ELDT - Entry Level Driver Training Skid Control and Recovery Questions and Answers 2 — Questions and Answers

Question 1: What is a 'drive wheel skid' and what causes it?

The front wheels losing traction due to understeering
The rear drive wheels losing traction, usually from excessive braking or acceleration (Correct answer)
The trailer wheels locking up
The steering wheel spinning freely

Correct answer: The rear drive wheels losing traction, usually from excessive braking or acceleration

A drive wheel skid occurs when the rear drive wheels lose traction and spin or lock up, typically caused by excessive braking force, sudden acceleration on slippery surfaces, or mechanical failure.

Drive wheel skids involve the tractor's rear drive axle wheels losing grip with the road surface. Common causes include: applying too much braking force which locks the drive wheels; sudden heavy acceleration on wet, icy, or gravel surfaces causing wheel spin; driving too fast for conditions; and mechanical issues like a seized brake. When drive wheels lose traction, the rear of the tractor can swing out sideways because there is no friction holding it straight. If uncorrected, this can lead to a jackknife where the tractor rotates relative to the trailer. The key to preventing drive wheel skids is smooth, gradual inputs on the throttle and brakes.

Question 2: What is the correct response to a front wheel skid?

Turn the steering wheel sharply in the opposite direction
Remove your foot from the brake and let the vehicle slow down until traction returns (Correct answer)
Apply more braking force
Shift to a higher gear

Correct answer: Remove your foot from the brake and let the vehicle slow down until traction returns

In a front wheel skid, the front tires have lost traction. The only way to regain steering is to let the wheels roll freely by removing your foot from the brake, allowing the tires to grip the road again.

Front wheel skids are particularly dangerous because the driver loses steering control entirely - the vehicle goes straight regardless of steering wheel position. Front wheel skids are typically caused by braking too hard on slippery surfaces or entering a curve too fast. The recovery is to remove all braking input and let the front wheels start rolling freely again. Once rolling, the tires regain their grip on the road and steering control returns. Do not pump the brakes or steer during the skid. After traction is restored, the driver should gently correct the vehicle's direction and reduce speed appropriately for the conditions.

Question 3: How does antilock braking system (ABS) help with skid prevention?

ABS makes the vehicle stop faster on all surfaces
ABS prevents wheel lockup during hard braking, allowing the driver to maintain steering control (Correct answer)
ABS eliminates the need for careful braking
ABS only works on the front wheels

Correct answer: ABS prevents wheel lockup during hard braking, allowing the driver to maintain steering control

ABS automatically modulates brake pressure to prevent wheel lockup during hard braking, which maintains tire traction and allows the driver to steer around obstacles while braking.

ABS (Antilock Braking System) uses sensors at each wheel to detect when a wheel is about to lock up. When lockup is detected, the system rapidly modulates (releases and reapplies) brake pressure at that wheel to keep it rotating. This is important because a rolling wheel has more traction than a sliding wheel. The primary benefit of ABS is maintaining steering control during emergency braking - the driver can steer around obstacles while braking hard. ABS does not necessarily reduce stopping distance (it can actually be longer on loose gravel or snow) but it prevents the skids that lead to jackknifes and loss of directional control.

Question 4: What is an oversteer skid and how should you correct it?

When the vehicle does not turn enough; add more steering input
When the rear of the vehicle slides outward in a turn; steer in the direction of the skid (Correct answer)
When the front tires wash out in a turn; brake harder
When the vehicle rolls over; no correction is possible

Correct answer: When the rear of the vehicle slides outward in a turn; steer in the direction of the skid

Oversteer occurs when the rear end swings wide in a turn. The correction is to steer in the direction the rear is sliding (countersteer) and ease off the throttle to transfer weight to the front tires.

Oversteer (also called 'loose' condition) happens when the rear tires lose traction before the front tires during a turn, causing the rear to swing outward. In a CMV, this is the beginning of a potential jackknife. To correct: (1) Ease off the accelerator to allow weight to shift forward; (2) Steer in the direction the rear is sliding (if the rear swings right, steer right) - this is countersteering; (3) Do not brake, as this will make the skid worse; (4) As the vehicle begins to straighten, gradually center the steering. Overcorrection is common and can cause the vehicle to fishtail in the opposite direction, so smooth, measured inputs are essential.

Question 5: Why are empty trucks more prone to skidding than loaded trucks?

Empty trucks have worse tires
Empty trucks have less weight pressing the tires against the road, resulting in less traction (Correct answer)
Empty trucks have more powerful brakes
Empty trucks are not more prone to skidding

Correct answer: Empty trucks have less weight pressing the tires against the road, resulting in less traction

Empty trucks have significantly less weight on the drive and trailer axles, which means less friction between the tires and road surface, making them more susceptible to wheel lockup and skidding.

The physics is straightforward: friction force (traction) is proportional to the weight pressing the tires against the road. An empty tractor-trailer combination weighs about 35,000 pounds versus 80,000 pounds loaded. With the same braking force applied, the lighter empty truck's wheels are much more likely to lock up because there is less weight creating friction between the tires and road. This is why empty trucks have shorter safe speeds in adverse conditions and require extra caution when braking. Modern brake systems can be adjusted for load, but the fundamental physics of reduced traction with less weight cannot be eliminated.

Question 6: What role does speed play in skid risk?

Speed has no effect on skid risk with modern tires
Higher speed dramatically increases both the likelihood and severity of skids (Correct answer)
Speed only matters on icy roads
Lower speed increases skid risk because of less momentum

Correct answer: Higher speed dramatically increases both the likelihood and severity of skids

Higher speed increases skid risk because tires have less time to maintain traction, stopping distances increase exponentially, and the force in any collision or recovery attempt is much greater.

Speed is the single most significant factor in skid risk and severity. The physics are compelling: stopping distance increases with the square of speed (doubling speed quadruples stopping distance); higher speed means tires must generate more friction force to change direction, increasing the chance they exceed available traction; centrifugal force in turns increases with the square of speed; and the energy that must be dissipated in a skid or crash increases with the square of speed. For CMVs with high centers of gravity, speed in turns is especially critical because the rollover threshold can be as low as 0.35g lateral force - well below what passenger cars can handle.

ELDT - Entry Level Driver Training Skid Control and Recovery Questions and Answers 2

What is a 'drive wheel skid' and what causes it?