Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Truck stopping distances?

David Bowman wrote:
...
dissipation
...
The geometry of the vehicle shape approximately requires that the
braking surface area scales as the second power of the linear size L
of the vehicle.

Nice try. Alas, "approximately requires" is not the same as
"requires".

Have you ever seen the brakes on a DC-10? Most people
haven't, but they're quite something. Each brake is
a giant Dagwood sandwich: Stator, rotor, stator, rotor,
stator et cetera. You wind up with the dissipation taking
place over lot more surface area than Dr. B's scaling
argument would suggest.

BTW the actuator on the DC-10 is pretty impressive, too.
Hydraulic rams mash the sandwich. There are nine rams:
three run by the #1 hydraulic system, three run by the
#2 hydraulic system, and three run by the #3 hydraulic
system. But we digress.

==============

But if you leave out the mention of dissipation, the
scaling argument holds up better. My best guess is that
trucks are limited by the strength of materials, namely
the strength of the rubber in the tires. See last week's
discussion of wide-track tires. Trucks don't have as
much tire area as they "should" in proportion to their
weight, relative to cars. They make up for it with
higher tire pressure. Lots higher.

The sideways force on each molecule of rubber is
proportionately higher. At some point you start
erasing the rubber right off the tire.

=============================================
Joseph Bellina wrote:
...
I suspect that the limiting factor is the temperature
of the brake pads, since if they get too hot, the friction coefficient
drops catastrophically.

That's true in principle, but stopping a truck
_once_ won't get the brakes that hot. This is
something they design for. You can even stop
a DC-10 from high speed (V1) _once_ without
incinerating anything. [You then need to wait
an hour or so for the brakes cool off before
you're allowed to try it again. There's a little
brake-energy meter in the cockpit.]