r.c.brown at ieee.org
Sun Nov 29 08:35:50 PST 2009
Kurt Nolte wrote:
> 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.
Probably also due a couple of other things as well:
The very high tire pressures typically run in truck/trailer tires and when unloaded there
is very little contact patch on the trailer tires plus with multiple tires per axle it is
easy to get one or more bouncing around.
Suspension (or lack thereof) does little to control the unsprung weight of the many wheels
and once they break loose, hard to stop them, especially if they hit a resonance.
And, from my limited experience in a big truck, I recall the brakes seemed to be more of
an on/off type of thing, a lot harder to modulate than in a passenger vehicle. And being
drum brakes, they are harder to modulate than a disc brake.
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