CDL · General Knowledge · 6 min read
Preventing Skids and Using ABS
- Identify the four primary causes of vehicle skids and explain how to recover from both drive-wheel and front-wheel skids.
- Explain how Antilock Braking Systems (ABS) operate, including required warning lamps, DOT equipment dates, and proper braking techniques.
- Differentiate between driving a vehicle with full, partial, or malfunctioning ABS and describe the limits of what ABS can do.
Understanding Skids and Their Causes
A skid happens whenever tires lose their grip on the road. Tires need traction to steer, stop, and move a commercial motor vehicle safely. When tires lose traction, the vehicle skids and can become difficult to steer or control.
A skid is caused in one of four distinct ways: over-braking, over-steering, over-acceleration, or driving too fast. Over-braking involves braking too hard and locking up the wheels. Over-steering means turning the wheels more sharply than the vehicle can actually turn. Over-acceleration means supplying too much power to the drive wheels, causing them to spin. Finally, driving too fast for road conditions causes most serious skids.
Retarders can also trigger a loss of traction. Skids can occur when using the speed retarder on slippery roads. The most common type of skid overall is one where the rear wheels lose traction through excessive braking or excessive acceleration.
Common mistakes
- Believing ABS always shortens stopping distance; its true purpose is maintaining steering control and preventing wheel lockup during hard braking.
- Braking differently just because a vehicle has ABS; use only the force needed to stop safely and stay in control.
- Assuming an ABS malfunction means all braking is lost; regular brakes still work, but the ABS should be serviced soon.
- Thinking ABS prevents all skids; ABS should help prevent brake-induced skids, but it will not prevent skids caused by spinning the drive wheels or going too fast in a turn.
Key points
- Skids are caused by over-braking, over-steering, over-acceleration, or driving too fast for road conditions, with speed being the primary factor in serious skids.
- To recover from a drive-wheel braking skid, release the brakes to let the wheels roll, then counter-steer quickly as the vehicle straightens out.
- In a front-wheel skid, the front end tends to go straight regardless of steering input; stop turning and/or braking so hard, and slow down as quickly as possible without skidding.
- ABS detects potential wheel lockup and adjusts brake pressure faster than a driver can respond, helping the driver maintain control during hard braking.
- DOT requires ABS on air-braked truck tractors built on or after March 1, 1997; other air-braked trucks, buses, trailers, and converter dollies built on or after March 1, 1998; and hydraulically braked trucks and buses with a GVWR of 10,000 pounds or more built on or after March 1, 1999.
- Tractors, trucks, and buses have yellow ABS malfunction lamps on the instrument panel; trailers have them on the left side, and dollies built on or after March 1, 1998, must have a lamp on the left side. A lamp that stays on after the bulb check or comes on while driving may indicate lost ABS control; regular brakes still work.
- ABS does not replace good driving habits, prevent power or turning skids, compensate for poor maintenance, or necessarily shorten stopping distances.
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