Rocky Mountain Doubles
Rocky Mountain doubles consist of a standard 48- or 53-foot semi-trailer pulled by a tractor, with a shorter 26- to 29-foot pup trailer attached via a converter dolly at the rear. Total combination length is typically 95 to 105 feet. The pup trailer is also called a B-trailer in some regional terminology. Rocky Mountain doubles are operated primarily in western states under LCV permit programs. The tractor pulls the lead semi-trailer through a standard fifth wheel coupling. The rear of the lead trailer has a pintle ring or hitch to which the converter dolly attaches. The dolly's fifth wheel receives the pup trailer's kingpin. The configuration produces more off-tracking than a standard semi but less than a turnpike double or triple. Under FMCSA's Freeze regulations established by the Surface Transportation Assistance Act of 1982 (STAA), Rocky Mountain doubles are permitted on the National Network and reasonable access roads. The gross vehicle weight for Rocky Mountain doubles can reach 105,500 lbs in states that have authorized this weight under special permits, though the federal bridge formula still applies to individual axle groups.