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 StyleStraight Type

FRAME DESIGNSC-channel rail sectionBy using a radius bend stress concentrations are avoided

BASIC FRAME TERMSYield Strength-highest stress material can take without permanent deformation (psi). Class 8 trucks use 110,000 psi, heat treated.Section Modulus-shape and size onlyResist bend moment (RBM).-SM x YSArea-total cross section (square inches)Applied Moment-point where load is appliedBending Moment-point at which it deflectsSafety Factor (Margin of Safety).-relationship of applied moment to RBM

Bending MomentHighest 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=9.5SM=10.7The 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 -inch steel whereas on/off road uses 5/16 or 3/8 steel.

Resist Bend Moment Equation

RBM = SM Yield StrengthRBM=10.7x110,000RBM=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 specd 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 AttachmentThree common methods of attaching components toThe frame are bolts, u-bolts, and clamp attachments.

FRAME CUTTING AND REPAIRGUIDELINES 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 MethodDrill 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 DIAGRAMFirstTire 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 pointonto the floor

FRAME LAYOUTUnequal offset side rail frame

Straight side rail frame alignmentreference points

Connecting alignment referencepoints

Locating Chassis Centerline

Checking Centerline Intersections

Comparing Lengths of Pair Diagonals

REPAIR OF FRAME, SIDERAILS, AND CROSS-MEMBERSAlways 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.