Music For The Masses
One of these days, I intend to start a band. It will presumably fall on one of those days after I learn to play an instrument (which also has no defined date attached to it). But eventually I'll be forced to do it because I already have a name picked out: Mass Moment of Inertia. It's one of those names only a few people will get, kind of like The Fratellis. The first time I heard that name, I recognized it immediately as the antagonist family of bank robbers in one of my favorite movies, The Goonies.
Most people will think it's some commentary on how society doesn't react to problems, or something way more cerebral than I can come up with. The mechanical geeks, like myself, will immediately realize the true and intended meaning. Mass moment of inertia represents the distance of an object's center of mass from a rotational axis. It's not likely to intrigue the public at large, but it's an extremely useful concept for car enthusiasts.
Mass moment of inertia is calculated by multiplying an object's center of mass by the square of the distance to its axis of rotation. If that sounds a lot like torque, that's because it is. Like torque, it's all about leverage, or what's called the 'moment arm.' The longer the wrench, the easier it is to turn a bolt. The same holds true for everything that rotates-except think of it from the bolt's point of view.
When it comes to cars, the things we think of most often in these terms are wheels. We all know that larger wheels are generally heavier, but they also position the mass further out from the axis of rotation, making it harder to accelerate and decelerate. But this isn't an versized-wheel-bashing rant. When it comes down to it, the same negative thing can be said about brake rotors.
Many enthusiasts agree that there's no such thing as too much brake on a car. In reality, more harm than good can be done with some of these giant brake kits. Most sports cars have no use for 15-inch rotors. They will never generate the kind of heat necessary to justify the performance sacrifice. Brake manufacturers do what they can to reduce weight with lighter calipers and aluminum hats, but in the end, all the mass is that iron rotor spinning in that giant radius. This is obviously why carbon-carbon and carbon-ceramic rotors are so attractive. They take the mass out of the most crucial part of the system. I'm not saying all aftermarket kits are overkill; most are truly advantageous. But to truly maximize braking performance, you should go with the smallest rotors you can get away with.
The concept of 'moments of inertia' doesn't just apply to things on a car that spin. Think about the entire car itself. We've all heard that, generally speaking, mid-engine vehicles corner better. We've all noticed the shorter bumper overhangs-and have you opened a new car's hood lately? It seems like the vehicle's front quarter is just empty space.
Trying to rotate a car is the same as spinning a wheel or brake disc around an axle. The further the mass is situated from the axis of rotation, the more difficult it is to rotate. Think of a barbell compared to a dumbbell of equal weight. Hold a 25-pound dumbbell and rotate your wrist back and forth... fairly easy. Do the same thing with a 25-pound barbell and it's a different story. It's tougher to get it rotating and also much more difficult to send it in the opposite direction. Same amount of weight, just a higher moment of inertia.
It's the same with a car. If all the weight is spread out at the ends, it becomes harder to rotate in a turn. It also explains why a poorly driven rear-engine car will fling itself off the outside of a turn like a flying hammer (Like a Flying Hammer... that will be my first album). The downside to a mid-engine car is that it will rotate fast, so once it starts to spin, it can all go bad very quickly. There's a reason why NSXs and Boxsters aren't tearing up the drift circuit.
Hopefully this will give a better understanding of why your trusted tuner is trying to talk you out of that plus-four wheel fitment you want so badly. If not, when you're in a bar with your buddies and my band's new hit single is played, you can be the cool guy with the story behind our name. If nothing else, maybe I can at least get you a free beer.