Re: Braking 101

[John Karasaki <johkar@teleport.com>]

At 11:44 AM 4/2/98 -0800, you wrote:
<snip>
>If I can lock up the existing brakes easily, what will be gained by
>the above?

It is true if in both cases all 4 wheels are locked up.  Then the only
thing stopping the car is the cf of the tires on the road.

But with ABS or a modulating right foot, you don't get to lock up.  Each
pulse allow the caliper and pads to have the same lenght of time to absorb
as much kenetic energy as possible.  Then the cf of the rotor/pads come
into play.  The reason the larger rotors are better is that they are bigger
fins to disapate heat.  This affects stopping at the most basic of levels
because the cf of the rotor/pad combination changes (for the worse) once
some critical elevated temperature is reached.

Once this elevated critical temperature is reached every brake pulse has
less braking effect than the one before -- and if you continue to try and
stop, it only gets worse.

Now why don't we all just lock up our brakes each time we go into turn one
at PIR?  Well, besides severly reducing overall tire life (due to flat
spotting of the tires), you actually don't stop as quickly if the wheels
are locked up as you do if you or the ABS is modulating the brakes
properly.  Why?  Because the cf between the tire and the road gets worse
too as critical temperatures are passed and because of other reduced force
"sliding effect" issues I don't remember from my Engineering Dynamics
classes of 10 years ago.

HTH in a basic way.

>It seems to me that the kinectic energy contained in the forward motion of
>the vehicle must be converted to heat and the bigger the mass (rotor) the
>better to absorb/dissipate the heat. (vented rotors). I suspect pad compound
>affects how much heat is transfered back to the caliper/brake fluid as well
>as how much friction is applied to the rotors.

Bingo.

>Obviously, tire adhesion is a major factor also as witnessed by the
>increased stopping distances on wet/ice vs dry.
>Anyway, now that I have exposed my ignorance, if some of you guys that know
>way more than I want to step in here and educate the rest of us, it would be
>appreciated.
>
>mike
>91 200q - UFOs that stop quickly
>85 F250 4x4, Xcab turbo diesel - 6800 lbs that takes a long time to stop.
>
>
Best Regards,

John P. Karasaki, P.E.
(Professional Mechanical Engineer that does mostly financial analysis now)