Downhill Braking


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 •The proponents of the old theory have rescinded it, there is now (almost) universal agreement that the proper way to brake on a downgrade is to intermittently apply all your service brakes in a way that will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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