[Phys-L] Re: Stopping Distance

[betwys1 at SBCGLOBAL.NET (Brian Whatcott)]

  At 06:00 AM 10/31/2005, Gene, you wrote:

> > In yesterday's Parade magazine there was a statement that large
> > trucks require more distance to stop than do light trucks and cars,
> > as much as 50% percent more distance.  Does anyone know the
> > explanation as to why this is?
> >
> > Gene

The thread that followed brought up several interesting
  points.
It was not mentioned (I don't think), that air pressure of
around 100 psi is used to hold off the wheel brakes.
This pressure can be dumped in about half a second after
  pedal pressure is applied.

    Double tires limit the size and effectiveness of the
  drums or disks that can be fitted.
Single wider tires are coming into favor (esp in Europe)
  for this reason, but they disallow  limping home on a
single flat of a pair.

Drum brakes can have a trailing and a leading shoe, in which
case the servo effect of the leading shoe is maintained in
  both the forward and reverse directions.
Twin leading shoes self servo in the forward direction only,
and naturally depend on the effective friction value
at the shoe so that fade is magnified.

It has been suggested (by lawyers in litigation) that about
1/5th of road accidents involve trucks. Hence truck braking
is an issue of some interest.

A Federal rule 571.121  for truck braking was written
  around 1995, but this was initially set aside, on the basis
  that new production which specified ABS could prove
  more hazardous than non ABS systems.

At any rate - though there are many more stipulations, we
  can at least take away two  parameters - a requirement to
  stop in 217 ft from 60 MPH on a road surface which was
  to provide a braking coefficient of 0.9.

Assuming constant deceleration leads to an estimated
  0.6 g approx from this requirement.

On the topic of stability - this comes in several flavors.
Dynamic stability requires more weight on the front of the
trailer than on the rear in order to prevent onset of
  fish-tailing.
You will notice that half laden trailers are loaded forward,
for this reason.

  However, an air brake just incapable of locking a rear axle
  when heavily laden  could lock and make the axle hop if
  half laden forward. This hopping loss of traction encourages the
onset of jack-knife  and then roll-over - i.e a destabilizing
effect.  I suspect that this would be an ABS added value.



Brian Whatcott    Altus OK    Eureka!