The Downhill Exit Ramp With a Loaded Trailer
You are driving a tractor pulling a 48-foot flatbed loaded with steel coils, grossing about 78,000 lb. You are on an interstate in light rain at night and your exit is a downhill right-hand ramp posted 25 mph with a decreasing-radius curve at the bottom. Your service brakes have been used heavily for the last two miles in stop-and-go traffic and you can smell hot brake lining. You are 200 feet from the ramp gore and traveling 55 mph. The ramp has a runaway truck ramp sign at the top.
1. What is the safest action right now, and what combination-vehicle physics drives that choice?
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Do not take the ramp at 55 mph. Your options, in order of safety:
- Continue past the exit and find a safer place to turn around. A missed exit costs minutes; a loaded combination entering a 25-mph decreasing-radius ramp at more than double the posted speed in the rain, with hot brakes, is a rollover and jackknife setup.
- If you must take it, brake firmly and early on the straight approach, get down to or below 25 mph before the gore, and use engine braking (downshift) to hold speed through the curve rather than riding the service brakes.
Why: a combination vehicle's trailer follows inside the tractor's path (off-tracking), so the trailer's rear wheels cut the corner and can strike the curb or leave the pavement on a tight right-hander. The trailer also amplifies the tractor's lateral motion (rearward amplification), so a quick steering correction at the tractor whips the trailer outward. Hot, wet brakes fade, and a loaded trailer's weight pushes the tractor (trailer push), especially downhill. Speed is the variable you control.
2. Suppose you take the ramp and feel the trailer begin to swing out as you enter the decreasing-radius section. What do you do with the steering and brakes?
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If the trailer starts to swing out:
- Ease off the accelerator smoothly - do not stab the brakes. Hard braking on a wet curve with a loaded trailer unloads the trailer tires and can trigger a jackknife.
- Hold the steering steady and look where you want the tractor to go. Do not countersteer sharply; a sharp correction at the tractor amplifies at the trailer and can swing it further.
- If you have room, let the rig straighten slightly by unwinding a little steering, then re-enter the curve more slowly.
- If a jackknife develops, release the brakes to let the wheels roll and regain traction, then steer to follow the trailer.
The governing idea: the trailer's motion lags and amplifies yours. Smooth, small inputs keep the trailer behind you; abrupt inputs put it beside you. Once you are through, pull over at the first safe spot, let the brakes cool, and check the load securement.