Ahh, racing. Where horsepower is good and weight is evil. Or is it? When you think about it, it's the weight on the tires that gives us traction. In the simple analysis, the friction force (F) between the tires and track is proportional to the weight on the tires (W) and friction coefficient (u) from F= Wu. So weight is good.
However, when you try to accelerate your car, weight becomes a problem. According to Sir Newton, force equals mass x acceleration, F= ma. Written another way, acceleration is proportional to the force divided by the mass, a= F/m. Notice we don't show "weight" here, only "mass," so what's the difference?
Picture a flywheel, 10 inches in diameter and 1/2 inch thick. Also picture a shaft, 1 inch
Here are the aluminum drums next to the stock iron drums. The aluminum is used on the heat
Mass is, well, mass. It's a measure of the amount of "matter" you have (count up all the protons, neutrons, and electrons in your car next time you're bored). Weight (W) is a function of mass and the acceleration of gravity (g), so W= mg. Now as long as we race only on Earth, the gravity is fairly constant, so weight and mass are routinely used to describe the same thing. However, they're not.
If we decided to hold the "Solar System Nationals" on, say, Jupiter (ignore for the moment that Jupiter doesn't really have a "surface" per se), the gravity would be much greater. If you hadn't changed the car since the last time you ran it on Earth, the mass you have to accelerate on Jupiter would be the same, but due to the increased gravity on Jupiter, the car's weight would now be much greater on Jupiter. Therefore, traction would be greatly improved, compared to Earth. Assuming the same power levels on Jupiter (yeah, I know...maybe if you brought your own air)--since you still have the same mass to accelerate, but now with more traction from the increased weight--your car could run much quicker on Jupiter than on Earth. However, don't expect any actual drag testing on Jupiter as part of this article, since we don't have that kind of travel budget.
It's difficult to see now with the drums sandblasted (and painted), but checking out the i
Using our old fishing scale (yes, we had a life before GTO restorations consumed every min
...and in this corner, weighing in at a featherweight 8 pounds, is the Aluminum Contender.
There's more: It's not really mass that's the problem--it's "inertia," which is a function of mass. Inertia can be described as an object's resistance to acceleration, so something with inertia (i.e. anything with mass) will resist being accelerated (or decelerated for that matter). More mass equals more inertia equals more resistance to movement (acceleration). You don't see fat guys winning the 100-meter Olympic sprints, and similarly, you don't see too many successful skinny Sumo wrestlers, for whom more inertia is a distinct advantage.
So far, we've really just been talking about stuff moving in a given direction. When it comes to rotating objects, it gets a bit more complicated. For a rotating object, its "rotational inertia" is determined by not only the mass, but also where the mass is in relation to the axis of rotation. If the bulk of the mass is located far from the axis of rotation, the object will have much greater rotational inertia.
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 drums, why not do a full rebuild of the braking system?
Here's an aluminum drum being test-fit on the author's '66 GTO. Aside from having to remov
Here's our friend Lyle mounting up the M&H slicks on the rolling frame to check for any wh
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?