Mack Truck Suspension, Steering Service Manual

This information provides design and function, specification and procedure details for axles and suspension for MACK vehicles. Note: We have attempted to cover as much information as possible. However, this information does not cover all the unique variations that a vehicle chassis may present. Note that illustrations are typical but may not reflect all the variations of assembly.

Contents:

  1. “Axle”, page 3
  2. “Liftable Axles”, page 3
  3. “Clearance Rear Wheels and Axles”, page 6
  4. “Axle Alignment”, page 7
  5. “Rear Axle Literature”, page 18
  6. “Rear Suspension”, page 19
  7. “Alignment and Adjustments”, page 20
  8. “U-Bolt Torque”, page 27
  9. “Air Suspension Height, Adjustment”, page 28
  10. “Ride Height and Pinion Angle Specifications”, page 29

Liftable Axles
Liftable axles (tags and pushers) can be added to increase the load-carrying capacity of a vehicle. When one or more liftable axle(s) are installed on a newly manufactured vehicle prior to the vehicle being delivered to the ultimate customer, the installer must ensure that the GAWR/GVWR, tire, rim and inflation pressure information is updated on an addendum to the incomplete vehicle document or vehicle certification label, as applicable.

Liftable Axle – Location
Installing a liftable axle requires tapping into the air system to supply air for various functions such as the lift axle air suspension, up and down air bags and lift axle brakes. The axle installer is responsible for ensuring that the air brake system continues to comply with the requirements of FMVSS/CMVSS 106, Brake Hoses, and 121, Air Brake Systems, as applicable (requirements for stopping distance, park brake grade holding ability, etc.), after the liftable axle(s) is installed.

The following example explains how to use the U.S. Federal Bridge Formula Table to locate a liftable axle on a chassis. The four axle dump truck in the following example is intended for a gross vehicle weight rating of 29,000 Kg (64,000 lb). The vehicle is equipped with an 8,165 Kg (18,000 lb) front axle, and a tandem rear axle of adequate capacity to carry the legal limit of 15,400 Kg (34,000 lb).

The U.S. Federal Bridge Formula Table shows that the spread between the centerline of the front steer axle and the centerline of the rearmost axle of the tandem for this four axle truck must be 0.01 Km (33 ft) In this configuration, the chassis will carry 8165 Kg (18,000 lb) on the front steer axle and 20,865 Kg (46,000 lb) on the rear tandem axles 8,165 + 20,865 = 29,030 Kg (18,000 + 46,000 = 64,000 lb).

Legally, however, only 15,400 Kg (34,000 lb) allowable on the rear axles for vehicles that operate on the national network of highways. For this chassis to legally carry 46,000 lb on the rear axles, a 5,443 Kg (12,000 lb) pusher axle must be added 15,400 + 5,443 = 20,843 Kg (12,000 + 34,000 = 46,000 lb). To properly locate the pusher axle, refer to the U.S. Federal Bridge Formula Table. For a three axle group (pusher axle plus tandem axles), the spread between the centerline of the pusher and the centerline of the rearmost axle of the tandem must be 13.5 ft. However, to ensure that the chassis configuration satisfies the U.S. Federal Bridge Formula, all groups of two or more axles on the chassis must be checked and verified to ensure that axle distance requirements are met.

Centering Rear Axles (Chassis Equipped with SS Suspensions)
To center rear axles equipped with SS suspensions, a measurement must be made between the frame and the brake drums on each axle. A convenient extension straightedge can be made from a straight piece of steel bar stock and attached to the frame rail with magnets. With a tape measure, measure from the straightedge to the brake drum at the three designated points shown in the illustration below.

The maximum allowable difference between the measurements taken at the front-rear and rear-rear brake drums to the straightedge must not exceed 1/4 in (6.4 mm), whether or not the chassis is equipped with a transverse torque rod. The maximum allowable difference between the left- and right-hand sides on the same axle, front-rear or rear-rear must not exceed 1/4 in (6.4 mm) if the chassis is equipped with a transverse torque rod, or 12.7 mm (1/2 in) if the chassis is NOT equipped with a transverse torque rod.

Centering Rear Axles (Chassis Equipped with AL Suspension and Fixed-Length Transverse Torque Rods)
The first step in proper axle alignment is verifying that the rear axles are properly centered on the chassis. Before any alignment measurements are taken, the chassis should be driven back and forth in a straight line several times to allow the suspension to move into its normal operating position. To verify that the rear axles are centered, perform the following steps:

  1. With the chassis parked on a level surface, block the front wheels to prevent the vehicle from moving, then release the parking brakes. (Air system pressure should be between 110 to 130 psi when performing these procedures.)
  2. Measure the ride height. Ride height gauges are available to accurately measure chassis ride height. Refer to “Air Suspension Height, Adjustment”, page 28 for information on using the gauges and measuring ride height.
  3. After the ride height has been checked and/or adjusted, determine if the axles are centered by measuring from the outside mof the frame rail to the inside edge of the spring clip (U-bolt) (Dimension “A” in the illustration below). The maximum allowable side-to-side difference on any one axle is 6.34 mm (1/4 in). Washer spacers can be added or removed from either side of the torque rod in order to obtain the correct axle positioning.
  4. After axle centering is verified or corrected, exhaust the air from the suspension and verify that no interference exists between the frame rail and any spring clip (U-bolt).

Note: Beginning approximately 3/19/01, a change in the spring seat and spacer resulted in a ride height dimension change from 5-1/4′′ to 4-7/8′′ on MAXAIRTM 40 air suspensions. Suspensions having a ride height of 4-7/8′′ are identified by an “X” marking on the spring cap. Always look for the “X” marking before adjusting ride height. Refer to service information in Function Group 7 for information on using the gauges and measuring ride height.

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