Full Cast or Composite Brake Drums... pros/cons

[Evan Frucht]

I'm looking into getting new brake drums.

not only do I have different brands to choose from but also I have a choice between "full cast brake drum" which is I assume is machined from one solid piece of metal, OR they also sell "composite" brake drums. same price, same brand. What's the deal?

thoughts?

also as a side note, how much does having cooling fins on a brake drum increase its cooling ability? is it so insignificant that it wont even matter?

 

[Evan Frucht]

Someone must know something I don't, but the info on this comparison is basically non existent in my searches online. Probably because most cars don't have the choice or option...

I think composite drums are supposed to be lighter (sort of guessing here,) but how do they compare in durabiliy... which one is more likely to warp or crack, for instance.

 

[Fran Blacker]

Someone must know something I don't, but the info on this comparison is basically non existent in my searches online. Probably because most cars don't have the choice or option...

I think composite drums are supposed to be lighter (sort of guessing here,) but how do they compare in durabiliy... which one is more likely to warp or crack, for instance.

Can you post a link to composite drums. Composite rotors heard of them on high end cars.

 

[Evan Frucht]

https://www.rockauto.com/en/catalog...318cid+v8,1240894,brake+&+wheel+hub,drum,1744

Can you post a link to composite drums. Composite rotors heard of them on high end cars.

link shows both composite and solid options

I went through the specs for each and they are the same, except for the fact of "full cast" or "composite"

the photos shows the difference

 

[69L48Z27]

Here’s what the internet says. I’ll buy it.

A composite brake drum for a motor vehicle or the like is comprised of a cast metal drum section, a stamped steel drum back, and a cast metal mounting face. ... The composite drum of the invention provides a reduction in weight and in the amount of machining required as compared with a full cast brake drum.

 

[69Bee]

From a machinist point of view, and someone who has turned craploads of both kinds, get the full cast drum. A full cast drum is going to be stronger than one that was cast onto a stamped steel shell. The composite drums and rotors require extra fixtures to prevent vibrations due to the thinness of the hub and dissimilar metals. The full cast rotor/drum only require the silence bands for vibrations, as well as the composite ones. I believe that it was '69 that Charger (and others) that had the 4 piston calipers with 2 piece (composite) rotors that were known to come apart.

Now, after the explanation between the two, so here is what's up. With the exception of front drums, I can't really remember when rear Mopar drums from mid '60s to '70 up were not composite. The cast drums on the rear might not allow enough stud to go thru the drum hub, whereas it will be fine with the composite hub. You don't mention what year, car, axle, or front/rear for that matter, so... What I have seen normally is cast front because of the hub, and composite rear. Hope that makes things a little clearer, instead of less.

 

[Photon440]

also as a side note, how much does having cooling fins on a brake drum increase its cooling ability? is it so insignificant that it wont even matter?

The extra surface area does help to shed heat, also the extra metal provides some measure of heat sink ability.

 

[Evan Frucht]

Here’s what the internet says. I’ll buy it.

A composite brake drum for a motor vehicle or the like is comprised of a cast metal drum section, a stamped steel drum back, and a cast metal mounting face. ... The composite drum of the invention provides a reduction in weight and in the amount of machining required as compared with a full cast brake drum.

Thanks. I gathered they are lighter and cheaper to produce... but there is no information online comparing them to the "full cast" type... as far as quality goes at least.

 

[BeepBeepRR]

 

[goodolboydws]

I'm looking into getting new brake drums.

not only do I have different brands to choose from but also I have a choice between "full cast brake drum" which is I assume is machined from one solid piece of metal, OR they also sell "composite" brake drums. same price, same brand. What's the deal?

thoughts?

also as a side note, how much does having cooling fins on a brake drum increase its cooling ability? is it so insignificant that it wont even matter?

Here's a link to one company that is making the kind that are STEEL with cast iron.

They use a steel cylinder exterior and cast iron into that-for the braking surface, Its then spun, to centrifically fill the mold cavities. Then machined..This brand says theirs are are 10-15% lighter and stronger than all cast iron, which seems plausible.
Personally, I'd think having an all steel exterior would at least make a less brittle piece than being all cast iron, but that's just me.
If they're good enough for big rigs and certain auto makers to be using them.....
https://www.durabrake.com/steel-shell-composite-brake-drum-breakthrough-in-pricing/

 

[goodolboydws]

Evan Fruct started this thread with a post including a question about whether cooling fins are effective or not, and I have data from a just-finished, complete brake job.

(I have to admit that this involved FORD brake drums on a '89 Crown Vic., but the procedure and results are applicable to other cast iron and composite brake drums)

Having 2 brake drums (one resurfaced original*, and one new composite) that I cleaned of rust and washed with solvent, and then coated with heat resistant paint, (POR15 high temp aluminum) I had to then HEAT these 2 drums to over 300F for at least 15 minutes for the paint to achieve its maximum durability.
Instead of using an open flame propane souce outside I ended up using a much more controllable heat source-our propane kitchen oven (which my wife surprisingly agreed to).
With the drums on open racks, and using a laser aimed thermometer "gun" for spot temperature accuracy in addition to the oven's controls, I had to raise the oven temperature to OVER 400F and let it stabilize at that temperature as measured on the drum center plates in order for the cooling fins to reach 300F, as the fins were shedding that much heat. The temperature slowly decreased as the the distance from the centerplate increased until it reached the fin area but it dropped most of the way in the fin area itself. This was in spite of the the newer drum having a 1" wide x aprox 7/16" thick solid rim, AND FEWER BUT THICKER FINS while the OEM drum had a deep "C" cross section rim edge.
*the OEM 1989 FORD brake drum was also composite.

 

[Photon440]

Fins shouldn't be shedding heat, they should be transferring it. If it's 400F in the oven, the extra surface area of the fins should be absorbing the heat and helping the drum to heat up faster.

 

[MoparLeo]

Think about what items ae still made of cast iron today and what are the properties they have that make them still relevant today.
Lets see...some cooking utensils. Is it for their strength or for their heat retaining properties ? With the exception of some big trucks, brake drums are not used very much today but steel rotors are universal...The things learned in the last 50 years about metallurgy and thermodynamics should help you with the answer. Heat has always been the enemy regardless of what material or system you are talking about. The goal is to cool it down, whether it is coolant, oil, brake fluid, trans fluid, bearings, tires, brakes, etc... Like someone pointd out already, the composite drum was used through out the seventies on rear drums because the wheel supported the face of the drums against the axle/hub so less material was needed and weight/balance/strength was easier to achieve with the lighter, thinner materials.

 

[1967coronet]

Interesting subject , I recall Buick in the 50s using a cast aluminum drum with a steel lining.
Popular with the hot rod crowd.