Jackknifing and Rollover Prevention
Two failure modes
A jackknife happens when the drive wheels lose traction: the trailer pushes the tractor around in a brake jackknife, or the tractor pulls away from the trailer in a power jackknife. A rollover happens when centrifugal force in a curve overturns a high-center-of-gravity load. Both are speed and traction problems.
Prevention
- Slow down before the curve, not in it. Braking mid-curve adds lateral force and can lock the drives.
- Obey ramp advisory speeds; they are set for the rollover threshold, not for a passenger car.
- Keep loads low and centered; a high or shifted load raises the center of gravity.
- Use smooth steering and progressive braking. On slippery roads, reduce speed and increase following distance.
- With multiple trailers, remember rearward amplification: the last trailer swings hardest.
Worked example
A tanker approaches a ramp posted 25 mph. At 40 mph, centrifugal force is roughly (40/25)² ≈ 2.6 times greater than at the advisory speed. The tanker's high center of gravity may not tolerate that, and the trailer can roll even though the tractor stays upright.
Traps
Thinking a heavy load makes a rollover less likely-a high load makes it more likely; braking hard in a curve to fix a fast entry; assuming an empty trailer cannot jackknife; ignoring the posted ramp speed because traffic is light.