Double-trailer rigs with a pair of 33-foot trailers would be “inherently more stable” and therefore safer than the twin 28s now running throughout the United States, according to John Woodrooffe, director of commercial vehicle research at the University of Michigan’s Transportation Research Institute.

Less-than-truckload carriers have proposed double 33s as a means to increase productivity and reduce the number of extra trucks that would otherwise be needed to handle increased freight volume projected for coming years. The proposed longer combination would stay within the current 80,000-pound legal limit for five-axle vehicles.

Woodrooffe spoke at the National Private Truck Council’s annual meeting in Cincinnati last week, and he touched on long combination vehicles. So I asked him about twin 33s compared to 28s. He said the 33-foot-long trailers’ extended wheelbases would make them more stable than twin 28s when being pulled by a truck-tractor, especially in turns. He has done computer simulations to prove this.

He said twin 28s “are among the most unstable combinations now out there.” Triple 28s are worse, and part of the problem is the single-tongue connection with two pivot points between leading and following trailers, he said. Drivers sometimes call the two combination types “wiggle wagons” because of the tendency of the last trailer to wander left and right at highway speeds.  

“It is true that the carriers that use them have done things to lessen those tendencies,” Woodrooffe said. These include assigning experienced drivers to them and using high-ratio steering gear in the tractor so it can’t make quick, sudden maneuvers that can upset a rear trailer.

In his address to the NPTC audience, the researcher recounted how long combination vehicles generally operate more safely than common tractor-trailers. But he said much of the credit goes to the special state and provincial programs that restrict operations and demand minimum safety records and training for drivers.

For example, in Alberta, Canada, trucks with twin 53-foot trailers and short triples have three to five times better safety records than single 53s because the province’s program declares it a “privilege” to operate the LCVs rather than a “right.” So, Woodrooffe said, truck operators are more attentive to safety.  

Special programs are not known to be part of the proposal for twin 33s, but it’s likely that motor carriers would agree to them in exchange for operating privileges.

Author

Tom Berg
Tom Berg

Tom Berg

Journalist since 1965, truck writer and editor since 1978. CDL-qualified; conducts road tests on new heavy-, medium- and light-duty tractors and trucks. Specializes in vocational and hybrid vehicles.

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Journalist since 1965, truck writer and editor since 1978. CDL-qualified; conducts road tests on new heavy-, medium- and light-duty tractors and trucks. Specializes in vocational and hybrid vehicles.

View Bio
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