Snub don't ride!
•Some drivers don't understand the severe
demands put on the brakes by long downhill runs. Suppose your doing 6 miles
with an average 6% downgrade. Runs like this are common out west. (In the
east too?) This is a 1900' change in elevation. A free fall from 1900'
results in a terminal velocity of 238 mph- neglecting air resistance. This
would be the velocity of your rig -neglecting air and rolling resistance-
if you didn't brake.
•Negotiating this grade is the same as
slowing down from 238 mph. This is like 16 stops from 60 mph (not 4, kinetic
energy varies with the square of the speed: (238 x 238) / (60 x 60) = 15.74
) Suppose you average 30 mph coming down, the run will then last for 12
minutes. Sixteen stops from 60 mph in 12 minutes is a lot of stopping.
Obviously your brakes had better be right and you had better use the right
braking technique if you want to make it safely to the bottom.
•In recent years there has been some erroneous
information going around about how to brake on long downhills. It was suggested
not too long ago that a continuous application of the brakes as opposed
to intermittent application or snubbing was the preferred method. THIS
IDEA IS COMPLETELY WRONG!
•The proponents of the old theory
have rescinded it, there is now (almost) universal agreement that the proper
way to brake on a downgrade is to intermittently apply all your service
brakes in a way that will reduce the speed of a fully loaded vehicle by
about 5 or 6 mph during each application. What is key here is not the speed
drop, this will depend on weight, grade and other factors, but air pressure,
you have got to get the application pressure high enough to get all
your brakes working.
•In theory, it doesn't make any
difference whether you ride or snub the brakes on the way down. The problem
is that you don't drive a theory, you drive a truck. In theory, the same
amount of heat is put into the braking system regardless of how you apply
the brakes. In practice, unless your brakes are in good condition, tractor-trailer
balance is right and the load is ideally located, the continuous application
of the brakes is likely to result in uneven drum and lining temperatures
and problems before you get to the bottom of the hill.
•Steady, low pressure application of the
brakes may not cause all the brakes on the vehicle to apply and may result
in some brakes -those with the lower activation pressures- doing more work
than others. Specifically, in many cases the tractor brakes will do too
much of the work while the trailer brakes loaf and you might then get fade
at the tractor axles.. Other brake problems can be aggravated by the low
and steady braking technique. What you want is all the brakes working
some of the time, not some of the brakes working all the time.
The application pressure must be high enough to ensure that all brake
chambers apply and that all linings make solid contact with the drums -
about 20 psi or higher.
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