from locking up that's the hard part.  I thought that this
conversation started with ABS, which simply modulates the pressure to the
brake cylinder to keep the tire from locking up (by measuring the speed of
the tire).  Trying to retrofit this into a older car is a legal
impossibility in all countries that I know of (apart from maybe some third
world ones...).  The problem is that ABS systems require very serious
testing and certification before they are allowed to be used on a road going
car, most of this testing is quite destructive and very expensive, hence
only worth while for a large production.  Putting an production line ABS
system into a new car (ie one it hasn't already been tested for) will
require this same expensive testing.  Which is not worth it at all.

PS: I'm not sure of the exact method used by the manufacturers for their ABS
systems but would imagine it's some kind of bang-bang controller, ie it
allows full brake pressure until the tires almost lock up, then bleeds the
pressure off and repeats the process.  I get this from the pulsing feeling
when heavily decelerating in most ABS fitted cars.

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