CDL Combination Vehicles Practice Test

• Carry the cargo as low as possible and take turns slowly
• Carry the cargo high over the axles and brake hard before turns
• Keep the trailer tandems slid fully forward and steer sharply
• Run the tires slightly under-inflated and stay in a low gear

Piling cargo high raises the centre of gravity and makes the rig easier to tip, and rollovers happen when a driver turns too fast. Keeping the load low and slowing down for corners, on-ramps and off-ramps are the two things that count most, and low cargo matters even more in a combination than in a straight truck.

• About ten times more likely
• About twice as likely
• About three times more likely
• An empty rig is the more likely of the two to roll

A fully loaded rig is roughly ten times more likely to roll over in a crash than an empty one, because the load sits high and raises the centre of gravity. Note that this is the opposite of stopping distance, where the empty rig is the harder one to handle.

• It can be rolled about three and a half times as easily as a five-axle tractor-semitrailer
• It will take three and a half times as long to stop as the tractor
• It off-tracks about three and a half feet more than the tractor on a corner
• It can be loaded to three and a half times the weight of the front trailer

Rearward amplification is the crack-the-whip effect measured. A figure of 2.0 means the rear trailer is twice as likely to turn over as the tractor, so a triple's 3.5 means the last trailer rolls about three and a half times as easily as a five-axle rig. It is a rollover number, not a stopping or off-tracking number.

• Redistribute the load so it is centred and spread out
• Leave it and simply reduce speed by 10 mph for the whole trip
• Slide the fifth wheel forward to level the trailer
• Move the load to the very front of the trailer instead

A load pushed to one side makes the trailer lean and makes a rollover more likely, so the fix is to centre the cargo and spread it out rather than to drive around the problem. Sliding the fifth wheel changes weight on the axles, not the side-to-side balance of the freight.

• The empty one, because stiff springs and strong brakes give poor traction
• The loaded one, because the extra weight has to be brought to a stop
• They stop in the same distance, since the brakes are sized to the axles
• The empty one, but only on wet pavement

Empty and lightly loaded rigs take longer to stop than loaded ones. The very stiff suspension springs and strong brakes give the tyres poor grip, so the wheels lock easily instead of slowing the vehicle, and a locked trailer wheel starts a jackknife rather than a stop.

• It is hard to stop smoothly and needs more distance than a fully loaded tractor-semitrailer
• It stops in about half the distance of a loaded tractor-semitrailer
• It stops in the same distance, because the brakes are unchanged
• It stops shortest of all, which is why bobtails may follow more closely

Tests have shown bobtails are very hard to stop smoothly and take longer to stop than a tractor-semitrailer loaded to its maximum gross weight. Drivers who assume a light vehicle stops quickly are the ones caught out by it.

• So braking can start early and a panic stop is never needed
• So the trailer brakes have time to warm up before they are needed
• So the trailer hand valve can be used instead of the foot brake
• So the rig can be stopped without downshifting

Room ahead is what lets a driver brake early and gently. A surprise that forces a hard stop is what locks the wheels of a combination rig, swings the trailer out and jackknifes the tractor, so the space is the real protection against a skid.

• When the trailer is empty or lightly loaded
• When the trailer is loaded to its maximum gross weight
• When the load is stacked high but centred
• When the tractor is pulling two loaded trailers

Locked trailer wheels have no grip, so the trailer swings out of line. That happens most readily on an empty or lightly loaded trailer, where there is little weight holding the tyres down. Weight in the trailer makes a jackknife less likely, even though it makes a rollover more likely.