Rear Pup Drifting on a Wet Downhill Ramp
You are pulling a set of 27-foot doubles at 60 mph on a downhill stretch of freeway where the right-hand ramp curves and drops. Each pup is loaded light, about 9,000 lb, and the pavement is wet. As you enter the ramp the first pup begins drifting toward the lane line, and in the mirror you can see the rear pup's tail swing out farther than the front one did. At the bottom you have to make a tight right turn into a truck stop; last trip your rear pup's right tires crossed the curb and clipped a sign. Behind you is a car that has been following closely for the last two miles, and there is no shoulder on the ramp. Your steering axle is at 11,600 lb, drive tandems at 25,400 lb, and both pup axles at about 9,000 lb.
1. Explain why the rear pup swings wider than the front one and why this particular combination of conditions makes it worse.
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This is rearward amplification: a movement at the steering wheel grows as it travels back through the combination. Each trailer pivots around its own axle, and each pivot lags and magnifies the one ahead of it. In a set of doubles the last trailer's rear corner can move two to three times as far sideways as the tractor's rear corner for the same steering input. Short wheelbases and light units make it worse.
Your conditions stack the effect. Nine-thousand-pound pups have little tire load pressing them into the pavement, so a small lateral force moves them easily. A downhill ramp unloads the front of the set as weight shifts, and wet pavement cuts available lateral grip on every axle at once.
Adding a steering correction while the motion is building is what turns sway into a jackknife or a rollover. The correction arrives at the rear trailer late and larger, so the driver chases it, overcorrects, and the amplitude grows each cycle. This is also why your light pups and your wet-road speed matter more than the tractor's weight: the tractor is planted, the pups are not.
2. The first pup has just begun to drift toward the lane line. What do you do in the next few seconds, and what do you deliberately not do?
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Do:
- Ease off the accelerator smoothly and let the combination slow on its own. Removing the drive force is what lets the pups' tires re-grip.
- Hold the steering wheel firmly and nearly straight. If the rear pup has already stepped out, a slight steer in the direction the tail is moving helps line the units back up.
- Turn on four-way flashers so the car behind you gets a warning.
- Keep your eyes up the road, not in the mirror. You cannot steer out of sway by watching it.
Do not:
- Do not brake hard. Hard braking transfers weight forward, lightens the rear pup further, and can lock trailer wheels on wet pavement.
- Do not steer sharply or saw the wheel. Each correction is amplified by the time it reaches the last trailer.
- Do not accelerate out of it. More speed increases the energy in the oscillation.
- Do not swing wide to give yourself room. That is a steering input of its own.
If the sway does not settle after you are off the throttle, slow gradually with light, steady brake pressure and get off the ramp at the first opportunity. The car behind you is a reason to signal early, not a reason to keep your speed up.
3. At the bottom you must make a tight right turn into the truck stop. Explain how off-tracking affects this combination and how you set up and execute the turn.
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Off-tracking is the difference between the path of the front axle and the path of the rear axles in a turn; the longer the combination and the tighter the turn, the further inside the rear units cut. With two 27-foot pups, the second pup's axle tracks well inside the tractor's path, and its rear corner also swings outside as the turn progresses.
Set up and execute:
- Start from as far left in your lane as is legal, widening the entry. The wider the entry, the less the rear cuts the corner.
- Begin the turn later than you would in a car and keep the tractor nearer the center line of the new road instead of hugging the right curb.
- Watch the rear pup in the mirrors through the whole turn. If its right side threatens the curb, sign, or a pole, stop and pull forward to re-set. Stopping is free; the sign is not.
- Watch the outside swing on the left at the start of the turn; that is what clips parked vehicles and fuel island posts.
- Keep speed very low. Off-tracking is geometry, but the light rear pup's path is also far more sensitive to speed on wet pavement.
If you cannot make the turn without the rear pup leaving your lane, do not take it; find another entrance.