syncronized_turbo at yahoo.co.uk
Sun Nov 29 00:39:49 PST 2009
With respect to an unloaded or partially loaded truck taking longer to
stop than a lightly loaded one, the same applies to buses and I know
some of the reasoning behind that.
We're in agreement that a locked tire skidding is going to stop you more
slowly than a properly braking, rolling tire, right? So you want to keep
your wheels rolling while you brake.
Heavy vehicles, (buses, semis, delivery trucks, dump trucks...) all have
brakes that are designed for their heaviest load. The size of the drums
and shoes is appropriate to stop a fully loaded, maximum-rated-weight
plus a percentage vehicle within a specified distance from a specified
speed. The amount of braking force they can lay down is /immense/, and
measures beyond what they would need in a less loaded situation.
These vehicles carry such a high percentage of their final mass in
cargo/passengers, unlike passenger cars and light duty trucks, that
their unloaded weight is a very small fraction of what the brakes are
designed to stop. Your maximum braking application pressure is thus
greatly reduced to keep you from locking up your tires.
True your inertia is also way down because of your reduced mass, and
your aerodynamic drag isn't going to change, but as it was explained to
me the increased stopping time from such a reduction of braking pressure
is greater than the reduction in stopping time garnered from the lower
overall mass of your vehicle.
Thus what I have been told, which seems consistent with the physics I know.
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