One option for a larger, yet...
One option for a larger, yet lightweight, rotor is a scalloped piece, like this cast-iron unit from Wilwood.
Any weight added at the wheels can be difficult to swallow because it is rotating weight. Not only does it add to the overall weight of the car, but it must also be spun, significantly increasing its inertia. Also, given the same amount of weight, the farther it is from the axle centerline, the more difficult it is to spin. That's why many racers have difficulty with the idea of adding taller rotors that are also heavier. But if you consider that the entire brake and caliper assembly fits inside the wheel and tire, which can be just as heavy, you realize that an incremental increase in rotor weight and diameter does not make a significant change in the rotational inertia at each wheel.
"When considering rotors, you have to consider what they actually do," Bush adds. "One advantage of the rotor diameter is to provide leverage to the caliper to slow the wheel. A bigger rotor will give more leverage and help with braking power. The trade-off here becomes higher rotating inertia from the larger diameter and added weight. But the extra weight can be a secondary advantage. Another function of the rotor is to absorb and dissipate heat. Overall weight and cooling vane areas are the primary considerations here. For example, a 48-vane, 12-pound rotor will cool and heat cycle far better than a 32-vane, 9-pound rotor. There is a never-ending battle between trying to run the lightest possible part and having it be heavy enough to endure the event. If the rotor is too light, it may crack or become heavily grooved. Inadequate cooling capability can also allow the pads and calipers to become overheated and possibly fade. However, if a rotor is too heavy, it will certainly cool and last longer, but it can also cost a little in terms of lap times.
Of course, a lighter caliper can only shave tenths off of your lap times if it is straight. Spencer points out that another advantage of beefier rotors isn't just longer life span, but also an increased ability to stay flat. "The inertia you get rid of by going with smaller, lighter rotors can result in slower lap times," he says. "Because when the rotors become warped, they drag. This leads to loss of control, inconsistency, overheating, and modulation problems, all of which ultimately affect lap time.
"When rotors get hot, they grow," he continues. "When they cool, there is contraction. This is a constant dynamic event. If you have rotors that are too light, the lower thermal capacity will cause it to heat up much faster. That means it's going to grow and distort, or warp, much faster than a larger rotor. When that happens, the rotor will get out of round, and part of it is going to be touching the pads as it spins. The drag this produces robs horsepower and slows you down all the way around the track. You think you may be cutting your lap times with the smaller rotors, but if they're causing more drag, that can just kill your performance.
"It's easy to tell if you have this problem. Just spin the wheel when the brakes are hot in pit lane. If the calipers are true to the disc face, that wheel should spin free. But if it makes half a turn and then slows dramatically or stops, then you know you have brake drag or something else wrong. Check your rotors. If they are out of round, meaning the rotor face isn't flat, you might want to try a bigger rotor that can handle more heat."