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.
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.
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.
E-production class Miata race cars have been using zero-offset wheels for a long time with great success, so we decided to look for a wheel with a +15mm offset, since we will ultimately run tires that are 20mm wider. The increased scrub radius of this more positive offset will put more load on the wheel bearings, but we're hoping the trade-off for extra track will improve lateral grip.
There are few 15x8 wheels on the market, even fewer in the offset we wanted. Work Wheels said it could make what we were looking for. Its history of building custom-sized endurance wheels convinced us we'd be getting a light and durable wheel.
After what seemed like a long wait, we received our Meister S1s, beautiful works of art that weigh almost 16 pounds a pop. A little surprising, but not a deal-breaker. Seeing as we've bent our share of unproven super-light wheels in our off-line lawn-mowing excursions, we're happy to stick with Work's products.
Even with all the measuring and hunting for offset-specific wheels, we still ended up with fender clearance issues. To cover up our artistic Sawzall expressionism, fender flares from a small Japanese boutique called Tuckin '99 were added. Tuckin '99, like many other boutique Japanese motorsports suppliers, built its business around a diehard set of Miata fans and club racers, and isn't really interested in scaling its business over the web. Parts are made in small runs and sold by word of mouth. As SCC is all about JDM, we purchased a set from Adrenaline Racing, in the JDM center of the world, Lake Oswego, Oregon.
Sprung weightTaking out sprung weight is far easier and subject to fewer compromises. It's also easier to remove and relocate this type of weight. Big-budget race teams will make everything as light as possible before adding weight back (in the form of lead bars in the floorboards) to satisfy the minimum regulations-all for the sake of better weight distribution. Since the Miata is already relatively well balanced, we opted against such extreme measures and focused on removing as much weight as we could from the edges to reduce the total moment of inertia.
Every removable body panel was pulled off to access the non-essential crap underneath: bumper supports, windows and crash supports, latches, and wiring for stuff we had long since thrown away. Axis Power Racing (APR) created a set of carbon fiber-wrapped Nomex honeycomb-cored replacement body panels-substantially lighter than the stock pieces-to shed further pounds. APR uses a vacuum infusion process to suck out as much excess resin as possible, minimizing weight. Unlike race car chassis components, these pieces are not baked in an autoclave for added rigidity since the panels we replaced are non-structural elements. The honeycomb adds enough rigidity to make the pieces more than just cosmetic and also allowed us to avoid the autoclave, significantly reducing cost.
The pieces were molded directly from the stock panels, but the hood was modified with a vent placed directly behind the radiator to evacuate hot air from the engine bay. This will be a big factor once we turbocharge the car and install various other heat exchangers in the nose. The front fenders were lined and reinforced with Kevlar to save the panels from internal damage due to road debris being thrown up by the tires. The only panels left stock were the rear quarter panels, which are part of the factory unibody. We sourced a Racing Beat Type 2 fiberglass nose to replace the factory piece as it saves a little weight, but also has added cooling ducts.
At 10 pounds per side, the MkI Miata pop-up headlights have a significant impact on polar moment. A time attack car still needs lights, but we didn't want the weight. Advanced Automotive Concepts has a fixed-mount headlight kit that shaves over 12 pounds. Other unnecessary parts (including the soft top, airbags, interior bits, seats, seat belts, stock battery and dash guts) found the dumpster. The stock side mirrors were replaced with carbon pieces sourced from APR Performance and the rear glass in the hardtop was chucked in favor of a Lexan replacement. Our final weight came down to 2018 pounds with a full tank of gas. Not the lightest Miata we've seen, but necessary weight (such as the cage) makes up the difference. Just in exterior body parts replaced, we've shed 154 pounds.
Now that Project Time Attack has spent some quality time with Jenny Craig, it's time for the 'roids. In our next installment, Project Time Attack will receive what is likely the best power-to-weight ratio of any of the monstrosities to come out of our garage.
| WELCOME TO WEIGHT WATCHERS | |||
| Items Removed | Weight (lbs) | Items Added | Weight (lbs) |
| Air conditioning | 36 | APR hard top | 9 |
| Aftermarket soft top and frame | 39 | APR hood | 9 |
| Passenger seat | 34 | APR doors (x2) | 19 |
| Driver seat | 34 | APR fenders (x2) | 2 |
| Spare tire and jack | 27 | APR trunk lid | 6 |
| Doors | 99 | APR bumper cover | 2 |
| 2 front fenders | 12 | Racing Beat nose | 11 |
| Trunk lid | 14 | Headlights | 8 |
| Rear bumper cover | 8 | APR airbag cover | 0 |
| Front bumper | 10 | APR Performance mirrors | 0.5 |
| Hood | 17 | Work Equip Meister S1 wheels (x4) | 62 |
| Wiper assembly | 5 | Nitto NT-01 205/50/15s tires (x4) | 80 |
| Headlights | 20 | Racetech seat | 22 |
| Airbag assembly | 8.5 | AWR cage | 82 |
| Stereo | 4.5 | KW coilovers | 34 |
| Carpets and interior | 4 | Odyssey PC680 | 15.5 |
| Front tow hooks | 3.5 | Total | 362 |
| Seat belt assembly (x2) | 10.5 | ||
| Intake assembly | 3 | ||
| Dash lightening | 28 | ||
| Stock mirrors | 1.5 | ||
| Dampers and springs (x4) | 45 | ||
| Battery | 21.5 | ||
| Steel wheels and tires | 152 | ||
| Total | 637 | ||
Photo Gallery: Project 1995 Mazda Miata Part 2 - Sport Compact Car
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