Skid Control and Recovery for Combination Vehicles
Skid Control and Recovery for Combination Vehicles tests whether you know what causes each kind of skid and the one recovery action the exam wants: get off the brakes and steer.
Why combinations skid
Locked wheels slide. In a combination the danger is worse because the tractor's drive axles may be lightly loaded (empty trailer or bobtail) and the trailer's brakes may apply later than the tractor's, so the trailer keeps pushing. Hard braking, speed on a slippery surface, and sudden steering are the usual triggers.
Kinds of skid and recovery
- Drive-axle (rear-wheel) skid: the tractor's rear steps out. Release the brakes so the wheels roll, and steer in the direction the rear is sliding - toward where you want the front to go. Do not add power.
- Front-wheel skid: steering stops working and the vehicle goes straight. Release the brake to let the front wheels roll again and regain steering.
- Trailer skid or jackknife: the trailer swings out of line. Release the brakes to let the trailer straighten, and steer to keep the tractor ahead of it. Do not grab the trailer hand valve.
- Acceleration skid: the drive wheels spin. Ease off the accelerator.
- Hydroplaning: occurs around 30 mph with about a tenth of an inch of water; underinflated tires hydroplane sooner.
Prevention
Slow down in rain, snow, and ice; brake earlier and more lightly; keep tires inflated and brakes adjusted; balance tractor and trailer braking; avoid engine braking on slippery surfaces, which can break the drive wheels loose.
Worked example
An empty van on wet pavement: the driver brakes for a signal and the drive axles lock. The tractor's rear slides right, so the nose swings left. The recovery is to release the brake pedal fully so the drive wheels roll, then steer right - toward the direction the rear is sliding - to bring the nose back onto the intended path. Standing on the pedal would keep the wheels locked and turn a skid into a jackknife.
Traps
- Keeping the brakes applied through the skid.
- Steering away from the skid.
- Using the trailer hand valve to straighten a jackknife.
- Believing ABS shortens every stopping distance; its main job is preserving steering control.