Commercial Vehicle Dynamics
December 12th 2022
This blog post familiarizes readers with some phenomena such as jackknifing, trailer swing, fishtailing, rollover, and offtracking, that can occur when operating longer combination vehicles.
Jackknifing occurs when the tractor's drive axles lock and lose traction, causing the tractor to skid, the forward inertia of the trailer can push the tractor from behind until it spins the tractor around and faces it backward like a pocketknife.
When a trailer's rear wheels lock or lose lateral traction, it swings to one side while the tractor moves forward on the road. This is known as a trailer swing. The difference between the trailer swing and jackknifing is that in trailer swing the trailer may return to its original position as the tractor continues forward on the roadway. This may occur when the trailer is unloaded or only lightly loaded, and a strong crosswind is present.
Fishtailing occurs when the rear wheels of a tractor lose traction and oversteer. The driver must counter-steer in the same direction as the skid to prevent the vehicle's rear end from sliding to one side. An opposite-direction skid will happen if the driver overcorrects. Fishtailing can be caused by surfaces with low friction such as rain, snow, and ice.
A rollover can happen when the vehicle's center of gravity moves outside the tires, causing the vehicle to tip over onto its side or roof. The lateral force caused by the weight shift of cargo that is not properly secured can cause a rollover.
Offtracking is the difference in paths of the frontmost inside wheel and rearmost inside wheel of a vehicle as it negotiates a turn. Offtracking occurs when the trailer wheels do not follow the same path as the tractor wheels. An example is a tractor-trailer turning at an intersection at a low speed.
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