Wilson livestock trailer is an example of a “wheels back” type exempt from the shock-absorbing RIG requirement under 393.86 of the motor carrier safety rules. Its rearmost axle and tire-wheel assemblies would keep a car from riding under the trailer’s structure, where its bumper covers most of the lateral area.
 - Photos: Tom Berg

Wilson livestock trailer is an example of a “wheels back” type exempt from the shock-absorbing RIG requirement under 393.86 of the motor carrier safety rules. Its rearmost axle and tire-wheel assemblies would keep a car from riding under the trailer’s structure, where its bumper covers most of the lateral area.

Photos: Tom Berg

The recent flap over rear impact guards, or RIGs, has got me looking at bumpers on the rear of trailers. All heavy trailers need some type of guard, and those with forward-set tandems need properly labeled, energy absorbing RIGs (see the previous Trailer Talk).

These are required on trailers built on or after January 26, 1998. I remember the runup to this, in the mid 1990s, when some fleet managers complained about the extra cost compared to simple “ICC bumpers” that were in use for more than 40 years.

I had little sympathy with the cost argument because a good friend of mine from college was killed when his car under-rode a stopped trailer in 1973. And I still pay attention whenever this subject comes up.

I’ve spotted variations in bumper design for those trailers exempt under the regulation (393.86, Sections 1 and 6 of the commercial vehicle safety code). These include pole trailers, pulpwood trailers, low chassis and special purpose vehicles, wheels back vehicles, and trailers towed in driveaway-towaway operations, the reg says.

None of those trailers need shock-absorbing RIGs because their low chassis height or rear-set axles and tire-wheel assemblies will stop a car from under-riding the trailers. Under-ride is what kills or badly injures motorists, often by decapitating them. That’s what happened to my friend.

At a recent event near Las Vegas (Kenworth’s unveiling of its new W990), I eyeballed the bumpers on trailers gathered for the ride-and-drive portion. All four had rear-set axles and their bumper designs varied in the ways they filled in the lateral area between their wheels.

Livestock trailer’s simple bumper meets the 1952 portion of the regulation. It would keep smaller and slower vehicles from under-riding the semi.
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Livestock trailer’s simple bumper meets the 1952 portion of the regulation. It would keep smaller and slower vehicles from under-riding the semi.

Common spread-axle flatbeds, like this Wilson aluminum unit, have a rear-set axle and wheels whose tire surfaces are within 12 inches of the trailer’s rear. Thus it’s exempt from needing an impact-absorbing guard.  This bumper appears stout, but it needn’t be energy absorbing as described by motor vehicle safety standards.
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Common spread-axle flatbeds, like this Wilson aluminum unit, have a rear-set axle and wheels whose tire surfaces are within 12 inches of the trailer’s rear. Thus it’s exempt from needing an impact-absorbing guard.  This bumper appears stout, but it needn’t be energy absorbing as described by motor vehicle safety standards.

Liftable tag axle on this Western curtain-sider meets the wheels-back definition, so the trailer also needs only a simple bumper.
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Liftable tag axle on this Western curtain-sider meets the wheels-back definition, so the trailer also needs only a simple bumper.

For clarity, I’m calling the less complex guards “bumpers.” Actually, the regulation calls all of these appendages “rear impact guards.”

These simple bumpers must be present on trailers built after Dec. 31, 1952. They are neither as strong nor as energy-absorbing as modern (since 1998) RIGs. Exceptions: truck-tractors, pole and plywood trailers, and vehicles in driveaway and towaway operations. Certain dimensions differ, including minimum distance from the ground – 24 inches for RIGs and 30 inches for bumpers.

Note that “pole trailers” are included as exemptions in both the older and newer rules, but “plywood trailers” are mentioned only in the older rule. Maybe plywood manufacturers actively lobbied federal safety authorities in 1950 or ’51, when the rule was written, and not in the mid ‘90s, when the more stringent reg was composed. And maybe pole makers were active at both times.

Wait – are trailers that carry particle board or wall board — more modern products with dimensions like plywood — also exempt? Maybe so, under the low-chassis or wheels-back provisions, if those types of trailers carry those boards. Y’know, a guy could go nuts trying to understand all of this.

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 trucks and trailers of all types.

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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 trucks and trailers of all types.

View Bio
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