Chapter 32
Vehicle Chassis Frame
Objectives (1 of 2)
• 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.
• Describe the different ways frame damage can
occur as a result of impact and overloading.
Objectives (2 of 2)
• Perform some basic chassis frame alignment
checks.
• Describe the various categories of frame damage
including diamond, twist, side-sway, sag, and bow.
• Explain how the chassis frame, side rails, and
cross-members can be repaired.
• List some guidelines to follow when using frame
repair hardware.
• Outline some basic frame welding techniques.
Frame C-channel
Basic Frame Terms
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Yield strength
Section modulus
Resist bend moment
Area
Applied moment
Bending moment
Safety factor
Maximum Bending Moment
Shop Talk
• Two truck frames with identical RBM can
perform very differently.
• RBM is calculated by factoring section
modulus with yield strength.
– If two trucks have identical RBM but one is
spec’d with a high section modulus but uses a
lower yield strength material, it will be more
rigid than a frame with high yield strength but
low section modulus.
Caution
• Steel frame rails that appear to the eye
similar can have drastically different yield
strengths.
• Always identify the frame material before
attempting any repairs.
Heat-treated Frame Caution Label
Shop Talk
• Although chassis frames are very strong,
care must be taken when lifting or moving
them, to avoid anything that may scratch, cut,
or damage an exposed frame assembly.
• Cushion all chain hoists or cable slings with a
section of heavy hose.
• If the frame rail is raised with a jack, place a
block of wood between the jack and the
frame rail.
Frame Load
Drilled Hole Placement
Cutting and Welding
Recommendations
Shop Talk
• The extreme front and extreme rear of a
frame rail is subjected to less stress during
normal operation than the sections in
between.
• A notable exception would be the stress to
which the rear of the frame rails are
subjected when a fully loaded dump box is
raised on a dump truck.
Caution
• Previously 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
• Two-element rail
– Consists of the main frame rail and a single inside
channel frame reinforcement
• Three-element rail
– Consists of the main frame rail and two frame
reinforcements, a single inside channel, and a single
outside channel frame reinforcement
• Four-element rail
– Consists of the main frame and three frame
reinforcements, a single inside channel, a single
outside channel, and a single inverted “L” outside
frame reinforcement
Fishplate and
Reinforcement Guidelines
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 section
modulus.
– This sudden increase in section modulus can
cause frame failures immediately adjacent to
the reinforced section.
Caution
• Additional reinforcement of the chassis frame
to support additional loading or to
concentrate a load, should not be made until
it has been fully verified that all other vehicle
components, such as the brake system,
steering system, suspension system can
properly and safely carry and support the
increased loading.
Shop Talk
• Frame straightening should be performed
only by a qualified frame alignment facility.
– Because impact damaged frames are
corrected by specialty technicians, this type of
frame servicing is not covered in this book.
Frame Damage
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Exceeding the gross vehicle weight rating
Uneven load distribution
Improper fifth wheel settings
Using the vehicle for purposes for which it
was not originally designed
• The use of special equipment for which the
frame was not designed
• Improper modification of the frame
Frame Damage Categories
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Diamond
Twist
Side-sway
Sag and bow
Frame Alignment
• Projecting a frame diagram
• Frame layout
Repairs
• Welding repairs
Caution
• A hardened steel frame weld that fails by
cracking cleanly through the center of the
weld profile often does so because the
incorrect filler wire (electrode) has been
used.
• A weld that fails by cracking clean to the
sides of the weld profile often does so due to
crystallization caused by overheating.
– Crystallization usually means that the welding
procedure has been performed too rapidly.
Repairs (1 of 7)
Repairs (2 of 7)
Repairs (3 of 7)
Repairs (4 of 7)
Repairs (5 of 7)
Repairs (6 of 7)
Repairs (7 of 7)
Shop Talk
• When cutting a frame, use a pencil or
soapstone to make all lines, points, or other
marks.
• Try to avoid the use of a scriber or tool that
will scratch the surface of the frame rail.
• Use a machinist’s square to project all points
from the webs to the upper flanges and to
measure inboard from the outside face of the
frame rails.
Shop Talk
• Cobalt high-speed drills are superior to
conventional high-speed drills for drilling
hardened frame rails.
• Drills should be sharpened to give 150
degrees included angle with 7–15 degrees lip
clearance.
Drilling
• A drill usually drills to a fractional oversize.
The best method of drilling a frame rail is to
use this method:
– Drill pilot hole.
– Drill to 1/8-inch under the nominal required
hole size.
– Taper ream to the exact nominal required hole
size.
Summary (1 of 6)
• 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 to which the truck is suited.
• The frame supports the cab, hood, and
powertrain components, along with the body
and payload.
Summary (2 of 6)
• 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).
• 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.
Summary (3 of 6)
• Hardened steel frame rails are formed from highstrength alloy steel, quenched and tempered (heattreated) to a minimum yield strength of 110,000 psi.
• Low SM frames provide greater flexibility and
therefore more ride forgiveness. High SM provides
the kind of rigidity and twist-resistance required by a
dump truck application.
• Most frames are available with either inside or
partial inside channel reinforcements or outside
reinforcements.
Summary (4 of 6)
• Reinforcements are used to increase rigidity
(section modulus) and provide a greater
resist bend moment (RBM) than can be
obtained by using a single mainframe rail.
• Cross-members are designed to connect the
frame rails.
– They provide rigidity and strength, along with
sufficient flexibility to withstand twisting and
bending stresses encountered when operating
on uneven terrain.
Summary (5 of 6)
• Attachments to the frame rails should be
made to the frame rail web and never to the
flanges, because stresses are highest in the
flange areas that are subjected to tensional
and compression loads.
• The most common frame rail used on heavyduty trucks is the C-channel design.
– Multiple frame rails can increase section
modulus and RBM.
Summary (6 of 6)
• A bent frame can decrease the control a
driver has over a vehicle during an
emergency.
• Frame damage can be generally categorized
as diamond, twist, sidesway, sag, and bow.
• Frame reinforcement can be either channel,
fishplate, or L-sectioned; they should be of
the same grade and thickness of steel and
sudden changes in section modulus are to be
avoided.