Composite rear drums?

[JTK]

My 2007 Cobalt is spec'd as having those on the back. For a minute I actually thought the things were plastic when I poked at them through the rim holes!?

I'm thinking it's just the flat back paint on them giving that appearance. From what I can tell, composite drums are a steel hub pressed/hot fitted into a cast drum? Would that be correct?

Joel

 

[Kestas]

Sounds right. Steel has better strength and corrosion resistance. Gray cast iron is a better brake surface. The small flakes of graphite in the cast iron matrix provide noise dampening.

I'm not sure, but perhaps the steel is inserted into the mold before the molten iron is poured.

Here's what a typical micrograph of grey iron looks like: http://www.doitpoms.ac.uk/miclib/micrograph.php?id=379

 

[74DartSport]

Back in the 80's GM used aluminum/iron drums. I wonder why that fell by the wayside. Quickie article.

 

[Kestas]

I'm guessing it's because aluminum exacerbated the brake fade problem that is characteristic with drum brakes. As I understand it, the brake fade came from the change in drum diameter as it heated up. This diameter growth resulted in a poor contact pattern with the shoes. Aluminum drums would grow even more (2.2X) than cast iron.

 

[johnd]

Now that I think about it, I don't know exactly what causes brake fade but I'm pretty sure it has little, if anything, to do with drum diameter. I am old enough to have grown up in the drum brake area and aluminum drums were touted as being not only lighter weight but dissipating heat better and were therefore more resistant to fade. They were typically on cars for "performance" applications. I remember the factory manual for my 1962 Pontiac Catalina listed them as an option.

 

[JTK]

Like Kestas says, I could see the whole machined braking surface of the drum not expanding at the same rate, giving you less or an erratic surface area to stop with. Then again... Why do they still use big - - - - - - drum brakes in tucks, busses, heavy-duty stuff?? There must be something good about them?

Joel