Since the front brakes do the majority of braking, upgrading the rear brakes is generally not required. One reason for considering a brake change is the necessity to change rear wheel bolt patterns when upgrading the front brakes from the early 4" bolt circle (BC) brake systems to the later 4.5" BC. Because all drum brake A-Body cars had 4" BCs, only the later model disc brake A-Body cars had axles with the 4.5" BC:
- 1973 - 1976 A-body 7-1/4" rear axle w/ 4.5" BC
- 1973 - 1976 A-body 8-1/4" rear axle w/ 4.5" BC
Other axles may used but they will require some minor modifications:
- All 8-1/4" F-Body axles (move spring perches & optionally shorten 2.94" overall)
- 1960-1972 8-3/4" B-Body axles (move spring perches & optionally shorten 3.6" overall)
- 1966-1970 9-3/4" B-Body axles (move spring perches & optionally shorten 3.536" overall)
The above axles may be optionally shortened is because the rear track (distance between tire centerlines) is a few inches shorter in the back than the front on A-Body cars. For wide rear wheels, a suitable backspacing can keep the tire centered in the wheelhouse.
See BigBlockMopar's Rear Axle Drum to Drum Measurements for more information. Please be aware that the driveshaft may have to be shortened (or lengthened) when switching to a different type of axle because the distance between the centerline of the axle shaft and the centerline of the rear universal joint varies between axle types.
Chrysler 8-3/4" Axle Information:
The early (pre-1972) A-Body drum brakes are difficult to upgrade because they used a 4" bolt circle that was unique to A-Body vehicles. Although there were other 4.5" BC brakes at that time which also were 1¾" or 2" wide (such as 11"x2" on some '65-73 full-size passenger vehicles and 10"x1¾" on '66-70 Chargers, Coronets, Belvederes, & Satellites), it is likely that custom axle shafts would have to be fabricated for use in A-Body axles . The largest production rear brake available in a 5x4½" bolt pattern was the 11"x3" (11"x2½" was available as well) in the same Chrysler full-size vehicles plus the '65-73 Imperials. Custom axle shafts would definitely be required for these much wider brakes.
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