Vehicle Chassis Frame

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Vehicle Chassis Frame

Objectives



Describe the chassis frame of a heavy
-
duty
truck.



Define the terms
yield strength, section
modulus (SM), and resist bend moment (RBM).



List the materials from which frame rails are
made and describe the characteristics of each.



Explain the elements of frame construction.

(Objectives cont.)



Explain how the chassis frame, side rails, and
cross
-
members can be repaired without
altering the frame dynamics.



Perform some basic chassis frame alignment
checks and project a frame
-
to
-
floor diagram.



Describe the various categories of frame
damage, including diamond, twist, sidesway,
sag, and bow.

INTRODUCTION


The chassis frame is the backbone of all
heavy
-
duty vehicles. The main body of the
frame on a highway tractor is shaped like a
ladder. In fact, we commonly refer to a heavy
-
duty truck frame as a ladder frame.

ROLE OF THE FRAME


The frame supports the cab, engine,
transmission, axles, and the various other
chassis components. The cross
-
members control
axial rotation (another way of saying twist) and
longitudinal motion of the rails. They reduce
torsional stress transmitted from one rail to the
other. Cross
-
members also are used for vehicle
component mounting and protecting the wires,
hoses, and tubing that are routed from one side
of the vehicle to the other.

Drop Style

Straight Type

FRAME DESIGNS

C
-
channel rail section

By using a radius bend stress

concentrations are avoided

BASIC FRAME TERMS


Yield Strength
-
highest stress material can
take without permanent deformation (psi).
Class 8 trucks use 110,000 psi, heat treated.


Section Modulus
-
shape and size only


Resist bend moment (RBM).
-
SM x YS


Area
-
total cross section (square inches)


Applied Moment
-
point where load is applied


Bending Moment
-
point at which it deflects


Safety Factor (Margin of Safety).
-
relationship of applied moment to RBM


Bending Moment

Highest load concentration

Section Modulus (SM)



If you take a plastic ruler and clamp it flat to a
bench with your hand and then twang it on the
unclamped end, you will note the following:



The ruler is highly flexible.


After twanging the ruler, it will continue to
oscillate for some time.



Now hold the ruler upright on the bench and
twang it. Notice how differently it performs.

B=3, D=10, b=2.75, d=9.5

SM=10.7

The higher the SM, the less flexible the frame tends to be.

Yield Strength


Yield strength is a measure of the material
strength of the frame. By definition, yield
strength is the maximum stress that a
material can withstand before it is
permanently deformed. Yield strength is
factored with SM to calculate RBM. On road
generally uses ¼
-
inch steel whereas on/off
road uses 5/16 or 3/8 steel.

Resist Bend Moment Equation


RBM = SM
×

Yield Strength

RBM=10.7x110,000

RBM=1,177,000 inch
-
pounds

Shop Talk


Two truck frames with identical RBM can
perform very differently. RBM is calculated by
factoring SM with yield strength. If two trucks
have identical RBM but one is spec’d with a
high SM but uses a lower yield strength
material, it will be more rigid than a frame
with high yield strength but low SM.

Steel Frames


Trucks are manufactured with frame rails of
mild steel (36,000 psi yield strength), high
-
strength low
-
alloy (HSLA) steel (50,000 psi
yield strength), or heat
-
treated steel (110,000
psi yield strength). On the inside of the frame
rail there should be stencil data that tells how
the frame is constructed. Most frames cannot
be welded or drilled.

Aluminum Alloy Frames


It is relatively easy to distinguish aluminum
frames from steel frames on the basis of their
greater web and flange thickness and
nonmagnetic properties. Frame material can
be identified by placing a magnet near the
frame. The magnet will be attracted to a steel
frame.

FRAME LOADING

Neutral fiber on C
-
channel rail.

U
-
Bolt and Clamp Attachment

Three common methods of attaching components to

The frame are bolts, u
-
bolts, and clamp attachments.


FRAME CUTTING AND REPAIR

GUIDELINES


When frames have to be cut and/or welded, it
is advisable to make the cut at an angle of
either 60 degrees (usually recommended) or
45 degrees. In this way, the RBM through any
given section will be minimally affected.

CROSS
-
MEMBERS


Cross
-
members are designed to connect the
frame rails. They provide rigidity and strength,
along with sufficient flexibility to withstand the
twisting and bending stresses encountered
when operating on uneven terrain. Stamped
C
-
section is a standard type of cross
-
member.

CAUTION:


When reassembling chassis components
previously assembled with Huck fasteners, it
is often unrealistic to install new Huck
fasteners because of accessibility. If you are
replacing Huck fasteners with bolts, ensure
that their hardness is consistent with the
original fasteners. This will usually, but not
always, be equivalent to an SAE Grade 8
fastener.

FRAME CONSTRUCTION


Frames may be single, double, or triple
construction. Additionally, most frames are
available with either inside or partial inside
channel reinforcements or outside
reinforcements. The reinforcements are used
to provide a greater RBM and SM over a
section of the frame.

MULTIELEMENT RAILS

Reinforcements

Alternate frame designs:

(A) I
-
beam, and (B) box.

DRILLING


Careful consideration should be given to the
number, location, and sizes of frame bolt
holes in the design of a vehicle. The number,
location, and sizes of additional bolt holes
installed to the frame subsequent to
manufacture of the vehicle can adversely
affect frame strength.

Drilling Method


Drill the pilot hole.


Drill to 1/8 inch under the nominal
required hole size.


Taper
-
ream the hole to the exact nominal
required hole size.

Shop Talk


When any type of frame reinforcement is
added, straight cut fishplates, L
-
sections, and
C
-
channels should be avoided because this
creates a sudden increase in SM. This
sudden increase in SM can cause frame
failures immediately adjacent to the
reinforced section.

STRESS CONCENTRATORS


In any modification of the chassis frame, the
addition of holes, reinforcements, welds,
clamps, splices, and so on, may cause an
increase in local stress in the frame at the
point of the modification. These local stress
concentrations can significantly affect the life
of the chassis frame.

CORROSION


Frame damage also may be caused by
corrosion caused by the contact between
dissimilar metals. If aluminum and steel, for
example, are allowed to come into direct
contact, galvanic corrosion can eat away both
materials. Aluminum is anodic with respect to
steel and will corrode in its presence.

FRAME DAMAGE CATEGORIES



Diamond :
when one frame rail has moved
ahead of the other.



Twist :
frame rails are twisted off a level
plane in relation to each other.




Sidesway:
usually caused by a sideswipe
-
type collision



Sag and bow:
Sag is downward bend, bow is
upward bend. Sag is usually the result of
overloading

Shop Talk


Frame straightening should be performed
only by a qualified frame alignment facility.
Because impact
-
damaged frames are
repaired by specialty technicians using
specialty equipment, this type of frame
servicing is not covered in this book.

FRAME ALIGNMENT

PROJECTING A FRAME DIAGRAM

First


Tire inflation.


Front end alignment.


No visual frame damage or bent axle housings.


Proper wheel and tire balance.


Tires and rims must be of the proper size and type
with no mismatching.


On disc wheel assemblies, the wheel discs should
be the same on all wheels.


Move the vehicle to a level floor, neutralize the
suspension (see Chapter 26), and ensure that the
front wheels are tracked as straight as possible.

Method of hanging a plumb bob

Marking alignment reference point

onto the floor

FRAME LAYOUT

Unequal offset side rail frame

Straight side rail frame alignment

reference points

Connecting alignment reference

points

Locating Chassis Centerline

Checking Centerline Intersections

Comparing Lengths of Pair Diagonals

REPAIR OF FRAME, SIDE

RAILS, AND CROSS
-
MEMBERS

Always make reinforcement plates

Longer than the tension flange edge

Welding Procedure


Welds on truck frames are common practice,
but they should be performed by a skilled
welder with some knowledge of how the
repair will affect the frame performance. The
heat
-
tempered frame rails used on most
highway tractors may be both repair
-
welded
and extended successfully, providing the
correct methods are used. Use a low
-
hydrogen welding electrode with a wire
tensile strength rating similar to the frame
rating. (E
-
11018 is ideal)

Preparing a Rail Crack for Welding

Grinding a V
-
groove

Finishing a two
-
side weld on a
frame rail

Summary



The chassis frame is the backbone of all
heavy duty trucks.



A truck frame is a dynamic component. It is
designed to flex when subjected to vehicle
loading and road forces. The extent to which
it can flex defines the type of operation that
the truck is suited to.

Summary (cont.)



The frame supports the cab, hood, and
powertrain components, along with the body
and payload.



The two main components of a ladder
-
type
frame are the two longitudinal members,
which are generally referred to as
rails.



Ultimate frame strength is measured for
comparative purposes by resist bend moment
(RBM).

Summary (cont.)



RBM is factored by section modulus and yield
strength.



Section modulus (SM) concerns the shape of
frame beams. High SM produces a more rigid
frame. Low SM produces higher flexibility.



Hardened steel frame rails are formed from
high
-
strength alloy steel, quenched and
tempered (heat
-
treated) to a minimum yield
strength of 110,000 psi.

Summary (cont.)



A bent frame can decrease the control a
driver has over a vehicle during an
emergency and increase the chances of an
accident occurring.



Frame damage can be generally categorized
as diamond, twist, sideway, sag, and bow.