Properly matching a truck body and/or accessory equipment to a chassis is important to ensure that the completed vehicle will perform as intended without adversely affecting handling characteristics or weight distribution. Typically, 60 – 70% of the body weight should be forward of the centerline of the rear axle(s). This percentage can be adjusted by either moving the center of gravity forward, which places more weight on the front axle, or moving the center of gravity rearward, which places more weight on the rear axle(s). The addition of a body, associated equipment and the payload should never result in the GAWRs and/or GVWR being exceeded.
When choosing a chassis for a body, the following must be considered:
- How much weight can be placed on the front and rear axles (GAWR).
- How much the vehicle can weigh, including the vehicle with full capacities of fuel, oil, coolant, etc., the driver and passenger if applicable, all associated equipment and the body's payload (GVWR).
- Curb or tare weight, or how much the chassis weighs before the body and/or equipment are installed. Tare weight includes the weight of all options, fuel, lubricants and coolants.
- Cab-to-axle (CA). This is the dimension from the back of the cab to the centerline of the rear axle, or the centerline of the rear tandem axle assembly.
- Wheelbase (WB). This is the dimension between the centerline of the front axle and the centerline of the rear axle (or the centerline of the tandem axle assembly). This dimension is important because it affects body installation, vehicle performance and whether a particular axle is overloaded.
- Back-of-cab (BOC). The distance between the back of the cab and the body.
- Body length (BL). This is the dimension from the front to the rear of the body.
- Overall vehicle length state regulated for straight trucks. If in doubt, contact the appropriate State Department of Transportation.
Vehicle wheelbase dimensions may be changed by moving the rear axle and suspension assembly to the new, desired location on the frame. When the axle assembly is moved, the suspension should be remounted to the frame, using as many existing drillings in the frame as possible. The number of new drillings in the frame rails should be limited. All unused drilled holes in the frame must be filled with a proper size bolt, nut and hardened washers. Tighten the hardware to proper specifications.
Wheelbase Changes and Driveshaft Length
Wheelbase changes affect driveshaft length, driveline angularity and size requirements. To avoid potential vibration problems and failures, the driveline for the new wheelbase dimension must maintain the correct driveshaft angle, size and length.
Wheelbase Changes and ABS/ATC Systems
An important factor in maintaining MVSS 121 complying brake timing is keeping the brake valves in the same relative position to the rear axle brake assemblies. Particularly with ABS/ATC chassis, the ABS/ATC components (modulator valve) for the rear axle must be moved to correspond with the increase or decrease in wheelbase length. The relationship between the rear axle and the modulator valve must be kept the same. Additionally, the service brake relay valve must be moved to maintain the same distance between the modulator valve and the relay valve. Do not cut and splice harnesses for the ABS/ATC speed sensors and modulator valves to compensate for changes in chassis wheelbase. Extension harnesses are available in 2, 4 and 6 foot lengths. Contact a MACK dealer, service dealer or parts dealer for necessary extension harness part numbers.
Wheelbase Changes and Steering Geometry
Changes to vehicle wheelbase will affect steering geometry (specifically, Ackerman angle), and may require a different cross-steering lever and cross-steering tube. For additional information, contact MACK Trucks, Inc. Product Support.
Wheelbase Changes on MACK Model Chassis Equipped with 105 mm (4.13 in) Frame Flanges
On models equipped with frame rails having 105 mm (4.13 in) flanges, the lower frame flange may have a relief cutout to provide clearance for the suspension trunnion, or for the transverse torque rod bracket. When changing wheelbase on one of these chassis, it will be necessary to cut a new relief in the frame flange to accommodate a new location of the trunnion and/or transverse torque rod bracket. To ensure a dimensionally correct relief cutout, templates are available through the MACK Parts System. Three of the templates are unique to the specific rear suspension model, and one template is for the transverse torque rod relief cutout.
A variety of methods can be used to secure the subframe assembly to the chassis frame. They include U-bolts, flexible attachments and bolted plates. When the subframe is installed, however, a mounting sill plate made of hardwood or other suitable material may be installed between the subframe and the chassis frame to protect the flange surfaces, and to allow for irregularities in the surfaces of the two frame members.
Bolted Methods of Attachment
The two bolted methods of attachment are rigid mounting and flexible mounting. Both of these methods include clips, brackets and other types of mountings which are bolted to non-critical areas of the frame rail web. The use of existing holes in the frame is encouraged. But when this is not possible, holes in the frame must be drilled in accordance with the frame drilling methods as outlined earlier in this section. As a rule, holes in the frame should be located no closer to the top and bottom frame flanges than existing holes that were drilled at the assembly plant.
Rigid types of mounting should be used for mounting vans or other similar types of bodies. A rigid type of mounting arrangement consists of a bolted plate or bracket welded to the subframe assembly and bolted to the chassis frame. Brackets must be bolted, not welded, to the chassis frame.
For torsionally stiff types of bodies, such as tanks or refuse bodies, the mounting must allow some flexing of the frame under normal driving conditions. Flexible mounting should be used. Flexible mounting is accomplished by using rubber mountings or spring-loaded brackets.
Subframes for Bridge Formula Mixers
Certain mixer applications require an extended tag axle for the chassis to meet the federal bridge formula (Boost-A-Load, Bridge Master, etc.). On these chassis, welding the subframe to the chassis frame rails is acceptable, providing the following installation criteria are met:
- Yield strength of the welding rods must be at least equal to the yield strength of the least strongest component of the assembly (either the chassis frame rail at 110,000 psi or the mixer subframe). Low hydrogen electrodes must be used, and they must comply with the applicable provisions of the American Welding Society (AWS) welding codes.
- Welding procedures must conform to those recommended by AWS to ensure frame material integrity. Design of the subframe must be tailored to the particular chassis frame configuration, and caution must be exercised to prevent weld stress concentrations and excessive heat effects in critical stress areas, especially at the front end of the subframe. For additional information on welding, refer to Frame Welding and Cutting in Frame Section.
- The subframe must be constructed of 9.5 mm (0.375 in) angle iron, with a minimum web dimension of 114.3 mm (4.5 in) and a minimum flange dimension of 101.6 mm (4.0 in).
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