General Knowledge: Basic Control

9 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.

Sources: 49 CFR 396.13, Brake capacity limits: achievable deceleration falls as mass rises for a fixed brake system, TC 21-305-20 para 12-3, TC 21-305-20 para 3-10, TC 21-305-20 para 3-12, TC 21-305-20 para 3-24, TC 21-305-20 para 3-9

What you need to know

396.13(a) makes being satisfied the vehicle is in safe operating condition the driver's first duty before driving, alongside reviewing and signing the last inspection report where required.

Before driving a motor vehicle, the driver shall: (a) Be satisfied that the motor vehicle is in safe operating condition.

49 CFR 396.13

In the ideal textbook case mass cancels out and distance is the same.

Real brakes do not scale with the load: a fixed brake system can only absorb so much energy so fast, so the heavier vehicle cannot reach the same deceleration and travels further. This is also why a heavy load makes brake fade arrive sooner on a descent. Brakes absorb energy at a finite rate, so a heavier vehicle achieves less deceleration from the same system.

*Why:* For tire-limited braking a = mu*g and d = v^2/(2*mu*g), independent of mass, which is the textbook result. But a real brake system has a maximum torque and a maximum rate of heat absorption. Once those bound the deceleration rather than tire friction, a = F_max/m, which falls as m rises, so d = v^2*m/(2*F_max) grows with mass. At maximum gross weight a heavy vehicle is in this brake-limited regime.

8 and 4, one hand each side just below center.

Two hands are specified for experienced drivers as much as for beginners.

The best and safest position of the hands on the steering wheel is one hand on each side of the wheel a little below the center (the 8 o’clock and 4 o'clock position).

TC 21-305-20 para 3-9

Firm but not tight.

A tight grip tires your arms and transmits every surface irregularity into a steering input you did not intend.

The grip on the wheel should be firm, but not tight.

TC 21-305-20 para 3-9

Hand-over-hand.

It gets a large amount of lock on quickly while keeping both hands on the wheel, which palming does not.

When turning a corner or making any other sharp turn, use the hand-over-hand steering method

TC 21-305-20 para 3-10

Seldom alone.

Engine braking, gear selection and a retarder do part of the work, which is what keeps the brakes cool enough to be there when you need them. Engine braking, gear selection and a retarder normally do part of the work.

Your brakes are used in all stopping procedures, but they are seldom used alone.

TC 21-305-20 para 3-12

Three rights and a left.

It takes longer on paper but removes the left turns across traffic that cause the trouble, and a large vehicle needs room for every one of them. It removes the left turns across traffic that cause the trouble.

The safest and often quickest way to turn around is to drive around the block, making three right turns and a final left turn. This eliminates the problems of most left turns.

TC 21-305-20 para 3-24

500 feet BOTH ways, because for part of the maneuver you are across the road and blocking it in both directions at once.

Select a place where there is at least 500 feet of clear visibility in each direction.

TC 21-305-20 para 3-24

Smooth, gradual application of power.

Knowing where yours begins to engage is what separates a clean start on a grade from a stall or a lurch.

A clutch provides the means to apply engine power to the wheels smoothly and gradually.

TC 21-305-20 para 12-3

Check yourself

• What must you do before driving under 392.7?

• Which needs more stopping distance, an empty tractor or a loaded one, and why?

• Where should your hands be on the steering wheel?

• How firmly should you hold the wheel?

• Which steering method does the manual specify for a sharp turn?

• What is the manual's rule on using the brakes alone to stop?

• What does the manual call the safest way to turn around?

• If you must turn around in the road, how much visibility do you need?

• What is the clutch for?

Answers are in the practice test for this topic.