are rear disc brakes so much heavier than drums?
I don't understand why having more braking capability is a 'problem', apart from additional unsprung weight?
Most of the braking in a car (or a bike) is performed by the front wheels. This is because under brakes there is a major transfer of vehicle weight to the front. This is why the front of a vehicle dips and the rear rises. If you stamp on the brakes, as in panic, there is a tendency for the front wheels to lock up because the weight of the vehicle has not had time to transfer and create the downward forces on the tyres for effective braking. Under correct controlled braking the brakes are applied progressively with increasing force as weight transfer increases the grip of the front tyres.
Because of the reduced weight on the rear tyres as the back of the car tends to lift you require less braking forces at the rear or you would lock up the rear wheels. Manufacturers go to a great deal of effort to get this balance between front and rear wheels correct. Usually smaller disks and callipers are used in all disk systems or less powerful rear brakes in all drum brake systems. Many light vehicles use disks at the front and the less powerful drum brakes at the rear. Drums also require lower hydraulic pressure and have the advantage of being very easy to use for handbrake systems.
Race cars often have a driver controlled proportioning valve to effectively limit the rear braking forces.
To return to the Morgan: It has been said that for road use rear discs would be so lightly used that if a car was only used on relatively short trips there may not be enough brake use to completely clear corrosion from the rear disks. The end result could be the build up of corroded patches on the rear disks that would eventually lead to uneven braking.
With a light weight car such as the Morgan rear drums are perfectly adequate for road use and cost less that rear disks (which would also need a separate handbrake system). For road use it is difficult to immagine the circumstances under which you would ever get any fade from the rear brakes. If you doubt me try touching the rear drums after a period of driving on a mountain road. I have and I can assure you that they do not get hot enough to cause fade.
Improved front brakes would probably be the first thing I would consider. Unless you constantly drive in a spirited manner
or intend to get involved in competition, perhaps the rear discs on the AR P4 are more of a fashion statement than a necessity.
Just my idea
