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Section 393.207—Suspension Systems
In response to a petition from the Truck Trailer Manufacturers Association (TTMA), FMCSA is amending § 393.207 to prohibit any device which is capable of dumping air individually from either of the two axle suspension systems on a semitrailer equipped with air-suspended ‘‘spread’’ or ‘‘split’’ tandem axles. TTMA indicated that the petition was not intended to prohibit:
(1) Devices that could exhaust air from both axle systems simultaneously, or
(2) lift axles on multi-axle units. The petition is available for review in the docket.
According to the TTMA, about 30,000 semitrailers are manufactured each year with split tandem axles and air suspensions. These axles are not
genuine tandems, but rather two single axles spaced at least 3,048 mm (10 feet) apart, the minimum separation required by the bridge formula [23 U.S.C. 127(a)] before each of them can carry the full 9,072 kg (20,000 pounds) allowed by Federal law. The TTMA estimates that 5,000 of these trailers are also equipped with valves to depressurize the
suspension system of one of the trailer axles, and sometimes of either axle. These valves are installed to compensate for problems created by the split tandem configuration. Normal tandems experience moderate tire
scrubbing in turns because the trailer pivots around a point that lies between the two axles. Tire scrubbing is more severe in split tandems because the pivot point is much farther from either axle. Dumping air pressure from the suspension system of the rear (or less often the leading) trailer axle reduces its load and allows the trailer to pivot
around the other axle with less resistance and tire scrubbing. The
TTMA’s own tests showed that if each axle in a split tandem is loaded to 8,845 kg (19,500 pounds) and pressure in the rear axle is dumped, the resulting weight shift will make the front axle 3,175 to 5,443 kg (7,000 to 12,000 pounds) heavier than the rear.
Dump valves were originally designed to aid maneuvering at 8 km/hour (5 mph) or less, mainly at terminals or other loading points. According to the TTMA, however, many drivers now activate them at higher speeds on streets and highways to turn corners more easily and to reduce tire wear. The TTMA also believes that suspension pressure is sometimes vented
accidentally because of wiring problems the moment the tractor hooks up to the trailer. In both cases, the inevitable weight shift often produces a load on the pressurized axle that exceeds the manufacturers’ ratings for that axle and its wheels, tires and brakes. In addition, the loaded axle frequently exceeds the single-axle weight limit.
FMCSA continues to believe the petition has merit, and the agency is
amending § 393.207 to prohibit controls of this type. Although § 393.3, which allows the use of equipment and accessories that do not decrease
operational safety, could be interpreted as prohibiting the use of equipment to disable the air suspension of one axle on a two-axle trailer, addressing this issue through rulemaking is the most appropriate course of action.
FMCSA has modified the language included in the 1997 NPRM in response
to comments from the ATA. ATA agreed with the proposal, but expressed
concern that motor carriers need the flexibility to exhaust air from the
suspension during low-speed turns. ATA believes this is necessary because there is significant resistance or tire scrubbing when drivers make low-speed turning maneuvers with a spread tandem trailer. FMCSA agrees and has revised the language to allow the exhausting of air if the controls are either located on the trailer, or the power unit and trailer combination are not capable of traveling at a speed greater than 10 miles per hour while the air is exhausted from the suspension system.
The above information is from the Federal Register from when Part 393.207 was changed. The Federal Register is the explanation of why a rule was interduced.
Be safe.