CDL Combination Vehicles TestDriving Combination Vehicles SafelyHard
When driving a combination vehicle, how does a sudden, hard application of the service brakes on a slippery surface typically affect the trailer?
- AIt causes the trailer to remain perfectly aligned with the tractor.
- BIt causes the trailer to lift off the ground due to inertia.
- CIt causes the trailer to push the tractor forward, increasing stopping distance.
- DIt causes the trailer wheels to lock up, leading to a jackknife.
Show answer & explanationAnswer & explanation
Correct answer: D. It causes the trailer wheels to lock up, leading to a jackknife.
A sudden, hard application of service brakes on a slippery surface can cause the trailer wheels to lock up. When the trailer wheels lock, the trailer will lose traction and tend to swing out to one side, pushing the tractor and creating a 'jackknife' situation, where the tractor and trailer fold in on each other.
Why the other options are wrong
- A. This is incorrect; a hard brake application on a slippery surface is very likely to cause the trailer to lose alignment, not maintain it.
- B. It is highly unlikely for a trailer to lift off the ground due to brake application; rather, it would slide or jackknife.
- C. While the trailer's momentum can push the tractor, the primary and most dangerous effect of locked trailer wheels is a jackknife, not just increased stopping distance.
Braking on Slippery Surfaces
Hard braking with a combination vehicle on a slippery surface can lock trailer wheels, causing the trailer to swing out and lead to a dangerous jackknife.
- Slippery surfaces reduce traction.
- Hard braking locks wheels.
- Locked trailer wheels cause jackknife.
Memory trick: Slippery brake? Jackknife ache!