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SELECTION TIPS
For
drum brakes
, the most common drive axle chamber size is a type 30. Heavy application will use type 36.
Front axle will typically use types 9 through 20.
For
Meritor air disc brakes
, a 5-1/2” arm length, 1-1/2”-28 spline is standard. The air chambers will vary from
type 20 to type 30. Type 24 and 30 are typical chamber sizes fro rear air disc brakes.
Please Note: Both wheelends for each axle should have compatible ASA part numbers! DO NOT mix different
Meritor part numbers on an axle because they may have different pistons and therefore different adjusted
lining clearances.
CHAMBER SIZE AND PISTON LENGTH
CAN SUBSTANTIALLY IMPACT STROKE LENGTH
All brakes have normal deflection, lining running clearance, and thermal expansion variances.
Below is an example of a “typical” drum brake setup running at 600 degrees braking temperature and the cor-
responding maximum stroke increments for each activity taking place in the brake:
.80” stroke = Drum thermal expansion
.72” stroke = Normal lining running clearance
.27” stroke = Deflection of brake and drum
1.79” stroke = Measurable to inspectors
(If 2.0” is the established acceptable stroke, the vehicle is within the legal limits.)
If the proper ASA part number with the corresponding piston length for that chamber application is NOT used,
the chamber push rod stroke and therefore the adjustment interval will be affected. To what degree depends
upon the length variance between the correct piston for that chamber
application and the wrong one.
Each change in length can represent up to a .10 inch difference in push rod stroke. The .10
difference,
in addition
to the above normal effects on stroke, could put the vehicle out of service
for an inspection violation.
When you are selecting an ASA, make sure that the chamber size corresponds to the ASA part number. Note:
ASA selection for air disc brakes is slightly different that for drum brakes. Please pay attention to the ADB
notations in the parts catalogs and wall charts.