|The MGA With An Attitude
To UPGRADE, or Not To Upgrade? Disc Brakes - BT-200A
On 7/21/2011, Mike Coe in Calgary, Alberta, Canada wrote:
"I'm restoring a 1956 ZB Magnette that I'd like to convert to front disc brakes.
I have the Green Adapter, but need all the other bits off an MGA. Some MGB parts will work though I understand. Is a four pot conversion available, or does one stay with the standard"?
In the end this page may fall under the category of "Can I talk you out of it"? This is not a cut and dried answer. It is a matter of personal choice, and a hell of a lot of people don't do it.
I have never driven a Magnette with drum brakes, let alone one with the disc brake conversion. I have driven MGAs both ways. For single shot stopping power the drum brakes work just fine, capable of locking up the wheels at any speed. The primary advantage of disc brakes is fade resistance with frequent use at high speed. I can imagine driving a Magnette may be something like driving my MGA with a 400 pound trailer attached. I do that a lot, and my trailer has over 100,000 miles on it, but I still run all drum brakes.
I have only encountered fade on my drum brakes on two occasions. Once was when running down hill on a twisty mountain road with lots of repetitive braking (while towing the luggage trailer). Solution to fade is of course to slow down and use more downshifting for engine braking. How much this bothers you may depend on how often you drive in lots of hills.
The other time was during a brisk road rally on all flat roads, chip rock surface, a 90 degree turn every mile for several miles, and trying to average 52 mph. The drill was to take corners at highest speed possible, race through the gears at full throttle and high engine speed up to about 60 or 65 mph, then stand on the brakes to make the next turn. This is the toughest abuse you can give the brakes without the assistance of down hill grade. After 3 or 4 hard braking streaks the drum brakes began to fade. By the 5th time they were getting uncomfortably mushy, and needed a bit of personal restraint. This required at least 5 minutes of harshest thrashing in the MGA, and it only happened once or twice in the past 25 years.
How much heat you put into the brakes on a single stop depends primarily on vehicle weight and top speed, as all of the energy of inertia must be turned into heat in the brakes. How much heat you put into the brakes on successive stops is limited by engine power, as there will be no more energy in the vehicle than can be input by the engine. If you do the same drill in the heavier Magnette with similar engine and similar brakes, it will be slower to accellerate and have lower top speed in the straights. Taking longer to get to the next corner means the engine will put more energy into the car, so the brakes will work harder to bring the speed down. While the Magnette could make 4 miles and 4 corners the MGA might make 5 miles and 5 corners. Total engine energy input and braking energy extraction may be about the same in the same time frame. So if you work at it hard enough you may be able to make the brakes fade.
Question is how often will it happen, and is it worth the effort to do the disc brake conversion? So far I haven't bothered to convert my MGA.1500, primarily because it never has brake fade during serious autocross competition (short timed runs).
From what chat I hear from Magnette owners, the desirable recipe is 1800 engine, lower final drive ratio, overdrive or 5-speed, and disc brakes. I still don't know how important disc brakes are on a Magnette, but that's what lots of people seem to want. Perhaps if I drove one for a while I'd want it too (or maybe not). Drum brakes have one advantage with less unsprung weight, which may be significant on a rough road.
I know the Magnette has tube shocks and a different upper control arm. Otherwise I think similar lower A-arm and coil springs (slightly thicker). The front brake backing plate has different orientation, but otherwise I think it uses the same brake parts as MGA 1500. Not sure what the king pin or swivel pin looks like, but guess it may be same or similar to the MGA. If the upper and lower trunnions are same as MGA, then the process of conversion to disc brakes would be nearly identical. There is plenty of that information on my web site.
It is very easy to convert MGA from drum to disc brakes, because the car was produced in both models. 99% of it is just bolt up the brake parts from the 1600 car into the 1500 chassis. The only part that needs tinkering is the frame bracket for the hose, and the mating brake pipe with different end fittings.
For some odd reason many people want to install MGB calipers on the MGA. This has no particular advantage, as they both have same cylinder bore, similar pads, and same braking effect. You can remove the guide pin from the MGA caliper and use the cheaper MGB pistons if cost is an issue. Trying to source cheaper MGB calipers as used parts is a lost cause, because then you need an expensive pair of special adapter plates to mount the calipers. There is a difference in offset of the brake disc between A and B, running in different plane.
Some folks solution to this issue is to install MGB kingpins and knuckles and all MGB parts outboard from there. That is not necessarily cheap, unless you might have a spare MGB parts car lying around. The MGB top trunnion is narrower, requiring a couple of shaft spacers to mate to the MGA upper arm. Otherwise it is possible to use MGB shock absorbers on the MGA by turning them around backward, installing the arms upside down, and modifying mounting of the rebound bumpers on the frame. The farther you go down the road to using MGB parts the more twisted it gets. Perhaps with the tube shocks on the Magnette you may only need a different upper control arm (or maybe not?).
One advantage to using MGB brake parts in the MGA is the availability of a wider variety of upgrade parts (speed shop and hot rod type stuff). You can then find alloy calipers which have a nice weight advantage, 4-pot calipers if you like, and any kind of brake pads. I don't know if you can find ventilated aluminum rotors, but before we go there I should ask how much money you want to spend on brakes?
A word of caution is due here. If you do anything at all to increase braking force on the front wheels you will screw up braking balance. The front wheels will then lock up before the rear brakes get up to full force, with the result being overall longer stopping distance. So lots of money spent on brakes can result in worse brakes if you don't do it right. Same thing can happen of you upgrade rear brakes. If the rear wheels lock up before front brakes are up to full force, you're just screwed. Then you then spend more money to install a proportioning valve to reduce rear braking force back to original level, offsetting any advantage from all the money spent on upgrades. The other way of managing mismatched braking balance front to rear is to install two brake master cylinders with an adjustable balance bar in between.
The only good way to upgrade brakes with more braking force is to increase braking force the same percentage at both ends of the car to maintain proper braking balance. If successful, that should result in lower pedal force requirement. You can get there very easily by installing a power brake booster. However, reducing pedal pressure does absolutely nothing to change the interface between tires and road, or the cause and effect stuff going on with the real working brake parts just inboard from the wheels.
Bottom line is, the best upgrade for brakes should somehow eliminate brake fade with frequently repeated braking. To that end, disc brakes may be a noble pursuit. Entirely fade free brakes brings to mind MGA Twin Cam all wheel disc brakes by Dunlop. Those have iron calipers, solid iron rotors (not vented), and single piston both sides, nothing particularly fancy. In other words, MG had the solution to this more than 50 years ago, so why are so many people trying to reinvent the wheel (or re-engineer the brakes)? Sometimes good enough really is good enough, and anything "better" is a waste of time and money.
If you can achieve fade free brakes, then you might consider reducing pedal pressure for personal comfort (power booster). If you want to stop in shorter distance, try larger and/or stickier tires. You wouldn't believe how quick my MGA will stop on hard pavement with all standard drum brakes and wide sticky race tires (and heavy enough foot).