5.1 Skids, Jackknife Prevention & Recovery

Every skid starts the same way: the tires are asked for more grip than the road can give. In a tank vehicle, a skid is especially dangerous because a high, shifting load can turn a slide into a jackknife or rollover. The AAMVA manual (8.3.5) gives the tank-specific warning, and the general and combination vehicle sections give the recovery steps.

The Tank Vehicle Rule

Don't over steer, over accelerate, or over brake. If you do, your vehicle may skid. On tank trailers, if your drive wheels or trailer wheels begin to skid, your vehicle may jackknife. When any vehicle starts to skid, you must take action to restore traction to the wheels.

What Causes a Skid

The general driving section lists four causes:

Drivers who match speed to conditions rarely have to over-brake or over-steer. Surge makes that more important in a tanker: liquid moving forward during braking, or outward in a turn, adds force right when the tires are already working hard.

Drive-Wheel Skids

The manual says the most common skid is one where the rear (drive) wheels lose traction through excessive braking or acceleration.

Acceleration skids

These usually happen on ice or snow. Taking your foot off the accelerator can easily stop them. If it is very slippery, push the clutch in; otherwise the engine can keep the wheels from rolling freely and regaining traction.

Braking skids

When the drive wheels lock, they have less traction than rolling wheels and usually slide sideways, trying to catch up with the front wheels. In a straight truck, the vehicle spins out. With a trailer, a drive-wheel skid can let the trailer push the towing vehicle sideways, causing a sudden jackknife.

Correcting a drive-wheel braking skid

1. Stop braking. This lets the rear wheels roll again and stops them from sliding.
2. Counter-steer. As the vehicle turns back on course, it tends to keep turning. Unless you turn the steering wheel quickly the other way, you may skid in the opposite direction.

The manual notes that learning to stay off the brake, turn the wheel quickly, push in the clutch, and counter-steer takes a lot of practice, ideally on a skid pad.

Front-Wheel Skids

Driving too fast for conditions causes most front-wheel skids. Other causes include worn front tire tread and cargo loaded so there is not enough weight on the front axle, which links back to compartment balance (Section 2.3).

In a front-wheel skid, the front end tends to go straight ahead no matter how much you turn the steering wheel. On a very slippery surface you may not be able to steer around a curve or turn. The only way to stop the skid is to let the vehicle slow down: stop turning and braking so hard, and slow down as quickly as possible without skidding.

Trailer Skids and Trailer Jackknife

The combination vehicles section describes a trailer jackknife: when trailer wheels lock up, the trailer tends to swing around. This is more likely when the trailer is empty or lightly loaded, which describes many tank trailers on the return trip.

Stopping a trailer skid

1. Recognize the skid. The earliest and best way is seeing it in your mirrors. Any time you brake hard, check the mirrors to make sure the trailer is staying where it should. Once the trailer swings out of your lane, it is very hard to prevent a jackknife.
2. Stop using the brake. Release the brakes to get traction back.
3. Do not use the trailer hand brake (if you have one) to "straighten out the rig." The manual says this is the wrong thing to do, because the brakes on the trailer wheels caused the skid in the first place.
4. Once the trailer wheels grip the road again, the trailer will start to follow the tractor and straighten out.

Why Tank Vehicles Are Prone to These Skids

How ABS Helps

Antilock braking systems (ABS) use wheel sensors and an electronic control unit to reduce brake pressure when a wheel is about to lock.

• ABS does not increase or decrease normal braking capability, and it does not necessarily shorten stopping distance. It helps you keep control and steer during hard braking.
• ABS on only the tractor helps you keep steering control, with less chance of jackknifing, but watch the trailer and let up on the brakes, if you can safely do so, if it begins to swing out.
• ABS on only the trailer makes the trailer less likely to swing out, but if you lose steering control or start a tractor jackknife, let up on the brakes, if you can safely do so, until you regain control.
• Malfunction lamps are yellow. Tractors, trucks, and buses have a lamp on the instrument panel. Trailers have a yellow lamp on the left side, on the front or rear corner. The lamp should come on for a bulb check and go out; on some older systems it stays on until you exceed 5 mph. If it stays on or comes on while driving, you may have lost ABS control.
• Brake as you always have. Use only the braking force necessary to stop safely and stay in control.

Prevention Checklist

• Match speed to the road surface (Section 4.3).
• Turn off the retarder on wet, icy, or snow-covered roads.
• Brake early and smoothly; hold steady pressure with partial loads.
• Slow before curves; no hard braking or quick steering in them.
• Check mirrors every time you brake hard.
• Watch empty and light trailers closely.

Common Test Traps

• "Pull the trailer hand brake to straighten a jackknife." Wrong. Release the brakes; the trailer brakes caused the skid.
• "Keep braking in a drive-wheel skid." Stop braking so the wheels can roll.
• "Most common skid?" The drive wheels losing traction from too much braking or acceleration.