Picture a thin flywheel and a long shaft. Let's say both the flywheel and the shaft have the same mass. You might then think they both have the same rotational inertia, but they don't. Because the mass of the shaft is located closer to the axis of rotation than the mass of the flywheel, the shaft will have significantly less rotational inertia (assuming you're spinning the shaft about its longitudinal axis, and not trying to twirl it like a baton, in which case it's a whole different story). In fact, the rotational inertia for a shaft is in proportion to the square of the diameter. Therefore, as the mass gets farther from the center of rotation, the rotational inertia increases significantly, hence the shape of a flywheel: thin, but big in diameter so as to locate most of the mass far from the rotational axis, thus maximizing rotational inertia for a given mass of flywheel.
If you're trying to spin something faster (rotational acceleration) at the same time you're trying to haul that same something in a given direction (linear acceleration), now BOTH the linear AND rotational inertias of that part combine to resist the accelerations, and therefore, more power is required to move the rotating part than a similar non-rotating part.
 Before installing the new...  Before installing the new drums, why not do a full rebuild of the braking system? |  Here's an aluminum drum being...  Here's an aluminum drum being test-fit on the author's '66 GTO. Aside from having to remove the wheels first, this could hardly even be considered a "bolt-on," since the aluminum drums just slip right on in place of the factory iron boat anchors. Of course, we cleaned and paint the alloy drums the factory red (for Rally I wheels), to stealth-ify our new-found speed secret. |  Here's our friend Lyle mounting...  Here's our friend Lyle mounting up the M&H slicks on the rolling frame to check for any wheel interference with the aluminum drums. The only issue we found is the possible need for slightly longer wheel studs, due to the thicker mounting flanges of the aluminum drums. Since our GTO will see regular strip duty (eventually), the addition of aftermarket axles (with longer wheel studs) in our future 12-bolt rearend will solve this issue. |
Now, why the physics (and astronomy) lesson? So we all understand why less inertia (not necessarily less weight) is good for auto racing, and reducing inertia smartly, by reducing the mass in key locations like rotating parts, can give more bang for the buck than just a general mass reduction. In other words, if you reduce the "remote" mass on a rotating part, like tread on a tire, your car will gain a greater acceleration potential than if you simply reduced the mass on a non-rotating part like, ahem, a driver.
Now, the truly hard-core wrecking yard hunter may know that GM built aluminium rear brake drums on selected vehicles (a partial list for wrecking yard hunting would include: '80-84 Buick LeSabre, '78-81 Century, '78-87 Regal, '81-85 Riviera, '82-92 Chevy Camaro, '86-89 Impala, '78-81 Malibu, '78-88 Monte Carlo, '80-85 Olds Delta 88, '78-82 Cutlass, '80-81 and '83-85 Pontiac Bonneville, '86 Parisienne, '82-92 Firebird, '78-85 LeMans , and some other applications). For those early A-, G-, or F-body Pontiacs with 9.5-inch rear drums, these later-model aluminium drums slip right on in place of the stock, heavy iron units! Being lazy, errr, efficient, we easily found a set of these aluminium drums for $50, after a few minutes of casual hunting on eBay.
Before you start thinking, "How on earth does an aluminium drum survive against the heat and friction of the brake shoes?" relax. The GM drums still use a cast-iron liner (to rub against the brake shoes). Where does the aluminium come in? At the axle flange mounting face and heat sink cast around the perimeter of the drums, so the brake drum heat sink mass reduction occurs at the most optimum location: the farthest point from the axis of rotation. This of course gives the greatest rotational inertia reduction.
How much mass savings are the aluminium drums worth? Our old '66 GTO iron brake drums weighed in at over 13 pounds each. The aluminium drums only pulled the scale to 8 pounds each. Total reduction for 2 drums= 10 pounds. It's not a huge amount, but definitely worth while, since the mass was reduced where it best reduces the rotating inertia of the drums. You might view the 5-pounds-per-drum mass reduction as equivalent to taking 5 pounds out of each rear wheel. If you're already running a flyweight Bogart or Monocoque wheel, how much would you pay for 5-pound-lighter wheels?
We never actually got the chance to do back-to-back drag tests between the iron and aluminium drums (or Earth and Jupiter), but our understanding of physics (and logic) says that the lightweight, low-inertia drums is a no-lose situation for racing. Just for fun, we ran an analysis on our handy computer, which predicted an e.t. reduction of a couple hundredths and mph increase of about .2 mph. That's not a huge amount, but how often are races won or lost by a hundredth? Every little bit helps, right?