When it comes to the track, overweight cars are as useful as a glass hammer. When you're talking about building a car to nail just one sizzling hot lap time, weight is everything. Unlike your average supermodel, you really can't have a time attack car that's too light. That was why we decided to turn the reasonably light 1995 R-package Miata into a featherweight time attack car.
Extra weight means slower lap times. Heavy cars require more force to accelerate, more tire to hang on in the corners and generate added momentum for brakes to deal with. Heavier cars need heavier wheels, tires, brakes and more power adders, which means more weight. It's a vicious cycle.
Instead of adding more mass to make an already heavy car perform, it's much easier to gut what you have, easing the burden on your existing hardware. With no power steering or windows, Project Time Attack Miata is a start, but the diet we had in mind meant stripping all but the most track-functional parts.
If you plan to compete, it's not as easy as calling the boys over and breaking out a 24-pack and the acetylene torch. Figuring out the class structure and acceptable reductions of the sanctioning body you'll be competing with might make the difference between racing in a class of tube-frame prototypes or full-interior Evos.
Our goal is to be fast, not pretty. We decided to spend just $9.90 painting our Racing Beat nose. This way we are a little less afraid of the inevitable off-track excursion during competition.
Most groups will move you into a faster class when you start removing weight from different parts of your car. Replacing doors with lightweight carbon fiber copies may land you in a faster class, while removing that same weight from your spare and jack may not. Different sanctioning bodies govern time attacks by power-to-weight ratios or points accrued from various modifications.
In NASA's Time Trials Series, Project Miata would fit somewhere in its TTS group for cars with a power-to-weight ratio of 8.1:1 down to 5.5:1. Assuming the car weighs 2300 pounds, we would have to be making over 400 wheel-hp to be kicked up to the unlimited class (TTU).
Unsprung weightFew variables affect acceleration, braking, turning, and part wear as much as unsprung weight. Unsprung weight is the mass of all items not supported by the suspension: wheels, tires, uprights, brake calipers and rotors, wheel bearings, and-if you want to get technical-some of the suspension, axles, control arms and any other part that is partially supported by the chassis on one side and connected to the wheels on the other. Yes, that includes brake lines and brake fluid.
While there are plenty of theories regarding the effects of removing unsprung versus sprung weight, it's generally accepted that eliminating unsprung weight is more effective. Reducing weight under each spring contributes to faster lap times by positively influencing such factors as rotational inertia, gyroscopic motion and how fast each corner of the car can react to road inputs and stay planted.
Rotational InertiaAs much as the physical weight of each unsprung corner matters, rotational inertia of wheels, tires or any rotating mass adds to the difficulty of accelerating and decelerating. Imagine you have a 10-speed bicycle with its rear wheel free-spinning. When its tire is filled with air, it's easier to get it turning or stopped than that same wheel with its tire filled with lead. This rotational inertia, or moment of inertia, is affected by the wheel's overall mass and even more by how far out on the radius that mass is distributed. A five-pound disk has less rotational inertia than a five-pound hoop of the same diameter. Even if you keep the weight the same, rotational inertia exponentially increases as the diameter increases. This concept holds true for anything being rotated about a given axis. As tires are the mass furthest out on a wheel, their weight has the biggest effect on the overall rotational inertia. On track or on the street, people often obsess over having ultra-light wheels without giving a thought to the weight of their tires.
The new Work wheels tuck nicely into the flares with the 205-series tire.When we go wider, we have to trim the flare a little more.
Brake rotors have the next biggest impact on rotational inertia. Even though some big brake upgrades may actually weigh less than cast OE components, larger rotors usually mean more rotational inertia. Two-piece rotors with thin steel or aluminum hats help compensate for the weight and moment of inertia generated by the larger parts, but mass at the radius ultimately matters most. This delicate balance of weight, rotational inertia and brake torque will be addressed in a future installment.
Making wheel and tire choicesThe vehicle formerly known as Project Miata rode on abnormally wide 245/45R15 Hoosier bias ply race tires. As Project Time Attack, it will ride on DOT radials to avoid points penalties during NASA and other time trial competitions. We had to find the right balance between grip, size and weight for the car's current characteristics. Problem is, Miatas came with donut-size wheels and wheel wells only a wee bit bigger. So finding a tire small yet wide enough was a dilemma. For now, the widest DOT tires we can run in the correct diameter are a 225/45R15.In addition to our desire to limit rotational inertia with the smallest wheels possible, plans for larger brakes affected our wheel and tire selection. To clear the brake upgrade we had in mind, the wheels had to be at least 15 inches in diameter. Instead of picking the wheel size, we started by finding all our favorite R-Compound tires that come in 15-inch sizes.
The limited selection made this pretty easy. We went for the Nitto NT-01 tire on account of its tread design, price and our past experience. Key advantages include the huge tread blocks and smooth radiused transitions between tread blocks and grooves that reduce squirm and scaling. Its compound also endures prolonged track temperatures while being hard enough to drive to and from the track without incurring excessive wear. Until the car requires more grip, we're going with a set of 205/50R15 Nitto NT01s.Most Miata track cars use 6.5- or 7-inch width wheels with 13- to 15-inch diameters. Since most of the weight of the wheel is in the hoop, as it is furthest out on the radius, the width of the hoop will contribute most to rotational inertia.Narrow wheels use less material in the hoops, which takes much of the weight out at the farthest radial distance. This comes at the cost of reducing tire width and contact patch. Stuffing wide tires onto narrow wheels isn't the answer either; pooched-out sidewalls would cause rollover issues, sloppy turn-in response and a deformed contact patch.
Finding a 15-inch wheel to fit a 205 tire isn't easy. A seven-inch width wheel is ideal. We knew our tire requirements would soon increase, so we looked for a 15x8 wheel anyway. With such a width, we had to make sure we could mount the wheels without touching the fenders on the outside and, more importantly, the control arms on the inside-fenders are easier to cut than control arms.
With our new wheels, the tire would contact the fender. Yes, we know the NT01 is mounted inside out. | 
It took us drawing straws to see who would make the first cut in the unfortunately mint rear fenders to accommodate the new Work wheels. | 
The cuts we made pushed the fender edge up to the top of the tire well. Some silicone and rivets, and they will be weatherproof. |