General Knowledge: Skid Control

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

Sources: Coulomb friction: F = mu * N, Friction circle: a tire's total force budget is shared between braking and cornering, Kinematics: braking distance d = v^2 / (2a), Newtonian mechanics: longitudinal load transfer under deceleration, TC 21-305-20 para 13-14, TC 21-305-20 para 13-15, TC 21-305-20 para 4-3

What you need to know

Braking distance grows with the square of speed, not in proportion to it.

Doubling speed multiplies the distance by four, so 60 feet becomes about 240 feet. This is why a speed that felt safe can become unrecoverable after a modest increase.

*Why:* Work done by the braking force equals the kinetic energy removed: F*d = (1/2)*m*v^2. With F = m*a this gives d = v^2/(2a). Holding a constant, d scales as v^2. Doubling v from 30 to 60 mph multiplies d by 2^2 = 4, so 60 ft becomes 240 ft.

Under braking, the vehicle's inertia pitches load onto the front axle and off the rear.

Less vertical force on a tire means less friction available before it locks. A locked rear wheel generates almost no sideways grip, which is how trailer swing and jackknifing begin. Less vertical force means less friction available, so the rear locks first.

*Why:* Under deceleration a, load transfers from rear to front by approximately m*a*h/L, where h is center-of-gravity height and L is wheelbase. Maximum friction at a tire is F = mu*N, so reducing the rear normal force N reduces the brake force the rear tires can take before sliding. Once sliding, the tire's available lateral force collapses, so the axle cannot resist yaw.

The friction a tire can deliver is proportional to the vertical force pressing it onto the road.

An empty trailer puts far less force on its tires, so a brake application that is fine when loaded can exceed the available friction and lock the wheels. Brakes on heavy vehicles are sized for the loaded case. Brake torque is set by air pressure and does not fall with the load, so the ratio of applied force to available grip rises.

*Why:* Maximum tire friction is F = mu*N, where N is the vertical load on the tire. Removing cargo reduces N roughly in proportion to the weight removed, which reduces maximum F by the same proportion. Brake torque, however, is set by air pressure and does not fall with the load, so the ratio of applied brake force to available friction rises and lockup occurs at a lower pedal effort.

A tire has one total force budget shared between braking and cornering.

Spend it all on braking and almost nothing is left for sideways grip, so a sliding tire cannot steer or resist a sideways push. This is the whole reason anti-lock braking exists: keeping the wheel rotating preserves the sideways grip that keeps the vehicle pointed where you aim it. That sideways force is what steers the vehicle and resists a skid.

*Why:* The resultant force a tire can transmit is bounded by mu*N in any direction, so longitudinal and lateral components satisfy roughly F_long^2 + F_lat^2 <= (mu*N)^2. Driving F_long to the limit forces F_lat towards zero. A fully locked tire is at the longitudinal limit, leaving no lateral capacity to resist yaw or follow a steer input.

Braking in a skid makes it worse, because it slows the wheels further and they are already sliding.

Easing off the accelerator without braking lets the tires start rolling again, which is what restores grip. Braking slows wheels that are already sliding.

If the vehicle starts to skid, the operator may be able to regain control if they ease up slowly on the accelerator and do not apply the brakes.

TC 21-305-20 para 4-3

You steer in the direction the rear is sliding, so to the right.

This points the front wheels where the vehicle is actually going and lets the rear catch up, instead of adding to the rotation.

At the same time, turn the steering wheel in the direction of the skid (see Figure 4-1). If the rear end of the vehicle is skidding to the right, turn the steering wheel to the right. If it is skidding to the left, turn the steering wheel to the left.

TC 21-305-20 para 4-3

Over-correcting throws the vehicle into a skid the other way, which is often worse than the first one because you are now moving the wheel in the opposite direction while the vehicle is already rotating.

Ease the wheel back toward center as the vehicle recovers.

Do not turn the steering wheel too sharply or keep it turned too long since this may cause the vehicle to start skidding in the opposite direction.

TC 21-305-20 para 4-3

Staying in gear keeps the engine connected to the wheels so it can slow the vehicle gradually and the drive wheels keep turning.

Neutral or a depressed clutch removes that control at the moment you need it.

Always make sure you keep the vehicle in gear.

TC 21-305-20 para 4-3

Nearly every skid traces back to doing something too fast for the available grip, whether that is speed, a stop, a start or a turn.

Equipment helps but it does not substitute for the speed you chose. On slippery surfaces, which is where almost all skids begin.

Almost all skids can be avoided if driving slowly enough and if the operator stops, starts, and turns slowly enough on slippery surfaces.

TC 21-305-20 para 4-3

Jackknifing starts with wheels that have stopped rolling.

A locked wheel cannot resist sideways force, so it comes around. Hard braking in an emergency is the usual trigger, which is why your following distance matters more than your reflexes. Normally from hard braking in an emergency or in hazardous conditions.

Skidding or locked wheels, normally due to hard braking in emergency or hazardous conditions, causes jackknifing. Locked or skidding wheels will tend to take the lead or come around. Tractor jackknifing occurs when the rear tractor wheels lock and skid to the left or right. To gain control, steer in the direction of the skid.

TC 21-305-20 para 13-14

Releasing the brakes gets the trailer wheels rolling again, and a rolling wheel can generate the sideways grip that pulls the trailer back in line.

The manual also warns that control must be regained immediately or not at all, so hesitation is what loses it.

Semitrailer jackknifing occurs when the semitrailer wheels lock and skid to the left or right. Control must be regained immediately if it is to be regained at all. To regain control, quickly release the brakes to get the wheels turning.

TC 21-305-20 para 13-15

Using it while driving brakes only the trailer, which is exactly how you make it skid or jackknife.

Using it for parking is also prohibited because air can leak out over time and release the brakes. Its only uses are preventing roll on an incline and coupling. Preventing roll while stopped, and during coupling, are its only uses.

DO NOT use the semitrailer hand brake in driving because this can cause the trailer to skid or jackknife. NEVER use the semitrailer hand brake for parking because all the air may leak out, unlocking the brakes

TC 21-305-20 para 13-15

Check yourself

• If your braking distance is 60 feet at 30 mph, at 60 mph it is about ______.

• Why does hard braking make the rear wheels of your tractor-trailer more likely to lock before the front wheels?

• Why can an empty or lightly loaded trailer lock its wheels more easily than a loaded one?

• Your tire that is sliding under braking also loses most of its ability to do what?

• How do you respond in the first moments of a skid?

• How do you steer when the rear of your vehicle skids to the right?

• What is the consequence if you steer too sharply or hold the correction too long during a skid?

• During skid recovery, what should you do with the transmission?

• What four things, done slowly enough, avoid almost all skids?

• What is the cause of jackknifing, and what normally brings it on?

• How do you respond when your semitrailer wheels lock and it swings out?

• When may your semitrailer hand brake be used?

Answers are in the practice test for this topic.