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Rear Disk Brake Conversion

Note: This is an older question that had actually "dropped off the board." We liked this new answer we received so much that we're running the question again. The original answer is also in the Archive. See also Cameron Lovre's article on this subject.

I have read many references to converting the rear drum brakes of an 1800S to disk brakes, but cannot seem to find any technical info on same. I have a 1967 1800S that is currently being restored from the ground up and would like to add the rear disk brakes. Can you get me in the right direction?
Bob Yapp
Rock Island, IL

Answer from a reader: I'm in latter stages of completing the conversion of my 1963 1800S into a vintage racer. One of the changes was converting the entire suspension system to a 1973 type... including the brakes.

First some comments:

1. The conversion of the rear brakes to drums does make a difference on a race course. However, the actual increase stopping power in a street application will not be very noticeable. The reason is that more than 70% of the brake system's stopping power comes from the front brakes. So why bother for racing? Well, during braking, the fore/aft weight transfer is much greater in a street car than in a race car. This is due to the stiffness of the front road springs. The standard 1800 front road spring is about 280 in/lb. However, a typical front road spring on a race car is 800 in/lb... and rides like a truck!

2. It's harder to find wheels with a late-type (4 1/4") bolt circle than the earlier type (4 1/2").

With all that said, here's how I converted the 1973, disc-braked rear axle to my 1963 1800S: the 1973 axle is actually a Dana Type 30 axle made under license by Spicer in England. This means that gear sets and limited slip units made for Dana 30 axles will work in the Volvo axle. I installed a limited slip unit and replaced all the bearings. The bearings were obtained from my local bearing supplier at a much lower cost than from Volvo.

A. Neither the 1973 axle assembly nor the 1963 car body were modified in any way.

B. The upper and lower axle control arms were modified to conform to the mounting points on my 1963 car's body. I started with the four control arms from the 1973 car. I found that their length was about 1-1/2" longer that those of the 1963. Also, the method of mount the arms to the body is not the same -- the 1963 has a threaded "pin" for the upper arm and an "eye" for the lower; the 1973 arrangement is "eye-eye". I cut the 1973 arms off, determined their inside diameters and fitted them with mild steel inserts. All the inserts were 2" long and drilled & tapped. The upper arms were fitted with threaded rod (from the hardware store) and the lower with high-strength ball-end (Heim) joints obtained from Pegasus (the race parts people).

C. The axle's brakes were tied together at the Tee fitting from the original 1963 axle that mounts on the LH side of the differential.

D. A Tilton variable pressure regulator was installed to control the amount of rear braking. However, a single pressure regulator from a 1970-73 1800E/ES will work as well.

E. A standard emergency brake cable from a 1973 1800 was used. The conversion takes place at the pull rod. The original 1963 pull rod was used. Attached to it was the nylon wheel emergency brake assembly from the 1973 car.

In conclusion, the conversion is great when it's completed, but it is time consuming and costly. It's a "must" for really competitive racing, but not for street use.
Del Lance
Blue Ridge Chapter - VCOA

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