Section

[Code of Federal Regulations]

[Title 49, Volume 5]

[Revised as of October 1, 2005]

From the U.S. Government Printing Office via GPO Access

[CITE: 49CFR393.71]

[Page 414-420]

TITLE 49--TRANSPORTATION

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 or ball-and-socket type coupling device

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.

(3) Saddle-mounted vehicles must be arranged such that the gross

weight of the vehicles is properly distributed to prevent undue

interference with the steering, braking, or maneuvering of the

combination of vehicles.

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 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. No motor vehicles or combination of

motor vehicles shall be towed in driveaway-towaway operations by means

other than a tow-bar, ball-and-socket type coupling device, saddle-mount

connections which meet the requirements of this section, or in the case

of a semi-trailer equipped with an upper coupler assembly, a fifth-wheel

meeting the requirements of Sec. 393.70.

(h) Requirements for tow-bars. Tow-bars shall comply with the

following requirements:

[[Page 415]]

(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 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______________________________________________________________________

[[Page 416]]

(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 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 Single

vehicle Front or Rear saddle-

mount front mount 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

[[Page 417]]

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 Single

vehicle Front or Rear saddle-

mount front mount mount

mount \1\

------------------------------------------------------------------------

Up to 5,000......................... 1.2 1.0 0.8 0.8

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.

[[Page 418]]

(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:

Minimum Test Load in Pounds

------------------------------------------------------------------------

Double or triple saddle-mount

-----------------------------------

Weight in pounds of heaviest towed Middle Single

vehicle Front or Rear saddle-

mount front mount 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

[[Page 419]]

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 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

[[Page 420]]

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; 70 FR 48054, Aug.

15, 2005]