4.1 Curves, Ramps, Grades & Intersection Turns

49 CFR 383.119(f) requires tank endorsement applicants to know the effects of road grade and curvature on vehicle handling with filled, half-filled, and empty tanks. Curves test the high center of gravity (Section 3.2), and hills test surge and braking at the same time. This section turns the manual's rules into a routine you can use on every ramp, hill, and intersection.

Curves: The Manual's Rule

For tank vehicles, the AAMVA manual (8.3.3) says:

Slow down before curves, then accelerate slightly through the curve. The posted speed for a curve may be too fast for a tank vehicle.

The general driving section adds that trucks with a high center of gravity can roll over at the posted speed limit for a curve, that braking in a curve is dangerous because it is easier to lock the wheels and cause a skid, and that you should be in a gear that lets you accelerate slightly in the curve.

Why slow down before the curve

• Tires have limited grip. Braking and turning both use grip. Doing both at once leaves less for either, so you can lock wheels and skid.
• Surge adds forces. Braking pushes the liquid forward, and turning pushes it outward. Together they load the tank and tires in two directions at once, and the manual warns that steering quickly while braking may roll the vehicle over.
• Speed decides the sideways force. The force needed to follow a curve grows with the square of speed (Section 3.2), so the speed you carry into the curve is what matters.

Why accelerate slightly through the curve

Light, steady power keeps the vehicle settled and avoids the weight shift and surge that come from coasting or braking in the turn. Slightly is the key word: accelerating hard in a curve adds speed and can spin drive wheels on slick roads.

A Routine for Every Curve and Ramp

Ramp Hazards That Catch Tank Drivers

• Speed after long highway driving. After hours at highway speed, a slower speed can feel slower than it is. Check the speedometer, not your sense of speed.
• Curves that tighten. Some ramps get sharper partway through. Enter slowly enough that a tighter section does not force braking in the curve.
• Downhill ramps. Gravity adds speed during the curve. Slow down more before entering and use the right gear.
• Banking changes. Ramps can flatten or change slope, reducing the help the road gives you.
• Traffic at the end. Queues at the bottom of a ramp can force a stop while you are still turning. Look ahead and slow early.

Grades With Filled, Half-Filled, and Empty Tanks

Going down a hill

The manual's mountain-driving rules apply to every heavy vehicle, and they are even more important with liquid on board:

1. Before starting down a hill, slow down and shift down to a speed you can control without using the brakes hard. Otherwise, the brakes can overheat and lose braking power.
2. On a long or steep downgrade, the brakes are only a supplement to engine braking. Once in the proper low gear:
   • Apply the brakes just hard enough to feel a definite slowdown.
   • When speed is about 5 mph below your safe speed, release the brakes. The application should last about 3 seconds.
   • When speed rises back to your safe speed, repeat.

Example: with a safe speed of 40 mph, do not brake until you reach 40 mph, brake to 35 mph, and release.

How the load changes the hill

• Full tank: the most weight and the most work for the brakes. Surge is small, but the heavy, top-heavy load makes speed control and curves on the grade the main concern.
• Half-filled tank: as the truck tips downhill and each brake application slows it, the liquid moves forward, then rebounds. Smooth, steady brake applications keep surge from pushing the truck. A curve at the bottom of a grade combines forward and sideways surge, so be at your curve speed before you reach it.
• Empty tank: light weight means less traction. Engine retarders can skid the drive wheels on slick surfaces (turn them off when the road is wet, icy, or snow covered), and empty tank vehicles may take longer to stop.

Going up and cresting a hill

Pulling uphill, liquid moves toward the rear, and shifting gears or lifting off the throttle lets it surge forward. Change speed smoothly. At the crest, the view ahead is limited, so do not arrive at the top too fast to handle a curve or stopped traffic on the other side.

Turns at Intersections

Turns are low speed, but surge and off-tracking still cause crashes and rollovers. When a vehicle goes around a corner, the rear wheels follow a shorter path than the front wheels; this is called off-tracking or cheating.

The manual's rules for right turns:

• Turn slowly to give yourself and others more time.
• If you cannot make the turn without swinging into another lane, turn wide as you complete the turn.
• Keep the rear of your vehicle close to the curb so other drivers cannot pass on the right.
• Don't turn wide to the left as you start the turn. A following driver may think you are turning left and try to pass you on the right.
• If you must cross into the oncoming lane, watch for oncoming vehicles and give them room, but don't back up for them.

For left turns, reach the center of the intersection before starting the turn so off-tracking does not hit another vehicle.

For tank vehicles, add:

• Keep the turn slow and steering smooth so the liquid does not slosh.
• Don't let the trailer wheels climb a curb. The jolt to one side of a top-heavy load is a rollover risk.
• Finish braking before you turn, and hold steady pressure if you must stop in the turn.

Common Test Traps

• "Should a tank vehicle take curves and ramps at the posted speed?" No. The posted speed may be too fast.
• "When should you brake for a curve?" Before it, while going straight.
• "What gear for a downgrade?" Select a low gear before starting down.
• "How do you make a tight right turn?" Keep the rear close to the curb and turn wide as you complete the turn, not by swinging left first.