Tail Swing on a Wet 6 Percent Grade with Two Miles Still to Go
You are 12 miles into a descent of a 6 percent grade on a two-lane mountain route in central Pennsylvania, running a triple at 79,000 pounds. The pavement is wet and covered with wet leaves in the shaded sections. The advisory sign at the top said 45 mph; you entered the grade at 50 mph in 9th gear and have been using the foot brake in 8-10 second applications, releasing between them. About halfway down, the third pup's tail begins to swing left and right in your right mirror, and the combination feels like it is pushing the tractor. Your air gauge shows 105 psi and falling slowly. The grade has another two miles to the bottom, where there is a stop sign at a crossroad.
1. What is happening, and what is your immediate brake and steering response?
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The rear of the set has lost lateral grip and is steering the tractor - the pups are pushing. Likely contributors: wet leaves under the light rear axles, weak or out-of-adjustment brakes on the rear pup or the dollies so the rear does less than its share of the slowing, and a set that is running too fast for the gear.
Immediate response:
- Release the brake pedal smoothly. A hard, held application on a slick surface is what lets the tail come around. Releasing lets the combination straighten.
- Hold the steering wheel steady. Do not counter-steer hard against the swing; small corrections only, and follow where the tail is taking the front. Sawing at the wheel whips the third pup.
- Downshift to a lower gear if the speed allows it, so the engine helps hold you back.
- Reapply light, steady pressure once the tail is behind you.
Never use the trailer hand valve to straighten or slow a swinging trailer. It applies the trailer brakes alone, which is exactly the input that folds a combination.
2. Why is the apply-and-release pattern you have been using part of the problem, and what is the correct technique for the rest of the grade?
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Repeated 8-10 second applications followed by release cycles the drums hot and then lets them cool unevenly, and every release hands the set back to whatever braking the rear trailers are or are not doing. On a loaded triple the pups are heavy enough to push the tractor the moment the drives stop pulling.
Correct technique for the remaining two miles:
- Get into the right gear at the top and stay there. A 6 percent grade at 79,000 pounds wants a gear that holds the set near the advisory speed without much brake at all.
- Use light, steady brake pressure to hold a target speed, rather than long hard stabs.
- If speed builds anyway, brake firmly enough to drop 5-10 mph, then hold that lower speed. Make each application deliberate, not reactive.
- Use the engine brake with care. On wet leaves, engine braking on the drive axles can break traction on the drives, which is its own way to lose the set.
- Do a brake check at the top of any grade - 5 mph, firm application - so you learn about a weak trailer brake before the descent, not halfway down it.
3. At the bottom you touch the drums: the tractor's are hot, the dollies' and the rear pup's are cool. What does that tell you, and what do you do?
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It tells you the rear brakes were not working during the descent, which is exactly why the set felt like it was pushing and why the tail got loose. The tractor was doing nearly all of the stopping.
Possible causes, in the order worth checking:
- Air not reaching the dollies: a kinked or crossed line, a coupler not fully seated, or a shutoff valve partly closed.
- Slack adjusters out of adjustment on the dolly or pup axles. With the brakes applied, pushrod travel beyond about 2 inches on a type 30 chamber means that brake is not doing its share.
- A failed relay or quick-release valve on a dolly, or a dolly that was simply never charged because its emergency line was not connected properly.
What you do: the set is out of service until the rear brakes work. Do not continue the run on the theory that the tractor stops it fine - it did not stop it fine on that grade, and the next one may be longer. Fix or get it to a shop, and note it on the inspection report so the same set does not go out that way again.