1985 SAAB 900 REAR DISC BRAKES


        "REAR DISC BRAKES!  Day before yesterday, I stopped by my friend Jason's salvage yard.  The B is safely back in her spot in my garage, and I don't want to do any welding till the weekend; so, I was looking at some options...

        Wandering around in the salvage yard with an MGB rear hub in my hand, I spotted a black 1985 Saab 900 2-door coupe that still had all its alloy wheels under it.  A bit of work & I confirmed that the bolt pattern for the wheels was the same as my B.  A bit more work and I confirmed that the rear rotor will slip over an MGB hub!

        So, armed with 2 old Saab rotors, their calipers and caliper mounting bolts, I headed home..."

Installing Rear Disc Brakes on an MGB

From This:                      To This:
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          discbrakes1001.jpg (25661 bytes)

Only Requires This:
discbrakes1002.jpg (26925 bytes) 
&, a few bolts!

PARTS LIST

 

PARTS #

PRICE

1985 Saab 900 Left Rotor

19620593
(Advance Auto)

$52.99*

1985 Saab 900 Right Rotor

19620496
(Advance Auto)

$52.99*

2-each 1985 Saab 900 calipers 

5672031
(Advance Auto)

$53.49 each

2-each Custom-built caliper mounting brackets for Saab calipers  

$100.00 pair

4-each MGB brake backing plate nuts and bolts

Already on MGB Rear End

 
4-each MGB brake drum retaining screws

323-255
(Moss Motors)

 
Saab caliper mounting bolts & straps

Acquired w/old calipers from salvage yard

 
Wilwood Spot Caliper

Right - WIL120-2280

$55.99

Wilwood Spot Caliper

Left - WIL120-2281

$55.99

  * with old core caliper.  Core charge $25.00 each


       First, I suppose any rear brake caliper could be used so long as you can design a bracket setup; heck, for that matter, a Midget caliper could be used.  The main issue, in my opinion, isn't the caliper; its the rotor.  It has to fit over the MGB rear hub.       

        Another thing about the rotor: the MGB front rotor is 10-3/4" in diameter where the Saab rear rotor is 10-1/2".  That's a pretty stout rotor for the rear brakes.  So, a proportioning valve will definitely be necessary.

 

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Laying the old 'test' Saab rotor from the salvage yard on the work bench next to an old MGB rear brake drum showed that the rotor is less than 1/4" thicker than the drum.


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And, the 2 small set screw holes that were used to hold the MGB drum to its hub can be used to hold the rotor to the hub by just drilling and threading 2 new holes in the MGB hub.

       The 2-pot Saab calipers are a bit smaller than original equipment 2-pot MGB front calipers, and about the same size as Midget calipers.  As I'm using Rover SD-1 4-pot calipers on the front of my car, the Saab rotors are sufficiently smaller to not overpower the front brakes (a real concern when building a 4-wheel disc brake setup).

        The MGB-GT V8 original front rotors are 10-3/4" in diameter where the Saab rotors are 10-1/2".  The rotors are basically the same size but the calipers are not meaning that the rear brake pads will have less surface area than the front.

        Now, on to the actual work of fitting the brakes.


        First, remove the entire MGB factory brake setup to include the backing plate.  Replace the 'back' (in relation to rear of car) 2 bolts that were used to attach the backing plate to the rear axle.  Hang onto the 2 'front' bolts; they'll be used to attach the caliper bracket to the axle.

        Second, after drilling and threading 2 small set screw holes in the MGB rear hub to hold the Saab rotor on it, slide the Saab rotor over the hub.  You can actually use either the MGB set screws or the Saab screws to hold the rotor to the hub.  I chose the MGB screws because they are longer.

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Third, a trip to my local machine shop resulted in purpose-built brackets.


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With the bracket made, attach them to the 2 front holes where the original backing plate was mounted using the original bolts.  This places the Saab calipers to the front of the axle.


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The Saab rotor fits under my car and turns without rubbing or binding on any of the suspension.  The only remaining issue is mounting the caliper to its purpose-built bracket.


rrdisc003.jpg (20108 bytes)
 

As the caliper bracket isn't visible from the front with the rotor installed, this photo shows the caliper mounted on its bracket from the rear.
        Oh,....



...the original MGB hard brake lines must be rerouted to align with the Saab calipers.  Luckily, the MGB master cylinder brake line fitting fits in the Saab caliper.
VOILA!  REAR DISC BRAKES ON AN MGB!  TOO EASY!!
PARK BRAKE - SQUARE 1: 

Now, for the emergency brake (oops! "parking brake") setup.

        The Saab integrates a mechanical parking brake in the front brake setup.  It's wrapped around the hub and clamps onto the caliper to push one side against the rotor.  That won't work with my set-up.

        One has to ask oneself, "For what do I need a parking brake?  To stop the car in the event of brake failure?  To ensure the car doesn't roll on an incline?"  Well, aside from the fact that parking brakes are required by most states, one can be used to stop a vehicle in the event of catastrophic brake failure or to keep a car parked on a hill or idling while in neutral from moving.  And, bracket racers can use a parking brake to lock the wheels on one axle while they apply power to the other axle.

        As dual brake systems have almost eliminated the need for a parking brake since its rare for both systems to go out at once, I decided that I want a parking brake to ensure the car doesn't move when on an incline or at the beginning of a race.  In the event of catastrophic brake failure of both systems, bury me in my MG!

        However, I must also meet Alabama guidelines concerning a parking brake:

"Section 32-5-212
Brakes.
Every motor vehicle when operated upon a highway shall be equipped with brakes adequate to control the movement of and to stop and to hold such vehicle, including two separate means of applying the brakes, each of which shall be effective to apply the brakes to at least two wheels and so constructed that no part which is liable to failure shall be common to two; except, that a motorcycle need be equipped with only one brake. All such brakes shall be maintained in good working order and shall conform to regulations not inconsistent with this section to be promulgated by the Director of Public Safety. Any person violating this section shall be guilty of a misdemeanor.

(Acts 1927, No. 347, p. 348; Code 1940, T. 36, 35.)"
 ______________________________________

          After reading the law, my daughter - the lawyer - informed me that a line lock won't work as my park brake; however, it will be a neat thing to just have for locking down the front tires as I turn up the rear ones to get a jump on Corvettes at traffic lights when they start believing their own press releases & think they're bad!

LINE LOCK:

       So, I trudged off to visit some of our local street rodders (no, not the Japanese 'buzz bomb' kids, the American muscle car guys my age!) to see what they use.  And, there, I learned more about 'line locks.'

        A 'line lock' is just a little electrical solenoid that, when activated by a button, maintains up to 3,000 pounds of hydraulic pressure on the chosen brakes (front or rear).  Most bracket racers place the 'line lock trigger' on the gear shift lever where its within easy reach of their thumb for staging out of the start lights.  They also usually integrate it into the front brakes so the rear wheels are free to start their run up without the car moving.

        So, while I'm not certain how I'm going to build a setup for my parking brake; I know I'll put a line lock in the front brake system (activated by a switch on the shifter) so I can lock them and apply power to the rear wheels for a quick take-off.  Corvettes be warned!
______________________________________

REAR PARK BRAKE:

       But, back to the parking brake issue.  Wilwood Engineering offers what it calls a 'mechanical spot caliper' for custom parking brake applications.  Wilwood describes it as:  

"The single piston floating Mechanical Spot Caliper is compact, lightweight and widely used in Parking Brake, Karting, Recreational and Light Industrial applications. Its two position, cam-actuated lever permits right or left, front or rear mounting, and accommodates two settings for leverage adjustment. The 1.2 pound Mechanical Spot fits rotors with diameters from 6.00" to 13.00", thickness from .25", and is supplied with pads installed. The caliper is available black or polished."

        The price is reasonable ($50-range) and they may be just what I'm looking for.  So, its on to a Wilwood representative for answers.

        And, pretty informative they were.  In addition to confirming that the Wilwood spot caliper would work in conjunction with the MGB parking brake cable setup.  A quick phone call to Summit Racing (1-800-230-3030) got them on order.

        Next is bracket construction. 

15 December 2005:  Okay, the brackets are finished.  We used a Pontiac Fiero bracket setup to get the angle right for the Wilwood sliding calipers.  Here are a couple of photos:

 

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        Now, I've got to slide up under the car & see how they'll interact with all the other stuff back there.

STAY TUNED!