Disc Brake Conversion

    I decided I wanted front disc brakes on the car.  The drum brakes stop the car just fine, but at high speeds, when you first hit the brakes, the car likes to pull just a little one way or the other, then stops straight.  The disc brakes will stop the car straight from the very first hit, and they will shed water better, while resisting fading.

    The brakes had to allow the use of the original size 13" wheels.  I did some checking, and found that rotors for a Dodge Colt were about the right size.  They are 9" diameter, which is about the biggest rotor you can fit in a 13" wheel.  The calipers are Mitsubishi and were used on the Dodge, along with several other makes of car. 

    To resist fade, I cross drilled the rotors as seen below.

    To make this work, I removed the rivets that held the drums to the front hubs.  I then removed the lugs, and put the hub in a brake lathe and machined a "pilot" on the back side of the hub for the rotor to locate on.  Then I put the hub in the Bridgeport and drilled 4 holes through the hub, and counter sunk them, so that I could use 4 grade 8 counter sunk head bolts, to bolt the rotors onto the back side of the hub. 

    You can see by the photos below that there is not a lot of room.  The photo on the right shows the top backing plate bolt less than .050" from the rotor.  This bolt head is ground down .125" prior to final installation to be sure there is plenty of clearance. 

Below you can see the back side of the new backing plate.  It is made of 1/4" steel for everything except the top caliper mount, (made of 1/2" steel) and the top of the backing plate where the top bolt goes, (made of 1/8" steel).  I welded the caliper mounts on to the back side of the backing plate, (inboard), and used 1/8" steel shim stock to make enough room for the caliper mounting surface to clear the rotor. 

Below you can see how the caliper mounts are 1/8" inboard of the backing plate.  This set up allows me to avoid using a spacer under the wheels, which would be required for the caliper to clear the wheel if I had moved the rotor further outboard.

    Clearance is tight.  On one side I had to grind down the part numbers cast into the rotor, so they would clear the lower backing plate bolts.  I only needed to grind around .020, but I just ground the numbers completely off.  The other side, I did not need to grind at all.

    Below you can see how I had to modify the caliper.  The ridge you see toward the top of the photo, is cut to slightly less than 1/2 original size.  This was done to clear my alloy wheels.  In original trim, the caliper would clear the factory steel wheels, but not aftermarket alloy wheels.  

    The next two photos show how I had to relocate one caliper mounting hole, (would have been right behind the steering arm).  Notice that I also cut the ear off, and chamfered the the other ear.  This chamfer lets the caliper clear the weld on the backing plate.  Like I said, this all fits rather tight!

    You can see that two different bolts are being used.  The original is a metric fine thread 12mm hex head bolt.  The bolt for the hole I relocated is a 7/16 fine thread socket head cap screw.  This is because there is room for the hex head on the top, but the bottom bolt has the steering arm right there.  The socket head cap screw is all that would fit, and the 7/16 is just slightly smaller than the 12mm. This makes the socket head on the 7/16 bolt about .040 smaller than the metric, and makes installing the bolt slightly easier.  I can't say it too much, it is TIGHT in there!

    In the photo below, you can see where I had to grind around 7/16" of the frame away.  When you turn the wheels to full lock, with the pads worn, this part of the frame would push the piston in on the caliper, causing me to have to "pump" the brake pedal back up.  Fitting a car with something for which it was never designed can have lots of little obstacles.

    I was very happy with the performance of the new disc brakes.  I do not notice any extra pedal effort, seems about the same as before.  I did have to install a proportioning valve, as the rear brakes were locking up way early.  I still need to do some more testing, and fine tuning, but the brakes are working GREAT right now.  Big improvement over the stock drums.  No matter how hard or fast you hit the brakes, and no matter how fast you are going, the car stops straight. 

    The next brake project will be to put discs on the rear.  That will have to wait for some time as I will also have to go to larger wheels to make it all work with the original early model rear suspension.