Alley Dock at the Skills Test: Off-Tracking Versus the Cone Line
Skills test, 08:15, light rain on wet asphalt. You are in a 45-foot, 102-inch-wide motorcoach: wheelbase 279 inches, rear overhang 96 inches, front overhang 60 inches. The course is a 26-foot-wide forward aisle and a 12-foot-wide dock 60 feet deep set at 90 degrees to the aisle. You must sight-side back into the dock and finish with the rear bumper within 3 feet of the barrier. Your first attempt: you stop with the left side of the bus about 8 feet from the near cone line, crank the wheel hard left, and back. Halfway in, the left rear corner is entering the dock, but the right rear of the coach is drifting toward the far cone row. The examiner says stop. You have already used one pull-up. A cone touch fails the skill outright; an extra pull-up is a point deduction.
1. Explain what the bus is doing geometrically and why this attempt is going wrong.
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Two things are happening at once, and both are predictable from the numbers.
1. Off-tracking. The rear axle does not follow the front axle; it cuts inside the turn. With a 279-inch (23.25 ft) wheelbase on a tight radius, the rear tires track several feet inside the path the front tires took. Starting only 8 feet off the near cone line and cranking hard left means the front swings wide and the rear axle cuts toward the near side - which is why the left rear corner gets into the dock mouth, but only after you have used up your room.
2. Rear overhang swing. The 96 inches behind the rear axle acts like a lever. When you steer hard left in reverse, the tail of the coach swings right, away from the turn - straight at the far cone row. With 8 feet of tail behind the pivot and a tight radius, that swing is over a foot before the axle has moved much.
The setup, not the backup, caused this. The bus was too close to the near cone line, so the only way to reach the dock was a hard, early crank, which maximized both off-tracking and tail swing. The correction is made in the aisle, not at the dock mouth.
2. Now: take the second pull-up or force the last 15 feet?
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Take the pull-up. The scoring math is unambiguous: an extra pull-up is a point deduction, a cone touch fails the skill outright and is scored as a failure of that exercise, which can end the test day. In real operations the arithmetic is even more lopsided - a pull-up costs ten seconds, a scraped coach or a struck pedestrian costs thousands.
The judgement point is where you pull forward to. Do not simply pull forward two feet and re-crank the same hard left; that re-creates the same geometry. Pull forward far enough to:
- straighten the coach in the aisle,
- re-position so the left rear corner has a margin of a foot or more from the near cone line, and
- start the turn later, letting the rear axle come closer to the dock mouth before the wheel goes left.
Then back slowly - walk-the-speed pace - with a stop-and-check if the tail starts drifting right. If the examiner permits, get out and look; a 30-second walk-around tells you more about the right rear corner than either mirror.
3. Walk through the set-up and the steering plan you will use on the next attempt, and explain the reference points you are watching.
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Set-up (in the aisle, before any backing):
- Pull past the dock far enough that the rear axle clears the dock mouth by roughly the depth you need, then stop straight.
- Position the coach wider from the near cone line - think 10 to 12 feet rather than 8 - so the turn can be gradual. A wider start converts a hard crank into a gentle arc, which reduces both off-tracking and tail swing.
- Stop, set the brake, and look at both mirrors before moving.
The backing plan:
- Back straight until the left rear corner is aligned with the near cone at the dock mouth, then begin turning left progressively, not all at once.
- Watch the left mirror for the gap between the left side of the coach and the near cone line - that gap should shrink slowly, not collapse.
- Watch the right mirror for the tail's swing toward the far cone row. If the right rear corner starts moving toward those cones, you are cranking too hard or too early: straighten the wheel, let the tail settle, then resume.
- Straighten the wheel as soon as the coach is aimed down the dock, then back straight to the barrier.
Reference discipline: mirrors show the sides and rear corners, not the whole path, and the wet asphalt hides cone shadows. Use slow speed, small corrections, and treat any doubt about the right rear corner as a reason to stop.