Good to see some more brake discussion. the people talking about boiling expand-
ing stuff OUT of the system are a worry though! Fluids (ie liquids ) are used in
brakesystems cos they are incompressible - when the stuff boils , it becomes, at
least partly, gaseous - which is compressible, and under load, your pedal will
go to the floor, not push back! The problems with drums were that when the
lining material has a high enough co-efficient of friction to be useful without
power assist, as the temperature rises, the co-efficient of friction drops way
off - also persisent heating can permanently ruin the material's ability to
recover, and it's stuffed. Also as the diameter of the drum goes up, the effects
of expansion due to heat get worse in terms of distance away from the shoe, and
no amount of pumping will get the shoe to stay in contact and do useful work.
I dont think you will find a drum brake much bigger than 11" anywhere - that
was the biggest ROLLS get before they went discs, and they were stopping a lot
of iron ! as others noted, it is easier for discs to lose heat than shrouded
drums inside wheels, and drums could also get distorted due to loads on the
wheels. I have never seen a drum damaged by corrosion ( I did a year in a BUSY
brake workshop once), but i have seen drums parted from their faces by crooked
shoes cutting clean through.
the major benefit of discs is that it costs less to stop the same amount of mass from the same speeds- the disc is cheap to make mostly (maybe venteds are $$$)
and the caliper is very simple - the seals are just square section rubber,
compared to the seals inside drum cylinders.
As i said once before I have only once in 30 years driving been conned into
having disc machined, and that didn't work anyway. BTW why do the poms tend
to stick with solid disc rotors, and others go vented???
