Section

[Code of Federal Regulations]
[Title 49, Volume 4]
[Revised as of October 1, 2003]
From the U.S. Government Printing Office via GPO Access
[CITE: 49CFR393.71]

[Page 1121-1127]

TITLE 49--TRANSPORTATION

CHAPTER III--FEDERAL MOTOR CARRIER SAFETY ADMINISTRATION,
DEPARTMENT OF TRANSPORTATION

PART 393--PARTS AND ACCESSORIES NECESSARY FOR SAFE OPERATION--Table of Contents

Subpart F--Coupling Devices and Towing Methods

Sec. 393.71 Coupling devices and towing methods, driveaway-towaway operations.

(a) Number in combination. (1) No more than three saddle-mounts may
be used in any combination.
(2) No more than one tow-bar may be used in any combination.
(3) When motor vehicles are towed by means of triple saddle-mounts,
the towed vehicles shall have brakes acting on all wheels which are in
contact with the roadway.
(b) Carrying vehicles on towing vehicle. (1) When adequately and
securely attached by means equivalent in security to that provided in
paragraph (j)(2) of this section, a motor vehicle or motor vehicles may
be full-mounted on the structure of a towing vehicle engaged in any
driveaway-towaway operation.
(2) No motor vehicle or motor vehicles may be full-mounted on a
towing vehicle unless the relationship of such full-mounted vehicles to
the rear axle or axles results in proper distribution of the total gross
weight of the vehicles and does not unduly interfere with the steering,
braking, or maneuvering of the towing vehicle, or otherwise contribute
to the unsafe operation of the vehicles comprising the combination.
(c) Carrying vehicles on towed vehicles. (1) When adequately and
securely attached by means equivalent in security to that provided in
paragraph (j)(2) of this section, a motor vehicle or motor vehicles may
be full-mounted on the structure of towed vehicles engaged in any
driveaway-towaway operation.
(2) No motor vehicle shall be full-mounted on a motor vehicle towed
by means of a tow-bar unless the towed vehicle is equipped with brakes
and is provided with means for effective application of brakes acting on
all wheels and is towed on its own wheels.
(3) No motor vehicle or motor vehicles shall be full-mounted on a
motor vehicle towed by means of a saddle-mount unless the center line of
the kingpin or equivalent means of attachment of such towed vehicle
shall be so located on the towing vehicle that the relationship to the
rear axle or axles results in proper distribution of the total gross
weight of the vehicles and does not unduly interfere with the steering,
braking, or maneuvering of

[[Page 1122]]

the towing vehicle or otherwise contribute to the unsafe operation of
vehicles comprising the combination; and unless a perpendicular to the
ground from the center of gravity of the full-mounted vehicles lies
forward of the center line of the rear axle of the saddle-mounted
vehicle.
(4) If a motor vehicle towed by means of a double saddle-mount has
any vehicle full-mounted on it, such saddle-mounted vehicle shall at all
times while so loaded have effective brakes acting on those wheels which
are in contact with the roadway.
(d) Bumper tow-bars on heavy vehicles prohibited. Tow-bars of the
type which depend upon the bumpers as a means of transmitting forces
between the vehicles shall not be used to tow a motor vehicle weighing
more than 5,000 pounds.
(e) Front wheels of saddle-mounted vehicles restrained. A motor
vehicle towed by means of a saddle-mount shall have the motion of the
front wheels restrained if under any condition of turning of such wheels
they will project beyond the widest part of either the towed or towing
vehicle.
(f) Vehicles to be towed in forward position. Unless the steering
mechanism is adequately locked in a straight-forward position, all motor
vehicles towed by means of a saddle-mount shall be towed with the front
end mounted on the towing vehicle.
(g) Means required for towing. (1) No motor vehicle or motor
vehicles shall be towed in driveaway-towaway operations by means other
than tow-bar or saddle-mount connections which shall meet the
requirements of this section.
(2) For the purpose of the regulations of this part:
(i) Coupling devices such as those used for towing house trailers
and employing ball and socket connections shall be considered as tow-
bars.
(ii) Motor vehicles or parts of motor vehicles adequately, securely,
and rigidly attached by devices meeting the requirements of paragraph
(n) of this section shall be considered as one vehicle in any position
in any combination.
(h) Requirements for tow-bars. Tow-bars shall comply with the
following requirements:
(1) Tow-bars, structural adequacy and mounting. Every tow-bar shall
be structurally adequate and properly installed and maintained. To
insure that it is structurally adequate, it must, at least, meet the
requirements of the following table:

------------------------------------------------------------------------
Longitudinal strength in tension and
compression \2\
-------------------------------------
New tow-
bars Strength as
Gross weight of towed vehicle acquired a beam (in
(pounds) \1\ and used any
All tow- by a motor direction
bars carrier concentrated
after load at
Sept. 30, center) ^2,3
1948
------------------------------------------------------------------------
Pounds

-------------
Less than 5,000................... 3,000 6,500 3,000
5,000 and over.................... .......... .......... ............
Less than 10,000.................. 6,000 (\1\ ) (\1\ )
10,000 and over................... .......... .......... ............
Less than 15,000.................. 9,000 (\1\ ) (\1\ )
------------------------------------------------------------------------
\1\ The required strength of tow-bars for towed vehicles of 15,000
pounds and over gross weight and of new tow-bars acquired and used
after Sept. 30, 1948, for towed vehicles of 5,000 pounds and over
gross weight shall be computed by means of the following formulae:
Longitudinal strength=gross weight of towed vehicle x 1.3. Strength as
a beam=gross weight of towed vehicle x 0.6.
\2\ In testing, the whole unit shall be tested with all clamps, joints,
and pins so mounted and fastened as to approximate conditions of
actual operation.
\3\ This test shall be applicable only to tow-bars which are, in normal
operation, subjected to a bending movement such as tow-bars for house
trailers.

(2) Tow-bars, jointed. The tow-bar shall be so constructed as to
freely permit motion in both horizontal and vertical planes between the
towed and towing vehicles. The means used to provide the motion shall be
such as to prohibit the transmission of stresses under normal operation
between the towed and towing vehicles, except along the longitudinal
axis of the tongue or tongues.
(3) Tow-bar fastenings. The means used to transmit the stresses to
the chassis or frames of the towed and towing vehicles may be either
temporary structures or bumpers or other integral parts of the vehicles:
Provided, however, That the means used shall be so constructed,
installed, and maintained that when tested as an assembly, failure in
such members shall not occur when the weakest new tow-bar which is
permissible under paragraph (h)(1) of this section is subjected to the
tests given therein.
(4) Means of adjusting length. On tow-bars, adjustable as to length,
the means used to make such adjustment

[[Page 1123]]

shall fit tightly and not result in any slackness or permit the tow-bar
to bend. With the tow-bar supported rigidly at both ends and with a load
of 50 pounds at the center, the sag, measured at the center, in any
direction shall not exceed 0.25 inch under any condition of adjustment
as to length.
(5) Method of clamping. Adequate means shall be provided for
securely fastening the tow-bar to the towed and towing vehicles.
(6) Tow-bar connection to steering mechanism. The tow-bar shall be
provided with suitable means of attachment to and actuation of the
steering mechanism, if any, of the towed vehicle. The attachment shall
provide for sufficient angularity of movement of the front wheels of the
towed vehicle so that it may follow substantially in the path of the
towing vehicle without cramping the tow-bar. The tow-bar shall be
provided with suitable joints to permit such movement.
(7) Tracking. The tow-bar shall be so designed, constructed,
maintained, and mounted as to cause the towed vehicle to follow
substantially in the path of the towing vehicle. Tow-bars of such design
on in our condition as to permit the towed vehicle to deviate more than
3 inches to either side of the path of a towing vehicle moving in a
straight line as measured from the center of the towing vehicle are
prohibited.
(8) Passenger car-trailer type couplings. Trailer couplings used for
driveaway-towaway operations of passenger car trailers shall conform to
Society of Automotive Engineers Standard No. J684c, ``Trailer Couplings
and Hitches--Automotive Type,'' July 1970. \1\
---------------------------------------------------------------------------

\1\ See footnote 1 to Sec. 393.24(c).
---------------------------------------------------------------------------

(9) Marking tow-bars. Every tow-bar acquired and used in driveaway-
towaway operations by a motor carrier shall be plainly marked with the
following certification of the manufacturer thereof (or words of
equivalent meaning):

This tow-bar complies with the requirements of the Federal Motor
Carrier Safety Administration for (maximum gross weight for which tow-
bar is manufactured) vehicles.
Allowable Maximum Gross Weight__________________________________________
Manufactured____________________________________________________________
(month and year)
by______________________________________________________________________
(name of manufacturer)

Tow-bar certification manufactured before the effective date of this
regulation must meet requirements in effect at the time of manufacture.
(10) Safety devices in case of tow-bar failure or disconnection. (i)
The towed vehicle shall be connected to the towing vehicle by a safety
device to prevent the towed vehicle from breaking loose in the event the
tow-bar fails or becomes disconnected. When safety chains or cables are
used as the safety device for that vehicle, at least two safety chains
or cables meeting the requirements of paragraph (h)(10)(ii) of this
section shall be used. The tensile strength of the safety device and the
means of attachment to the vehicles shall be at least equivalent to the
corresponding longitudinal strength for tow-bars required in the table
of paragraph (h)(1) of this section. If safety chains or cables are used
as the safety device, the required strength shall be the combined
strength of the combination of chains and cables.
(ii) If chains or cables are used as the safety device, they shall
be crossed and attached to the vehicles near the points of bumper
attachments to the chassis of the vehicles. The length of chain used
shall be no more than necessary to permit free turning of the vehicles.
The chains shall be attached to the tow-bar at the point of crossing or
as close to that point as is practicable.
(iii) A safety device other than safety chains or cables must
provide strength, security of attachment, and directional stability
equal to, or greater than, that provided by safety chains or cables
installed in accordance with paragraph (h)(10)(ii) of this section. A
safety device other than safety chains or cables must be designed,
constructed, and installed so that, if the tow-bar fails or becomes
disconnected, the tow-bar will not drop to the ground.
(i) [Reserved]
(j) Requirements for upper-half of saddle-mounts. The upper-half of
any saddle-mount shall comply with the following requirements:
(1) Upper-half connection to towed vehicle. The upper-half shall be
securely attached to the frame or axle of the towed vehicle by means of
U-bolts or

[[Page 1124]]

other means providing at least equivalent security.
(2) U-bolts or other attachments. U-bolts used to attach the upper
half to the towed vehicle shall be made of steel rod, free of defects,
so shaped as to avoid at any point a radius of less than 1 inch:
Provided, however, That a lesser radius may be utilized if the U-bolt is
so fabricated as not to cause more than 5 percent reduction in cross-
sectional area at points of curvature, in which latter event the minimum
radius shall be one-sixteenth inch. U-bolts shall have a diameter not
less than required by the following table:

Diameter of U-Bolts in Inches
------------------------------------------------------------------------
Double or triple saddle-mount
-------------------------------------
Weight in pounds of heaviest towed Middle
vehicle Front or Rear Single
mount front mount saddle-
mount mount \1\
------------------------------------------------------------------------
Up to 5,000....................... 0.625 0.5625 0.500 0.500
5,000 and over.................... 0.6875 0.625 0.5625 0.5625
------------------------------------------------------------------------
\1\ The total weight of all the vehicles being towed shall govern. If
other devices are used to accomplish the same purposes as U-bolts they
shall have at least equivalent strength of U-bolts made of mild steel.
Cast iron shall not be used for clamps or any other holding devices.

(3) U-bolts and points of support, location. The distance between
the most widely separated U-bolts shall not be less than 9 inches. The
distance between the widely separated points where the upper-half
supports the towed vehicle shall not be less than 9 inches, except that
saddle-mounts employing ball and socket joints shall employ a device
which clamps the axle of the towed vehicle throughout a length of not
less than 5 inches.
(4) Cradle-type upper-halves, specifications. Upper-halves of the
cradle-type using vertical members to restrain the towed vehicle from
relative movement in the direction of motion of the vehicles shall be
substantially constructed and adequate for the purpose. Such cradle-
mounts shall be equipped with at least one bolt or equivalent means to
provide against relative vertical movement between the upper-half and
the towed vehicle. Bolts, if used, shall be at least one-half inch in
diameter. Devices using equivalent means shall have at least equivalent
strength. The means used to provide against relative vertical motion
between the upper-half and the towed vehicle shall be such as not to
permit a relative motion of over one-half inch. The distance between the
most widely separated points of support between the upper-half and the
towed vehicle shall be at least 9 inches.
(5) Lateral movement of towed vehicle. (i) Towed vehicles having a
straight axle or an axle having a drop of less than 3 inches, unless the
saddle-mount is constructed in accordance with paragraph (m)(2) of this
section, shall be securely fastened by means of chains or cables to the
upper-half so as to insure against relative lateral motion between the
towed vehicle and the upper-half. The chains or cables shall be at least
\3/16\-inch diameter and secured by bolts of at least equal diameter.
(ii) Towed vehicles with an axle with a drop of 3 inches or more, or
connected by a saddle-mount constructed in accordance with paragraph
(m)(2) of this section, need not be restrained by chains or cables
provided that the upper-half is so designed as to provide against such
relative motion.
(iii) Chains or cables shall not be required if the upper-half is so
designed as positively to provide against lateral movement of the axle.
(k) Requirements for lower half of saddle-mounts. The lower half of
any saddle-mount shall comply with the following requirements:
(1) U-bolts or other attachments. U-bolts used to attach the lower
half to the towing vehicle shall be made of steel rod, free of defects,
so shaped as to avoid at any point a radius of less than 1 inch:
Provided, however, That a lesser radius may be utilized if the U-bolt is
so fabricated as not to cause more than 5 percent reduction in cross-
sectional area at points of curvature, in which latter event the minimum
radius shall be one-sixteenth inch. U-bolts shall have a total cross-
sectional area not less than as required by the following table:

Total Cross-Sectional Area of U-Bolts in Square Inches
------------------------------------------------------------------------
Double or triple saddle-mount
-------------------------------------
Weight in pounds of heaviest towed Middle
vehicle Front or Rear Single
mount front mount saddle-
mount mount \1\
------------------------------------------------------------------------
Up to 5,000....................... 1.2 1.0 0.8 0.8

[[Page 1125]]

5,000 and over.................... 1.4 1.2 1.0 1.0
------------------------------------------------------------------------
\1\ The total weight of all the vehicles being towed shall govern. If
other devices are used to accomplish the same purposes as U-bolts they
shall have at least equivalent strength of U-bolts made of mild steel.
Cast iron shall not be used for clamps or any other holding devices.

(2) Shifting. Adequate provision shall be made by design and
installation to provide against relative movement between the lower-half
and the towing vehicle especially during periods of rapid acceleration
and deceleration. To insure against shifting, designs of the tripod type
shall be equipped with adequate and securely fastened hold-back chains
or similar devices.
(3) Swaying. (i) Adequate provision shall be made by design and
installation to provide against swaying or lateral movement of the towed
vehicle relative to the towing vehicle. To insure against swaying,
lower-halves designed with cross-members attached to but separable from
vertical members shall have such cross-members fastened to the vertical
members by at least two bolts on each side. Such bolts shall be of at
least equivalent cross-sectional area as those required for U-bolts for
the corresponding saddle-mount as given in the table in paragraph (k)(1)
of this section. The minimum distance between the most widely separated
points of support of the cross-member by the vertical member shall be
three inches as measured in a direction parallel to the longitudinal
axis of the towing vehicle.
(ii) The lower-half shall have a bearing surface on the frame of the
towing vehicle of such dimensions that the pressure exerted by the
lower-half upon the frame of the towing vehicle shall not exceed 200
pounds per square inch under any conditions of static loading. Hardwood
blocks or blocks of other suitable material, such as hard rubber,
aluminum or brakelining, if used between the lower half and the frame of
the towing vehicle shall be at least \1/2\ inch thick, 3 inches wide,
and a combined length of 6 inches.
(iii) Under no condition shall the highest point of support of the
towed vehicle by the upper-half be more than 24 inches, measured
vertically, above the top of the frame of the towing vehicle, measured
at the point where the lower-half rests on the towing vehicle.
(4) Wood blocks. (i) Hardwood blocks of good quality may be used to
build up the height of the front end of the towed vehicle, provided that
the total height of such wood blocks shall not exceed 8 inches and not
over two separate pieces are placed upon each other to obtain such
height; however, hardwood blocks, not over 4 in number, to a total
height not to exceed 14 inches, may be used if the total cross-sectional
area of the U-bolts used to attach the lower-half of the towing vehicle
is at least 50 percent greater than that required by the table contained
in paragraph (k)(1) of this section, or, if other devices are used in
lieu of U-bolts, they shall provide for as great a resistance to bending
as is provided by the larger U-bolts above prescribed.
(ii) Hardwood blocks must be at least 4 inches in width and the
surfaces between blocks or block and lower-half or block and upper-half
shall be planed and so installed and maintained as to minimize any
tendency of the towed vehicle to sway or rock.
(5) Cross-member, general requirements. The cross-member, which is
that part of the lower-half used to distribute the weight of the towed
vehicle equally to each member of the frame of the towing vehicle, if
used, shall be structurally adequate and properly installed and
maintained adequately to perform this function.
(6) Cross-member, use of wood. No materials, other than suitable
metals, shall be used as the cross-member, and wood may not be used
structurally in any manner that will result in its being subject to
tensile stresses. Wood may be used in cross-members if supported
throughout its length by suitable metal cross-members.
(7) Lower half strength. The lower half shall be capable of
supporting the loads given in the following table. For the purpose of
test, the saddle-mount shall be mounted as normally operated and the
load applied through the upper half:

[[Page 1126]]

Minimum Test Load in Pounds
------------------------------------------------------------------------
Double or triple saddle-mount
-------------------------------------
Weight in pounds of heaviest towed Middle
vehicle Front or Rear Single
mount front mount saddle-
mount mount \1\
------------------------------------------------------------------------
Up to 5,000....................... 15,000 10,000 5,000 5,000
5,000 and over.................... 30,000 20,000 10,000 10,000
------------------------------------------------------------------------
\1\ The total weight of all the vehicles being towed shall govern.

(l) Requirements for kingpins of saddle-mounts. The kingpin of any
saddle-mount shall comply with the following requirements:
(1) Kingpin size. (i) Kingpins shall be constructed of steel
suitable for the purpose, free of defects, and having a diameter not
less than required by the following table:

Diameter of Solid Kingpin in Inches
----------------------------------------------------------------------------------------------------------------
Double or triple saddle-mount
-----------------------------------------------------------------------------
Front mount Middle or front Rear mount Single saddle-
Weight in pounds of heaviest towed -------------------- mount -------------------- mount \1\
vehicle -------------------- -----------------
Mild H.T.S.\2\ Mild Mild H.T.S.\2\ Mild
steel steel H.T.S.\2\ steel steel H.T.S.
----------------------------------------------------------------------------------------------------------------
Up to 5,000....................... 1.125 1.000 1.000 0.875 0.875 0.750 0.875 0.750
5,000 and over.................... 1.500 1.125 1.250 1.000 1.000 0.875 1.000 0.875
----------------------------------------------------------------------------------------------------------------
\1\ The total weight of all the vehicles being towed shall govern.
\2\ High-tensile steel is steel having a minimum ultimate strength of 65,000 pounds per square inch.

(ii) If a ball and socket joint is used in place of a kingpin, the
diameter of the neck of the ball shall be at least equal to the diameter
of the corresponding solid kingpin given in the above table. If hollow
kingpins are used, the metallic cross-sectional area shall be at least
equal to the cross-sectional area of the corresponding solid kingpin.
(2) Kingpin fit. If a kingpin bushing is not used, the king-pin
shall fit snugly into the upper and lower-halves but shall not bind.
Those portions of the upper or lower-halves in moving contact with the
kingpin shall be smoothly machined with no rough or sharp edges. The
bearing surface thus provided shall not be less in depth than the radius
of the kingpin.
(3) Kingpin bushing on saddle-mounts. The kingpin of all new saddle-
mounts acquired and used shall be snugly enclosed in a bushing at least
along such length of the kingpin as may be in moving contact with either
the upper or lower-halves. The bearing surface thus provided shall not
be less in depth than the radius of the kingpin.
(4) Kingpin to restrain vertical motion. The kingpin shall be so
designed and installed as to restrain the upper-half from moving in a
vertical direction relative to the lower-half.
(m) Additional requirements for saddle-mounts. Saddle-mounts shall
comply with the following requirements:
(1) Bearing surface between upper and lower-halves. The upper and
lower-halves shall be so constructed and connected that the bearing
surface between the two halves shall not be less than 16 square inches
under any conditions of angularity between the towing and towed
vehicles: Provided, however, That saddle-mounts using a ball and socket
joint shall have a ball of such dimension that the static bearing load
shall not exceed 800 pounds per square inch, based on the projected
cross-sectional area of the ball: And further provided, That saddle-
mounts having the upper-half supported by ball, taper, or roller-
bearings shall not have such bearings loaded beyond the limits
prescribed for such bearings by the manufacturer thereof. The upper-half
shall rest evenly and smoothly upon the lower-half and the contact
surfaces shall be lubricated and maintained so that there shall be a
minimum of frictional resistance between the parts.
(2) Saddle-mounts, angularity. All saddle-mounts acquired and used
shall provide for angularity between the towing and towed vehicles due
to vertical curvatures of the highway. Such means shall not depend upon
either the looseness or deformation of the parts of either the saddle-
mount or

[[Page 1127]]

the vehicles to provide for such angularity.
(3) Tracking. The saddle-mount shall be so designed, constructed,
maintained, and installed that the towed vehicle or vehicles will follow
substantially in the path of the towing vehicle without swerving. Towed
vehicles shall not deviate more than 3 inches to either side of the path
of the towing vehicle when moving in a straight line.
(4) Prevention of frame bending. Where necessary, provision shall be
made to prevent the bending of the frame of the towing vehicle by
insertion of suitable blocks inside the frame channel to prevent
kinking. The saddle-mount shall not be so located as to cause
deformation of the frame by reason of cantilever action.
(5) Extension of frame. No saddle-mount shall be located at a point
to the rear of the frame of a towing vehicle.
(6) Nuts, secured. All nuts used on bolts, U-bolts, king-pins, or in
any other part of the saddle-mount shall be secured against accidental
disconnection by means of cotter-keys, lock-washers, double nuts, safety
nuts, or equivalent means. Parts shall be so designed and installed that
nuts shall be fully engaged.
(7) Inspection of all parts. The saddle-mount shall be so designed
that it may be disassembled and each separate part inspected for worn,
bent, cracked, broken, or missing parts.
(8) Saddle-mounts, marking. Every new saddle-mount acquired and used
in driveaway-towaway operations by a motor carrier shall have the upper-
half and the lower-half separately marked with the following
certification of the manufacturer thereof (or words of equivalent
meaning).

This saddle-mount complies with the requirements of the Federal
Motor Carrier Safety Administration for vehicles up to 5,000 pounds (or
over 5,000 pounds):

Manufactured____________________________________________________________
(Month and year)
by______________________________________________________________________
(Name of manufacturer)

(n) Requirements for devices used to connect motor vehicles or parts
of motor vehicles together to form one vehicle--(1) Front axle
attachment. The front axle of one motor vehicle intended to be coupled
with another vehicle as defined in paragraph (g)(2)(ii) of this section
shall be attached with U-bolts meeting the requirements of paragraph
(j)(2) of this section.
(2) Rear axle attachment. The rear axle of one vehicle shall be
coupled to the frame of the other vehicle by means of a connecting
device which when in place forms a rectangle. The device shall be
composed of two pieces, top and bottom. The device shall be made of 4-
inch by \1/2\-inch steel bar bent to shape and shall have the corners
reinforced with a plate at least 3 inches by \1/2\ inch by 8 inches
long. The device shall be bolted together with \3/4\-inch bolts and at
least three shall be used on each side. Wood may be used as spacers to
keep the frames apart and it shall be at least 4 inches square.

(Sec. 12, 80 Stat. 931; 49 U.S.C. 1651 note; section 6 of the Department
of Transportation Act, 49 U.S.C. 1655, and the delegations of authority
at 49 CFR 1.48 and 389.4)

[33 FR 19735, Dec. 25, 1968, as amended at 35 FR 10907, July 7, 1970; 37
FR 21440, Oct. 11, 1972; 53 FR 49400, Dec. 7, 1988]