talked to a racing engineer yesterday, here's a summary :
The performance of a drumbrake does not hold a lineair relationship to the diameter and lining width,
Yes, larger sized drums create more friction,
more friction equals more absorption of kinetic energy,
but more friction also equals generating more heat.
Drums are very poor in loosing build up heat,
so heat fading can start occurring awfully quick inside a drum.
The drum only absorbs kinetic energy for as long as it can absorb heat.
The thinner the cast of the drum, the sooner it will become saturated with heat and brake fading sets in.
The thicker, thus heavier, the cast of the drum, the more rotational mass your car has.
So it's the same story as disc brakes here : better performance is found by means of better cooling power, creating more rotational mass.
Brake performance on trackdays :
for disks its all about
cooling capacity
for drums its all about
heat absorbing capacity
Drums at the rear on trackdays will have little to do, for when you brake, you'll be braking late thus full stopping power is used,
which means 80-90% of the weight of the car will be travelling to the front, giving the rear brakes very little inert weight to bring to a hold.
Brake bias is a result of the front and rear brakes working in harmony together : the rear brakes should only deliver brake torque after the front brakes already do so....and same story the amount of brake torque. The timegap between b/f and the relative amount of torque b/f, thats braking balance.
....
So the rear brakes do so little that high-performance "fade-resistant"- brakeshoes will not be noticeable other than in your wallet.
In fact, if the brakeshoes deliver more friction, the amount of heat increases, as a result to that the brakes will even fade sooner !
---there went my nr1 motivation for a disc conversion----
Disc brakes in the rear are mainly esthetical, unless it's needed to bring balance to the back when using ultra-fast response callipers with grippy pads/discs in the front.
---since I can't get anything through the Mot larger than 258m, non-grooved, non-perforated and OEM callipers that will not be a problem-----
To end the conclusion on which type of drum is better : it depends on it's capacity to absorb heat. It seems, but that's a guess, that if the older type drum (180mm) is wider by 27mm, it might have more mass and thus should absorb heat longer. The slightly smaller friction surface should also indicate a slower buildup of heat.
----okay, there went my plans on suffering a 180mm drum in favor of a 203mm---
Bought the guy another
and went home feeling