In the early days of sports car racing, most cars were based on stock, street vehicles. In the late 1940s through the 1950s, you could race your Jaguar, MG or Triumph after removing hub caps and taping over headlights. No race car trailers, tow rigs, race tires or safety gear. Life was simple. By the 1960s and the advent of pro racing series based on stock street cars, like the Trans Am Series, preparation was getting more sophisticated, more difficult and more expensive and the process is no simpler today.
Let’s assume you are building a car from scratch. But even if you purchase an existing car, many of the same principles will apply. Building a stock-based race car is a lot of work and requires significant commitment.
ENGINE & DRIVELINE
Let’s start with the engine and driveline. We aren’t getting into building here, but only what needs to be done. For the engine, a rebuild is critical. Even if you must run stock cams, valvetrain, pistons, etc., the engine should be blueprinted to extract both maximum performance and reliability. Replace all items prone to wear like valves, valve guides, bearings, camshaft, pistons, rings. The crankshaft should be checked and replaced if needed. It will cheaper to do this upfront rather than wait, since an engine failure can cause catastrophic and expensive damage. Be sure to replace all expendable items like spark plug wires, belts, timing chain or belt, radiator hoses, engine mounts, battery, alternator, water pump and oil pump. Make sure the radiator is flowing properly, or replace it. This is especially important in hot, summer climates.
On the driveline, the transmission should be rebuilt with new bearings and bushings. Replace gears if needed. Make sure shifting is smooth and quick. Shift linkage likely will need replacement. It sucks to DNF due to a broken shifter cable. Axles, wheel bearings and CV joints should be replaced. Third member bearings and gears should be replaced and the assembly should be blueprinted. There are two reasons for all the cost and trouble. First, reliability improves dramatically. The expense of getting to a race is considerable. Starting out with weak links in the system will lead to frustrating and possibly expensive DNFs. The second reason is power. By using new, blueprinted parts with high-grade synthetic lubricants, parasitic drag will be reduced considerably, which is like adding horsepower.
The next issue is weight. Yes, of course spec classes — most racing classes in fact — have a minimum weight. That’s a good thing, but just because your racecar is very close to the minimum weight rule, you still have a lot of work to do. Being complacent about weight means a midpack finish. If you are above minimum weight, you will never be competitive.
When I had the great pleasure of working with Smokey Yunick at Circle Track magazine, one evening over dinner, we were discussing the effect of weight on lap times. As always, Smokey had an answer, which was accurate because he tested it. On a half-mile paved oval track, with a late-model stock car ( 2,700 pounds, 600 horsepower), adding 50 pounds cost a half second a lap. Lap times without the additional weight were in the mid 18-second range. With an extra 50 pounds, lap times increased to the low 19-second range. Imagine what an extra 50 pounds would do to lap times on a 2.5-mile road course where times are in the 1:40 range.
But wait! There’s more! While being at minimum weight is crucial, where that weight rests in the car is even more important. If you can remove 100 pounds of weight beyond the minimum weight rule from the body structure, especially from the top half of the car, then place ballast as low as possible, you will lower the center of gravity by a finite amount. You can also position the weight fore and aft and left to right to improve static weight distribution. Why is that important? To fully understand this would require a course in race car dynamics, but here is the 3-by-5 card version.
The center of gravity is the single point in a vehicle where the car would balance if suspended from that point. The center of gravity has a distance above the ground, and lateral and longitudinal points within the wheelbase and track width of the car. This is important because the center of gravity location not only determines static weight distribution, but also dynamic weight distribution, which is determined by how much weight is transferred and where it goes during cornering, braking and acceleration. The higher the center of gravity is above ground, the more weight is transferred forward while braking, rearward while accelerating and laterally while cornering. The dynamic weight distribution has a major effect on tire traction.
Simply stated, less weight transfer means more overall tire traction. But it is also important to get the lateral and longitudinal weight distribution as close to ideal as possible. It can be difficult to get a 50/50 left to right weight distribution on a sports car or sedan with the driver offset, usually to the left, but it is worth the effort to do so. A 50/50 lateral weight distribution allows equal traction in left and right turns, and provides better brake balance under at-the-limit braking conditions. Keep in mind that the weight on a tire is proportional to tire traction, but as weight increases, tire traction increases at a slower rate. This means a net loss of traction anytime weight is transferred. For example, if a car has 45 percent right side weight — always weigh a car and determine weight distribution with the driver’s weight on board — and 5 percent of the weight is transferred laterally while cornering, then the car will have 50 percent dynamic weight in left turns, but a 40 percent to 60 percent right to left distribution in right turns. While traction improves significantly in left turns, it decreases even more in right turns meaning that overall, there is a net loss in cornering speed (traction) around the entire track and that results in slower lap times. The obvious exception is on an oval track where left weight bias is desirable, at least to point.
Optimum longitudinal weight distribution depends on the location of the drive wheels. Rear-drive cars are best in the 52 percent to 56 percent rear weight range, with higher horsepower cars benefiting from higher rear weight percentages. Front-drive cars often have well over 60 percent front static weight. A front distribution of 55 percent is more desirable, with higher power cars benefiting from more front weight bias. However, more weight to the front will hurt braking performance, so finding the best compromise is difficult at best, and often nearly impossible on a spec-class car, front or rear drive. All-wheel-drive cars should be near 50/50 weight distribution, with slightly more rear weight as power goes up.
The only way to change the location of the center of gravity is to physically move weight in the car. An approximation of the height of the center of gravity above ground is the centerline of the engine crankshaft. So one way to lower the center of gravity is run the lowest possible ride height without any part of the car bottoming out on the race track at speed. This has other possible benefits including improved camber change on independent suspensions.
Since getting the car as light as possible is a primary goal, let’s look at ways to do that. But first, study the rules to see what is allowed and what components can be modified or replaced with aftermarket parts. Again, follow the rules. If it is not specified as allowed, it probably is not allowed.
- Trim excess interior sheet metal
- Remove unnecessary wiring, gauges and instruments
- Remove heaters, cores and associated parts
- Replace the battery with a lighter, smaller one
- Remove or trim non-structural panels
- Remove unnecessary exterior trim
- Run lighter wheels if allowed
- Remove all undercoating and sound-deadening
- Sandblast all body panels and interior surfaces, repaint with a minimum of material
- Replace side windows with Lexan if allowed
- Remove unnecessary locks and window mechanisms
- If a passenger seat is required, replace with a lightweight seat if allowed
- Replace non-critical fasteners with lighter fasteners
- Make sure all hardware is high quality, the correct size and that bolts are just long enough to capture the nut with minimal threads protruding from the nut
Now that the car is as light as possible, ideally you will be adding weight to the car. Be sure to have the driver in place or the driver’s weight in the driver’s seat so that you get the most accurate corner weights. With the car on scales, start placing components in locations that improve weight distribution. If you have to add weight, and ballast is not allowed, add roll cage bracing where the weight will improve weight distribution. Keep extra bars low and place them as far to the passenger side of the car as possible. Under no circumstances should you compromise safety. Well-conceived bar placement should improve the structural integrity of the roll cage and improve safety. A byproduct of this is an increase structural rigidity of the platform, which will reduce chassis flex also. Chassis flex makes the whole car an undamped spring which is very difficult to tune. In the 1970s, I drove a formula car with a monocoque chassis that flexed so much that increasing spring rates had no effect on the handling.
Other items in the car can be relocated. Can the battery be moved to the trunk? Can you move fuel lines to the passenger side of the car, and can they be routed through tubing as a safety feature? Can the fuel tank be moved toward the passenger side and to the rear of the car? Even a half inch can make a difference. Look at your car. You can find many more places to remove weight or move items for better weight distribution.
The suspension is the most important area of the car for improved performance. Any item that can be replaced with aftermarket racing parts should be. All items in the suspension and steering – ball joints, tie rod ends, bushings, wheel bearings, U-joints, CV joints, driveshafts should all be replaced or at least rebuilt when stock items must be used. The steering box should be rebuilt. There should be no free play in the steering, and bump steer should be eliminated. Bump steer occurs when the steering tie-rod geometry does not match the suspension geometry causing one tire to go into toe-in or toe-out over one-wheel bumps, which results in unwanted steering inputs and speed-robbing tire scrub.
It is critical to use the correct spring rates for your car, your experience and track conditions. Most often, the tendency is to use springs that are too stiff for optimum tire contact control. We’ll cover this in a later issue of Speed News. The job of the spring includes allowing the tires to conform to the racing surface over bumps and keeping the chassis from bottoming over bumps. Erring on the soft side makes the car easier to drive at the limits of tire adhesion.
Since most stock and spec classes have ride height limitations, you want a spring that allows the lowest ride height possible. But if there are no restrictions, you need a spring that first, is soft enough to allow the tire contact patch to stay on the racing surface and second, stiff enough to keep the suspension from bottoming out the shock absorbers and the chassis from bottoming on the race track.
If you are allowed to run adjustable ride height coilover springs/shocks or struts, do so. You have more spring rate options and the adjustable ride heights will make setting corner weights infinitely easier. You will also have adjustable shock valving, another bonus.
Shocks must be tuned for the specifics of you car – spring rates, track surface smoothness and handling characteristics. Shocks are very important. They need to be the correct length and shaft travel for your situation. The shock absorber shaft should be near the mid-point of shaft travel when the car is at ride height. If the shock is too compressed at ride height, the shock could bottom out, causing internal damage. A solid, progressive-rate bump stop is a big help. Keep in mind that shocks affect tire traction by controlling the springs over bumps — high-speed valving — and by affecting how fast weight is transferred during the initial transitional phases of cornering braking and accelerating. Tuning shocks is an art and a science. It pays to make friends with a shock company representative.
Many spec classes use a specific suspension package with non-adjustable antiroll bars. But if allowed, having an adjustable antiroll bar — at least one — is very important. The first job of the antiroll bar is to limit chassis roll. Chassis roll is detrimental due the associated camber change. Any camber change due to body roll requires negative camber settings, and that means that while chassis roll is taking place, not all of the tire contact patch is on the track surface. The more negative camber being used, the more of a problem this becomes, making transitional handling a challenge.
The second job of the antiroll bar is to fine tune roll-couple distribution, or the amount of weight being transferred during cornering at the front relative to the rear. A stiffer bar at one end means more weight transfer at the other end. If a car understeers, a stiffer rear bar or softer front bar will reduce, eliminate understeer or cause oversteer. Having one bar with adjusting holes or an adjustable slider makes fine tuning handling balance much simpler.
One criterion: solid. Solid bushings, preferably rod ends or heim joints, but at least metal, are best. Solid, hard plastic bushings like Delrin, work well but wear faster. The goal is to minimize compliance in the suspension. When a bushing can be compressed, suspension settings must compensate. But even then, there is a vague feel to the car until all the compression has occurred. The more the bushing can compress, the greater the time needed for compression and the more vague the car feels. Once the compression is maxed out, the car has taken a set and feedback to the driver is greatly improved.
The braking system is a crucial part of the overall package. If you are allowed to upgrade brakes, do so. If not, check them often and change pads, rotors, calipers and fluid as needed. And that could be often. When I drove a Camaro in the Firestone Firehawk endurance series, we had to use stock brakes. We had to replace everything after every race. Often, the brakes would be marginal by the end of three-hour event. At Mazda Raceway Laguna Seca, I suffered complete brake failure with less than five minutes remaining in a three-hour race, ending up in a sand trap at the end of the straightaway. And I was nursing the brakes.
The important elements for a well-engineered brake system include rotor size, pad area, caliper piston configuration, area and number, master cylinder sizes and either a brake bias valve that can be adjusted, or a dual-master-cylinder setup with a balance bar. Also, brake fluid is critical. Brake fluid is hygroscopic, meaning that it absorbs water vapor quickly from the atmosphere. Use small cans of fluid and keep them tightly sealed when not using the can. When water vapor is absorbed into the fluid, the boiling point of the fluid drops quickly. When the fluid boils, the brake pedal will at first get spongy, then go to the floor with little or no braking force. It is easier to purge brake lines and master cylinders with new fluid than getting into the fence and watching the checkered flag from a sand trap.
We’ve discussed weight saving and the cockpit is prime area for that. But there are other considerations as well. Seat mounting is a safety and comfort factor for the driver. Seats or seat rails need rigid mounting points for security. Never bolt a seat to a sheet metal floor pan. This is not an area to skimp on. Follow the seat manufacturer’s recommendations. Safety restraints are another crucial area. Mounts must be anchored and aligned properly. Follow the restraint manufacturer’s directions or look in the NASA rule book for proper mounting procedures.
Gauges, controls, mirrors and switches are important considerations. Make sure the driver can see and/or access all controls, etc. when strapped into the driver seat. Not being able to reach something like a fire bottle actuator or ignition switch can lead to serious problems. Interior mirrors should be adjustable from the driver seat if possible. Everything in the cockpit should be securely mounted so that in a crash, nothing in the cockpit can become a projectile.
Obviously, building a production-based racecar is a major commitment. We have hit many of the highlights, but there is still more to consider. Here is one closing thought about using professionals for the heavy lifting: I know how to weld, but I am far from a professional. One of my standing rules is never drive a car with a roll cage or suspension where I did any welding. Obviously, no one else should drive it either. If you have the skills, great. If not let pros do the critical stuff.