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.
Mopar A-body disc brake systems have had chronic problems with premature rear lockup under heavy braking and there are two options for correction: proportioning valves and smaller rear wheel cylinders. Dan Stern is extremely knowledgeable about about Chrysler vehicles and he very kindly provided the following rear brake wheel cylinder information. The following wheel cylinders fit all Chrysler rear brakes:
|
Wheel Cylinder Bore Size |
Part Numbers |
Typical Applications and Notes |
| 15/16" |
|
|
| 7/8" |
|
|
| 13/16" |
|
|
| 3/4" |
|
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Upgrading 65-72 A-Body 8-3/4" axles to 4-1/2" BC
For high horsepower applications, it is difficult to find a stronger OEM axle than the 8-3/4" that came with the early A-Body cars. When the front end is upgraded to 4.5" BC disc brakes, it makes sense to change the axle's bolt pattern at the same time to avoid having two different sets of wheels on the car.
Although redrilling the axle flange and brake drum is one option, a better solution is to replace the axle shafts with a suitable 8-3/4" axle shaft from another body style. Since all rear drum brake backing plates from the other body styles all mount to the axle flange with the same bolt pattern, this would also be the ideal time to install the rear brake you require. OEM axle shafts would have to be cut down and resplined to fit the A-Body 8-3/4" axle but aftermarket axle shafts are available to fit a wide variety of axles and brakes.
Because of the relatively low demand for these custom axles, axle manufacturers haven't (yet) a set up standard off-the-shelf parts for this conversion. Most require detailed measurements of the axle and brakes to be able to fabricate the correct part. The following manufacturers can provide conversion kits that include axle shafts, bearings, and studs:
Disc Brakes
Although the 1974-1975 Imperials were equipped with rear disc brakes, the scarcity of these parts makes this option an unrecommended upgrade. Aftermarket disc brake conversions are a more realistic upgrade if rear disc brakes are desired but they provide minimal additional braking over the rear drum brakes.
http://www.markwilliams.com/detail.aspx?ID=1717
Still to be added:
- ready-made axle shaft PNs for the various 8-3/4" axle and brake combinations (Moser, Strange, etc)
- rear wheel disc brake upgrades
- rear brake interchanges